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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.util.concurrent.*; |
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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.Map; |
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import java.util.WeakHashMap; |
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|
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/** |
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* Abstract base class for tasks that run within a {@link |
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* ForkJoinPool}. A 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" ForkJoinTask begins execution when submitted to a |
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* {@link ForkJoinPool}. Once started, it will usually in turn start |
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* other subtasks. As indicated by the name of this class, many |
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* programs using ForkJoinTasks employ only methods {@code fork} and |
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* {@code join}, or derivatives such as {@code invokeAll}. However, |
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* this class also provides a number of other methods that can come |
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* into play in advanced usages, as well as extension mechanics that |
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* allow support of new forms of fork/join processing. |
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* |
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* <p>A ForkJoinTask is a lightweight form of {@link Future}. The |
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* efficiency of ForkJoinTasks stems from a set of restrictions (that |
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* are only partially statically enforceable) reflecting their |
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* intended use as computational tasks calculating pure functions or |
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* operating on purely isolated objects. The primary coordination |
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* mechanisms are {@link #fork}, that arranges asynchronous execution, |
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* and {@link #join}, that doesn't proceed until the task's result has |
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* been computed. Computations should avoid {@code synchronized} |
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* methods or blocks, and should minimize other blocking |
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* synchronization apart from joining other tasks or using |
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* synchronizers such as Phasers that are advertised to cooperate with |
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* fork/join scheduling. Tasks should also not perform blocking IO, |
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* and should ideally access variables that are completely independent |
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* of those accessed by other running tasks. Minor breaches of these |
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* restrictions, for example using shared output streams, may be |
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* tolerable in practice, but frequent use may result in poor |
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* performance, and the potential to indefinitely stall if the number |
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* of threads not waiting for IO or other external synchronization |
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* becomes exhausted. This usage restriction is in part enforced by |
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* not permitting checked exceptions such as {@code IOExceptions} |
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* to be thrown. However, computations may still encounter unchecked |
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* exceptions, that are rethrown to callers attempting join |
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* them. These exceptions may additionally include |
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* RejectedExecutionExceptions stemming from internal resource |
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* exhaustion such as failure to allocate 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 #helpJoin} enables callers to actively |
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* execute other tasks while awaiting joins, which is sometimes more |
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* efficient but only applies when all subtasks are known to be |
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* strictly tree-structured. Method {@link #invoke} is semantically |
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* equivalent to {@code fork(); join()} but always attempts to |
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* begin execution in the current thread. The "<em>quiet</em>" forms |
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* of 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 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. 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 others |
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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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* ClassCastException. |
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* |
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* <p>Most base support methods are {@code final} because their |
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* implementations are intrinsically tied to the underlying |
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* lightweight task scheduling framework, and so cannot be overridden. |
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* Developers creating new basic styles of fork/join processing should |
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* minimally implement {@code protected} methods |
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* {@link #exec}, {@link #setRawResult}, and |
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* {@link #getRawResult}, while also introducing an abstract |
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* computational method that can be implemented in its subclasses, |
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* possibly relying on other {@code protected} methods provided |
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* by this class. |
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* |
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* <p>ForkJoinTasks should perform relatively small amounts of |
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* computations, otherwise splitting into smaller tasks. As a very |
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* rough rule of thumb, a task should perform more than 100 and less |
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* than 10000 basic computational steps. If tasks are too big, then |
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* parallelism cannot improve throughput. If too small, then memory |
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* and internal task maintenance overhead may overwhelm processing. |
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* |
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* <p>ForkJoinTasks are {@code Serializable}, which enables them |
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* to be used in extensions such as remote execution frameworks. It is |
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* in general sensible to serialize tasks only before or after, but |
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* not during execution. Serialization is not relied on during |
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* 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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* Run control status bits packed into a single int to minimize |
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* footprint and to ensure atomicity (via CAS). Status is |
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* initially zero, and takes on nonnegative values until |
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* completed, upon which status holds COMPLETED. CANCELLED, or |
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* EXCEPTIONAL, which use the top 3 bits. Tasks undergoing |
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* blocking waits by other threads have SIGNAL_MASK bits set -- |
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* bit 15 for external (nonFJ) waits, and the rest a count of |
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* waiting FJ threads. (This representation relies on |
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* ForkJoinPool max thread limits). Completion of a stolen task |
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* with SIGNAL_MASK bits set awakens waiter via notifyAll. Even |
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* though suboptimal for some purposes, we use basic builtin |
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* wait/notify to take advantage of "monitor inflation" in JVMs |
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* that we would otherwise need to emulate to avoid adding further |
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* per-task bookkeeping overhead. Note that bits 16-28 are |
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* currently unused. Also value 0x80000000 is available as spare |
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* completion value. |
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*/ |
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volatile int status; // accessed directly by pool and workers |
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|
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static final int COMPLETION_MASK = 0xe0000000; |
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static final int NORMAL = 0xe0000000; // == mask |
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static final int CANCELLED = 0xc0000000; |
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static final int EXCEPTIONAL = 0xa0000000; |
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static final int SIGNAL_MASK = 0x0000ffff; |
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static final int INTERNAL_SIGNAL_MASK = 0x00007fff; |
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static final int EXTERNAL_SIGNAL = 0x00008000; // top bit of low word |
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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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// within-package utilities |
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|
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/** |
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* Gets current worker thread, or null if not a worker thread. |
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*/ |
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static ForkJoinWorkerThread getWorker() { |
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Thread t = Thread.currentThread(); |
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return ((t instanceof ForkJoinWorkerThread) ? |
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(ForkJoinWorkerThread) t : null); |
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} |
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|
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final boolean casStatus(int cmp, int val) { |
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return UNSAFE.compareAndSwapInt(this, statusOffset, cmp, val); |
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} |
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|
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/** |
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* Workaround for not being able to rethrow unchecked exceptions. |
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*/ |
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static void rethrowException(Throwable ex) { |
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if (ex != null) |
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UNSAFE.throwException(ex); |
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} |
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|
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// Setting 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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* |
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* @param completion one of NORMAL, CANCELLED, EXCEPTIONAL |
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*/ |
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final void setCompletion(int completion) { |
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ForkJoinPool pool = getPool(); |
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if (pool != null) { |
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int s; // Clear signal bits while setting completion status |
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do {} while ((s = status) >= 0 && !casStatus(s, completion)); |
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|
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if ((s & SIGNAL_MASK) != 0) { |
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if ((s &= INTERNAL_SIGNAL_MASK) != 0) |
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pool.updateRunningCount(s); |
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synchronized (this) { notifyAll(); } |
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} |
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} |
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else |
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externallySetCompletion(completion); |
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} |
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|
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/** |
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* Version of setCompletion for non-FJ threads. Leaves signal |
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* bits for unblocked threads to adjust, and always notifies. |
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*/ |
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private void externallySetCompletion(int completion) { |
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int s; |
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do {} while ((s = status) >= 0 && |
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!casStatus(s, (s & SIGNAL_MASK) | completion)); |
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synchronized (this) { notifyAll(); } |
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} |
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|
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/** |
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* Sets status to indicate normal completion. |
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*/ |
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final void setNormalCompletion() { |
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// Try typical fast case -- single CAS, no signal, not already done. |
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// Manually expand casStatus to improve chances of inlining it |
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if (!UNSAFE.compareAndSwapInt(this, statusOffset, 0, NORMAL)) |
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setCompletion(NORMAL); |
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} |
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|
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// internal waiting and notification |
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|
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/** |
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* Performs the actual monitor wait for awaitDone. |
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*/ |
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private void doAwaitDone() { |
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// Minimize lock bias and in/de-flation effects by maximizing |
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// chances of waiting inside sync |
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try { |
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while (status >= 0) |
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synchronized (this) { if (status >= 0) wait(); } |
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} catch (InterruptedException ie) { |
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onInterruptedWait(); |
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} |
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} |
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|
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/** |
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* Performs the actual timed monitor wait for awaitDone. |
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*/ |
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private void doAwaitDone(long startTime, long nanos) { |
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synchronized (this) { |
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try { |
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while (status >= 0) { |
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long nt = nanos - (System.nanoTime() - startTime); |
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if (nt <= 0) |
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break; |
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wait(nt / 1000000, (int) (nt % 1000000)); |
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} |
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} catch (InterruptedException ie) { |
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onInterruptedWait(); |
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} |
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} |
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} |
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|
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// Awaiting completion |
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|
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/** |
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* Sets status to indicate there is joiner, then waits for join, |
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* surrounded with pool notifications. |
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* |
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* @return status upon exit |
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*/ |
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private int awaitDone(ForkJoinWorkerThread w, |
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boolean maintainParallelism) { |
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ForkJoinPool pool = (w == null) ? null : w.pool; |
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int s; |
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while ((s = status) >= 0) { |
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if (casStatus(s, (pool == null) ? s|EXTERNAL_SIGNAL : s+1)) { |
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if (pool == null || !pool.preJoin(this, maintainParallelism)) |
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doAwaitDone(); |
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if (((s = status) & INTERNAL_SIGNAL_MASK) != 0) |
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adjustPoolCountsOnUnblock(pool); |
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break; |
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} |
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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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* Timed version of awaitDone |
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* |
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* @return status upon exit |
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*/ |
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private int awaitDone(ForkJoinWorkerThread w, long nanos) { |
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ForkJoinPool pool = (w == null) ? null : w.pool; |
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int s; |
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while ((s = status) >= 0) { |
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if (casStatus(s, (pool == null) ? s|EXTERNAL_SIGNAL : s+1)) { |
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long startTime = System.nanoTime(); |
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if (pool == null || !pool.preJoin(this, false)) |
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doAwaitDone(startTime, nanos); |
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if ((s = status) >= 0) { |
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adjustPoolCountsOnCancelledWait(pool); |
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s = status; |
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} |
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if (s < 0 && (s & INTERNAL_SIGNAL_MASK) != 0) |
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adjustPoolCountsOnUnblock(pool); |
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break; |
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} |
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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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* Notifies pool that thread is unblocked. Called by signalled |
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* threads when woken by non-FJ threads (which is atypical). |
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*/ |
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private void adjustPoolCountsOnUnblock(ForkJoinPool pool) { |
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int s; |
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do {} while ((s = status) < 0 && !casStatus(s, s & COMPLETION_MASK)); |
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if (pool != null && (s &= INTERNAL_SIGNAL_MASK) != 0) |
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pool.updateRunningCount(s); |
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} |
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|
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/** |
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* Notifies pool to adjust counts on cancelled or timed out wait. |
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*/ |
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private void adjustPoolCountsOnCancelledWait(ForkJoinPool pool) { |
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if (pool != null) { |
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int s; |
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while ((s = status) >= 0 && (s & INTERNAL_SIGNAL_MASK) != 0) { |
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if (casStatus(s, s - 1)) { |
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pool.updateRunningCount(1); |
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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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* Handles interruptions during waits. |
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*/ |
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private void onInterruptedWait() { |
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ForkJoinWorkerThread w = getWorker(); |
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if (w == null) |
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Thread.currentThread().interrupt(); // re-interrupt |
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else if (w.isTerminating()) |
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cancelIgnoringExceptions(); |
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// else if FJworker, ignore interrupt |
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} |
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|
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// Recording and reporting exceptions |
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|
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private void setDoneExceptionally(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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* Throws the exception associated with status s. |
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* |
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* @throws the exception |
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*/ |
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private void reportException(int s) { |
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if ((s &= COMPLETION_MASK) < NORMAL) { |
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if (s == CANCELLED) |
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throw new CancellationException(); |
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else |
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rethrowException(exceptionMap.get(this)); |
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} |
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} |
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|
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/** |
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* Returns result or throws exception using j.u.c.Future conventions. |
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* Only call when {@code isDone} known to be true. |
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*/ |
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private V reportFutureResult() |
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throws ExecutionException, InterruptedException { |
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int s = status & COMPLETION_MASK; |
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if (s < NORMAL) { |
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Throwable ex; |
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if (s == CANCELLED) |
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throw new CancellationException(); |
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if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null) |
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throw new ExecutionException(ex); |
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if (Thread.interrupted()) |
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throw new InterruptedException(); |
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} |
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return getRawResult(); |
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} |
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|
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/** |
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* Returns result or throws exception using j.u.c.Future conventions |
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* with timeouts. |
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*/ |
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private V reportTimedFutureResult() |
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throws InterruptedException, ExecutionException, TimeoutException { |
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Throwable ex; |
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int s = status & COMPLETION_MASK; |
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if (s == NORMAL) |
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return getRawResult(); |
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if (s == CANCELLED) |
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throw new CancellationException(); |
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if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null) |
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throw new ExecutionException(ex); |
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if (Thread.interrupted()) |
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throw new InterruptedException(); |
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throw new TimeoutException(); |
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} |
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|
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// internal execution methods |
408 |
|
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/** |
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* Calls exec, recording completion, and rethrowing exception if |
411 |
* encountered. Caller should normally check status before calling. |
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* |
413 |
* @return true if completed normally |
414 |
*/ |
415 |
private boolean tryExec() { |
416 |
try { // try block must contain only call to exec |
417 |
if (!exec()) |
418 |
return false; |
419 |
} catch (Throwable rex) { |
420 |
setDoneExceptionally(rex); |
421 |
rethrowException(rex); |
422 |
return false; // not reached |
423 |
} |
424 |
setNormalCompletion(); |
425 |
return true; |
426 |
} |
427 |
|
428 |
/** |
429 |
* Main execution method used by worker threads. Invokes |
430 |
* base computation unless already complete. |
431 |
*/ |
432 |
final void quietlyExec() { |
433 |
if (status >= 0) { |
434 |
try { |
435 |
if (!exec()) |
436 |
return; |
437 |
} catch (Throwable rex) { |
438 |
setDoneExceptionally(rex); |
439 |
return; |
440 |
} |
441 |
setNormalCompletion(); |
442 |
} |
443 |
} |
444 |
|
445 |
/** |
446 |
* Calls exec(), recording but not rethrowing exception. |
447 |
* Caller should normally check status before calling. |
448 |
* |
449 |
* @return true if completed normally |
450 |
*/ |
451 |
private boolean tryQuietlyInvoke() { |
452 |
try { |
453 |
if (!exec()) |
454 |
return false; |
455 |
} catch (Throwable rex) { |
456 |
setDoneExceptionally(rex); |
457 |
return false; |
458 |
} |
459 |
setNormalCompletion(); |
460 |
return true; |
461 |
} |
462 |
|
463 |
/** |
464 |
* Cancels, ignoring any exceptions it throws. |
465 |
*/ |
466 |
final void cancelIgnoringExceptions() { |
467 |
try { |
468 |
cancel(false); |
469 |
} catch (Throwable ignore) { |
470 |
} |
471 |
} |
472 |
|
473 |
/** |
474 |
* Main implementation of helpJoin |
475 |
*/ |
476 |
private int busyJoin(ForkJoinWorkerThread w) { |
477 |
int s; |
478 |
ForkJoinTask<?> t; |
479 |
while ((s = status) >= 0 && (t = w.scanWhileJoining(this)) != null) |
480 |
t.quietlyExec(); |
481 |
return (s >= 0) ? awaitDone(w, false) : s; // block if no work |
482 |
} |
483 |
|
484 |
// public methods |
485 |
|
486 |
/** |
487 |
* Arranges to asynchronously execute this task. While it is not |
488 |
* necessarily enforced, it is a usage error to fork a task more |
489 |
* than once unless it has completed and been reinitialized. This |
490 |
* method may be invoked only from within ForkJoinTask |
491 |
* computations (as may be determined using method {@link |
492 |
* #inForkJoinPool}). Attempts to invoke in other contexts result |
493 |
* in exceptions or errors, possibly including ClassCastException. |
494 |
* |
495 |
* @return {@code this}, to simplify usage. |
496 |
*/ |
497 |
public final ForkJoinTask<V> fork() { |
498 |
((ForkJoinWorkerThread) Thread.currentThread()) |
499 |
.pushTask(this); |
500 |
return this; |
501 |
} |
502 |
|
503 |
/** |
504 |
* Returns the result of the computation when it is ready. |
505 |
* This method differs from {@code get} in that abnormal |
506 |
* completion results in RuntimeExceptions or Errors, not |
507 |
* ExecutionExceptions. |
508 |
* |
509 |
* @return the computed result |
510 |
*/ |
511 |
public final V join() { |
512 |
ForkJoinWorkerThread w = getWorker(); |
513 |
if (w == null || status < 0 || !w.unpushTask(this) || !tryExec()) |
514 |
reportException(awaitDone(w, true)); |
515 |
return getRawResult(); |
516 |
} |
517 |
|
518 |
/** |
519 |
* Commences performing this task, awaits its completion if |
520 |
* necessary, and return its result. |
521 |
* |
522 |
* @throws Throwable (a RuntimeException, Error, or unchecked |
523 |
* exception) if the underlying computation did so |
524 |
* @return the computed result |
525 |
*/ |
526 |
public final V invoke() { |
527 |
if (status >= 0 && tryExec()) |
528 |
return getRawResult(); |
529 |
else |
530 |
return join(); |
531 |
} |
532 |
|
533 |
/** |
534 |
* Forks both tasks, returning when {@code isDone} holds for |
535 |
* both of them or an exception is encountered. This method may be |
536 |
* invoked only from within ForkJoinTask computations (as may be |
537 |
* determined using method {@link #inForkJoinPool}). Attempts to |
538 |
* invoke in other contexts result in exceptions or errors, |
539 |
* possibly including ClassCastException. |
540 |
* |
541 |
* @param t1 one task |
542 |
* @param t2 the other task |
543 |
* @throws NullPointerException if t1 or t2 are null |
544 |
* @throws RuntimeException or Error if either task did so |
545 |
*/ |
546 |
public static void invokeAll(ForkJoinTask<?>t1, ForkJoinTask<?> t2) { |
547 |
t2.fork(); |
548 |
t1.invoke(); |
549 |
t2.join(); |
550 |
} |
551 |
|
552 |
/** |
553 |
* Forks the given tasks, returning when {@code isDone} holds |
554 |
* for all of them. If any task encounters an exception, others |
555 |
* may be cancelled. This method may be invoked only from within |
556 |
* ForkJoinTask computations (as may be determined using method |
557 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
558 |
* result in exceptions or errors, possibly including |
559 |
* ClassCastException. |
560 |
* |
561 |
* @param tasks the array of tasks |
562 |
* @throws NullPointerException if tasks or any element are null |
563 |
* @throws RuntimeException or Error if any task did so |
564 |
*/ |
565 |
public static void invokeAll(ForkJoinTask<?>... tasks) { |
566 |
Throwable ex = null; |
567 |
int last = tasks.length - 1; |
568 |
for (int i = last; i >= 0; --i) { |
569 |
ForkJoinTask<?> t = tasks[i]; |
570 |
if (t == null) { |
571 |
if (ex == null) |
572 |
ex = new NullPointerException(); |
573 |
} |
574 |
else if (i != 0) |
575 |
t.fork(); |
576 |
else { |
577 |
t.quietlyInvoke(); |
578 |
if (ex == null) |
579 |
ex = t.getException(); |
580 |
} |
581 |
} |
582 |
for (int i = 1; i <= last; ++i) { |
583 |
ForkJoinTask<?> t = tasks[i]; |
584 |
if (t != null) { |
585 |
if (ex != null) |
586 |
t.cancel(false); |
587 |
else { |
588 |
t.quietlyJoin(); |
589 |
if (ex == null) |
590 |
ex = t.getException(); |
591 |
} |
592 |
} |
593 |
} |
594 |
if (ex != null) |
595 |
rethrowException(ex); |
596 |
} |
597 |
|
598 |
/** |
599 |
* Forks all tasks in the collection, returning when |
600 |
* {@code isDone} holds for all of them. If any task |
601 |
* encounters an exception, others may be cancelled. This method |
602 |
* may be invoked only from within ForkJoinTask computations (as |
603 |
* may be determined using method {@link |
604 |
* #inForkJoinPool}). Attempts to invoke in other contexts result |
605 |
* in exceptions or errors, possibly including ClassCastException. |
606 |
* |
607 |
* @param tasks the collection of tasks |
608 |
* @return the tasks argument, to simplify usage |
609 |
* @throws NullPointerException if tasks or any element are null |
610 |
* @throws RuntimeException or Error if any task did so |
611 |
*/ |
612 |
public static <T extends ForkJoinTask<?>> Collection<T> invokeAll(Collection<T> tasks) { |
613 |
if (!(tasks instanceof List<?>)) { |
614 |
invokeAll(tasks.toArray(new ForkJoinTask<?>[tasks.size()])); |
615 |
return tasks; |
616 |
} |
617 |
@SuppressWarnings("unchecked") |
618 |
List<? extends ForkJoinTask<?>> ts = |
619 |
(List<? extends ForkJoinTask<?>>) tasks; |
620 |
Throwable ex = null; |
621 |
int last = ts.size() - 1; |
622 |
for (int i = last; i >= 0; --i) { |
623 |
ForkJoinTask<?> t = ts.get(i); |
624 |
if (t == null) { |
625 |
if (ex == null) |
626 |
ex = new NullPointerException(); |
627 |
} |
628 |
else if (i != 0) |
629 |
t.fork(); |
630 |
else { |
631 |
t.quietlyInvoke(); |
632 |
if (ex == null) |
633 |
ex = t.getException(); |
634 |
} |
635 |
} |
636 |
for (int i = 1; i <= last; ++i) { |
637 |
ForkJoinTask<?> t = ts.get(i); |
638 |
if (t != null) { |
639 |
if (ex != null) |
640 |
t.cancel(false); |
641 |
else { |
642 |
t.quietlyJoin(); |
643 |
if (ex == null) |
644 |
ex = t.getException(); |
645 |
} |
646 |
} |
647 |
} |
648 |
if (ex != null) |
649 |
rethrowException(ex); |
650 |
return tasks; |
651 |
} |
652 |
|
653 |
/** |
654 |
* Returns {@code true} if the computation performed by this task |
655 |
* has completed (or has been cancelled). |
656 |
* |
657 |
* @return {@code true} if this computation has completed |
658 |
*/ |
659 |
public final boolean isDone() { |
660 |
return status < 0; |
661 |
} |
662 |
|
663 |
/** |
664 |
* Returns {@code true} if this task was cancelled. |
665 |
* |
666 |
* @return {@code true} if this task was cancelled |
667 |
*/ |
668 |
public final boolean isCancelled() { |
669 |
return (status & COMPLETION_MASK) == CANCELLED; |
670 |
} |
671 |
|
672 |
/** |
673 |
* Asserts that the results of this task's computation will not be |
674 |
* used. If a cancellation occurs before attempting to execute this |
675 |
* task, execution will be suppressed, {@link #isCancelled} |
676 |
* will report true, and {@link #join} will result in a |
677 |
* {@code CancellationException} being thrown. Otherwise, when |
678 |
* cancellation races with completion, there are no guarantees |
679 |
* about whether {@code isCancelled} will report {@code true}, |
680 |
* whether {@code join} will return normally or via an exception, |
681 |
* or whether these behaviors will remain consistent upon repeated |
682 |
* invocation. |
683 |
* |
684 |
* <p>This method may be overridden in subclasses, but if so, must |
685 |
* still ensure that these minimal properties hold. In particular, |
686 |
* the cancel method itself must not throw exceptions. |
687 |
* |
688 |
* <p> This method is designed to be invoked by <em>other</em> |
689 |
* tasks. To terminate the current task, you can just return or |
690 |
* throw an unchecked exception from its computation method, or |
691 |
* invoke {@link #completeExceptionally}. |
692 |
* |
693 |
* @param mayInterruptIfRunning this value is ignored in the |
694 |
* default implementation because tasks are not in general |
695 |
* cancelled via interruption |
696 |
* |
697 |
* @return {@code true} if this task is now cancelled |
698 |
*/ |
699 |
public boolean cancel(boolean mayInterruptIfRunning) { |
700 |
setCompletion(CANCELLED); |
701 |
return (status & COMPLETION_MASK) == CANCELLED; |
702 |
} |
703 |
|
704 |
/** |
705 |
* Returns {@code true} if this task threw an exception or was cancelled. |
706 |
* |
707 |
* @return {@code true} if this task threw an exception or was cancelled |
708 |
*/ |
709 |
public final boolean isCompletedAbnormally() { |
710 |
return (status & COMPLETION_MASK) < NORMAL; |
711 |
} |
712 |
|
713 |
/** |
714 |
* Returns the exception thrown by the base computation, or a |
715 |
* CancellationException if cancelled, or null if none or if the |
716 |
* method has not yet completed. |
717 |
* |
718 |
* @return the exception, or {@code null} if none |
719 |
*/ |
720 |
public final Throwable getException() { |
721 |
int s = status & COMPLETION_MASK; |
722 |
if (s >= NORMAL) |
723 |
return null; |
724 |
if (s == CANCELLED) |
725 |
return new CancellationException(); |
726 |
return exceptionMap.get(this); |
727 |
} |
728 |
|
729 |
/** |
730 |
* Completes this task abnormally, and if not already aborted or |
731 |
* cancelled, causes it to throw the given exception upon |
732 |
* {@code join} and related operations. This method may be used |
733 |
* to induce exceptions in asynchronous tasks, or to force |
734 |
* completion of tasks that would not otherwise complete. Its use |
735 |
* in other situations is likely to be wrong. This method is |
736 |
* overridable, but overridden versions must invoke {@code super} |
737 |
* implementation to maintain guarantees. |
738 |
* |
739 |
* @param ex the exception to throw. If this exception is |
740 |
* not a RuntimeException or Error, the actual exception thrown |
741 |
* will be a RuntimeException with cause ex. |
742 |
*/ |
743 |
public void completeExceptionally(Throwable ex) { |
744 |
setDoneExceptionally((ex instanceof RuntimeException) || |
745 |
(ex instanceof Error) ? ex : |
746 |
new RuntimeException(ex)); |
747 |
} |
748 |
|
749 |
/** |
750 |
* Completes this task, and if not already aborted or cancelled, |
751 |
* returning a {@code null} result upon {@code join} and related |
752 |
* operations. This method may be used to provide results for |
753 |
* asynchronous tasks, or to provide alternative handling for |
754 |
* tasks that would not otherwise complete normally. Its use in |
755 |
* other situations is likely to be wrong. This method is |
756 |
* overridable, but overridden versions must invoke {@code super} |
757 |
* implementation to maintain guarantees. |
758 |
* |
759 |
* @param value the result value for this task |
760 |
*/ |
761 |
public void complete(V value) { |
762 |
try { |
763 |
setRawResult(value); |
764 |
} catch (Throwable rex) { |
765 |
setDoneExceptionally(rex); |
766 |
return; |
767 |
} |
768 |
setNormalCompletion(); |
769 |
} |
770 |
|
771 |
public final V get() throws InterruptedException, ExecutionException { |
772 |
ForkJoinWorkerThread w = getWorker(); |
773 |
if (w == null || status < 0 || !w.unpushTask(this) || !tryQuietlyInvoke()) |
774 |
awaitDone(w, true); |
775 |
return reportFutureResult(); |
776 |
} |
777 |
|
778 |
public final V get(long timeout, TimeUnit unit) |
779 |
throws InterruptedException, ExecutionException, TimeoutException { |
780 |
long nanos = unit.toNanos(timeout); |
781 |
ForkJoinWorkerThread w = getWorker(); |
782 |
if (w == null || status < 0 || !w.unpushTask(this) || !tryQuietlyInvoke()) |
783 |
awaitDone(w, nanos); |
784 |
return reportTimedFutureResult(); |
785 |
} |
786 |
|
787 |
/** |
788 |
* Possibly executes other tasks until this task is ready, then |
789 |
* returns the result of the computation. This method may be more |
790 |
* efficient than {@code join}, but is only applicable when |
791 |
* there are no potential dependencies between continuation of the |
792 |
* current task and that of any other task that might be executed |
793 |
* while helping. (This usually holds for pure divide-and-conquer |
794 |
* tasks). This method may be invoked only from within |
795 |
* ForkJoinTask computations (as may be determined using method |
796 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
797 |
* result in exceptions or errors, possibly including |
798 |
* ClassCastException. |
799 |
* |
800 |
* @return the computed result |
801 |
*/ |
802 |
public final V helpJoin() { |
803 |
ForkJoinWorkerThread w = (ForkJoinWorkerThread) Thread.currentThread(); |
804 |
if (status < 0 || !w.unpushTask(this) || !tryExec()) |
805 |
reportException(busyJoin(w)); |
806 |
return getRawResult(); |
807 |
} |
808 |
|
809 |
/** |
810 |
* Possibly executes other tasks until this task is ready. This |
811 |
* method may be invoked only from within ForkJoinTask |
812 |
* computations (as may be determined using method {@link |
813 |
* #inForkJoinPool}). Attempts to invoke in other contexts result |
814 |
* in exceptions or errors, possibly including ClassCastException. |
815 |
*/ |
816 |
public final void quietlyHelpJoin() { |
817 |
if (status >= 0) { |
818 |
ForkJoinWorkerThread w = |
819 |
(ForkJoinWorkerThread) Thread.currentThread(); |
820 |
if (!w.unpushTask(this) || !tryQuietlyInvoke()) |
821 |
busyJoin(w); |
822 |
} |
823 |
} |
824 |
|
825 |
/** |
826 |
* Joins this task, without returning its result or throwing an |
827 |
* exception. This method may be useful when processing |
828 |
* collections of tasks when some have been cancelled or otherwise |
829 |
* known to have aborted. |
830 |
*/ |
831 |
public final void quietlyJoin() { |
832 |
if (status >= 0) { |
833 |
ForkJoinWorkerThread w = getWorker(); |
834 |
if (w == null || !w.unpushTask(this) || !tryQuietlyInvoke()) |
835 |
awaitDone(w, true); |
836 |
} |
837 |
} |
838 |
|
839 |
/** |
840 |
* Commences performing this task and awaits its completion if |
841 |
* necessary, without returning its result or throwing an |
842 |
* exception. This method may be useful when processing |
843 |
* collections of tasks when some have been cancelled or otherwise |
844 |
* known to have aborted. |
845 |
*/ |
846 |
public final void quietlyInvoke() { |
847 |
if (status >= 0 && !tryQuietlyInvoke()) |
848 |
quietlyJoin(); |
849 |
} |
850 |
|
851 |
/** |
852 |
* Possibly executes tasks until the pool hosting the current task |
853 |
* {@link ForkJoinPool#isQuiescent}. This method may be of use in |
854 |
* designs in which many tasks are forked, but none are explicitly |
855 |
* joined, instead executing them until all are processed. |
856 |
*/ |
857 |
public static void helpQuiesce() { |
858 |
((ForkJoinWorkerThread) Thread.currentThread()) |
859 |
.helpQuiescePool(); |
860 |
} |
861 |
|
862 |
/** |
863 |
* Resets the internal bookkeeping state of this task, allowing a |
864 |
* subsequent {@code fork}. This method allows repeated reuse of |
865 |
* this task, but only if reuse occurs when this task has either |
866 |
* never been forked, or has been forked, then completed and all |
867 |
* outstanding joins of this task have also completed. Effects |
868 |
* under any other usage conditions are not guaranteed, and are |
869 |
* almost surely wrong. This method may be useful when executing |
870 |
* pre-constructed trees of subtasks in loops. |
871 |
*/ |
872 |
public void reinitialize() { |
873 |
if ((status & COMPLETION_MASK) == EXCEPTIONAL) |
874 |
exceptionMap.remove(this); |
875 |
status = 0; |
876 |
} |
877 |
|
878 |
/** |
879 |
* Returns the pool hosting the current task execution, or null |
880 |
* if this task is executing outside of any ForkJoinPool. |
881 |
* |
882 |
* @return the pool, or {@code null} if none |
883 |
*/ |
884 |
public static ForkJoinPool getPool() { |
885 |
Thread t = Thread.currentThread(); |
886 |
return (t instanceof ForkJoinWorkerThread) ? |
887 |
((ForkJoinWorkerThread) t).pool : null; |
888 |
} |
889 |
|
890 |
/** |
891 |
* Returns {@code true} if the current thread is executing as a |
892 |
* ForkJoinPool computation. |
893 |
* |
894 |
* @return {@code true} if the current thread is executing as a |
895 |
* ForkJoinPool computation, or false otherwise |
896 |
*/ |
897 |
public static boolean inForkJoinPool() { |
898 |
return Thread.currentThread() instanceof ForkJoinWorkerThread; |
899 |
} |
900 |
|
901 |
/** |
902 |
* Tries to unschedule this task for execution. This method will |
903 |
* typically succeed if this task is the most recently forked task |
904 |
* by the current thread, and has not commenced executing in |
905 |
* another thread. This method may be useful when arranging |
906 |
* alternative local processing of tasks that could have been, but |
907 |
* were not, stolen. This method may be invoked only from within |
908 |
* ForkJoinTask computations (as may be determined using method |
909 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
910 |
* result in exceptions or errors, possibly including |
911 |
* ClassCastException. |
912 |
* |
913 |
* @return {@code true} if unforked |
914 |
*/ |
915 |
public boolean tryUnfork() { |
916 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
917 |
.unpushTask(this); |
918 |
} |
919 |
|
920 |
/** |
921 |
* Returns an estimate of the number of tasks that have been |
922 |
* forked by the current worker thread but not yet executed. This |
923 |
* value may be useful for heuristic decisions about whether to |
924 |
* fork other tasks. |
925 |
* |
926 |
* @return the number of tasks |
927 |
*/ |
928 |
public static int getQueuedTaskCount() { |
929 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
930 |
.getQueueSize(); |
931 |
} |
932 |
|
933 |
/** |
934 |
* Returns an estimate of how many more locally queued tasks are |
935 |
* held by the current worker thread than there are other worker |
936 |
* threads that might steal them. This value may be useful for |
937 |
* heuristic decisions about whether to fork other tasks. In many |
938 |
* usages of ForkJoinTasks, at steady state, each worker should |
939 |
* aim to maintain a small constant surplus (for example, 3) of |
940 |
* tasks, and to process computations locally if this threshold is |
941 |
* exceeded. |
942 |
* |
943 |
* @return the surplus number of tasks, which may be negative |
944 |
*/ |
945 |
public static int getSurplusQueuedTaskCount() { |
946 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
947 |
.getEstimatedSurplusTaskCount(); |
948 |
} |
949 |
|
950 |
// Extension methods |
951 |
|
952 |
/** |
953 |
* Returns the result that would be returned by {@link #join}, even |
954 |
* if this task completed abnormally, or {@code null} if this task |
955 |
* is not known to have been completed. This method is designed |
956 |
* to aid debugging, as well as to support extensions. Its use in |
957 |
* any other context is discouraged. |
958 |
* |
959 |
* @return the result, or {@code null} if not completed |
960 |
*/ |
961 |
public abstract V getRawResult(); |
962 |
|
963 |
/** |
964 |
* Forces the given value to be returned as a result. This method |
965 |
* is designed to support extensions, and should not in general be |
966 |
* called otherwise. |
967 |
* |
968 |
* @param value the value |
969 |
*/ |
970 |
protected abstract void setRawResult(V value); |
971 |
|
972 |
/** |
973 |
* Immediately performs the base action of this task. This method |
974 |
* is designed to support extensions, and should not in general be |
975 |
* called otherwise. The return value controls whether this task |
976 |
* is considered to be done normally. It may return false in |
977 |
* asynchronous actions that require explicit invocations of |
978 |
* {@link #complete} to become joinable. It may throw exceptions |
979 |
* to indicate abnormal exit. |
980 |
* |
981 |
* @return {@code true} if completed normally |
982 |
* @throws Error or RuntimeException if encountered during computation |
983 |
*/ |
984 |
protected abstract boolean exec(); |
985 |
|
986 |
/** |
987 |
* Returns, but does not unschedule or execute, a task queued by |
988 |
* the current thread but not yet executed, if one is immediately |
989 |
* available. There is no guarantee that this task will actually |
990 |
* be polled or executed next. Conversely, this method may return |
991 |
* null even if a task exists but cannot be accessed without |
992 |
* contention with other threads. This method is designed |
993 |
* primarily to support extensions, and is unlikely to be useful |
994 |
* otherwise. This method may be invoked only from within |
995 |
* ForkJoinTask computations (as may be determined using method |
996 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
997 |
* result in exceptions or errors, possibly including |
998 |
* ClassCastException. |
999 |
* |
1000 |
* @return the next task, or {@code null} if none are available |
1001 |
*/ |
1002 |
protected static ForkJoinTask<?> peekNextLocalTask() { |
1003 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
1004 |
.peekTask(); |
1005 |
} |
1006 |
|
1007 |
/** |
1008 |
* Unschedules and returns, without executing, the next task |
1009 |
* queued by the current thread but not yet executed. This method |
1010 |
* is designed primarily to support extensions, and is unlikely to |
1011 |
* be useful otherwise. This method may be invoked only from |
1012 |
* within ForkJoinTask computations (as may be determined using |
1013 |
* method {@link #inForkJoinPool}). Attempts to invoke in other |
1014 |
* contexts result in exceptions or errors, possibly including |
1015 |
* ClassCastException. |
1016 |
* |
1017 |
* @return the next task, or {@code null} if none are available |
1018 |
*/ |
1019 |
protected static ForkJoinTask<?> pollNextLocalTask() { |
1020 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
1021 |
.pollLocalTask(); |
1022 |
} |
1023 |
|
1024 |
/** |
1025 |
* Unschedules and returns, without executing, the next task |
1026 |
* queued by the current thread but not yet executed, if one is |
1027 |
* available, or if not available, a task that was forked by some |
1028 |
* other thread, if available. Availability may be transient, so a |
1029 |
* {@code null} result does not necessarily imply quiescence |
1030 |
* of the pool this task is operating in. This method is designed |
1031 |
* primarily to support extensions, and is unlikely to be useful |
1032 |
* otherwise. This method may be invoked only from within |
1033 |
* ForkJoinTask computations (as may be determined using method |
1034 |
* {@link #inForkJoinPool}). Attempts to invoke in other contexts |
1035 |
* result in exceptions or errors, possibly including |
1036 |
* ClassCastException. |
1037 |
* |
1038 |
* @return a task, or {@code null} if none are available |
1039 |
*/ |
1040 |
protected static ForkJoinTask<?> pollTask() { |
1041 |
return ((ForkJoinWorkerThread) Thread.currentThread()) |
1042 |
.pollTask(); |
1043 |
} |
1044 |
|
1045 |
/** |
1046 |
* Adaptor for Runnables. This implements RunnableFuture |
1047 |
* to be compliant with AbstractExecutorService constraints |
1048 |
* when used in ForkJoinPool. |
1049 |
*/ |
1050 |
static final class AdaptedRunnable<T> extends ForkJoinTask<T> |
1051 |
implements RunnableFuture<T> { |
1052 |
final Runnable runnable; |
1053 |
final T resultOnCompletion; |
1054 |
T result; |
1055 |
AdaptedRunnable(Runnable runnable, T result) { |
1056 |
if (runnable == null) throw new NullPointerException(); |
1057 |
this.runnable = runnable; |
1058 |
this.resultOnCompletion = result; |
1059 |
} |
1060 |
public T getRawResult() { return result; } |
1061 |
public void setRawResult(T v) { result = v; } |
1062 |
public boolean exec() { |
1063 |
runnable.run(); |
1064 |
result = resultOnCompletion; |
1065 |
return true; |
1066 |
} |
1067 |
public void run() { invoke(); } |
1068 |
private static final long serialVersionUID = 5232453952276885070L; |
1069 |
} |
1070 |
|
1071 |
/** |
1072 |
* Adaptor for Callables |
1073 |
*/ |
1074 |
static final class AdaptedCallable<T> extends ForkJoinTask<T> |
1075 |
implements RunnableFuture<T> { |
1076 |
final Callable<T> callable; |
1077 |
T result; |
1078 |
AdaptedCallable(Callable<T> callable) { |
1079 |
if (callable == null) throw new NullPointerException(); |
1080 |
this.callable = callable; |
1081 |
} |
1082 |
public T getRawResult() { return result; } |
1083 |
public void setRawResult(T v) { result = v; } |
1084 |
public boolean exec() { |
1085 |
try { |
1086 |
result = callable.call(); |
1087 |
return true; |
1088 |
} catch (Error err) { |
1089 |
throw err; |
1090 |
} catch (RuntimeException rex) { |
1091 |
throw rex; |
1092 |
} catch (Exception ex) { |
1093 |
throw new RuntimeException(ex); |
1094 |
} |
1095 |
} |
1096 |
public void run() { invoke(); } |
1097 |
private static final long serialVersionUID = 2838392045355241008L; |
1098 |
} |
1099 |
|
1100 |
/** |
1101 |
* Returns a new ForkJoinTask that performs the {@code run} |
1102 |
* method of the given Runnable as its action, and returns a null |
1103 |
* result upon {@code join}. |
1104 |
* |
1105 |
* @param runnable the runnable action |
1106 |
* @return the task |
1107 |
*/ |
1108 |
public static ForkJoinTask<Void> adapt(Runnable runnable) { |
1109 |
return new AdaptedRunnable<Void>(runnable, null); |
1110 |
} |
1111 |
|
1112 |
/** |
1113 |
* Returns a new ForkJoinTask that performs the {@code run} |
1114 |
* method of the given Runnable as its action, and returns the |
1115 |
* given result upon {@code join}. |
1116 |
* |
1117 |
* @param runnable the runnable action |
1118 |
* @param result the result upon completion |
1119 |
* @return the task |
1120 |
*/ |
1121 |
public static <T> ForkJoinTask<T> adapt(Runnable runnable, T result) { |
1122 |
return new AdaptedRunnable<T>(runnable, result); |
1123 |
} |
1124 |
|
1125 |
/** |
1126 |
* Returns a new ForkJoinTask that performs the {@code call} |
1127 |
* method of the given Callable as its action, and returns its |
1128 |
* result upon {@code join}, translating any checked |
1129 |
* exceptions encountered into {@code RuntimeException}. |
1130 |
* |
1131 |
* @param callable the callable action |
1132 |
* @return the task |
1133 |
*/ |
1134 |
public static <T> ForkJoinTask<T> adapt(Callable<T> callable) { |
1135 |
return new AdaptedCallable<T>(callable); |
1136 |
} |
1137 |
|
1138 |
// Serialization support |
1139 |
|
1140 |
private static final long serialVersionUID = -7721805057305804111L; |
1141 |
|
1142 |
/** |
1143 |
* Save the state to a stream. |
1144 |
* |
1145 |
* @serialData the current run status and the exception thrown |
1146 |
* during execution, or {@code null} if none |
1147 |
* @param s the stream |
1148 |
*/ |
1149 |
private void writeObject(java.io.ObjectOutputStream s) |
1150 |
throws java.io.IOException { |
1151 |
s.defaultWriteObject(); |
1152 |
s.writeObject(getException()); |
1153 |
} |
1154 |
|
1155 |
/** |
1156 |
* Reconstitute the instance from a stream. |
1157 |
* |
1158 |
* @param s the stream |
1159 |
*/ |
1160 |
private void readObject(java.io.ObjectInputStream s) |
1161 |
throws java.io.IOException, ClassNotFoundException { |
1162 |
s.defaultReadObject(); |
1163 |
status &= ~INTERNAL_SIGNAL_MASK; // clear internal signal counts |
1164 |
status |= EXTERNAL_SIGNAL; // conservatively set external signal |
1165 |
Object ex = s.readObject(); |
1166 |
if (ex != null) |
1167 |
setDoneExceptionally((Throwable) ex); |
1168 |
} |
1169 |
|
1170 |
// Unsafe mechanics |
1171 |
|
1172 |
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
1173 |
private static final long statusOffset = |
1174 |
objectFieldOffset("status", ForkJoinTask.class); |
1175 |
|
1176 |
private static long objectFieldOffset(String field, Class<?> klazz) { |
1177 |
try { |
1178 |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1179 |
} catch (NoSuchFieldException e) { |
1180 |
// Convert Exception to corresponding Error |
1181 |
NoSuchFieldError error = new NoSuchFieldError(field); |
1182 |
error.initCause(e); |
1183 |
throw error; |
1184 |
} |
1185 |
} |
1186 |
|
1187 |
/** |
1188 |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1189 |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1190 |
* into a jdk. |
1191 |
* |
1192 |
* @return a sun.misc.Unsafe |
1193 |
*/ |
1194 |
private static sun.misc.Unsafe getUnsafe() { |
1195 |
try { |
1196 |
return sun.misc.Unsafe.getUnsafe(); |
1197 |
} catch (SecurityException se) { |
1198 |
try { |
1199 |
return java.security.AccessController.doPrivileged |
1200 |
(new java.security |
1201 |
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
1202 |
public sun.misc.Unsafe run() throws Exception { |
1203 |
java.lang.reflect.Field f = sun.misc |
1204 |
.Unsafe.class.getDeclaredField("theUnsafe"); |
1205 |
f.setAccessible(true); |
1206 |
return (sun.misc.Unsafe) f.get(null); |
1207 |
}}); |
1208 |
} catch (java.security.PrivilegedActionException e) { |
1209 |
throw new RuntimeException("Could not initialize intrinsics", |
1210 |
e.getCause()); |
1211 |
} |
1212 |
} |
1213 |
} |
1214 |
} |