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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package jsr166e; |
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|
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import java.io.Serializable; |
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import java.util.Collection; |
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import java.util.List; |
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import java.util.RandomAccess; |
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import java.lang.ref.WeakReference; |
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import java.lang.ref.ReferenceQueue; |
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import java.util.concurrent.Callable; |
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import java.util.concurrent.CancellationException; |
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import java.util.concurrent.ExecutionException; |
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import java.util.concurrent.Future; |
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import java.util.concurrent.RejectedExecutionException; |
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import java.util.concurrent.RunnableFuture; |
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import java.util.concurrent.TimeUnit; |
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import java.util.concurrent.TimeoutException; |
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import java.util.concurrent.locks.ReentrantLock; |
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import java.lang.reflect.Constructor; |
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|
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/** |
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* Abstract base class for tasks that run within a {@link ForkJoinPool}. |
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* A {@code ForkJoinTask} is a thread-like entity that is much |
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* lighter weight than a normal thread. Huge numbers of tasks and |
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* subtasks may be hosted by a small number of actual threads in a |
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* ForkJoinPool, at the price of some usage limitations. |
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* |
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* <p>A "main" {@code ForkJoinTask} begins execution when it is |
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* explicitly submitted to a {@link ForkJoinPool}, or, if not already |
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* engaged in a ForkJoin computation, commenced in the {@link |
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* ForkJoinPool#commonPool} via {@link #fork}, {@link #invoke}, or |
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* related methods. Once started, it will usually in turn start other |
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* subtasks. As indicated by the name of this class, many programs |
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* using {@code ForkJoinTask} employ only methods {@link #fork} and |
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* {@link #join}, or derivatives such as {@link |
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* #invokeAll(ForkJoinTask...) invokeAll}. However, this class also |
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* provides a number of other methods that can come into play in |
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* advanced usages, as well as extension mechanics that allow support |
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* of new forms of fork/join processing. |
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* |
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* <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}. |
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* The efficiency of {@code ForkJoinTask}s stems from a set of |
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* restrictions (that are only partially statically enforceable) |
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* reflecting their main use as computational tasks calculating pure |
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* functions or operating on purely isolated objects. The primary |
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* coordination mechanisms are {@link #fork}, that arranges |
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* asynchronous execution, and {@link #join}, that doesn't proceed |
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* until the task's result has been computed. Computations should |
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* ideally avoid {@code synchronized} methods or blocks, and should |
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* minimize other blocking synchronization apart from joining other |
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* tasks or using synchronizers such as Phasers that are advertised to |
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* cooperate with fork/join scheduling. Subdividable tasks should also |
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* not perform blocking IO, and should ideally access variables that |
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* are completely independent of those accessed by other running |
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* tasks. These guidelines are loosely enforced by not permitting |
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* checked exceptions such as {@code IOExceptions} to be |
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* thrown. However, computations may still encounter unchecked |
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* exceptions, that are rethrown to callers attempting to join |
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* them. These exceptions may additionally include {@link |
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* RejectedExecutionException} stemming from internal resource |
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* exhaustion, such as failure to allocate internal task |
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* queues. Rethrown exceptions behave in the same way as regular |
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* exceptions, but, when possible, contain stack traces (as displayed |
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* for example using {@code ex.printStackTrace()}) of both the thread |
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* that initiated the computation as well as the thread actually |
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* encountering the exception; minimally only the latter. |
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* |
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* <p>It is possible to define and use ForkJoinTasks that may block, |
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* but doing do requires three further considerations: (1) Completion |
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* of few if any <em>other</em> tasks should be dependent on a task |
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* that blocks on external synchronization or IO. Event-style async |
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* tasks that are never joined (for example, those subclassing {@link |
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* CountedCompleter}) often fall into this category. (2) To minimize |
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* resource impact, tasks should be small; ideally performing only the |
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* (possibly) blocking action. (3) Unless the {@link |
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* ForkJoinPool.ManagedBlocker} API is used, or the number of possibly |
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* blocked tasks is known to be less than the pool's {@link |
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* ForkJoinPool#getParallelism} level, the pool cannot guarantee that |
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* enough threads will be available to ensure progress or good |
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* performance. |
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* |
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* <p>The primary method for awaiting completion and extracting |
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* results of a task is {@link #join}, but there are several variants: |
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* The {@link Future#get} methods support interruptible and/or timed |
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* waits for completion and report results using {@code Future} |
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* conventions. Method {@link #invoke} is semantically |
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* equivalent to {@code fork(); join()} but always attempts to begin |
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* execution in the current thread. The "<em>quiet</em>" forms of |
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* these methods do not extract results or report exceptions. These |
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* may be useful when a set of tasks are being executed, and you need |
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* to delay processing of results or exceptions until all complete. |
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* Method {@code invokeAll} (available in multiple versions) |
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* performs the most common form of parallel invocation: forking a set |
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* of tasks and joining them all. |
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* |
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* <p>In the most typical usages, a fork-join pair act like a call |
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* (fork) and return (join) from a parallel recursive function. As is |
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* the case with other forms of recursive calls, returns (joins) |
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* should be performed innermost-first. For example, {@code a.fork(); |
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* b.fork(); b.join(); a.join();} is likely to be substantially more |
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* efficient than joining {@code a} before {@code b}. |
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* |
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* <p>The execution status of tasks may be queried at several levels |
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* of detail: {@link #isDone} is true if a task completed in any way |
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* (including the case where a task was cancelled without executing); |
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* {@link #isCompletedNormally} is true if a task completed without |
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* cancellation or encountering an exception; {@link #isCancelled} is |
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* true if the task was cancelled (in which case {@link #getException} |
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* returns a {@link java.util.concurrent.CancellationException}); and |
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* {@link #isCompletedAbnormally} is true if a task was either |
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* cancelled or encountered an exception, in which case {@link |
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* #getException} will return either the encountered exception or |
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* {@link java.util.concurrent.CancellationException}. |
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* |
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* <p>The ForkJoinTask class is not usually directly subclassed. |
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* Instead, you subclass one of the abstract classes that support a |
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* particular style of fork/join processing, typically {@link |
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* RecursiveAction} for most computations that do not return results, |
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* {@link RecursiveTask} for those that do, and {@link |
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* CountedCompleter} for those in which completed actions trigger |
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* other actions. Normally, a concrete ForkJoinTask subclass declares |
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* fields comprising its parameters, established in a constructor, and |
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* then defines a {@code compute} method that somehow uses the control |
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* methods supplied by this base class. |
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* |
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* <p>Method {@link #join} and its variants are appropriate for use |
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* only when completion dependencies are acyclic; that is, the |
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* parallel computation can be described as a directed acyclic graph |
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* (DAG). Otherwise, executions may encounter a form of deadlock as |
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* tasks cyclically wait for each other. However, this framework |
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* supports other methods and techniques (for example the use of |
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* {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that |
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* may be of use in constructing custom subclasses for problems that |
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* are not statically structured as DAGs. To support such usages a |
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* ForkJoinTask may be atomically <em>tagged</em> with a {@code short} |
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* value using {@link #setForkJoinTaskTag} or {@link |
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* #compareAndSetForkJoinTaskTag} and checked using {@link |
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* #getForkJoinTaskTag}. The ForkJoinTask implementation does not use |
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* these {@code protected} methods or tags for any purpose, but they |
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* may be of use in the construction of specialized subclasses. For |
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* example, parallel graph traversals can use the supplied methods to |
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* avoid revisiting nodes/tasks that have already been processed. |
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* (Method names for tagging are bulky in part to encourage definition |
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* of methods that reflect their usage patterns.) |
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* |
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* <p>Most base support methods are {@code final}, to prevent |
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* overriding of implementations that are intrinsically tied to the |
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* underlying lightweight task scheduling framework. Developers |
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* creating new basic styles of fork/join processing should minimally |
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* implement {@code protected} methods {@link #exec}, {@link |
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* #setRawResult}, and {@link #getRawResult}, while also introducing |
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* an abstract computational method that can be implemented in its |
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* subclasses, possibly relying on other {@code protected} methods |
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* provided by this class. |
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* |
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* <p>ForkJoinTasks should perform relatively small amounts of |
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* computation. Large tasks should be split into smaller subtasks, |
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* usually via recursive decomposition. As a very rough rule of thumb, |
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* a task should perform more than 100 and less than 10000 basic |
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* computational steps, and should avoid indefinite looping. If tasks |
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* are too big, then parallelism cannot improve throughput. If too |
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* small, then memory and internal task maintenance overhead may |
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* overwhelm processing. |
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* |
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* <p>This class provides {@code adapt} methods for {@link Runnable} |
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* and {@link Callable}, that may be of use when mixing execution of |
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* {@code ForkJoinTasks} with other kinds of tasks. When all tasks are |
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* of this form, consider using a pool constructed in <em>asyncMode</em>. |
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* |
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* <p>ForkJoinTasks are {@code Serializable}, which enables them to be |
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* used in extensions such as remote execution frameworks. It is |
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* sensible to serialize tasks only before or after, but not during, |
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* execution. Serialization is not relied on during execution itself. |
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* |
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* @since 1.7 |
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* @author Doug Lea |
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*/ |
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public abstract class ForkJoinTask<V> implements Future<V>, Serializable { |
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|
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/* |
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* See the internal documentation of class ForkJoinPool for a |
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* general implementation overview. ForkJoinTasks are mainly |
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* responsible for maintaining their "status" field amidst relays |
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* to methods in ForkJoinWorkerThread and ForkJoinPool. |
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* |
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* The methods of this class are more-or-less layered into |
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* (1) basic status maintenance |
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* (2) execution and awaiting completion |
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* (3) user-level methods that additionally report results. |
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* This is sometimes hard to see because this file orders exported |
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* methods in a way that flows well in javadocs. |
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*/ |
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|
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/* |
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* The status field holds run control status bits packed into a |
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* single int to minimize footprint and to ensure atomicity (via |
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* CAS). Status is initially zero, and takes on nonnegative |
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* values until completed, upon which status (anded with |
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* DONE_MASK) holds value NORMAL, CANCELLED, or EXCEPTIONAL. Tasks |
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* undergoing blocking waits by other threads have the SIGNAL bit |
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* set. Completion of a stolen task with SIGNAL set awakens any |
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* waiters via notifyAll. Even though suboptimal for some |
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* purposes, we use basic builtin wait/notify to take advantage of |
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* "monitor inflation" in JVMs that we would otherwise need to |
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* emulate to avoid adding further per-task bookkeeping overhead. |
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* We want these monitors to be "fat", i.e., not use biasing or |
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* thin-lock techniques, so use some odd coding idioms that tend |
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* to avoid them, mainly by arranging that every synchronized |
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* block performs a wait, notifyAll or both. |
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* |
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* These control bits occupy only (some of) the upper half (16 |
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* bits) of status field. The lower bits are used for user-defined |
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* tags. |
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*/ |
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|
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/** The run status of this task */ |
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volatile int status; // accessed directly by pool and workers |
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static final int DONE_MASK = 0xf0000000; // mask out non-completion bits |
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static final int NORMAL = 0xf0000000; // must be negative |
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static final int CANCELLED = 0xc0000000; // must be < NORMAL |
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static final int EXCEPTIONAL = 0x80000000; // must be < CANCELLED |
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static final int SIGNAL = 0x00010000; // must be >= 1 << 16 |
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static final int SMASK = 0x0000ffff; // short bits for tags |
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|
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/** |
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* Marks completion and wakes up threads waiting to join this |
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* task. |
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* |
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* @param completion one of NORMAL, CANCELLED, EXCEPTIONAL |
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* @return completion status on exit |
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*/ |
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private int setCompletion(int completion) { |
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for (int s;;) { |
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if ((s = status) < 0) |
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return s; |
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if (U.compareAndSwapInt(this, STATUS, s, s | completion)) { |
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if ((s >>> 16) != 0) |
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synchronized (this) { notifyAll(); } |
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return completion; |
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} |
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} |
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} |
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|
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/** |
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* Primary execution method for stolen tasks. Unless done, calls |
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* exec and records status if completed, but doesn't wait for |
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* completion otherwise. |
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* |
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* @return status on exit from this method |
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*/ |
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final int doExec() { |
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int s; boolean completed; |
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if ((s = status) >= 0) { |
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try { |
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completed = exec(); |
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} catch (Throwable rex) { |
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return setExceptionalCompletion(rex); |
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} |
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if (completed) |
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s = setCompletion(NORMAL); |
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} |
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return s; |
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} |
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|
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/** |
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* Tries to set SIGNAL status unless already completed. Used by |
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* ForkJoinPool. Other variants are directly incorporated into |
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* externalAwaitDone etc. |
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* |
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* @return true if successful |
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*/ |
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final boolean trySetSignal() { |
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int s = status; |
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return s >= 0 && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL); |
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} |
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|
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/** |
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* Blocks a non-worker-thread until completion. |
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* @return status upon completion |
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*/ |
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private int externalAwaitDone() { |
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int s; |
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boolean interrupted = false; |
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if ((s = status) >= 0 && ForkJoinPool.tryUnsubmitFromCommonPool(this)) |
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s = doExec(); |
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while (s >= 0) { |
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if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) { |
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synchronized (this) { |
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if (status >= 0) { |
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try { |
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wait(); |
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} catch (InterruptedException ie) { |
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interrupted = true; |
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} |
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} |
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else |
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notifyAll(); |
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} |
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} |
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s = status; |
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} |
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if (interrupted) |
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Thread.currentThread().interrupt(); |
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return s; |
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} |
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|
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/** |
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* Blocks a non-worker-thread until completion or interruption. |
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*/ |
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private int externalInterruptibleAwaitDone() throws InterruptedException { |
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if (Thread.interrupted()) |
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throw new InterruptedException(); |
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int s; |
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if ((s = status) >= 0 && ForkJoinPool.tryUnsubmitFromCommonPool(this)) |
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s = doExec(); |
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while (s >= 0) { |
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if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) { |
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synchronized (this) { |
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if (status >= 0) |
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wait(); |
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else |
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notifyAll(); |
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} |
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} |
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s = status; |
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} |
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return s; |
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} |
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|
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/** |
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* Implementation for join, get, quietlyJoin. Directly handles |
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* only cases of already-completed, external wait, and |
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* unfork+exec. Others are relayed to ForkJoinPool.awaitJoin. |
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* |
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* @return status upon completion |
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*/ |
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private int doJoin() { |
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int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w; |
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return (s = status) < 0 ? s : |
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((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? |
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(w = (wt = (ForkJoinWorkerThread)t).workQueue). |
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tryUnpush(this) && (s = doExec()) < 0 ? s : |
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wt.pool.awaitJoin(w, this) : |
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externalAwaitDone(); |
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} |
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|
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/** |
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* Implementation for invoke, quietlyInvoke. |
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* |
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* @return status upon completion |
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*/ |
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private int doInvoke() { |
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int s; Thread t; ForkJoinWorkerThread wt; |
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return (s = doExec()) < 0 ? s : |
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((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? |
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(wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, this) : |
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externalAwaitDone(); |
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} |
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|
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// Exception table support |
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|
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/** |
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* Table of exceptions thrown by tasks, to enable reporting by |
369 |
* callers. Because exceptions are rare, we don't directly keep |
370 |
* them with task objects, but instead use a weak ref table. Note |
371 |
* that cancellation exceptions don't appear in the table, but are |
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* instead recorded as status values. |
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* |
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* Note: These statics are initialized below in static block. |
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*/ |
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private static final ExceptionNode[] exceptionTable; |
377 |
private static final ReentrantLock exceptionTableLock; |
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private static final ReferenceQueue<Object> exceptionTableRefQueue; |
379 |
|
380 |
/** |
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* Fixed capacity for exceptionTable. |
382 |
*/ |
383 |
private static final int EXCEPTION_MAP_CAPACITY = 32; |
384 |
|
385 |
/** |
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* Key-value nodes for exception table. The chained hash table |
387 |
* uses identity comparisons, full locking, and weak references |
388 |
* for keys. The table has a fixed capacity because it only |
389 |
* maintains task exceptions long enough for joiners to access |
390 |
* them, so should never become very large for sustained |
391 |
* periods. However, since we do not know when the last joiner |
392 |
* completes, we must use weak references and expunge them. We do |
393 |
* so on each operation (hence full locking). Also, some thread in |
394 |
* any ForkJoinPool will call helpExpungeStaleExceptions when its |
395 |
* pool becomes isQuiescent. |
396 |
*/ |
397 |
static final class ExceptionNode extends WeakReference<ForkJoinTask<?>> { |
398 |
final Throwable ex; |
399 |
ExceptionNode next; |
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final long thrower; // use id not ref to avoid weak cycles |
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ExceptionNode(ForkJoinTask<?> task, Throwable ex, ExceptionNode next) { |
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super(task, exceptionTableRefQueue); |
403 |
this.ex = ex; |
404 |
this.next = next; |
405 |
this.thrower = Thread.currentThread().getId(); |
406 |
} |
407 |
} |
408 |
|
409 |
/** |
410 |
* Records exception and sets status. |
411 |
* |
412 |
* @return status on exit |
413 |
*/ |
414 |
final int recordExceptionalCompletion(Throwable ex) { |
415 |
int s; |
416 |
if ((s = status) >= 0) { |
417 |
int h = System.identityHashCode(this); |
418 |
final ReentrantLock lock = exceptionTableLock; |
419 |
lock.lock(); |
420 |
try { |
421 |
expungeStaleExceptions(); |
422 |
ExceptionNode[] t = exceptionTable; |
423 |
int i = h & (t.length - 1); |
424 |
for (ExceptionNode e = t[i]; ; e = e.next) { |
425 |
if (e == null) { |
426 |
t[i] = new ExceptionNode(this, ex, t[i]); |
427 |
break; |
428 |
} |
429 |
if (e.get() == this) // already present |
430 |
break; |
431 |
} |
432 |
} finally { |
433 |
lock.unlock(); |
434 |
} |
435 |
s = setCompletion(EXCEPTIONAL); |
436 |
} |
437 |
return s; |
438 |
} |
439 |
|
440 |
/** |
441 |
* Records exception and possibly propagates |
442 |
* |
443 |
* @return status on exit |
444 |
*/ |
445 |
private int setExceptionalCompletion(Throwable ex) { |
446 |
int s = recordExceptionalCompletion(ex); |
447 |
if ((s & DONE_MASK) == EXCEPTIONAL) |
448 |
internalPropagateException(ex); |
449 |
return s; |
450 |
} |
451 |
|
452 |
/** |
453 |
* Hook for exception propagation support for tasks with completers. |
454 |
*/ |
455 |
void internalPropagateException(Throwable ex) { |
456 |
} |
457 |
|
458 |
/** |
459 |
* Cancels, ignoring any exceptions thrown by cancel. Used during |
460 |
* worker and pool shutdown. Cancel is spec'ed not to throw any |
461 |
* exceptions, but if it does anyway, we have no recourse during |
462 |
* shutdown, so guard against this case. |
463 |
*/ |
464 |
static final void cancelIgnoringExceptions(ForkJoinTask<?> t) { |
465 |
if (t != null && t.status >= 0) { |
466 |
try { |
467 |
t.cancel(false); |
468 |
} catch (Throwable ignore) { |
469 |
} |
470 |
} |
471 |
} |
472 |
|
473 |
/** |
474 |
* Removes exception node and clears status |
475 |
*/ |
476 |
private void clearExceptionalCompletion() { |
477 |
int h = System.identityHashCode(this); |
478 |
final ReentrantLock lock = exceptionTableLock; |
479 |
lock.lock(); |
480 |
try { |
481 |
ExceptionNode[] t = exceptionTable; |
482 |
int i = h & (t.length - 1); |
483 |
ExceptionNode e = t[i]; |
484 |
ExceptionNode pred = null; |
485 |
while (e != null) { |
486 |
ExceptionNode next = e.next; |
487 |
if (e.get() == this) { |
488 |
if (pred == null) |
489 |
t[i] = next; |
490 |
else |
491 |
pred.next = next; |
492 |
break; |
493 |
} |
494 |
pred = e; |
495 |
e = next; |
496 |
} |
497 |
expungeStaleExceptions(); |
498 |
status = 0; |
499 |
} finally { |
500 |
lock.unlock(); |
501 |
} |
502 |
} |
503 |
|
504 |
/** |
505 |
* Returns a rethrowable exception for the given task, if |
506 |
* available. To provide accurate stack traces, if the exception |
507 |
* was not thrown by the current thread, we try to create a new |
508 |
* exception of the same type as the one thrown, but with the |
509 |
* recorded exception as its cause. If there is no such |
510 |
* constructor, we instead try to use a no-arg constructor, |
511 |
* followed by initCause, to the same effect. If none of these |
512 |
* apply, or any fail due to other exceptions, we return the |
513 |
* recorded exception, which is still correct, although it may |
514 |
* contain a misleading stack trace. |
515 |
* |
516 |
* @return the exception, or null if none |
517 |
*/ |
518 |
private Throwable getThrowableException() { |
519 |
if ((status & DONE_MASK) != EXCEPTIONAL) |
520 |
return null; |
521 |
int h = System.identityHashCode(this); |
522 |
ExceptionNode e; |
523 |
final ReentrantLock lock = exceptionTableLock; |
524 |
lock.lock(); |
525 |
try { |
526 |
expungeStaleExceptions(); |
527 |
ExceptionNode[] t = exceptionTable; |
528 |
e = t[h & (t.length - 1)]; |
529 |
while (e != null && e.get() != this) |
530 |
e = e.next; |
531 |
} finally { |
532 |
lock.unlock(); |
533 |
} |
534 |
Throwable ex; |
535 |
if (e == null || (ex = e.ex) == null) |
536 |
return null; |
537 |
if (false && e.thrower != Thread.currentThread().getId()) { |
538 |
Class<? extends Throwable> ec = ex.getClass(); |
539 |
try { |
540 |
Constructor<?> noArgCtor = null; |
541 |
Constructor<?>[] cs = ec.getConstructors();// public ctors only |
542 |
for (int i = 0; i < cs.length; ++i) { |
543 |
Constructor<?> c = cs[i]; |
544 |
Class<?>[] ps = c.getParameterTypes(); |
545 |
if (ps.length == 0) |
546 |
noArgCtor = c; |
547 |
else if (ps.length == 1 && ps[0] == Throwable.class) |
548 |
return (Throwable)(c.newInstance(ex)); |
549 |
} |
550 |
if (noArgCtor != null) { |
551 |
Throwable wx = (Throwable)(noArgCtor.newInstance()); |
552 |
wx.initCause(ex); |
553 |
return wx; |
554 |
} |
555 |
} catch (Exception ignore) { |
556 |
} |
557 |
} |
558 |
return ex; |
559 |
} |
560 |
|
561 |
/** |
562 |
* Poll stale refs and remove them. Call only while holding lock. |
563 |
*/ |
564 |
private static void expungeStaleExceptions() { |
565 |
for (Object x; (x = exceptionTableRefQueue.poll()) != null;) { |
566 |
if (x instanceof ExceptionNode) { |
567 |
ForkJoinTask<?> key = ((ExceptionNode)x).get(); |
568 |
ExceptionNode[] t = exceptionTable; |
569 |
int i = System.identityHashCode(key) & (t.length - 1); |
570 |
ExceptionNode e = t[i]; |
571 |
ExceptionNode pred = null; |
572 |
while (e != null) { |
573 |
ExceptionNode next = e.next; |
574 |
if (e == x) { |
575 |
if (pred == null) |
576 |
t[i] = next; |
577 |
else |
578 |
pred.next = next; |
579 |
break; |
580 |
} |
581 |
pred = e; |
582 |
e = next; |
583 |
} |
584 |
} |
585 |
} |
586 |
} |
587 |
|
588 |
/** |
589 |
* If lock is available, poll stale refs and remove them. |
590 |
* Called from ForkJoinPool when pools become quiescent. |
591 |
*/ |
592 |
static final void helpExpungeStaleExceptions() { |
593 |
final ReentrantLock lock = exceptionTableLock; |
594 |
if (lock.tryLock()) { |
595 |
try { |
596 |
expungeStaleExceptions(); |
597 |
} finally { |
598 |
lock.unlock(); |
599 |
} |
600 |
} |
601 |
} |
602 |
|
603 |
/** |
604 |
* A version of "sneaky throw" to relay exceptions |
605 |
*/ |
606 |
static void rethrow(final Throwable ex) { |
607 |
if (ex != null) { |
608 |
if (ex instanceof Error) |
609 |
throw (Error)ex; |
610 |
if (ex instanceof RuntimeException) |
611 |
throw (RuntimeException)ex; |
612 |
throw uncheckedThrowable(ex, RuntimeException.class); |
613 |
} |
614 |
} |
615 |
|
616 |
/** |
617 |
* The sneaky part of sneaky throw, relying on generics |
618 |
* limitations to evade compiler complaints about rethrowing |
619 |
* unchecked exceptions |
620 |
*/ |
621 |
@SuppressWarnings("unchecked") static <T extends Throwable> |
622 |
T uncheckedThrowable(final Throwable t, final Class<T> c) { |
623 |
return (T)t; // rely on vacuous cast |
624 |
} |
625 |
|
626 |
/** |
627 |
* Throws exception, if any, associated with the given status. |
628 |
*/ |
629 |
private void reportException(int s) { |
630 |
if (s == CANCELLED) |
631 |
throw new CancellationException(); |
632 |
if (s == EXCEPTIONAL) |
633 |
rethrow(getThrowableException()); |
634 |
} |
635 |
|
636 |
// public methods |
637 |
|
638 |
/** |
639 |
* Arranges to asynchronously execute this task in the pool the |
640 |
* current task is running in, if applicable, or using the {@link |
641 |
* ForkJoinPool#commonPool} if not {@link #inForkJoinPool}. While |
642 |
* it is not necessarily enforced, it is a usage error to fork a |
643 |
* task more than once unless it has completed and been |
644 |
* reinitialized. Subsequent modifications to the state of this |
645 |
* task or any data it operates on are not necessarily |
646 |
* consistently observable by any thread other than the one |
647 |
* executing it unless preceded by a call to {@link #join} or |
648 |
* related methods, or a call to {@link #isDone} returning {@code |
649 |
* true}. |
650 |
* |
651 |
* @return {@code this}, to simplify usage |
652 |
*/ |
653 |
public final ForkJoinTask<V> fork() { |
654 |
Thread t; |
655 |
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) |
656 |
((ForkJoinWorkerThread)t).workQueue.push(this); |
657 |
else |
658 |
ForkJoinPool.submitToCommonPool(this); |
659 |
return this; |
660 |
} |
661 |
|
662 |
/** |
663 |
* Returns the result of the computation when it {@link #isDone is |
664 |
* done}. This method differs from {@link #get()} in that |
665 |
* abnormal completion results in {@code RuntimeException} or |
666 |
* {@code Error}, not {@code ExecutionException}, and that |
667 |
* interrupts of the calling thread do <em>not</em> cause the |
668 |
* method to abruptly return by throwing {@code |
669 |
* InterruptedException}. |
670 |
* |
671 |
* @return the computed result |
672 |
*/ |
673 |
public final V join() { |
674 |
int s; |
675 |
if ((s = doJoin() & DONE_MASK) != NORMAL) |
676 |
reportException(s); |
677 |
return getRawResult(); |
678 |
} |
679 |
|
680 |
/** |
681 |
* Commences performing this task, awaits its completion if |
682 |
* necessary, and returns its result, or throws an (unchecked) |
683 |
* {@code RuntimeException} or {@code Error} if the underlying |
684 |
* computation did so. |
685 |
* |
686 |
* @return the computed result |
687 |
*/ |
688 |
public final V invoke() { |
689 |
int s; |
690 |
if ((s = doInvoke() & DONE_MASK) != NORMAL) |
691 |
reportException(s); |
692 |
return getRawResult(); |
693 |
} |
694 |
|
695 |
/** |
696 |
* Forks the given tasks, returning when {@code isDone} holds for |
697 |
* each task or an (unchecked) exception is encountered, in which |
698 |
* case the exception is rethrown. If more than one task |
699 |
* encounters an exception, then this method throws any one of |
700 |
* these exceptions. If any task encounters an exception, the |
701 |
* other may be cancelled. However, the execution status of |
702 |
* individual tasks is not guaranteed upon exceptional return. The |
703 |
* status of each task may be obtained using {@link |
704 |
* #getException()} and related methods to check if they have been |
705 |
* cancelled, completed normally or exceptionally, or left |
706 |
* unprocessed. |
707 |
* |
708 |
* @param t1 the first task |
709 |
* @param t2 the second task |
710 |
* @throws NullPointerException if any task is null |
711 |
*/ |
712 |
public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) { |
713 |
int s1, s2; |
714 |
t2.fork(); |
715 |
if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL) |
716 |
t1.reportException(s1); |
717 |
if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL) |
718 |
t2.reportException(s2); |
719 |
} |
720 |
|
721 |
/** |
722 |
* Forks the given tasks, returning when {@code isDone} holds for |
723 |
* each task or an (unchecked) exception is encountered, in which |
724 |
* case the exception is rethrown. If more than one task |
725 |
* encounters an exception, then this method throws any one of |
726 |
* these exceptions. If any task encounters an exception, others |
727 |
* may be cancelled. However, the execution status of individual |
728 |
* tasks is not guaranteed upon exceptional return. The status of |
729 |
* each task may be obtained using {@link #getException()} and |
730 |
* related methods to check if they have been cancelled, completed |
731 |
* normally or exceptionally, or left unprocessed. |
732 |
* |
733 |
* @param tasks the tasks |
734 |
* @throws NullPointerException if any task is null |
735 |
*/ |
736 |
public static void invokeAll(ForkJoinTask<?>... tasks) { |
737 |
Throwable ex = null; |
738 |
int last = tasks.length - 1; |
739 |
for (int i = last; i >= 0; --i) { |
740 |
ForkJoinTask<?> t = tasks[i]; |
741 |
if (t == null) { |
742 |
if (ex == null) |
743 |
ex = new NullPointerException(); |
744 |
} |
745 |
else if (i != 0) |
746 |
t.fork(); |
747 |
else if (t.doInvoke() < NORMAL && ex == null) |
748 |
ex = t.getException(); |
749 |
} |
750 |
for (int i = 1; i <= last; ++i) { |
751 |
ForkJoinTask<?> t = tasks[i]; |
752 |
if (t != null) { |
753 |
if (ex != null) |
754 |
t.cancel(false); |
755 |
else if (t.doJoin() < NORMAL) |
756 |
ex = t.getException(); |
757 |
} |
758 |
} |
759 |
if (ex != null) |
760 |
rethrow(ex); |
761 |
} |
762 |
|
763 |
/** |
764 |
* Forks all tasks in the specified collection, returning when |
765 |
* {@code isDone} holds for each task or an (unchecked) exception |
766 |
* is encountered, in which case the exception is rethrown. If |
767 |
* more than one task encounters an exception, then this method |
768 |
* throws any one of these exceptions. If any task encounters an |
769 |
* exception, others may be cancelled. However, the execution |
770 |
* status of individual tasks is not guaranteed upon exceptional |
771 |
* return. The status of each task may be obtained using {@link |
772 |
* #getException()} and related methods to check if they have been |
773 |
* cancelled, completed normally or exceptionally, or left |
774 |
* unprocessed. |
775 |
* |
776 |
* @param tasks the collection of tasks |
777 |
* @return the tasks argument, to simplify usage |
778 |
* @throws NullPointerException if tasks or any element are null |
779 |
*/ |
780 |
public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) { |
781 |
if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) { |
782 |
invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()])); |
783 |
return tasks; |
784 |
} |
785 |
@SuppressWarnings("unchecked") |
786 |
List<? extends ForkJoinTask<?>> ts = |
787 |
(List<? extends ForkJoinTask<?>>) tasks; |
788 |
Throwable ex = null; |
789 |
int last = ts.size() - 1; |
790 |
for (int i = last; i >= 0; --i) { |
791 |
ForkJoinTask<?> t = ts.get(i); |
792 |
if (t == null) { |
793 |
if (ex == null) |
794 |
ex = new NullPointerException(); |
795 |
} |
796 |
else if (i != 0) |
797 |
t.fork(); |
798 |
else if (t.doInvoke() < NORMAL && ex == null) |
799 |
ex = t.getException(); |
800 |
} |
801 |
for (int i = 1; i <= last; ++i) { |
802 |
ForkJoinTask<?> t = ts.get(i); |
803 |
if (t != null) { |
804 |
if (ex != null) |
805 |
t.cancel(false); |
806 |
else if (t.doJoin() < NORMAL) |
807 |
ex = t.getException(); |
808 |
} |
809 |
} |
810 |
if (ex != null) |
811 |
rethrow(ex); |
812 |
return tasks; |
813 |
} |
814 |
|
815 |
/** |
816 |
* Attempts to cancel execution of this task. This attempt will |
817 |
* fail if the task has already completed or could not be |
818 |
* cancelled for some other reason. If successful, and this task |
819 |
* has not started when {@code cancel} is called, execution of |
820 |
* this task is suppressed. After this method returns |
821 |
* successfully, unless there is an intervening call to {@link |
822 |
* #reinitialize}, subsequent calls to {@link #isCancelled}, |
823 |
* {@link #isDone}, and {@code cancel} will return {@code true} |
824 |
* and calls to {@link #join} and related methods will result in |
825 |
* {@code CancellationException}. |
826 |
* |
827 |
* <p>This method may be overridden in subclasses, but if so, must |
828 |
* still ensure that these properties hold. In particular, the |
829 |
* {@code cancel} method itself must not throw exceptions. |
830 |
* |
831 |
* <p>This method is designed to be invoked by <em>other</em> |
832 |
* tasks. To terminate the current task, you can just return or |
833 |
* throw an unchecked exception from its computation method, or |
834 |
* invoke {@link #completeExceptionally}. |
835 |
* |
836 |
* @param mayInterruptIfRunning this value has no effect in the |
837 |
* default implementation because interrupts are not used to |
838 |
* control cancellation. |
839 |
* |
840 |
* @return {@code true} if this task is now cancelled |
841 |
*/ |
842 |
public boolean cancel(boolean mayInterruptIfRunning) { |
843 |
return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED; |
844 |
} |
845 |
|
846 |
public final boolean isDone() { |
847 |
return status < 0; |
848 |
} |
849 |
|
850 |
public final boolean isCancelled() { |
851 |
return (status & DONE_MASK) == CANCELLED; |
852 |
} |
853 |
|
854 |
/** |
855 |
* Returns {@code true} if this task threw an exception or was cancelled. |
856 |
* |
857 |
* @return {@code true} if this task threw an exception or was cancelled |
858 |
*/ |
859 |
public final boolean isCompletedAbnormally() { |
860 |
return status < NORMAL; |
861 |
} |
862 |
|
863 |
/** |
864 |
* Returns {@code true} if this task completed without throwing an |
865 |
* exception and was not cancelled. |
866 |
* |
867 |
* @return {@code true} if this task completed without throwing an |
868 |
* exception and was not cancelled |
869 |
*/ |
870 |
public final boolean isCompletedNormally() { |
871 |
return (status & DONE_MASK) == NORMAL; |
872 |
} |
873 |
|
874 |
/** |
875 |
* Returns the exception thrown by the base computation, or a |
876 |
* {@code CancellationException} if cancelled, or {@code null} if |
877 |
* none or if the method has not yet completed. |
878 |
* |
879 |
* @return the exception, or {@code null} if none |
880 |
*/ |
881 |
public final Throwable getException() { |
882 |
int s = status & DONE_MASK; |
883 |
return ((s >= NORMAL) ? null : |
884 |
(s == CANCELLED) ? new CancellationException() : |
885 |
getThrowableException()); |
886 |
} |
887 |
|
888 |
/** |
889 |
* Completes this task abnormally, and if not already aborted or |
890 |
* cancelled, causes it to throw the given exception upon |
891 |
* {@code join} and related operations. This method may be used |
892 |
* to induce exceptions in asynchronous tasks, or to force |
893 |
* completion of tasks that would not otherwise complete. Its use |
894 |
* in other situations is discouraged. This method is |
895 |
* overridable, but overridden versions must invoke {@code super} |
896 |
* implementation to maintain guarantees. |
897 |
* |
898 |
* @param ex the exception to throw. If this exception is not a |
899 |
* {@code RuntimeException} or {@code Error}, the actual exception |
900 |
* thrown will be a {@code RuntimeException} with cause {@code ex}. |
901 |
*/ |
902 |
public void completeExceptionally(Throwable ex) { |
903 |
setExceptionalCompletion((ex instanceof RuntimeException) || |
904 |
(ex instanceof Error) ? ex : |
905 |
new RuntimeException(ex)); |
906 |
} |
907 |
|
908 |
/** |
909 |
* Completes this task, and if not already aborted or cancelled, |
910 |
* returning the given value as the result of subsequent |
911 |
* invocations of {@code join} and related operations. This method |
912 |
* may be used to provide results for asynchronous tasks, or to |
913 |
* provide alternative handling for tasks that would not otherwise |
914 |
* complete normally. Its use in other situations is |
915 |
* discouraged. This method is overridable, but overridden |
916 |
* versions must invoke {@code super} implementation to maintain |
917 |
* guarantees. |
918 |
* |
919 |
* @param value the result value for this task |
920 |
*/ |
921 |
public void complete(V value) { |
922 |
try { |
923 |
setRawResult(value); |
924 |
} catch (Throwable rex) { |
925 |
setExceptionalCompletion(rex); |
926 |
return; |
927 |
} |
928 |
setCompletion(NORMAL); |
929 |
} |
930 |
|
931 |
/** |
932 |
* Completes this task normally without setting a value. The most |
933 |
* recent value established by {@link #setRawResult} (or {@code |
934 |
* null} by default) will be returned as the result of subsequent |
935 |
* invocations of {@code join} and related operations. |
936 |
* |
937 |
* @since 1.8 |
938 |
*/ |
939 |
public final void quietlyComplete() { |
940 |
setCompletion(NORMAL); |
941 |
} |
942 |
|
943 |
/** |
944 |
* Waits if necessary for the computation to complete, and then |
945 |
* retrieves its result. |
946 |
* |
947 |
* @return the computed result |
948 |
* @throws CancellationException if the computation was cancelled |
949 |
* @throws ExecutionException if the computation threw an |
950 |
* exception |
951 |
* @throws InterruptedException if the current thread is not a |
952 |
* member of a ForkJoinPool and was interrupted while waiting |
953 |
*/ |
954 |
public final V get() throws InterruptedException, ExecutionException { |
955 |
int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ? |
956 |
doJoin() : externalInterruptibleAwaitDone(); |
957 |
Throwable ex; |
958 |
if ((s &= DONE_MASK) == CANCELLED) |
959 |
throw new CancellationException(); |
960 |
if (s == EXCEPTIONAL && (ex = getThrowableException()) != null) |
961 |
throw new ExecutionException(ex); |
962 |
return getRawResult(); |
963 |
} |
964 |
|
965 |
/** |
966 |
* Waits if necessary for at most the given time for the computation |
967 |
* to complete, and then retrieves its result, if available. |
968 |
* |
969 |
* @param timeout the maximum time to wait |
970 |
* @param unit the time unit of the timeout argument |
971 |
* @return the computed result |
972 |
* @throws CancellationException if the computation was cancelled |
973 |
* @throws ExecutionException if the computation threw an |
974 |
* exception |
975 |
* @throws InterruptedException if the current thread is not a |
976 |
* member of a ForkJoinPool and was interrupted while waiting |
977 |
* @throws TimeoutException if the wait timed out |
978 |
*/ |
979 |
public final V get(long timeout, TimeUnit unit) |
980 |
throws InterruptedException, ExecutionException, TimeoutException { |
981 |
if (Thread.interrupted()) |
982 |
throw new InterruptedException(); |
983 |
// Messy in part because we measure in nanosecs, but wait in millisecs |
984 |
int s; long ns, ms; |
985 |
if ((s = status) >= 0 && (ns = unit.toNanos(timeout)) > 0L) { |
986 |
long deadline = System.nanoTime() + ns; |
987 |
ForkJoinPool p = null; |
988 |
ForkJoinPool.WorkQueue w = null; |
989 |
Thread t = Thread.currentThread(); |
990 |
if (t instanceof ForkJoinWorkerThread) { |
991 |
ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t; |
992 |
p = wt.pool; |
993 |
w = wt.workQueue; |
994 |
s = p.helpJoinOnce(w, this); // no retries on failure |
995 |
} |
996 |
boolean canBlock = false; |
997 |
boolean interrupted = false; |
998 |
try { |
999 |
while ((s = status) >= 0) { |
1000 |
if (w != null && w.runState < 0) |
1001 |
cancelIgnoringExceptions(this); |
1002 |
else if (!canBlock) { |
1003 |
if (p == null || p.tryCompensate(this, null)) |
1004 |
canBlock = true; |
1005 |
} |
1006 |
else { |
1007 |
if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L && |
1008 |
U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) { |
1009 |
synchronized (this) { |
1010 |
if (status >= 0) { |
1011 |
try { |
1012 |
wait(ms); |
1013 |
} catch (InterruptedException ie) { |
1014 |
if (p == null) |
1015 |
interrupted = true; |
1016 |
} |
1017 |
} |
1018 |
else |
1019 |
notifyAll(); |
1020 |
} |
1021 |
} |
1022 |
if ((s = status) < 0 || interrupted || |
1023 |
(ns = deadline - System.nanoTime()) <= 0L) |
1024 |
break; |
1025 |
} |
1026 |
} |
1027 |
} finally { |
1028 |
if (p != null && canBlock) |
1029 |
p.incrementActiveCount(); |
1030 |
} |
1031 |
if (interrupted) |
1032 |
throw new InterruptedException(); |
1033 |
} |
1034 |
if ((s &= DONE_MASK) != NORMAL) { |
1035 |
Throwable ex; |
1036 |
if (s == CANCELLED) |
1037 |
throw new CancellationException(); |
1038 |
if (s != EXCEPTIONAL) |
1039 |
throw new TimeoutException(); |
1040 |
if ((ex = getThrowableException()) != null) |
1041 |
throw new ExecutionException(ex); |
1042 |
} |
1043 |
return getRawResult(); |
1044 |
} |
1045 |
|
1046 |
/** |
1047 |
* Joins this task, without returning its result or throwing its |
1048 |
* exception. This method may be useful when processing |
1049 |
* collections of tasks when some have been cancelled or otherwise |
1050 |
* known to have aborted. |
1051 |
*/ |
1052 |
public final void quietlyJoin() { |
1053 |
doJoin(); |
1054 |
} |
1055 |
|
1056 |
/** |
1057 |
* Commences performing this task and awaits its completion if |
1058 |
* necessary, without returning its result or throwing its |
1059 |
* exception. |
1060 |
*/ |
1061 |
public final void quietlyInvoke() { |
1062 |
doInvoke(); |
1063 |
} |
1064 |
|
1065 |
/** |
1066 |
* Possibly executes tasks until the pool hosting the current task |
1067 |
* {@link ForkJoinPool#isQuiescent is quiescent}. This method may |
1068 |
* be of use in designs in which many tasks are forked, but none |
1069 |
* are explicitly joined, instead executing them until all are |
1070 |
* processed. |
1071 |
*/ |
1072 |
public static void helpQuiesce() { |
1073 |
Thread t; |
1074 |
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) { |
1075 |
ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t; |
1076 |
wt.pool.helpQuiescePool(wt.workQueue); |
1077 |
} |
1078 |
else |
1079 |
ForkJoinPool.externalHelpQuiescePool(); |
1080 |
} |
1081 |
|
1082 |
/** |
1083 |
* Resets the internal bookkeeping state of this task, allowing a |
1084 |
* subsequent {@code fork}. This method allows repeated reuse of |
1085 |
* this task, but only if reuse occurs when this task has either |
1086 |
* never been forked, or has been forked, then completed and all |
1087 |
* outstanding joins of this task have also completed. Effects |
1088 |
* under any other usage conditions are not guaranteed. |
1089 |
* This method may be useful when executing |
1090 |
* pre-constructed trees of subtasks in loops. |
1091 |
* |
1092 |
* <p>Upon completion of this method, {@code isDone()} reports |
1093 |
* {@code false}, and {@code getException()} reports {@code |
1094 |
* null}. However, the value returned by {@code getRawResult} is |
1095 |
* unaffected. To clear this value, you can invoke {@code |
1096 |
* setRawResult(null)}. |
1097 |
*/ |
1098 |
public void reinitialize() { |
1099 |
if ((status & DONE_MASK) == EXCEPTIONAL) |
1100 |
clearExceptionalCompletion(); |
1101 |
else |
1102 |
status = 0; |
1103 |
} |
1104 |
|
1105 |
/** |
1106 |
* Returns the pool hosting the current task execution, or null |
1107 |
* if this task is executing outside of any ForkJoinPool. |
1108 |
* |
1109 |
* @see #inForkJoinPool |
1110 |
* @return the pool, or {@code null} if none |
1111 |
*/ |
1112 |
public static ForkJoinPool getPool() { |
1113 |
Thread t = Thread.currentThread(); |
1114 |
return (t instanceof ForkJoinWorkerThread) ? |
1115 |
((ForkJoinWorkerThread) t).pool : null; |
1116 |
} |
1117 |
|
1118 |
/** |
1119 |
* Returns {@code true} if the current thread is a {@link |
1120 |
* ForkJoinWorkerThread} executing as a ForkJoinPool computation. |
1121 |
* |
1122 |
* @return {@code true} if the current thread is a {@link |
1123 |
* ForkJoinWorkerThread} executing as a ForkJoinPool computation, |
1124 |
* or {@code false} otherwise |
1125 |
*/ |
1126 |
public static boolean inForkJoinPool() { |
1127 |
return Thread.currentThread() instanceof ForkJoinWorkerThread; |
1128 |
} |
1129 |
|
1130 |
/** |
1131 |
* Tries to unschedule this task for execution. This method will |
1132 |
* typically (but is not guaranteed to) succeed if this task is |
1133 |
* the most recently forked task by the current thread, and has |
1134 |
* not commenced executing in another thread. This method may be |
1135 |
* useful when arranging alternative local processing of tasks |
1136 |
* that could have been, but were not, stolen. |
1137 |
* |
1138 |
* @return {@code true} if unforked |
1139 |
*/ |
1140 |
public boolean tryUnfork() { |
1141 |
Thread t; |
1142 |
return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? |
1143 |
((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) : |
1144 |
ForkJoinPool.tryUnsubmitFromCommonPool(this); |
1145 |
} |
1146 |
|
1147 |
/** |
1148 |
* Returns an estimate of the number of tasks that have been |
1149 |
* forked by the current worker thread but not yet executed. This |
1150 |
* value may be useful for heuristic decisions about whether to |
1151 |
* fork other tasks. |
1152 |
* |
1153 |
* @return the number of tasks |
1154 |
*/ |
1155 |
public static int getQueuedTaskCount() { |
1156 |
Thread t; |
1157 |
return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? |
1158 |
((ForkJoinWorkerThread)t).workQueue.queueSize() : |
1159 |
ForkJoinPool.getEstimatedSubmitterQueueLength(); |
1160 |
} |
1161 |
|
1162 |
/** |
1163 |
* Returns an estimate of how many more locally queued tasks are |
1164 |
* held by the current worker thread than there are other worker |
1165 |
* threads that might steal them, or zero if this thread is not |
1166 |
* operating in a ForkJoinPool. This value may be useful for |
1167 |
* heuristic decisions about whether to fork other tasks. In many |
1168 |
* usages of ForkJoinTasks, at steady state, each worker should |
1169 |
* aim to maintain a small constant surplus (for example, 3) of |
1170 |
* tasks, and to process computations locally if this threshold is |
1171 |
* exceeded. |
1172 |
* |
1173 |
* @return the surplus number of tasks, which may be negative |
1174 |
*/ |
1175 |
public static int getSurplusQueuedTaskCount() { |
1176 |
/* |
1177 |
* The aim of this method is to return a cheap heuristic guide |
1178 |
* for task partitioning when programmers, frameworks, tools, |
1179 |
* or languages have little or no idea about task granularity. |
1180 |
* In essence by offering this method, we ask users only about |
1181 |
* tradeoffs in overhead vs expected throughput and its |
1182 |
* variance, rather than how finely to partition tasks. |
1183 |
* |
1184 |
* In a steady state strict (tree-structured) computation, |
1185 |
* each thread makes available for stealing enough tasks for |
1186 |
* other threads to remain active. Inductively, if all threads |
1187 |
* play by the same rules, each thread should make available |
1188 |
* only a constant number of tasks. |
1189 |
* |
1190 |
* The minimum useful constant is just 1. But using a value of |
1191 |
* 1 would require immediate replenishment upon each steal to |
1192 |
* maintain enough tasks, which is infeasible. Further, |
1193 |
* partitionings/granularities of offered tasks should |
1194 |
* minimize steal rates, which in general means that threads |
1195 |
* nearer the top of computation tree should generate more |
1196 |
* than those nearer the bottom. In perfect steady state, each |
1197 |
* thread is at approximately the same level of computation |
1198 |
* tree. However, producing extra tasks amortizes the |
1199 |
* uncertainty of progress and diffusion assumptions. |
1200 |
* |
1201 |
* So, users will want to use values larger, but not much |
1202 |
* larger than 1 to both smooth over transient shortages and |
1203 |
* hedge against uneven progress; as traded off against the |
1204 |
* cost of extra task overhead. We leave the user to pick a |
1205 |
* threshold value to compare with the results of this call to |
1206 |
* guide decisions, but recommend values such as 3. |
1207 |
* |
1208 |
* When all threads are active, it is on average OK to |
1209 |
* estimate surplus strictly locally. In steady-state, if one |
1210 |
* thread is maintaining say 2 surplus tasks, then so are |
1211 |
* others. So we can just use estimated queue length. |
1212 |
* However, this strategy alone leads to serious mis-estimates |
1213 |
* in some non-steady-state conditions (ramp-up, ramp-down, |
1214 |
* other stalls). We can detect many of these by further |
1215 |
* considering the number of "idle" threads, that are known to |
1216 |
* have zero queued tasks, so compensate by a factor of |
1217 |
* (#idle/#active) threads. |
1218 |
*/ |
1219 |
Thread t; ForkJoinWorkerThread wt; |
1220 |
return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? |
1221 |
(wt = (ForkJoinWorkerThread)t).workQueue.queueSize() - wt.pool.idlePerActive() : |
1222 |
0; |
1223 |
} |
1224 |
|
1225 |
// Extension methods |
1226 |
|
1227 |
/** |
1228 |
* Returns the result that would be returned by {@link #join}, even |
1229 |
* if this task completed abnormally, or {@code null} if this task |
1230 |
* is not known to have been completed. This method is designed |
1231 |
* to aid debugging, as well as to support extensions. Its use in |
1232 |
* any other context is discouraged. |
1233 |
* |
1234 |
* @return the result, or {@code null} if not completed |
1235 |
*/ |
1236 |
public abstract V getRawResult(); |
1237 |
|
1238 |
/** |
1239 |
* Forces the given value to be returned as a result. This method |
1240 |
* is designed to support extensions, and should not in general be |
1241 |
* called otherwise. |
1242 |
* |
1243 |
* @param value the value |
1244 |
*/ |
1245 |
protected abstract void setRawResult(V value); |
1246 |
|
1247 |
/** |
1248 |
* Immediately performs the base action of this task and returns |
1249 |
* true if, upon return from this method, this task is guaranteed |
1250 |
* to have completed normally. This method may return false |
1251 |
* otherwise, to indicate that this task is not necessarily |
1252 |
* complete (or is not known to be complete), for example in |
1253 |
* asynchronous actions that require explicit invocations of |
1254 |
* completion methods. This method may also throw an (unchecked) |
1255 |
* exception to indicate abnormal exit. This method is designed to |
1256 |
* support extensions, and should not in general be called |
1257 |
* otherwise. |
1258 |
* |
1259 |
* @return {@code true} if this task is known to have completed normally |
1260 |
*/ |
1261 |
protected abstract boolean exec(); |
1262 |
|
1263 |
/** |
1264 |
* Returns, but does not unschedule or execute, a task queued by |
1265 |
* the current thread but not yet executed, if one is immediately |
1266 |
* available and the current thread is operating in a |
1267 |
* ForkJoinPool. There is no guarantee that this task will |
1268 |
* actually be polled or executed next. Conversely, this method |
1269 |
* may return null even if a task exists but cannot be accessed |
1270 |
* without contention with other threads. This method is designed |
1271 |
* primarily to support extensions, and is unlikely to be useful |
1272 |
* otherwise. |
1273 |
* |
1274 |
* @return the next task, or {@code null} if none are available |
1275 |
*/ |
1276 |
protected static ForkJoinTask<?> peekNextLocalTask() { |
1277 |
Thread t; |
1278 |
return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? |
1279 |
((ForkJoinWorkerThread)t).workQueue.peek() : |
1280 |
null; |
1281 |
} |
1282 |
|
1283 |
/** |
1284 |
* Unschedules and returns, without executing, the next task |
1285 |
* queued by the current thread but not yet executed, if the |
1286 |
* current thread is operating in a ForkJoinPool. This method is |
1287 |
* designed primarily to support extensions, and is unlikely to be |
1288 |
* useful otherwise. |
1289 |
* |
1290 |
* @return the next task, or {@code null} if none are available |
1291 |
*/ |
1292 |
protected static ForkJoinTask<?> pollNextLocalTask() { |
1293 |
Thread t; |
1294 |
return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? |
1295 |
((ForkJoinWorkerThread)t).workQueue.nextLocalTask() : |
1296 |
null; |
1297 |
} |
1298 |
|
1299 |
/** |
1300 |
* If the current thread is operating in a ForkJoinPool, |
1301 |
* unschedules and returns, without executing, the next task |
1302 |
* queued by the current thread but not yet executed, if one is |
1303 |
* available, or if not available, a task that was forked by some |
1304 |
* other thread, if available. Availability may be transient, so a |
1305 |
* {@code null} result does not necessarily imply quiescence of |
1306 |
* the pool this task is operating in. This method is designed |
1307 |
* primarily to support extensions, and is unlikely to be useful |
1308 |
* otherwise. |
1309 |
* |
1310 |
* @return a task, or {@code null} if none are available |
1311 |
*/ |
1312 |
protected static ForkJoinTask<?> pollTask() { |
1313 |
Thread t; ForkJoinWorkerThread wt; |
1314 |
return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ? |
1315 |
(wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) : |
1316 |
null; |
1317 |
} |
1318 |
|
1319 |
// tag operations |
1320 |
|
1321 |
/** |
1322 |
* Returns the tag for this task. |
1323 |
* |
1324 |
* @return the tag for this task |
1325 |
* @since 1.8 |
1326 |
*/ |
1327 |
public final short getForkJoinTaskTag() { |
1328 |
return (short)status; |
1329 |
} |
1330 |
|
1331 |
/** |
1332 |
* Atomically sets the tag value for this task. |
1333 |
* |
1334 |
* @param tag the tag value |
1335 |
* @return the previous value of the tag |
1336 |
* @since 1.8 |
1337 |
*/ |
1338 |
public final short setForkJoinTaskTag(short tag) { |
1339 |
for (int s;;) { |
1340 |
if (U.compareAndSwapInt(this, STATUS, s = status, |
1341 |
(s & ~SMASK) | (tag & SMASK))) |
1342 |
return (short)s; |
1343 |
} |
1344 |
} |
1345 |
|
1346 |
/** |
1347 |
* Atomically conditionally sets the tag value for this task. |
1348 |
* Among other applications, tags can be used as visit markers |
1349 |
* in tasks operating on graphs, as in methods that check: {@code |
1350 |
* if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))} |
1351 |
* before processing, otherwise exiting because the node has |
1352 |
* already been visited. |
1353 |
* |
1354 |
* @param e the expected tag value |
1355 |
* @param tag the new tag value |
1356 |
* @return true if successful; i.e., the current value was |
1357 |
* equal to e and is now tag. |
1358 |
* @since 1.8 |
1359 |
*/ |
1360 |
public final boolean compareAndSetForkJoinTaskTag(short e, short tag) { |
1361 |
for (int s;;) { |
1362 |
if ((short)(s = status) != e) |
1363 |
return false; |
1364 |
if (U.compareAndSwapInt(this, STATUS, s, |
1365 |
(s & ~SMASK) | (tag & SMASK))) |
1366 |
return true; |
1367 |
} |
1368 |
} |
1369 |
|
1370 |
/** |
1371 |
* Adaptor for Runnables. This implements RunnableFuture |
1372 |
* to be compliant with AbstractExecutorService constraints |
1373 |
* when used in ForkJoinPool. |
1374 |
*/ |
1375 |
static final class AdaptedRunnable<T> extends ForkJoinTask<T> |
1376 |
implements RunnableFuture<T> { |
1377 |
final Runnable runnable; |
1378 |
T result; |
1379 |
AdaptedRunnable(Runnable runnable, T result) { |
1380 |
if (runnable == null) throw new NullPointerException(); |
1381 |
this.runnable = runnable; |
1382 |
this.result = result; // OK to set this even before completion |
1383 |
} |
1384 |
public final T getRawResult() { return result; } |
1385 |
public final void setRawResult(T v) { result = v; } |
1386 |
public final boolean exec() { runnable.run(); return true; } |
1387 |
public final void run() { invoke(); } |
1388 |
private static final long serialVersionUID = 5232453952276885070L; |
1389 |
} |
1390 |
|
1391 |
/** |
1392 |
* Adaptor for Runnables without results |
1393 |
*/ |
1394 |
static final class AdaptedRunnableAction extends ForkJoinTask<Void> |
1395 |
implements RunnableFuture<Void> { |
1396 |
final Runnable runnable; |
1397 |
AdaptedRunnableAction(Runnable runnable) { |
1398 |
if (runnable == null) throw new NullPointerException(); |
1399 |
this.runnable = runnable; |
1400 |
} |
1401 |
public final Void getRawResult() { return null; } |
1402 |
public final void setRawResult(Void v) { } |
1403 |
public final boolean exec() { runnable.run(); return true; } |
1404 |
public final void run() { invoke(); } |
1405 |
private static final long serialVersionUID = 5232453952276885070L; |
1406 |
} |
1407 |
|
1408 |
/** |
1409 |
* Adaptor for Callables |
1410 |
*/ |
1411 |
static final class AdaptedCallable<T> extends ForkJoinTask<T> |
1412 |
implements RunnableFuture<T> { |
1413 |
final Callable<? extends T> callable; |
1414 |
T result; |
1415 |
AdaptedCallable(Callable<? extends T> callable) { |
1416 |
if (callable == null) throw new NullPointerException(); |
1417 |
this.callable = callable; |
1418 |
} |
1419 |
public final T getRawResult() { return result; } |
1420 |
public final void setRawResult(T v) { result = v; } |
1421 |
public final boolean exec() { |
1422 |
try { |
1423 |
result = callable.call(); |
1424 |
return true; |
1425 |
} catch (Error err) { |
1426 |
throw err; |
1427 |
} catch (RuntimeException rex) { |
1428 |
throw rex; |
1429 |
} catch (Exception ex) { |
1430 |
throw new RuntimeException(ex); |
1431 |
} |
1432 |
} |
1433 |
public final void run() { invoke(); } |
1434 |
private static final long serialVersionUID = 2838392045355241008L; |
1435 |
} |
1436 |
|
1437 |
/** |
1438 |
* Returns a new {@code ForkJoinTask} that performs the {@code run} |
1439 |
* method of the given {@code Runnable} as its action, and returns |
1440 |
* a null result upon {@link #join}. |
1441 |
* |
1442 |
* @param runnable the runnable action |
1443 |
* @return the task |
1444 |
*/ |
1445 |
public static ForkJoinTask<?> adapt(Runnable runnable) { |
1446 |
return new AdaptedRunnableAction(runnable); |
1447 |
} |
1448 |
|
1449 |
/** |
1450 |
* Returns a new {@code ForkJoinTask} that performs the {@code run} |
1451 |
* method of the given {@code Runnable} as its action, and returns |
1452 |
* the given result upon {@link #join}. |
1453 |
* |
1454 |
* @param runnable the runnable action |
1455 |
* @param result the result upon completion |
1456 |
* @return the task |
1457 |
*/ |
1458 |
public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) { |
1459 |
return new AdaptedRunnable<T>(runnable, result); |
1460 |
} |
1461 |
|
1462 |
/** |
1463 |
* Returns a new {@code ForkJoinTask} that performs the {@code call} |
1464 |
* method of the given {@code Callable} as its action, and returns |
1465 |
* its result upon {@link #join}, translating any checked exceptions |
1466 |
* encountered into {@code RuntimeException}. |
1467 |
* |
1468 |
* @param callable the callable action |
1469 |
* @return the task |
1470 |
*/ |
1471 |
public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) { |
1472 |
return new AdaptedCallable<T>(callable); |
1473 |
} |
1474 |
|
1475 |
// Serialization support |
1476 |
|
1477 |
private static final long serialVersionUID = -7721805057305804111L; |
1478 |
|
1479 |
/** |
1480 |
* Saves this task to a stream (that is, serializes it). |
1481 |
* |
1482 |
* @serialData the current run status and the exception thrown |
1483 |
* during execution, or {@code null} if none |
1484 |
*/ |
1485 |
private void writeObject(java.io.ObjectOutputStream s) |
1486 |
throws java.io.IOException { |
1487 |
s.defaultWriteObject(); |
1488 |
s.writeObject(getException()); |
1489 |
} |
1490 |
|
1491 |
/** |
1492 |
* Reconstitutes this task from a stream (that is, deserializes it). |
1493 |
*/ |
1494 |
private void readObject(java.io.ObjectInputStream s) |
1495 |
throws java.io.IOException, ClassNotFoundException { |
1496 |
s.defaultReadObject(); |
1497 |
Object ex = s.readObject(); |
1498 |
if (ex != null) |
1499 |
setExceptionalCompletion((Throwable)ex); |
1500 |
} |
1501 |
|
1502 |
// Unsafe mechanics |
1503 |
private static final sun.misc.Unsafe U; |
1504 |
private static final long STATUS; |
1505 |
static { |
1506 |
exceptionTableLock = new ReentrantLock(); |
1507 |
exceptionTableRefQueue = new ReferenceQueue<Object>(); |
1508 |
exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY]; |
1509 |
try { |
1510 |
U = getUnsafe(); |
1511 |
STATUS = U.objectFieldOffset |
1512 |
(ForkJoinTask.class.getDeclaredField("status")); |
1513 |
} catch (Exception e) { |
1514 |
throw new Error(e); |
1515 |
} |
1516 |
} |
1517 |
|
1518 |
/** |
1519 |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1520 |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1521 |
* into a jdk. |
1522 |
* |
1523 |
* @return a sun.misc.Unsafe |
1524 |
*/ |
1525 |
private static sun.misc.Unsafe getUnsafe() { |
1526 |
try { |
1527 |
return sun.misc.Unsafe.getUnsafe(); |
1528 |
} catch (SecurityException se) { |
1529 |
try { |
1530 |
return java.security.AccessController.doPrivileged |
1531 |
(new java.security |
1532 |
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1533 |
public sun.misc.Unsafe run() throws Exception { |
1534 |
java.lang.reflect.Field f = sun.misc |
1535 |
.Unsafe.class.getDeclaredField("theUnsafe"); |
1536 |
f.setAccessible(true); |
1537 |
return (sun.misc.Unsafe) f.get(null); |
1538 |
}}); |
1539 |
} catch (java.security.PrivilegedActionException e) { |
1540 |
throw new RuntimeException("Could not initialize intrinsics", |
1541 |
e.getCause()); |
1542 |
} |
1543 |
} |
1544 |
} |
1545 |
} |