ForkJoinTask.java revision 2805
1771N/A * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 1771N/A * This code is free software; you can redistribute it and/or modify it 1771N/A * under the terms of the GNU General Public License version 2 only, as 2362N/A * published by the Free Software Foundation. Oracle designates this 1771N/A * particular file as subject to the "Classpath" exception as provided 2362N/A * by Oracle in the LICENSE file that accompanied this code. 1771N/A * This code is distributed in the hope that it will be useful, but WITHOUT 1771N/A * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 1771N/A * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 1771N/A * version 2 for more details (a copy is included in the LICENSE file that 1771N/A * You should have received a copy of the GNU General Public License version 1771N/A * 2 along with this work; if not, write to the Free Software Foundation, 1771N/A * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 2362N/A * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 2362N/A * or visit www.oracle.com if you need additional information or have any 1771N/A * This file is available under and governed by the GNU General Public 1771N/A * License version 2 only, as published by the Free Software Foundation. 1771N/A * However, the following notice accompanied the original version of this 1771N/A * Written by Doug Lea with assistance from members of JCP JSR-166 1771N/A * Expert Group and released to the public domain, as explained at 1771N/A * Abstract base class for tasks that run within a {@link ForkJoinPool}. 1771N/A * A {@code ForkJoinTask} is a thread-like entity that is much 1771N/A * lighter weight than a normal thread. Huge numbers of tasks and 1771N/A * subtasks may be hosted by a small number of actual threads in a 1771N/A * ForkJoinPool, at the price of some usage limitations. 1771N/A * <p>A "main" {@code ForkJoinTask} begins execution when submitted 1771N/A * to a {@link ForkJoinPool}. Once started, it will usually in turn 1771N/A * start other subtasks. As indicated by the name of this class, 1771N/A * many programs using {@code ForkJoinTask} employ only methods 1771N/A * {@link #fork} and {@link #join}, or derivatives such as {@link 2805N/A * #invokeAll(ForkJoinTask...) invokeAll}. However, this class also 2805N/A * provides a number of other methods that can come into play in 2805N/A * advanced usages, as well as extension mechanics that allow 1771N/A * <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}. 1771N/A * The efficiency of {@code ForkJoinTask}s stems from a set of 1771N/A * restrictions (that are only partially statically enforceable) 1771N/A * reflecting their intended use as computational tasks calculating 1771N/A * pure functions or operating on purely isolated objects. The 1771N/A * primary coordination mechanisms are {@link #fork}, that arranges 1771N/A * asynchronous execution, and {@link #join}, that doesn't proceed 1771N/A * until the task's result has been computed. Computations should 1771N/A * avoid {@code synchronized} methods or blocks, and should minimize 1771N/A * other blocking synchronization apart from joining other tasks or 1771N/A * using synchronizers such as Phasers that are advertised to 1771N/A * cooperate with fork/join scheduling. Tasks should also not perform 1771N/A * blocking IO, and should ideally access variables that are 1771N/A * completely independent of those accessed by other running 1771N/A * tasks. Minor breaches of these restrictions, for example using 1771N/A * shared output streams, may be tolerable in practice, but frequent 1771N/A * use may result in poor performance, and the potential to 1771N/A * indefinitely stall if the number of threads not waiting for IO or 1771N/A * other external synchronization becomes exhausted. This usage 1771N/A * restriction is in part enforced by not permitting checked 1771N/A * exceptions such as {@code IOExceptions} to be thrown. However, 1771N/A * computations may still encounter unchecked exceptions, that are 1771N/A * rethrown to callers attempting to join them. These exceptions may 1771N/A * additionally include {@link RejectedExecutionException} stemming 1771N/A * from internal resource exhaustion, such as failure to allocate 1771N/A * <p>The primary method for awaiting completion and extracting 1771N/A * results of a task is {@link #join}, but there are several variants: 1771N/A * The {@link Future#get} methods support interruptible and/or timed 1771N/A * waits for completion and report results using {@code Future} 2754N/A * conventions. Method {@link #invoke} is semantically 1771N/A * equivalent to {@code fork(); join()} but always attempts to begin 1771N/A * execution in the current thread. The "<em>quiet</em>" forms of 1771N/A * these methods do not extract results or report exceptions. These 1771N/A * may be useful when a set of tasks are being executed, and you need 1771N/A * to delay processing of results or exceptions until all complete. 1771N/A * Method {@code invokeAll} (available in multiple versions) 1771N/A * performs the most common form of parallel invocation: forking a set 1771N/A * of tasks and joining them all. 1771N/A * <p>The execution status of tasks may be queried at several levels 1771N/A * of detail: {@link #isDone} is true if a task completed in any way 1771N/A * (including the case where a task was cancelled without executing); 1771N/A * {@link #isCompletedNormally} is true if a task completed without 1771N/A * cancellation or encountering an exception; {@link #isCancelled} is 1771N/A * true if the task was cancelled (in which case {@link #getException} 1771N/A * returns a {@link java.util.concurrent.CancellationException}); and 1771N/A * {@link #isCompletedAbnormally} is true if a task was either 1771N/A * cancelled or encountered an exception, in which case {@link 1771N/A * #getException} will return either the encountered exception or 1771N/A * {@link java.util.concurrent.CancellationException}. 1771N/A * <p>The ForkJoinTask class is not usually directly subclassed. 1771N/A * Instead, you subclass one of the abstract classes that support a 1771N/A * RecursiveAction} for computations that do not return results, or 1771N/A * {@link RecursiveTask} for those that do. Normally, a concrete 1771N/A * ForkJoinTask subclass declares fields comprising its parameters, 1771N/A * established in a constructor, and then defines a {@code compute} 1771N/A * method that somehow uses the control methods supplied by this base 1771N/A * class. While these methods have {@code public} access (to allow 1771N/A * instances of different task subclasses to call each other's 1771N/A * methods), some of them may only be called from within other 1771N/A * ForkJoinTasks (as may be determined using method {@link 1771N/A * #inForkJoinPool}). Attempts to invoke them in other contexts 1771N/A * result in exceptions or errors, possibly including 2754N/A * {@code ClassCastException}. 1771N/A * <p>Most base support methods are {@code final}, to prevent 1771N/A * overriding of implementations that are intrinsically tied to the 1771N/A * underlying lightweight task scheduling framework. Developers 1771N/A * creating new basic styles of fork/join processing should minimally 1771N/A * implement {@code protected} methods {@link #exec}, {@link 1771N/A * #setRawResult}, and {@link #getRawResult}, while also introducing 1771N/A * an abstract computational method that can be implemented in its 1771N/A * subclasses, possibly relying on other {@code protected} methods 1771N/A * <p>ForkJoinTasks should perform relatively small amounts of 1771N/A * computation. Large tasks should be split into smaller subtasks, 1771N/A * usually via recursive decomposition. As a very rough rule of thumb, 1771N/A * a task should perform more than 100 and less than 10000 basic 1771N/A * computational steps. If tasks are too big, then parallelism cannot 1771N/A * improve throughput. If too small, then memory and internal task 1771N/A * maintenance overhead may overwhelm processing. 1771N/A * <p>This class provides {@code adapt} methods for {@link Runnable} 1771N/A * and {@link Callable}, that may be of use when mixing execution of 2754N/A * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are 2754N/A * of this form, consider using a pool constructed in <em>asyncMode</em>. 1771N/A * <p>ForkJoinTasks are {@code Serializable}, which enables them to be 1771N/A * used in extensions such as remote execution frameworks. It is 1771N/A * sensible to serialize tasks only before or after, but not during, 1771N/A * execution. Serialization is not relied on during execution itself. 2754N/A * See the internal documentation of class ForkJoinPool for a 2754N/A * general implementation overview. ForkJoinTasks are mainly 2754N/A * responsible for maintaining their "status" field amidst relays 2754N/A * to methods in ForkJoinWorkerThread and ForkJoinPool. The 2754N/A * methods of this class are more-or-less layered into (1) basic 2754N/A * status maintenance (2) execution and awaiting completion (3) 2754N/A * user-level methods that additionally report results. This is 2754N/A * sometimes hard to see because this file orders exported methods 2754N/A * in a way that flows well in javadocs. In particular, most 2754N/A * join mechanics are in method quietlyJoin, below. 2754N/A * The status field holds run control status bits packed into a 2754N/A * single int to minimize footprint and to ensure atomicity (via 2754N/A * CAS). Status is initially zero, and takes on nonnegative 2754N/A * values until completed, upon which status holds value 2754N/A * NORMAL, CANCELLED, or EXCEPTIONAL. Tasks undergoing blocking 2754N/A * waits by other threads have the SIGNAL bit set. Completion of 2754N/A * a stolen task with SIGNAL set awakens any waiters via 2754N/A * notifyAll. Even though suboptimal for some purposes, we use 2754N/A * inflation" in JVMs that we would otherwise need to emulate to 2754N/A * avoid adding further per-task bookkeeping overhead. We want 2754N/A * these monitors to be "fat", i.e., not use biasing or thin-lock 2754N/A * techniques, so use some odd coding idioms that tend to avoid 2754N/A /** The run status of this task */ 1771N/A volatile int status;
// accessed directly by pool and workers 1771N/A * Table of exceptions thrown by tasks, to enable reporting by 1771N/A * callers. Because exceptions are rare, we don't directly keep 1771N/A * them with task objects, but instead use a weak ref table. Note 1771N/A * that cancellation exceptions don't appear in the table, but are 1771N/A * instead recorded as status values. 1771N/A * TODO: Use ConcurrentReferenceHashMap 2754N/A // Maintaining completion status 2754N/A * Marks completion and wakes up threads waiting to join this task, 2754N/A * also clearing signal request bits. 1771N/A * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL 2754N/A * Records exception and sets exceptional completion. 2754N/A * Blocks a worker thread until completion. Called only by 2754N/A * pool. Currently unused -- pool-based waits use timeout 2754N/A int s;
// the odd construction reduces lock bias effects 2754N/A * Blocks a worker thread until completed or timed out. Called 2754N/A * Blocks a non-worker-thread until completion. 2754N/A * Unless done, calls exec and records status if completed, but 2754N/A * doesn't wait for completion otherwise. Primary execution method 2754N/A * for ForkJoinWorkerThread. 1771N/A * Arranges to asynchronously execute this task. While it is not 1771N/A * necessarily enforced, it is a usage error to fork a task more 1771N/A * than once unless it has completed and been reinitialized. 1771N/A * Subsequent modifications to the state of this task or any data 1771N/A * it operates on are not necessarily consistently observable by 1771N/A * any thread other than the one executing it unless preceded by a 1771N/A * call to {@link #join} or related methods, or a call to {@link 1771N/A * #isDone} returning {@code true}. 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * @return {@code this}, to simplify usage 1771N/A * Returns the result of the computation when it {@link #isDone is done}. 1771N/A * This method differs from {@link #get()} in that 1771N/A * abnormal completion results in {@code RuntimeException} or 1771N/A * {@code Error}, not {@code ExecutionException}. 1771N/A * @return the computed result 1771N/A * Commences performing this task, awaits its completion if 2754N/A * necessary, and returns its result, or throws an (unchecked) 2754N/A * {@code RuntimeException} or {@code Error} if the underlying 1771N/A * @return the computed result 1771N/A * Forks the given tasks, returning when {@code isDone} holds for 1771N/A * each task or an (unchecked) exception is encountered, in which 2754N/A * case the exception is rethrown. If more than one task 2754N/A * encounters an exception, then this method throws any one of 2754N/A * these exceptions. If any task encounters an exception, the 2754N/A * other may be cancelled. However, the execution status of 2754N/A * individual tasks is not guaranteed upon exceptional return. The 2754N/A * status of each task may be obtained using {@link 2754N/A * #getException()} and related methods to check if they have been 2754N/A * cancelled, completed normally or exceptionally, or left 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * @param t2 the second task 1771N/A * @throws NullPointerException if any task is null 1771N/A * Forks the given tasks, returning when {@code isDone} holds for 1771N/A * each task or an (unchecked) exception is encountered, in which 2754N/A * case the exception is rethrown. If more than one task 2754N/A * encounters an exception, then this method throws any one of 2754N/A * these exceptions. If any task encounters an exception, others 2754N/A * may be cancelled. However, the execution status of individual 2754N/A * tasks is not guaranteed upon exceptional return. The status of 2754N/A * each task may be obtained using {@link #getException()} and 2754N/A * related methods to check if they have been cancelled, completed 2754N/A * normally or exceptionally, or left unprocessed. 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * @throws NullPointerException if any task is null 1771N/A * Forks all tasks in the specified collection, returning when 1771N/A * {@code isDone} holds for each task or an (unchecked) exception 2754N/A * is encountered, in which case the exception is rethrown. If 2754N/A * more than one task encounters an exception, then this method 2754N/A * throws any one of these exceptions. If any task encounters an 2754N/A * exception, others may be cancelled. However, the execution 2754N/A * status of individual tasks is not guaranteed upon exceptional 2754N/A * return. The status of each task may be obtained using {@link 2754N/A * #getException()} and related methods to check if they have been 2754N/A * cancelled, completed normally or exceptionally, or left 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * @param tasks the collection of tasks 1771N/A * @return the tasks argument, to simplify usage 1771N/A * @throws NullPointerException if tasks or any element are null 1771N/A * Attempts to cancel execution of this task. This attempt will 1771N/A * fail if the task has already completed, has already been 1771N/A * cancelled, or could not be cancelled for some other reason. If 1771N/A * successful, and this task has not started when cancel is 1771N/A * called, execution of this task is suppressed, {@link 1771N/A * #isCancelled} will report true, and {@link #join} will result 1771N/A * in a {@code CancellationException} being thrown. 1771N/A * <p>This method may be overridden in subclasses, but if so, must 1771N/A * still ensure that these minimal properties hold. In particular, 1771N/A * the {@code cancel} method itself must not throw exceptions. 1771N/A * <p>This method is designed to be invoked by <em>other</em> 1771N/A * tasks. To terminate the current task, you can just return or 1771N/A * throw an unchecked exception from its computation method, or 1771N/A * invoke {@link #completeExceptionally}. 1771N/A * @param mayInterruptIfRunning this value is ignored in the 1771N/A * default implementation because tasks are not 1771N/A * cancelled via interruption 1771N/A * @return {@code true} if this task is now cancelled 2754N/A * Cancels, ignoring any exceptions thrown by cancel. Used during 2754N/A * worker and pool shutdown. Cancel is spec'ed not to throw any 2754N/A * exceptions, but if it does anyway, we have no recourse during 2754N/A * shutdown, so guard against this case. 2754N/A * Cancels if current thread is a terminating worker thread, 2754N/A * ignoring any exceptions thrown by cancel. 1771N/A * Returns {@code true} if this task threw an exception or was cancelled. 1771N/A * @return {@code true} if this task threw an exception or was cancelled 1771N/A * Returns {@code true} if this task completed without throwing an 1771N/A * exception and was not cancelled. 1771N/A * @return {@code true} if this task completed without throwing an 1771N/A * exception and was not cancelled 1771N/A * Returns the exception thrown by the base computation, or a 1771N/A * {@code CancellationException} if cancelled, or {@code null} if 1771N/A * none or if the method has not yet completed. 1771N/A * @return the exception, or {@code null} if none 1771N/A * Completes this task abnormally, and if not already aborted or 1771N/A * cancelled, causes it to throw the given exception upon 1771N/A * {@code join} and related operations. This method may be used 1771N/A * to induce exceptions in asynchronous tasks, or to force 1771N/A * completion of tasks that would not otherwise complete. Its use 1771N/A * in other situations is discouraged. This method is 1771N/A * overridable, but overridden versions must invoke {@code super} 1771N/A * implementation to maintain guarantees. 1771N/A * @param ex the exception to throw. If this exception is not a 1771N/A * {@code RuntimeException} or {@code Error}, the actual exception 1771N/A * thrown will be a {@code RuntimeException} with cause {@code ex}. 1771N/A * Completes this task, and if not already aborted or cancelled, 2754N/A * returning the given value as the result of subsequent 2754N/A * invocations of {@code join} and related operations. This method 2754N/A * may be used to provide results for asynchronous tasks, or to 2754N/A * provide alternative handling for tasks that would not otherwise 2754N/A * complete normally. Its use in other situations is 2754N/A * discouraged. This method is overridable, but overridden 2754N/A * versions must invoke {@code super} implementation to maintain 1771N/A * @param value the result value for this task 2805N/A * Waits if necessary for the computation to complete, and then 2805N/A * @return the computed result 2805N/A * @throws CancellationException if the computation was cancelled 2805N/A * @throws ExecutionException if the computation threw an 2805N/A * @throws InterruptedException if the current thread is not a 2805N/A * member of a ForkJoinPool and was interrupted while waiting 2805N/A synchronized (
this) {
// interruptible form of awaitDone 2805N/A * Waits if necessary for at most the given time for the computation 2805N/A * to complete, and then retrieves its result, if available. 2805N/A * @param timeout the maximum time to wait 2805N/A * @param unit the time unit of the timeout argument 2805N/A * @return the computed result 2805N/A * @throws CancellationException if the computation was cancelled 2805N/A * @throws ExecutionException if the computation threw an 2805N/A * @throws InterruptedException if the current thread is not a 2805N/A * member of a ForkJoinPool and was interrupted while waiting 2805N/A * @throws TimeoutException if the wait timed out 2754N/A * Timed wait loop intermixes cases for FJ (pool != null) and 2754N/A * non FJ threads. For FJ, decrement pool count but don't try 2754N/A * for replacement; increment count on completion. For non-FJ, 2754N/A * deal with interrupts. This is messy, but a little less so 2754N/A * than is splitting the FJ and nonFJ cases. 2754N/A boolean dec =
false;
// true if pool count decremented 2754N/A * Joins this task, without returning its result or throwing its 1771N/A * exception. This method may be useful when processing 1771N/A * collections of tasks when some have been cancelled or otherwise 1771N/A * Commences performing this task and awaits its completion if 2754N/A * necessary, without returning its result or throwing its 1771N/A * Possibly executes tasks until the pool hosting the current task 1771N/A * {@link ForkJoinPool#isQuiescent is quiescent}. This method may 1771N/A * be of use in designs in which many tasks are forked, but none 1771N/A * are explicitly joined, instead executing them until all are 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * Resets the internal bookkeeping state of this task, allowing a 1771N/A * subsequent {@code fork}. This method allows repeated reuse of 1771N/A * this task, but only if reuse occurs when this task has either 1771N/A * never been forked, or has been forked, then completed and all 1771N/A * outstanding joins of this task have also completed. Effects 1771N/A * under any other usage conditions are not guaranteed. 1771N/A * This method may be useful when executing 1771N/A * pre-constructed trees of subtasks in loops. 1771N/A * Returns the pool hosting the current task execution, or null 1771N/A * if this task is executing outside of any ForkJoinPool. 1771N/A * @return the pool, or {@code null} if none 1771N/A * Returns {@code true} if the current thread is executing as a 1771N/A * ForkJoinPool computation. 1771N/A * @return {@code true} if the current thread is executing as a 1771N/A * ForkJoinPool computation, or false otherwise 1771N/A * Tries to unschedule this task for execution. This method will 1771N/A * typically succeed if this task is the most recently forked task 1771N/A * by the current thread, and has not commenced executing in 1771N/A * another thread. This method may be useful when arranging 1771N/A * alternative local processing of tasks that could have been, but 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * @return {@code true} if unforked 1771N/A * Returns an estimate of the number of tasks that have been 1771N/A * forked by the current worker thread but not yet executed. This 1771N/A * value may be useful for heuristic decisions about whether to 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * @return the number of tasks 1771N/A * Returns an estimate of how many more locally queued tasks are 1771N/A * held by the current worker thread than there are other worker 1771N/A * threads that might steal them. This value may be useful for 1771N/A * heuristic decisions about whether to fork other tasks. In many 1771N/A * usages of ForkJoinTasks, at steady state, each worker should 1771N/A * aim to maintain a small constant surplus (for example, 3) of 1771N/A * tasks, and to process computations locally if this threshold is 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * @return the surplus number of tasks, which may be negative 1771N/A * Returns the result that would be returned by {@link #join}, even 1771N/A * if this task completed abnormally, or {@code null} if this task 1771N/A * is not known to have been completed. This method is designed 1771N/A * to aid debugging, as well as to support extensions. Its use in 1771N/A * any other context is discouraged. 1771N/A * @return the result, or {@code null} if not completed 1771N/A * Forces the given value to be returned as a result. This method 1771N/A * is designed to support extensions, and should not in general be 1771N/A * Immediately performs the base action of this task. This method 1771N/A * is designed to support extensions, and should not in general be 1771N/A * called otherwise. The return value controls whether this task 1771N/A * is considered to be done normally. It may return false in 1771N/A * asynchronous actions that require explicit invocations of 1771N/A * {@link #complete} to become joinable. It may also throw an 1771N/A * (unchecked) exception to indicate abnormal exit. 1771N/A * @return {@code true} if completed normally 1771N/A * Returns, but does not unschedule or execute, a task queued by 1771N/A * the current thread but not yet executed, if one is immediately 1771N/A * available. There is no guarantee that this task will actually 1771N/A * be polled or executed next. Conversely, this method may return 1771N/A * null even if a task exists but cannot be accessed without 1771N/A * contention with other threads. This method is designed 1771N/A * primarily to support extensions, and is unlikely to be useful 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * @return the next task, or {@code null} if none are available 1771N/A * Unschedules and returns, without executing, the next task 1771N/A * queued by the current thread but not yet executed. This method 1771N/A * is designed primarily to support extensions, and is unlikely to 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * @return the next task, or {@code null} if none are available 1771N/A * Unschedules and returns, without executing, the next task 1771N/A * queued by the current thread but not yet executed, if one is 1771N/A * available, or if not available, a task that was forked by some 1771N/A * other thread, if available. Availability may be transient, so a 1771N/A * {@code null} result does not necessarily imply quiescence 1771N/A * of the pool this task is operating in. This method is designed 1771N/A * primarily to support extensions, and is unlikely to be useful 1771N/A * <p>This method may be invoked only from within {@code 1771N/A * ForkJoinTask} computations (as may be determined using method 1771N/A * {@link #inForkJoinPool}). Attempts to invoke in other contexts 1771N/A * result in exceptions or errors, possibly including {@code 1771N/A * @return a task, or {@code null} if none are available 1771N/A * Adaptor for Runnables. This implements RunnableFuture 1771N/A * to be compliant with AbstractExecutorService constraints 1771N/A * when used in ForkJoinPool. 1771N/A * Returns a new {@code ForkJoinTask} that performs the {@code run} 1771N/A * method of the given {@code Runnable} as its action, and returns 1771N/A * a null result upon {@link #join}. 1771N/A * @param runnable the runnable action 1771N/A * Returns a new {@code ForkJoinTask} that performs the {@code run} 1771N/A * method of the given {@code Runnable} as its action, and returns 1771N/A * the given result upon {@link #join}. 1771N/A * @param runnable the runnable action 1771N/A * @param result the result upon completion 1771N/A * Returns a new {@code ForkJoinTask} that performs the {@code call} 1771N/A * method of the given {@code Callable} as its action, and returns 1771N/A * its result upon {@link #join}, translating any checked exceptions 1771N/A * encountered into {@code RuntimeException}. 1771N/A * @param callable the callable action 2754N/A * Saves the state to a stream (that is, serializes it). 1771N/A * @serialData the current run status and the exception thrown 1771N/A * during execution, or {@code null} if none 2754N/A * Reconstitutes the instance from a stream (that is, deserializes it). 1771N/A // Convert Exception to corresponding Error