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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/licenses/publicdomain |
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*/ |
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|
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package jsr166y; |
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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.Collections; |
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import java.util.List; |
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import java.util.RandomAccess; |
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import java.util.Map; |
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import java.util.WeakHashMap; |
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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.Executor; |
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import java.util.concurrent.ExecutorService; |
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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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|
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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 submitted |
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* to a {@link ForkJoinPool}. Once started, it will usually in turn |
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* start other subtasks. As indicated by the name of this class, |
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* many programs using {@code ForkJoinTask} employ only methods |
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* {@link #fork} and {@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 |
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* support 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 intended use as computational tasks calculating |
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* pure functions or operating on purely isolated objects. The |
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* primary 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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* avoid {@code synchronized} methods or blocks, and should minimize |
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* other blocking synchronization apart from joining other tasks or |
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* using synchronizers such as Phasers that are advertised to |
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* cooperate with fork/join scheduling. Tasks should also not perform |
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* blocking IO, and should ideally access variables that are |
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* completely independent of those accessed by other running |
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* tasks. Minor breaches of these restrictions, for example using |
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* shared output streams, may be tolerable in practice, but frequent |
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* use may result in poor performance, and the potential to |
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* indefinitely stall if the number of threads not waiting for IO or |
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* other external synchronization becomes exhausted. This usage |
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* restriction is in part enforced by not permitting checked |
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* exceptions such as {@code IOExceptions} to be thrown. However, |
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* computations may still encounter unchecked exceptions, that are |
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* rethrown to callers attempting to join them. These exceptions may |
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* additionally include {@link RejectedExecutionException} stemming |
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* from internal resource exhaustion, such as failure to allocate |
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* internal task queues. |
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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>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 computations that do not return results, or |
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* {@link RecursiveTask} for those that do. Normally, a concrete |
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* ForkJoinTask subclass declares fields comprising its parameters, |
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* established in a constructor, and then defines a {@code compute} |
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* method that somehow uses the control methods supplied by this base |
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* class. While these methods have {@code public} access (to allow |
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* instances of different task subclasses to call each other's |
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* methods), some of them may only be called from within other |
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* ForkJoinTasks (as may be determined using method {@link |
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* #inForkJoinPool}). Attempts to invoke them in other contexts |
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* result in exceptions or errors, possibly including |
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* {@code ClassCastException}. |
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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. |
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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. The |
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* methods of this class are more-or-less layered into (1) basic |
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* status maintenance (2) execution and awaiting completion (3) |
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* user-level methods that additionally report results. This is |
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* sometimes hard to see because this file orders exported methods |
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* in a way that flows well in javadocs. In particular, most |
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* join mechanics are in method quietlyJoin, below. |
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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 holds value |
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* NORMAL, CANCELLED, or EXCEPTIONAL. Tasks undergoing blocking |
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* waits by other threads have the SIGNAL bit set. Completion of |
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* a stolen task with SIGNAL set awakens any waiters via |
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* notifyAll. Even though suboptimal for some purposes, we use |
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* basic builtin wait/notify to take advantage of "monitor |
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* inflation" in JVMs that we would otherwise need to emulate to |
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* avoid adding further per-task bookkeeping overhead. We want |
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* these monitors to be "fat", i.e., not use biasing or thin-lock |
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* techniques, so use some odd coding idioms that tend to avoid |
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* them. |
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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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|
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private static final int NORMAL = -1; |
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private static final int CANCELLED = -2; |
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private static final int EXCEPTIONAL = -3; |
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private static final int SIGNAL = 1; |
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|
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/** |
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* Table of exceptions thrown by tasks, to enable reporting by |
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* callers. Because exceptions are rare, we don't directly keep |
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* them with task objects, but instead use a weak ref table. Note |
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* that cancellation exceptions don't appear in the table, but are |
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* instead recorded as status values. |
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* TODO: Use ConcurrentReferenceHashMap |
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*/ |
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static final Map<ForkJoinTask<?>, Throwable> exceptionMap = |
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Collections.synchronizedMap |
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(new WeakHashMap<ForkJoinTask<?>, Throwable>()); |
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|
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// Maintaining completion status |
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|
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/** |
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* Marks completion and wakes up threads waiting to join this task, |
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* also clearing signal request bits. |
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* |
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* @param completion one of NORMAL, CANCELLED, EXCEPTIONAL |
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*/ |
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private void setCompletion(int completion) { |
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int s; |
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while ((s = status) >= 0) { |
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if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) { |
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if (s != 0) |
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synchronized (this) { notifyAll(); } |
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break; |
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} |
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} |
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} |
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|
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/** |
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* Records exception and sets exceptional completion. |
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* |
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* @return status on exit |
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*/ |
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private void setExceptionalCompletion(Throwable rex) { |
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exceptionMap.put(this, rex); |
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setCompletion(EXCEPTIONAL); |
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} |
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|
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/** |
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* Blocks a worker thread until completed or timed out. Called |
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* only by pool. |
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*/ |
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final void internalAwaitDone(long millis, int nanos) { |
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if (status >= 0) { |
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try { // the odd construction reduces lock bias effects |
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synchronized (this) { |
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if (status > 0 || |
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UNSAFE.compareAndSwapInt(this, statusOffset, |
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0, SIGNAL)) |
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wait(millis, nanos); |
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} |
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} catch (InterruptedException ie) { |
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cancelIfTerminating(); |
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} |
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} |
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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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*/ |
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private void externalAwaitDone() { |
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if (status >= 0) { |
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boolean interrupted = false; |
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synchronized(this) { |
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int s; |
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while ((s = status) >= 0) { |
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if (s == 0 && |
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!UNSAFE.compareAndSwapInt(this, statusOffset, |
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0, SIGNAL)) |
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continue; |
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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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} |
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if (interrupted) |
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Thread.currentThread().interrupt(); |
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} |
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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 or timeout. |
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*/ |
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private void externalInterruptibleAwaitDone(boolean timed, long nanos) |
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throws InterruptedException { |
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if (Thread.interrupted()) |
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throw new InterruptedException(); |
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if (status >= 0) { |
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long startTime = timed ? System.nanoTime() : 0L; |
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synchronized(this) { |
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int s; |
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while ((s = status) >= 0) { |
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long nt; |
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if (s == 0 && |
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!UNSAFE.compareAndSwapInt(this, statusOffset, |
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0, SIGNAL)) |
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continue; |
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else if (!timed) |
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wait(); |
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else if ((nt = nanos - (System.nanoTime()-startTime)) > 0L) |
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wait(nt / 1000000, (int)(nt % 1000000)); |
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else |
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break; |
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} |
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} |
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} |
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} |
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|
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/** |
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* Unless done, calls exec and records status if completed, but |
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* doesn't wait for completion otherwise. Primary execution method |
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* for ForkJoinWorkerThread. |
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*/ |
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final void quietlyExec() { |
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try { |
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if (status < 0 || !exec()) |
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return; |
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} catch (Throwable rex) { |
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setExceptionalCompletion(rex); |
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return; |
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} |
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setCompletion(NORMAL); // must be outside try block |
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} |
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|
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// public methods |
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|
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/** |
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* Arranges to asynchronously execute this task. While it is not |
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* necessarily enforced, it is a usage error to fork a task more |
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* than once unless it has completed and been reinitialized. |
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* Subsequent modifications to the state of this task or any data |
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* it operates on are not necessarily consistently observable by |
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* any thread other than the one executing it unless preceded by a |
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* call to {@link #join} or related methods, or a call to {@link |
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* #isDone} returning {@code true}. |
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* |
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* <p>This method may be invoked only from within {@code |
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* ForkJoinPool} computations (as may be determined using method |
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* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
339 |
* result in exceptions or errors, possibly including {@code |
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* ClassCastException}. |
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* |
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* @return {@code this}, to simplify usage |
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*/ |
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public final ForkJoinTask<V> fork() { |
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((ForkJoinWorkerThread) Thread.currentThread()) |
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.pushTask(this); |
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return this; |
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} |
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|
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/** |
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* Returns the result of the computation when it {@link #isDone is |
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* done}. This method differs from {@link #get()} in that |
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* abnormal completion results in {@code RuntimeException} or |
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* {@code Error}, not {@code ExecutionException}, and that |
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* interrupts of the calling thread do <em>not</em> cause the |
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* method to abruptly return by throwing {@code |
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* InterruptedException}. |
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* |
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* @return the computed result |
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*/ |
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public final V join() { |
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quietlyJoin(); |
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Throwable ex; |
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if (status < NORMAL && (ex = getException()) != null) |
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UNSAFE.throwException(ex); |
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return getRawResult(); |
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} |
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|
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/** |
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* Commences performing this task, awaits its completion if |
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* necessary, and returns its result, or throws an (unchecked) |
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* {@code RuntimeException} or {@code Error} if the underlying |
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* computation did so. |
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* |
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* @return the computed result |
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*/ |
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public final V invoke() { |
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quietlyInvoke(); |
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Throwable ex; |
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if (status < NORMAL && (ex = getException()) != null) |
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UNSAFE.throwException(ex); |
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return getRawResult(); |
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} |
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|
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/** |
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* Forks the given tasks, returning when {@code isDone} holds for |
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* each task or an (unchecked) exception is encountered, in which |
388 |
* case the exception is rethrown. If more than one task |
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* encounters an exception, then this method throws any one of |
390 |
* these exceptions. If any task encounters an exception, the |
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* other may be cancelled. However, the execution status of |
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* individual tasks is not guaranteed upon exceptional return. The |
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* status of each task may be obtained using {@link |
394 |
* #getException()} and related methods to check if they have been |
395 |
* cancelled, completed normally or exceptionally, or left |
396 |
* unprocessed. |
397 |
* |
398 |
* <p>This method may be invoked only from within {@code |
399 |
* ForkJoinPool} computations (as may be determined using method |
400 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
401 |
* result in exceptions or errors, possibly including {@code |
402 |
* ClassCastException}. |
403 |
* |
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* @param t1 the first task |
405 |
* @param t2 the second task |
406 |
* @throws NullPointerException if any task is null |
407 |
*/ |
408 |
public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) { |
409 |
t2.fork(); |
410 |
t1.invoke(); |
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t2.join(); |
412 |
} |
413 |
|
414 |
/** |
415 |
* Forks the given tasks, returning when {@code isDone} holds for |
416 |
* each task or an (unchecked) exception is encountered, in which |
417 |
* case the exception is rethrown. If more than one task |
418 |
* encounters an exception, then this method throws any one of |
419 |
* these exceptions. If any task encounters an exception, others |
420 |
* may be cancelled. However, the execution status of individual |
421 |
* tasks is not guaranteed upon exceptional return. The status of |
422 |
* each task may be obtained using {@link #getException()} and |
423 |
* related methods to check if they have been cancelled, completed |
424 |
* normally or exceptionally, or left unprocessed. |
425 |
* |
426 |
* <p>This method may be invoked only from within {@code |
427 |
* ForkJoinPool} computations (as may be determined using method |
428 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
429 |
* result in exceptions or errors, possibly including {@code |
430 |
* ClassCastException}. |
431 |
* |
432 |
* @param tasks the tasks |
433 |
* @throws NullPointerException if any task is null |
434 |
*/ |
435 |
public static void invokeAll(ForkJoinTask<?>... tasks) { |
436 |
Throwable ex = null; |
437 |
int last = tasks.length - 1; |
438 |
for (int i = last; i >= 0; --i) { |
439 |
ForkJoinTask<?> t = tasks[i]; |
440 |
if (t == null) { |
441 |
if (ex == null) |
442 |
ex = new NullPointerException(); |
443 |
} |
444 |
else if (i != 0) |
445 |
t.fork(); |
446 |
else { |
447 |
t.quietlyInvoke(); |
448 |
if (ex == null && t.status < NORMAL) |
449 |
ex = t.getException(); |
450 |
} |
451 |
} |
452 |
for (int i = 1; i <= last; ++i) { |
453 |
ForkJoinTask<?> t = tasks[i]; |
454 |
if (t != null) { |
455 |
if (ex != null) |
456 |
t.cancel(false); |
457 |
else { |
458 |
t.quietlyJoin(); |
459 |
if (ex == null && t.status < NORMAL) |
460 |
ex = t.getException(); |
461 |
} |
462 |
} |
463 |
} |
464 |
if (ex != null) |
465 |
UNSAFE.throwException(ex); |
466 |
} |
467 |
|
468 |
/** |
469 |
* Forks all tasks in the specified collection, returning when |
470 |
* {@code isDone} holds for each task or an (unchecked) exception |
471 |
* is encountered, in which case the exception is rethrown. If |
472 |
* more than one task encounters an exception, then this method |
473 |
* throws any one of these exceptions. If any task encounters an |
474 |
* exception, others may be cancelled. However, the execution |
475 |
* status of individual tasks is not guaranteed upon exceptional |
476 |
* return. The status of each task may be obtained using {@link |
477 |
* #getException()} and related methods to check if they have been |
478 |
* cancelled, completed normally or exceptionally, or left |
479 |
* unprocessed. |
480 |
* |
481 |
* <p>This method may be invoked only from within {@code |
482 |
* ForkJoinPool} computations (as may be determined using method |
483 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
484 |
* result in exceptions or errors, possibly including {@code |
485 |
* ClassCastException}. |
486 |
* |
487 |
* @param tasks the collection of tasks |
488 |
* @return the tasks argument, to simplify usage |
489 |
* @throws NullPointerException if tasks or any element are null |
490 |
*/ |
491 |
public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) { |
492 |
if (!(tasks instanceof RandomAccess) || !(tasks instanceof List<?>)) { |
493 |
invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()])); |
494 |
return tasks; |
495 |
} |
496 |
@SuppressWarnings("unchecked") |
497 |
List<? extends ForkJoinTask<?>> ts = |
498 |
(List<? extends ForkJoinTask<?>>) tasks; |
499 |
Throwable ex = null; |
500 |
int last = ts.size() - 1; |
501 |
for (int i = last; i >= 0; --i) { |
502 |
ForkJoinTask<?> t = ts.get(i); |
503 |
if (t == null) { |
504 |
if (ex == null) |
505 |
ex = new NullPointerException(); |
506 |
} |
507 |
else if (i != 0) |
508 |
t.fork(); |
509 |
else { |
510 |
t.quietlyInvoke(); |
511 |
if (ex == null && t.status < NORMAL) |
512 |
ex = t.getException(); |
513 |
} |
514 |
} |
515 |
for (int i = 1; i <= last; ++i) { |
516 |
ForkJoinTask<?> t = ts.get(i); |
517 |
if (t != null) { |
518 |
if (ex != null) |
519 |
t.cancel(false); |
520 |
else { |
521 |
t.quietlyJoin(); |
522 |
if (ex == null && t.status < NORMAL) |
523 |
ex = t.getException(); |
524 |
} |
525 |
} |
526 |
} |
527 |
if (ex != null) |
528 |
UNSAFE.throwException(ex); |
529 |
return tasks; |
530 |
} |
531 |
|
532 |
/** |
533 |
* Attempts to cancel execution of this task. This attempt will |
534 |
* fail if the task has already completed or could not be |
535 |
* cancelled for some other reason. If successful, and this task |
536 |
* has not started when {@code cancel} is called, execution of |
537 |
* this task is suppressed. After this method returns |
538 |
* successfully, unless there is an intervening call to {@link |
539 |
* #reinitialize}, subsequent calls to {@link #isCancelled}, |
540 |
* {@link #isDone}, and {@code cancel} will return {@code true} |
541 |
* and calls to {@link #join} and related methods will result in |
542 |
* {@code CancellationException}. |
543 |
* |
544 |
* <p>This method may be overridden in subclasses, but if so, must |
545 |
* still ensure that these properties hold. In particular, the |
546 |
* {@code cancel} method itself must not throw exceptions. |
547 |
* |
548 |
* <p>This method is designed to be invoked by <em>other</em> |
549 |
* tasks. To terminate the current task, you can just return or |
550 |
* throw an unchecked exception from its computation method, or |
551 |
* invoke {@link #completeExceptionally}. |
552 |
* |
553 |
* @param mayInterruptIfRunning this value has no effect in the |
554 |
* default implementation because interrupts are not used to |
555 |
* control cancellation. |
556 |
* |
557 |
* @return {@code true} if this task is now cancelled |
558 |
*/ |
559 |
public boolean cancel(boolean mayInterruptIfRunning) { |
560 |
setCompletion(CANCELLED); |
561 |
return status == CANCELLED; |
562 |
} |
563 |
|
564 |
/** |
565 |
* Cancels, ignoring any exceptions thrown by cancel. Used during |
566 |
* worker and pool shutdown. Cancel is spec'ed not to throw any |
567 |
* exceptions, but if it does anyway, we have no recourse during |
568 |
* shutdown, so guard against this case. |
569 |
*/ |
570 |
final void cancelIgnoringExceptions() { |
571 |
try { |
572 |
cancel(false); |
573 |
} catch (Throwable ignore) { |
574 |
} |
575 |
} |
576 |
|
577 |
/** |
578 |
* Cancels if current thread is a terminating worker thread, |
579 |
* ignoring any exceptions thrown by cancel. |
580 |
*/ |
581 |
final void cancelIfTerminating() { |
582 |
Thread t = Thread.currentThread(); |
583 |
if ((t instanceof ForkJoinWorkerThread) && |
584 |
((ForkJoinWorkerThread) t).isTerminating()) { |
585 |
try { |
586 |
cancel(false); |
587 |
} catch (Throwable ignore) { |
588 |
} |
589 |
} |
590 |
} |
591 |
|
592 |
public final boolean isDone() { |
593 |
return status < 0; |
594 |
} |
595 |
|
596 |
public final boolean isCancelled() { |
597 |
return status == CANCELLED; |
598 |
} |
599 |
|
600 |
/** |
601 |
* Returns {@code true} if this task threw an exception or was cancelled. |
602 |
* |
603 |
* @return {@code true} if this task threw an exception or was cancelled |
604 |
*/ |
605 |
public final boolean isCompletedAbnormally() { |
606 |
return status < NORMAL; |
607 |
} |
608 |
|
609 |
/** |
610 |
* Returns {@code true} if this task completed without throwing an |
611 |
* exception and was not cancelled. |
612 |
* |
613 |
* @return {@code true} if this task completed without throwing an |
614 |
* exception and was not cancelled |
615 |
*/ |
616 |
public final boolean isCompletedNormally() { |
617 |
return status == NORMAL; |
618 |
} |
619 |
|
620 |
/** |
621 |
* Returns the exception thrown by the base computation, or a |
622 |
* {@code CancellationException} if cancelled, or {@code null} if |
623 |
* none or if the method has not yet completed. |
624 |
* |
625 |
* @return the exception, or {@code null} if none |
626 |
*/ |
627 |
public final Throwable getException() { |
628 |
int s = status; |
629 |
return ((s >= NORMAL) ? null : |
630 |
(s == CANCELLED) ? new CancellationException() : |
631 |
exceptionMap.get(this)); |
632 |
} |
633 |
|
634 |
/** |
635 |
* Completes this task abnormally, and if not already aborted or |
636 |
* cancelled, causes it to throw the given exception upon |
637 |
* {@code join} and related operations. This method may be used |
638 |
* to induce exceptions in asynchronous tasks, or to force |
639 |
* completion of tasks that would not otherwise complete. Its use |
640 |
* in other situations is discouraged. This method is |
641 |
* overridable, but overridden versions must invoke {@code super} |
642 |
* implementation to maintain guarantees. |
643 |
* |
644 |
* @param ex the exception to throw. If this exception is not a |
645 |
* {@code RuntimeException} or {@code Error}, the actual exception |
646 |
* thrown will be a {@code RuntimeException} with cause {@code ex}. |
647 |
*/ |
648 |
public void completeExceptionally(Throwable ex) { |
649 |
setExceptionalCompletion((ex instanceof RuntimeException) || |
650 |
(ex instanceof Error) ? ex : |
651 |
new RuntimeException(ex)); |
652 |
} |
653 |
|
654 |
/** |
655 |
* Completes this task, and if not already aborted or cancelled, |
656 |
* returning the given value as the result of subsequent |
657 |
* invocations of {@code join} and related operations. This method |
658 |
* may be used to provide results for asynchronous tasks, or to |
659 |
* provide alternative handling for tasks that would not otherwise |
660 |
* complete normally. Its use in other situations is |
661 |
* discouraged. This method is overridable, but overridden |
662 |
* versions must invoke {@code super} implementation to maintain |
663 |
* guarantees. |
664 |
* |
665 |
* @param value the result value for this task |
666 |
*/ |
667 |
public void complete(V value) { |
668 |
try { |
669 |
setRawResult(value); |
670 |
} catch (Throwable rex) { |
671 |
setExceptionalCompletion(rex); |
672 |
return; |
673 |
} |
674 |
setCompletion(NORMAL); |
675 |
} |
676 |
|
677 |
/** |
678 |
* Waits if necessary for the computation to complete, and then |
679 |
* retrieves its result. |
680 |
* |
681 |
* @return the computed result |
682 |
* @throws CancellationException if the computation was cancelled |
683 |
* @throws ExecutionException if the computation threw an |
684 |
* exception |
685 |
* @throws InterruptedException if the current thread is not a |
686 |
* member of a ForkJoinPool and was interrupted while waiting |
687 |
*/ |
688 |
public final V get() throws InterruptedException, ExecutionException { |
689 |
Thread t = Thread.currentThread(); |
690 |
if (t instanceof ForkJoinWorkerThread) |
691 |
quietlyJoin(); |
692 |
else |
693 |
externalInterruptibleAwaitDone(false, 0L); |
694 |
int s = status; |
695 |
if (s != NORMAL) { |
696 |
Throwable ex; |
697 |
if (s == CANCELLED) |
698 |
throw new CancellationException(); |
699 |
if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null) |
700 |
throw new ExecutionException(ex); |
701 |
} |
702 |
return getRawResult(); |
703 |
} |
704 |
|
705 |
/** |
706 |
* Waits if necessary for at most the given time for the computation |
707 |
* to complete, and then retrieves its result, if available. |
708 |
* |
709 |
* @param timeout the maximum time to wait |
710 |
* @param unit the time unit of the timeout argument |
711 |
* @return the computed result |
712 |
* @throws CancellationException if the computation was cancelled |
713 |
* @throws ExecutionException if the computation threw an |
714 |
* exception |
715 |
* @throws InterruptedException if the current thread is not a |
716 |
* member of a ForkJoinPool and was interrupted while waiting |
717 |
* @throws TimeoutException if the wait timed out |
718 |
*/ |
719 |
public final V get(long timeout, TimeUnit unit) |
720 |
throws InterruptedException, ExecutionException, TimeoutException { |
721 |
long nanos = unit.toNanos(timeout); |
722 |
Thread t = Thread.currentThread(); |
723 |
if (t instanceof ForkJoinWorkerThread) |
724 |
((ForkJoinWorkerThread)t).joinTask(this, true, nanos); |
725 |
else |
726 |
externalInterruptibleAwaitDone(true, nanos); |
727 |
int s = status; |
728 |
if (s != NORMAL) { |
729 |
Throwable ex; |
730 |
if (s == CANCELLED) |
731 |
throw new CancellationException(); |
732 |
if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null) |
733 |
throw new ExecutionException(ex); |
734 |
throw new TimeoutException(); |
735 |
} |
736 |
return getRawResult(); |
737 |
} |
738 |
|
739 |
/** |
740 |
* Joins this task, without returning its result or throwing its |
741 |
* exception. This method may be useful when processing |
742 |
* collections of tasks when some have been cancelled or otherwise |
743 |
* known to have aborted. |
744 |
*/ |
745 |
public final void quietlyJoin() { |
746 |
Thread t; |
747 |
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) { |
748 |
ForkJoinWorkerThread w = (ForkJoinWorkerThread) t; |
749 |
if (status >= 0) { |
750 |
if (w.unpushTask(this)) { |
751 |
boolean completed; |
752 |
try { |
753 |
completed = exec(); |
754 |
} catch (Throwable rex) { |
755 |
setExceptionalCompletion(rex); |
756 |
return; |
757 |
} |
758 |
if (completed) { |
759 |
setCompletion(NORMAL); |
760 |
return; |
761 |
} |
762 |
} |
763 |
w.joinTask(this, false, 0L); |
764 |
} |
765 |
} |
766 |
else |
767 |
externalAwaitDone(); |
768 |
} |
769 |
|
770 |
/** |
771 |
* Commences performing this task and awaits its completion if |
772 |
* necessary, without returning its result or throwing its |
773 |
* exception. |
774 |
*/ |
775 |
public final void quietlyInvoke() { |
776 |
if (status >= 0) { |
777 |
boolean completed; |
778 |
try { |
779 |
completed = exec(); |
780 |
} catch (Throwable rex) { |
781 |
setExceptionalCompletion(rex); |
782 |
return; |
783 |
} |
784 |
if (completed) |
785 |
setCompletion(NORMAL); |
786 |
else |
787 |
quietlyJoin(); |
788 |
} |
789 |
} |
790 |
|
791 |
/** |
792 |
* Possibly executes tasks until the pool hosting the current task |
793 |
* {@link ForkJoinPool#isQuiescent is quiescent}. This method may |
794 |
* be of use in designs in which many tasks are forked, but none |
795 |
* are explicitly joined, instead executing them until all are |
796 |
* processed. |
797 |
* |
798 |
* <p>This method may be invoked only from within {@code |
799 |
* ForkJoinPool} computations (as may be determined using method |
800 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
801 |
* result in exceptions or errors, possibly including {@code |
802 |
* ClassCastException}. |
803 |
*/ |
804 |
public static void helpQuiesce() { |
805 |
((ForkJoinWorkerThread) Thread.currentThread()) |
806 |
.helpQuiescePool(); |
807 |
} |
808 |
|
809 |
/** |
810 |
* Resets the internal bookkeeping state of this task, allowing a |
811 |
* subsequent {@code fork}. This method allows repeated reuse of |
812 |
* this task, but only if reuse occurs when this task has either |
813 |
* never been forked, or has been forked, then completed and all |
814 |
* outstanding joins of this task have also completed. Effects |
815 |
* under any other usage conditions are not guaranteed. |
816 |
* This method may be useful when executing |
817 |
* pre-constructed trees of subtasks in loops. |
818 |
* |
819 |
* <p>Upon completion of this method, {@code isDone()} reports |
820 |
* {@code false}, and {@code getException()} reports {@code |
821 |
* null}. However, the value returned by {@code getRawResult} is |
822 |
* unaffected. To clear this value, you can invoke {@code |
823 |
* setRawResult(null)}. |
824 |
*/ |
825 |
public void reinitialize() { |
826 |
if (status == EXCEPTIONAL) |
827 |
exceptionMap.remove(this); |
828 |
status = 0; |
829 |
} |
830 |
|
831 |
/** |
832 |
* Returns the pool hosting the current task execution, or null |
833 |
* if this task is executing outside of any ForkJoinPool. |
834 |
* |
835 |
* @see #inForkJoinPool |
836 |
* @return the pool, or {@code null} if none |
837 |
*/ |
838 |
public static ForkJoinPool getPool() { |
839 |
Thread t = Thread.currentThread(); |
840 |
return (t instanceof ForkJoinWorkerThread) ? |
841 |
((ForkJoinWorkerThread) t).pool : null; |
842 |
} |
843 |
|
844 |
/** |
845 |
* Returns {@code true} if the current thread is a {@link |
846 |
* ForkJoinWorkerThread} executing as a ForkJoinPool computation. |
847 |
* |
848 |
* @return {@code true} if the current thread is a {@link |
849 |
* ForkJoinWorkerThread} executing as a ForkJoinPool computation, |
850 |
* or {@code false} otherwise |
851 |
*/ |
852 |
public static boolean inForkJoinPool() { |
853 |
return Thread.currentThread() instanceof ForkJoinWorkerThread; |
854 |
} |
855 |
|
856 |
/** |
857 |
* Tries to unschedule this task for execution. This method will |
858 |
* typically succeed if this task is the most recently forked task |
859 |
* by the current thread, and has not commenced executing in |
860 |
* another thread. This method may be useful when arranging |
861 |
* alternative local processing of tasks that could have been, but |
862 |
* were not, stolen. |
863 |
* |
864 |
* <p>This method may be invoked only from within {@code |
865 |
* ForkJoinPool} computations (as may be determined using method |
866 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
867 |
* result in exceptions or errors, possibly including {@code |
868 |
* ClassCastException}. |
869 |
* |
870 |
* @return {@code true} if unforked |
871 |
*/ |
872 |
public boolean tryUnfork() { |
873 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
874 |
.unpushTask(this); |
875 |
} |
876 |
|
877 |
/** |
878 |
* Returns an estimate of the number of tasks that have been |
879 |
* forked by the current worker thread but not yet executed. This |
880 |
* value may be useful for heuristic decisions about whether to |
881 |
* fork other tasks. |
882 |
* |
883 |
* <p>This method may be invoked only from within {@code |
884 |
* ForkJoinPool} computations (as may be determined using method |
885 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
886 |
* result in exceptions or errors, possibly including {@code |
887 |
* ClassCastException}. |
888 |
* |
889 |
* @return the number of tasks |
890 |
*/ |
891 |
public static int getQueuedTaskCount() { |
892 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
893 |
.getQueueSize(); |
894 |
} |
895 |
|
896 |
/** |
897 |
* Returns an estimate of how many more locally queued tasks are |
898 |
* held by the current worker thread than there are other worker |
899 |
* threads that might steal them. This value may be useful for |
900 |
* heuristic decisions about whether to fork other tasks. In many |
901 |
* usages of ForkJoinTasks, at steady state, each worker should |
902 |
* aim to maintain a small constant surplus (for example, 3) of |
903 |
* tasks, and to process computations locally if this threshold is |
904 |
* exceeded. |
905 |
* |
906 |
* <p>This method may be invoked only from within {@code |
907 |
* ForkJoinPool} computations (as may be determined using method |
908 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
909 |
* result in exceptions or errors, possibly including {@code |
910 |
* ClassCastException}. |
911 |
* |
912 |
* @return the surplus number of tasks, which may be negative |
913 |
*/ |
914 |
public static int getSurplusQueuedTaskCount() { |
915 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
916 |
.getEstimatedSurplusTaskCount(); |
917 |
} |
918 |
|
919 |
// Extension methods |
920 |
|
921 |
/** |
922 |
* Returns the result that would be returned by {@link #join}, even |
923 |
* if this task completed abnormally, or {@code null} if this task |
924 |
* is not known to have been completed. This method is designed |
925 |
* to aid debugging, as well as to support extensions. Its use in |
926 |
* any other context is discouraged. |
927 |
* |
928 |
* @return the result, or {@code null} if not completed |
929 |
*/ |
930 |
public abstract V getRawResult(); |
931 |
|
932 |
/** |
933 |
* Forces the given value to be returned as a result. This method |
934 |
* is designed to support extensions, and should not in general be |
935 |
* called otherwise. |
936 |
* |
937 |
* @param value the value |
938 |
*/ |
939 |
protected abstract void setRawResult(V value); |
940 |
|
941 |
/** |
942 |
* Immediately performs the base action of this task. This method |
943 |
* is designed to support extensions, and should not in general be |
944 |
* called otherwise. The return value controls whether this task |
945 |
* is considered to be done normally. It may return false in |
946 |
* asynchronous actions that require explicit invocations of |
947 |
* {@link #complete} to become joinable. It may also throw an |
948 |
* (unchecked) exception to indicate abnormal exit. |
949 |
* |
950 |
* @return {@code true} if completed normally |
951 |
*/ |
952 |
protected abstract boolean exec(); |
953 |
|
954 |
/** |
955 |
* Returns, but does not unschedule or execute, a task queued by |
956 |
* the current thread but not yet executed, if one is immediately |
957 |
* available. There is no guarantee that this task will actually |
958 |
* be polled or executed next. Conversely, this method may return |
959 |
* null even if a task exists but cannot be accessed without |
960 |
* contention with other threads. This method is designed |
961 |
* primarily to support extensions, and is unlikely to be useful |
962 |
* otherwise. |
963 |
* |
964 |
* <p>This method may be invoked only from within {@code |
965 |
* ForkJoinPool} computations (as may be determined using method |
966 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
967 |
* result in exceptions or errors, possibly including {@code |
968 |
* ClassCastException}. |
969 |
* |
970 |
* @return the next task, or {@code null} if none are available |
971 |
*/ |
972 |
protected static ForkJoinTask<?> peekNextLocalTask() { |
973 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
974 |
.peekTask(); |
975 |
} |
976 |
|
977 |
/** |
978 |
* Unschedules and returns, without executing, the next task |
979 |
* queued by the current thread but not yet executed. This method |
980 |
* is designed primarily to support extensions, and is unlikely to |
981 |
* be useful otherwise. |
982 |
* |
983 |
* <p>This method may be invoked only from within {@code |
984 |
* ForkJoinPool} computations (as may be determined using method |
985 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
986 |
* result in exceptions or errors, possibly including {@code |
987 |
* ClassCastException}. |
988 |
* |
989 |
* @return the next task, or {@code null} if none are available |
990 |
*/ |
991 |
protected static ForkJoinTask<?> pollNextLocalTask() { |
992 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
993 |
.pollLocalTask(); |
994 |
} |
995 |
|
996 |
/** |
997 |
* Unschedules and returns, without executing, the next task |
998 |
* queued by the current thread but not yet executed, if one is |
999 |
* available, or if not available, a task that was forked by some |
1000 |
* other thread, if available. Availability may be transient, so a |
1001 |
* {@code null} result does not necessarily imply quiescence |
1002 |
* of the pool this task is operating in. This method is designed |
1003 |
* primarily to support extensions, and is unlikely to be useful |
1004 |
* otherwise. |
1005 |
* |
1006 |
* <p>This method may be invoked only from within {@code |
1007 |
* ForkJoinPool} computations (as may be determined using method |
1008 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
1009 |
* result in exceptions or errors, possibly including {@code |
1010 |
* ClassCastException}. |
1011 |
* |
1012 |
* @return a task, or {@code null} if none are available |
1013 |
*/ |
1014 |
protected static ForkJoinTask<?> pollTask() { |
1015 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
1016 |
.pollTask(); |
1017 |
} |
1018 |
|
1019 |
/** |
1020 |
* Adaptor for Runnables. This implements RunnableFuture |
1021 |
* to be compliant with AbstractExecutorService constraints |
1022 |
* when used in ForkJoinPool. |
1023 |
*/ |
1024 |
static final class AdaptedRunnable<T> extends ForkJoinTask<T> |
1025 |
implements RunnableFuture<T> { |
1026 |
final Runnable runnable; |
1027 |
final T resultOnCompletion; |
1028 |
T result; |
1029 |
AdaptedRunnable(Runnable runnable, T result) { |
1030 |
if (runnable == null) throw new NullPointerException(); |
1031 |
this.runnable = runnable; |
1032 |
this.resultOnCompletion = result; |
1033 |
} |
1034 |
public T getRawResult() { return result; } |
1035 |
public void setRawResult(T v) { result = v; } |
1036 |
public boolean exec() { |
1037 |
runnable.run(); |
1038 |
result = resultOnCompletion; |
1039 |
return true; |
1040 |
} |
1041 |
public void run() { invoke(); } |
1042 |
private static final long serialVersionUID = 5232453952276885070L; |
1043 |
} |
1044 |
|
1045 |
/** |
1046 |
* Adaptor for Callables |
1047 |
*/ |
1048 |
static final class AdaptedCallable<T> extends ForkJoinTask<T> |
1049 |
implements RunnableFuture<T> { |
1050 |
final Callable<? extends T> callable; |
1051 |
T result; |
1052 |
AdaptedCallable(Callable<? extends T> callable) { |
1053 |
if (callable == null) throw new NullPointerException(); |
1054 |
this.callable = callable; |
1055 |
} |
1056 |
public T getRawResult() { return result; } |
1057 |
public void setRawResult(T v) { result = v; } |
1058 |
public boolean exec() { |
1059 |
try { |
1060 |
result = callable.call(); |
1061 |
return true; |
1062 |
} catch (Error err) { |
1063 |
throw err; |
1064 |
} catch (RuntimeException rex) { |
1065 |
throw rex; |
1066 |
} catch (Exception ex) { |
1067 |
throw new RuntimeException(ex); |
1068 |
} |
1069 |
} |
1070 |
public void run() { invoke(); } |
1071 |
private static final long serialVersionUID = 2838392045355241008L; |
1072 |
} |
1073 |
|
1074 |
/** |
1075 |
* Returns a new {@code ForkJoinTask} that performs the {@code run} |
1076 |
* method of the given {@code Runnable} as its action, and returns |
1077 |
* a null result upon {@link #join}. |
1078 |
* |
1079 |
* @param runnable the runnable action |
1080 |
* @return the task |
1081 |
*/ |
1082 |
public static ForkJoinTask<?> adapt(Runnable runnable) { |
1083 |
return new AdaptedRunnable<Void>(runnable, null); |
1084 |
} |
1085 |
|
1086 |
/** |
1087 |
* Returns a new {@code ForkJoinTask} that performs the {@code run} |
1088 |
* method of the given {@code Runnable} as its action, and returns |
1089 |
* the given result upon {@link #join}. |
1090 |
* |
1091 |
* @param runnable the runnable action |
1092 |
* @param result the result upon completion |
1093 |
* @return the task |
1094 |
*/ |
1095 |
public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) { |
1096 |
return new AdaptedRunnable<T>(runnable, result); |
1097 |
} |
1098 |
|
1099 |
/** |
1100 |
* Returns a new {@code ForkJoinTask} that performs the {@code call} |
1101 |
* method of the given {@code Callable} as its action, and returns |
1102 |
* its result upon {@link #join}, translating any checked exceptions |
1103 |
* encountered into {@code RuntimeException}. |
1104 |
* |
1105 |
* @param callable the callable action |
1106 |
* @return the task |
1107 |
*/ |
1108 |
public static <T> ForkJoinTask<T> adapt(Callable<? extends T> callable) { |
1109 |
return new AdaptedCallable<T>(callable); |
1110 |
} |
1111 |
|
1112 |
// Serialization support |
1113 |
|
1114 |
private static final long serialVersionUID = -7721805057305804111L; |
1115 |
|
1116 |
/** |
1117 |
* Saves the state to a stream (that is, serializes it). |
1118 |
* |
1119 |
* @serialData the current run status and the exception thrown |
1120 |
* during execution, or {@code null} if none |
1121 |
* @param s the stream |
1122 |
*/ |
1123 |
private void writeObject(java.io.ObjectOutputStream s) |
1124 |
throws java.io.IOException { |
1125 |
s.defaultWriteObject(); |
1126 |
s.writeObject(getException()); |
1127 |
} |
1128 |
|
1129 |
/** |
1130 |
* Reconstitutes the instance from a stream (that is, deserializes it). |
1131 |
* |
1132 |
* @param s the stream |
1133 |
*/ |
1134 |
private void readObject(java.io.ObjectInputStream s) |
1135 |
throws java.io.IOException, ClassNotFoundException { |
1136 |
s.defaultReadObject(); |
1137 |
Object ex = s.readObject(); |
1138 |
if (ex != null) |
1139 |
setExceptionalCompletion((Throwable) ex); |
1140 |
} |
1141 |
|
1142 |
// Unsafe mechanics |
1143 |
|
1144 |
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
1145 |
private static final long statusOffset = |
1146 |
objectFieldOffset("status", ForkJoinTask.class); |
1147 |
|
1148 |
private static long objectFieldOffset(String field, Class<?> klazz) { |
1149 |
try { |
1150 |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1151 |
} catch (NoSuchFieldException e) { |
1152 |
// Convert Exception to corresponding Error |
1153 |
NoSuchFieldError error = new NoSuchFieldError(field); |
1154 |
error.initCause(e); |
1155 |
throw error; |
1156 |
} |
1157 |
} |
1158 |
|
1159 |
/** |
1160 |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1161 |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1162 |
* into a jdk. |
1163 |
* |
1164 |
* @return a sun.misc.Unsafe |
1165 |
*/ |
1166 |
private static sun.misc.Unsafe getUnsafe() { |
1167 |
try { |
1168 |
return sun.misc.Unsafe.getUnsafe(); |
1169 |
} catch (SecurityException se) { |
1170 |
try { |
1171 |
return java.security.AccessController.doPrivileged |
1172 |
(new java.security |
1173 |
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1174 |
public sun.misc.Unsafe run() throws Exception { |
1175 |
java.lang.reflect.Field f = sun.misc |
1176 |
.Unsafe.class.getDeclaredField("theUnsafe"); |
1177 |
f.setAccessible(true); |
1178 |
return (sun.misc.Unsafe) f.get(null); |
1179 |
}}); |
1180 |
} catch (java.security.PrivilegedActionException e) { |
1181 |
throw new RuntimeException("Could not initialize intrinsics", |
1182 |
e.getCause()); |
1183 |
} |
1184 |
} |
1185 |
} |
1186 |
} |