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/*
* Written by Doug Lea with assistance from members of JCP JSR-166 Expert Group and released to the
* public domain, as explained at http://creativecommons.org/publicdomain/zero/1.0/
*/
package alluxio.concurrent.jsr;
import java.io.Serializable;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.lang.reflect.Constructor;
import java.util.Collection;
import java.util.List;
import java.util.Objects;
import java.util.RandomAccess;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.ReentrantLock;
/**
* Abstract base class for tasks that run within a {@link ForkJoinPool}. A {@code ForkJoinTask} is a
* thread-like entity that is much lighter weight than a normal thread. Huge numbers of tasks and
* subtasks may be hosted by a small number of actual threads in a ForkJoinPool, at the price of
* some usage limitations.
*
*
* A "main" {@code ForkJoinTask} begins execution when it is explicitly submitted to a
* {@link ForkJoinPool}, or, if not already engaged in a ForkJoin computation, commenced in the
* {@link ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or related methods. Once
* started, it will usually in turn start other subtasks. As indicated by the name of this class,
* many programs using {@code ForkJoinTask} employ only methods {@link #fork} and {@link #join}, or
* derivatives such as {@link #invokeAll(ForkJoinTask...) invokeAll}. However, this class also
* provides a number of other methods that can come into play in advanced usages, as well as
* extension mechanics that allow support of new forms of fork/join processing.
*
*
* A {@code ForkJoinTask} is a lightweight form of {@link Future}. The efficiency of
* {@code ForkJoinTask}s stems from a set of restrictions (that are only partially statically
* enforceable) reflecting their main use as computational tasks calculating pure functions or
* operating on purely isolated objects. The primary coordination mechanisms are {@link #fork}, that
* arranges asynchronous execution, and {@link #join}, that doesn't proceed until the task's result
* has been computed. Computations should ideally avoid {@code synchronized} methods or blocks, and
* should minimize other blocking synchronization apart from joining other tasks or using
* synchronizers such as Phasers that are advertised to cooperate with fork/join scheduling.
* Subdividable tasks should also not perform blocking I/O, and should ideally access variables that
* are completely independent of those accessed by other running tasks. These guidelines are loosely
* enforced by not permitting checked exceptions such as {@code IOExceptions} to be thrown. However,
* computations may still encounter unchecked exceptions, that are rethrown to callers attempting to
* join them. These exceptions may additionally include {@link RejectedExecutionException} stemming
* from internal resource exhaustion, such as failure to allocate internal task queues. Rethrown
* exceptions behave in the same way as regular exceptions, but, when possible, contain stack traces
* (as displayed for example using {@code ex.printStackTrace()}) of both the thread that initiated
* the computation as well as the thread actually encountering the exception; minimally only the
* latter.
*
*
* It is possible to define and use ForkJoinTasks that may block, but doing so requires three
* further considerations: (1) Completion of few if any other tasks should be dependent on
* a task that blocks on external synchronization or I/O. Event-style async tasks that are never
* joined (for example, those subclassing {@link CountedCompleter}) often fall into this category.
* (2) To minimize resource impact, tasks should be small; ideally performing only the (possibly)
* blocking action. (3) Unless the {@link ForkJoinPool.ManagedBlocker} API is used, or the number of
* possibly blocked tasks is known to be less than the pool's {@link ForkJoinPool#getParallelism}
* level, the pool cannot guarantee that enough threads will be available to ensure progress or good
* performance.
*
*
* The primary method for awaiting completion and extracting results of a task is {@link #join}, but
* there are several variants: The {@link Future#get} methods support interruptible and/or timed
* waits for completion and report results using {@code Future} conventions. Method {@link #invoke}
* is semantically equivalent to {@code fork(); join()} but always attempts to begin execution in
* the current thread. The "quiet" forms of these methods do not extract results or report
* exceptions. These may be useful when a set of tasks are being executed, and you need to delay
* processing of results or exceptions until all complete. Method {@code invokeAll} (available in
* multiple versions) performs the most common form of parallel invocation: forking a set of tasks
* and joining them all.
*
*
* In the most typical usages, a fork-join pair act like a call (fork) and return (join) from a
* parallel recursive function. As is the case with other forms of recursive calls, returns (joins)
* should be performed innermost-first. For example, {@code a.fork();
* b.fork(); b.join(); a.join();} is likely to be substantially more efficient than joining
* {@code a} before {@code b}.
*
*
* The execution status of tasks may be queried at several levels of detail: {@link #isDone} is true
* if a task completed in any way (including the case where a task was cancelled without executing);
* {@link #isCompletedNormally} is true if a task completed without cancellation or encountering an
* exception; {@link #isCancelled} is true if the task was cancelled (in which case
* {@link #getException} returns a {@link java.util.concurrent.CancellationException}); and
* {@link #isCompletedAbnormally} is true if a task was either cancelled or encountered an
* exception, in which case {@link #getException} will return either the encountered exception or
* {@link java.util.concurrent.CancellationException}.
*
*
* The ForkJoinTask class is not usually directly subclassed. Instead, you subclass one of the
* abstract classes that support a particular style of fork/join processing, typically
* {@link java.util.concurrent.RecursiveAction} for most computations that do not return results,
* {@link java.util.concurrent.RecursiveTask} for those that do, and
* {@link java9.util.concurrent.CountedCompleter} for those in which completed actions trigger other
* actions. Normally, a concrete ForkJoinTask subclass declares fields comprising its parameters,
* established in a constructor, and then defines a {@code compute} method that somehow uses the
* control methods supplied by this base class.
*
*
* Method {@link #join} and its variants are appropriate for use only when completion dependencies
* are acyclic; that is, the parallel computation can be described as a directed acyclic graph
* (DAG). Otherwise, executions may encounter a form of deadlock as tasks cyclically wait for each
* other. However, this framework supports other methods and techniques (for example the use of
* {@link java.util.concurrent.Phaser}, {@link #helpQuiesce}, and {@link #complete}) that may be of
* use in constructing custom subclasses for problems that are not statically structured as DAGs. To
* support such usages, a ForkJoinTask may be atomically tagged with a {@code short} value
* using {@link #setForkJoinTaskTag} or {@link #compareAndSetForkJoinTaskTag} and checked using
* {@link #getForkJoinTaskTag}. The ForkJoinTask implementation does not use these {@code protected}
* methods or tags for any purpose, but they may be of use in the construction of specialized
* subclasses. For example, parallel graph traversals can use the supplied methods to avoid
* revisiting nodes/tasks that have already been processed. (Method names for tagging are bulky in
* part to encourage definition of methods that reflect their usage patterns.)
*
*
* Most base support methods are {@code final}, to prevent overriding of implementations that are
* intrinsically tied to the underlying lightweight task scheduling framework. Developers creating
* new basic styles of fork/join processing should minimally implement {@code protected} methods
* {@link #exec}, {@link #setRawResult}, and {@link #getRawResult}, while also introducing an
* abstract computational method that can be implemented in its subclasses, possibly relying on
* other {@code protected} methods provided by this class.
*
*
* ForkJoinTasks should perform relatively small amounts of computation. Large tasks should be split
* into smaller subtasks, usually via recursive decomposition. As a very rough rule of thumb, a task
* should perform more than 100 and less than 10000 basic computational steps, and should avoid
* indefinite looping. If tasks are too big, then parallelism cannot improve throughput. If too
* small, then memory and internal task maintenance overhead may overwhelm processing.
*
*
* This class provides {@code adapt} methods for {@link Runnable} and {@link Callable}, that may be
* of use when mixing execution of {@code ForkJoinTasks} with other kinds of tasks. When all tasks
* are of this form, consider using a pool constructed in asyncMode.
*
*
* ForkJoinTasks are {@code Serializable}, which enables them to be used in extensions such as
* remote execution frameworks. It is sensible to serialize tasks only before or after, but not
* during, execution. Serialization is not relied on during execution itself.
*
* @since 1.7
* @author Doug Lea
*/
public abstract class ForkJoinTask implements Future, Serializable {
// CVS rev. 1.117
/*
* See the internal documentation of class ForkJoinPool for a general implementation overview.
* ForkJoinTasks are mainly responsible for maintaining their "status" field amidst relays to
* methods in ForkJoinWorkerThread and ForkJoinPool.
*
* The methods of this class are more-or-less layered into (1) basic status maintenance (2)
* execution and awaiting completion (3) user-level methods that additionally report results. This
* is sometimes hard to see because this file orders exported methods in a way that flows well in
* javadocs.
*/
/**
* The status field holds run control status bits packed into a single int to minimize footprint
* and to ensure atomicity (via CAS). Status is initially zero, and takes on nonnegative values
* until completed, upon which status (anded with DONE_MASK) holds value NORMAL, CANCELLED, or
* EXCEPTIONAL. Tasks undergoing blocking waits by other threads have the SIGNAL bit set.
* Completion of a stolen task with SIGNAL set awakens any waiters via notifyAll. Even though
* suboptimal for some purposes, we use basic builtin wait/notify to take advantage of "monitor
* inflation" in JVMs that we would otherwise need to emulate to avoid adding further per-task
* bookkeeping overhead. We want these monitors to be "fat", i.e., not use biasing or thin-lock
* techniques, so use some odd coding idioms that tend to avoid them, mainly by arranging that
* every synchronized block performs a wait, notifyAll or both.
*
* These control bits occupy only (some of) the upper half (16 bits) of status field. The lower
* bits are used for user-defined tags.
*/
static final int DONE_MASK = 0xf0000000; // mask out non-completion bits
static final int NORMAL = 0xf0000000; // must be negative
static final int CANCELLED = 0xc0000000; // must be < NORMAL
static final int EXCEPTIONAL = 0x80000000; // must be < CANCELLED
static final int SIGNAL = 0x00010000; // must be >= 1 << 16
static final int SMASK = 0x0000ffff; // short bits for tags
/**
* Hash table of exceptions thrown by tasks, to enable reporting by callers. Because exceptions
* are rare, we don't directly keep them with task objects, but instead use a weak ref table. Note
* that cancellation exceptions don't appear in the table, but are instead recorded as status
* values.
*
* The exception table has a fixed capacity.
*/
private static final ExceptionNode[] exceptionTable = new ExceptionNode[32];
/** Lock protecting access to exceptionTable. */
private static final ReentrantLock exceptionTableLock = new ReentrantLock();
/** Reference queue of stale exceptionally completed tasks. */
private static final ReferenceQueue> exceptionTableRefQueue =
new ReferenceQueue<>();
private static final long serialVersionUID = -7721805057305804111L;
// Unsafe mechanics
private static final sun.misc.Unsafe U = UnsafeAccess.unsafe;
private static final long STATUS;
static {
try {
STATUS = U.objectFieldOffset(ForkJoinTask.class.getDeclaredField("status"));
} catch (Exception e) {
throw new ExceptionInInitializerError(e);
}
}
/** The run status of this task */
volatile int status; // accessed directly by pool and workers
// Exception table support
/**
* Cancels, ignoring any exceptions thrown by cancel. Used during worker and pool shutdown. Cancel
* is spec'ed not to throw any exceptions, but if it does anyway, we have no recourse during
* shutdown, so guard against this case.
*/
static final void cancelIgnoringExceptions(ForkJoinTask t) {
if (t != null && t.status >= 0) {
try {
t.cancel(false);
} catch (Throwable ignore) {
}
}
}
/**
* Polls stale refs and removes them. Call only while holding lock.
*/
private static void expungeStaleExceptions() {
for (Object x; (x = exceptionTableRefQueue.poll()) != null;) {
if (x instanceof ExceptionNode) {
ExceptionNode[] t = exceptionTable;
int i = ((ExceptionNode) x).hashCode & (t.length - 1);
ExceptionNode e = t[i];
ExceptionNode pred = null;
while (e != null) {
ExceptionNode next = e.next;
if (e == x) {
if (pred == null)
t[i] = next;
else
pred.next = next;
break;
}
pred = e;
e = next;
}
}
}
}
/**
* If lock is available, polls stale refs and removes them. Called from ForkJoinPool when pools
* become quiescent.
*/
static final void helpExpungeStaleExceptions() {
final ReentrantLock lock = exceptionTableLock;
if (lock.tryLock()) {
try {
expungeStaleExceptions();
} finally {
lock.unlock();
}
}
}
/**
* A version of "sneaky throw" to relay exceptions.
*/
static void rethrow(Throwable ex) {
ForkJoinTask.uncheckedThrow(ex);
}
/**
* The sneaky part of sneaky throw, relying on generics limitations to evade compiler complaints
* about rethrowing unchecked exceptions.
*/
@SuppressWarnings("unchecked")
static void uncheckedThrow(Throwable t) throws T {
if (t != null)
throw (T) t; // rely on vacuous cast
else
throw new Error("Unknown Exception");
}
/**
* Forks the given tasks, returning when {@code isDone} holds for each task or an (unchecked)
* exception is encountered, in which case the exception is rethrown. If more than one task
* encounters an exception, then this method throws any one of these exceptions. If any task
* encounters an exception, the other may be cancelled. However, the execution status of
* individual tasks is not guaranteed upon exceptional return. The status of each task may be
* obtained using {@link #getException()} and related methods to check if they have been
* cancelled, completed normally or exceptionally, or left unprocessed.
*
* @param t1 the first task
* @param t2 the second task
* @throws NullPointerException if any task is null
*/
public static void invokeAll(ForkJoinTask t1, ForkJoinTask t2) {
int s1, s2;
t2.fork();
if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL)
t1.reportException(s1);
if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL)
t2.reportException(s2);
}
/**
* Forks the given tasks, returning when {@code isDone} holds for each task or an (unchecked)
* exception is encountered, in which case the exception is rethrown. If more than one task
* encounters an exception, then this method throws any one of these exceptions. If any task
* encounters an exception, others may be cancelled. However, the execution status of individual
* tasks is not guaranteed upon exceptional return. The status of each task may be obtained using
* {@link #getException()} and related methods to check if they have been cancelled, completed
* normally or exceptionally, or left unprocessed.
*
* @param tasks the tasks
* @throws NullPointerException if any task is null
*/
public static void invokeAll(ForkJoinTask... tasks) {
Throwable ex = null;
int last = tasks.length - 1;
for (int i = last; i >= 0; --i) {
ForkJoinTask t = tasks[i];
if (t == null) {
if (ex == null)
ex = new NullPointerException();
} else if (i != 0)
t.fork();
else if (t.doInvoke() < NORMAL && ex == null)
ex = t.getException();
}
for (int i = 1; i <= last; ++i) {
ForkJoinTask t = tasks[i];
if (t != null) {
if (ex != null)
t.cancel(false);
else if (t.doJoin() < NORMAL)
ex = t.getException();
}
}
if (ex != null)
rethrow(ex);
}
/**
* Forks all tasks in the specified collection, returning when {@code isDone} holds for each task
* or an (unchecked) exception is encountered, in which case the exception is rethrown. If more
* than one task encounters an exception, then this method throws any one of these exceptions. If
* any task encounters an exception, others may be cancelled. However, the execution status of
* individual tasks is not guaranteed upon exceptional return. The status of each task may be
* obtained using {@link #getException()} and related methods to check if they have been
* cancelled, completed normally or exceptionally, or left unprocessed.
*
* @param tasks the collection of tasks
* @param the type of the values returned from the tasks
* @return the tasks argument, to simplify usage
* @throws NullPointerException if tasks or any element are null
*/
public static > Collection invokeAll(Collection tasks) {
if (!(tasks instanceof RandomAccess) || !(tasks instanceof List)) {
invokeAll(tasks.toArray(new ForkJoinTask[tasks.size()]));
return tasks;
}
List> ts = (List>) tasks;
Throwable ex = null;
int last = ts.size() - 1;
for (int i = last; i >= 0; --i) {
ForkJoinTask t = ts.get(i);
if (t == null) {
if (ex == null)
ex = new NullPointerException();
} else if (i != 0)
t.fork();
else if (t.doInvoke() < NORMAL && ex == null)
ex = t.getException();
}
for (int i = 1; i <= last; ++i) {
ForkJoinTask t = ts.get(i);
if (t != null) {
if (ex != null)
t.cancel(false);
else if (t.doJoin() < NORMAL)
ex = t.getException();
}
}
if (ex != null)
rethrow(ex);
return tasks;
}
/**
* Possibly executes tasks until the pool hosting the current task
* {@linkplain ForkJoinPool#isQuiescent is quiescent}. This method may be of use in designs in
* which many tasks are forked, but none are explicitly joined, instead executing them until all
* are processed.
*/
public static void helpQuiesce() {
Thread t;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
ForkJoinWorkerThread wt = (ForkJoinWorkerThread) t;
wt.pool.helpQuiescePool(wt.workQueue);
} else
ForkJoinPool.quiesceCommonPool();
}
/**
* Returns the pool hosting the current thread, or {@code null} if the current thread is executing
* outside of any ForkJoinPool.
*
*
* This method returns {@code null} if and only if {@link #inForkJoinPool} returns {@code false}.
*
* @return the pool, or {@code null} if none
*/
public static ForkJoinPool getPool() {
Thread t = Thread.currentThread();
return (t instanceof ForkJoinWorkerThread) ? ((ForkJoinWorkerThread) t).pool : null;
}
/**
* Returns {@code true} if the current thread is a {@link ForkJoinWorkerThread} executing as a
* ForkJoinPool computation.
*
* @return {@code true} if the current thread is a {@link ForkJoinWorkerThread} executing as a
* ForkJoinPool computation, or {@code false} otherwise
*/
public static boolean inForkJoinPool() {
return Thread.currentThread() instanceof ForkJoinWorkerThread;
}
/**
* Returns an estimate of the number of tasks that have been forked by the current worker thread
* but not yet executed. This value may be useful for heuristic decisions about whether to fork
* other tasks.
*
* @return the number of tasks
*/
public static int getQueuedTaskCount() {
Thread t;
ForkJoinPool.WorkQueue q;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
q = ((ForkJoinWorkerThread) t).workQueue;
else
q = ForkJoinPool.commonSubmitterQueue();
return (q == null) ? 0 : q.queueSize();
}
/**
* Returns an estimate of how many more locally queued tasks are held by the current worker thread
* than there are other worker threads that might steal them, or zero if this thread is not
* operating in a ForkJoinPool. This value may be useful for heuristic decisions about whether to
* fork other tasks. In many usages of ForkJoinTasks, at steady state, each worker should aim to
* maintain a small constant surplus (for example, 3) of tasks, and to process computations
* locally if this threshold is exceeded.
*
* @return the surplus number of tasks, which may be negative
*/
public static int getSurplusQueuedTaskCount() {
return ForkJoinPool.getSurplusQueuedTaskCount();
}
/**
* Returns, but does not unschedule or execute, a task queued by the current thread but not yet
* executed, if one is immediately available. There is no guarantee that this task will actually
* be polled or executed next. Conversely, this method may return null even if a task exists but
* cannot be accessed without contention with other threads. This method is designed primarily to
* support extensions, and is unlikely to be useful otherwise.
*
* @return the next task, or {@code null} if none are available
*/
protected static ForkJoinTask peekNextLocalTask() {
Thread t;
ForkJoinPool.WorkQueue q;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
q = ((ForkJoinWorkerThread) t).workQueue;
else
q = ForkJoinPool.commonSubmitterQueue();
return (q == null) ? null : q.peek();
}
/**
* Unschedules and returns, without executing, the next task queued by the current thread but not
* yet executed, if the current thread is operating in a ForkJoinPool. This method is designed
* primarily to support extensions, and is unlikely to be useful otherwise.
*
* @return the next task, or {@code null} if none are available
*/
protected static ForkJoinTask pollNextLocalTask() {
Thread t;
return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
? ((ForkJoinWorkerThread) t).workQueue.nextLocalTask()
: null;
}
// public methods
/**
* If the current thread is operating in a ForkJoinPool, unschedules and returns, without
* executing, the next task queued by the current thread but not yet executed, if one is
* available, or if not available, a task that was forked by some other thread, if available.
* Availability may be transient, so a {@code null} result does not necessarily imply quiescence
* of the pool this task is operating in. This method is designed primarily to support extensions,
* and is unlikely to be useful otherwise.
*
* @return a task, or {@code null} if none are available
*/
protected static ForkJoinTask pollTask() {
Thread t;
ForkJoinWorkerThread wt;
return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
? (wt = (ForkJoinWorkerThread) t).pool.nextTaskFor(wt.workQueue)
: null;
}
/**
* If the current thread is operating in a ForkJoinPool, unschedules and returns, without
* executing, a task externally submitted to the pool, if one is available. Availability may be
* transient, so a {@code null} result does not necessarily imply quiescence of the pool. This
* method is designed primarily to support extensions, and is unlikely to be useful otherwise.
*
* @return a task, or {@code null} if none are available
* @since 9
*/
protected static ForkJoinTask pollSubmission() {
Thread t;
return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
? ((ForkJoinWorkerThread) t).pool.pollSubmission()
: null;
}
/**
* Returns a new {@code ForkJoinTask} that performs the {@code run} method of the given
* {@code Runnable} as its action, and returns a null result upon {@link #join}.
*
* @param runnable the runnable action
* @return the task
*/
public static ForkJoinTask adapt(Runnable runnable) {
return new AdaptedRunnableAction(runnable);
}
/**
* Returns a new {@code ForkJoinTask} that performs the {@code run} method of the given
* {@code Runnable} as its action, and returns the given result upon {@link #join}.
*
* @param runnable the runnable action
* @param result the result upon completion
* @param the type of the result
* @return the task
*/
public static ForkJoinTask adapt(Runnable runnable, T result) {
return new AdaptedRunnable(runnable, result);
}
/**
* Returns a new {@code ForkJoinTask} that performs the {@code call} method of the given
* {@code Callable} as its action, and returns its result upon {@link #join}, translating any
* checked exceptions encountered into {@code RuntimeException}.
*
* @param callable the callable action
* @param the type of the callable's result
* @return the task
*/
public static ForkJoinTask adapt(Callable callable) {
return new AdaptedCallable(callable);
}
/**
* Marks completion and wakes up threads waiting to join this task.
*
* @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
* @return completion status on exit
*/
private int setCompletion(int completion) {
for (int s;;) {
if ((s = status) < 0)
return s;
if (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
if ((s >>> 16) != 0)
synchronized (this) {
notifyAll();
}
return completion;
}
}
}
/**
* Primary execution method for stolen tasks. Unless done, calls exec and records status if
* completed, but doesn't wait for completion otherwise.
*
* @return status on exit from this method
*/
final int doExec() {
int s;
boolean completed;
if ((s = status) >= 0) {
try {
completed = exec();
} catch (Throwable rex) {
return setExceptionalCompletion(rex);
}
if (completed)
s = setCompletion(NORMAL);
}
return s;
}
/**
* If not done, sets SIGNAL status and performs Object.wait(timeout). This task may or may not be
* done on exit. Ignores interrupts.
*
* @param timeout using Object.wait conventions.
*/
final void internalWait(long timeout) {
int s;
if ((s = status) >= 0 && // force completer to issue notify
U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
synchronized (this) {
if (status >= 0)
try {
wait(timeout);
} catch (InterruptedException ie) {
}
else
notifyAll();
}
}
}
/**
* Blocks a non-worker-thread until completion.
*
* @return status upon completion
*/
private int externalAwaitDone() {
int s = ((this instanceof CountedCompleter) ? // try helping
ForkJoinPool.common.externalHelpComplete((CountedCompleter) this, 0)
: ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0);
if (s >= 0 && (s = status) >= 0) {
boolean interrupted = false;
do {
if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
synchronized (this) {
if (status >= 0) {
try {
wait(0L);
} catch (InterruptedException ie) {
interrupted = true;
}
} else
notifyAll();
}
}
} while ((s = status) >= 0);
if (interrupted)
Thread.currentThread().interrupt();
}
return s;
}
/**
* Blocks a non-worker-thread until completion or interruption.
*/
private int externalInterruptibleAwaitDone() throws InterruptedException {
int s;
if (Thread.interrupted())
throw new InterruptedException();
if ((s = status) >= 0 && (s = ((this instanceof CountedCompleter)
? ForkJoinPool.common.externalHelpComplete((CountedCompleter) this, 0)
: ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0)) >= 0) {
while ((s = status) >= 0) {
if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
synchronized (this) {
if (status >= 0)
wait(0L);
else
notifyAll();
}
}
}
}
return s;
}
/**
* Implementation for join, get, quietlyJoin. Directly handles only cases of already-completed,
* external wait, and unfork+exec. Others are relayed to ForkJoinPool.awaitJoin.
*
* @return status upon completion
*/
private int doJoin() {
int s;
Thread t;
ForkJoinWorkerThread wt;
ForkJoinPool.WorkQueue w;
return (s = status) < 0 ? s
: ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
? (w = (wt = (ForkJoinWorkerThread) t).workQueue).tryUnpush(this) && (s = doExec()) < 0
? s
: wt.pool.awaitJoin(w, this, 0L)
: externalAwaitDone();
}
/**
* Implementation for invoke, quietlyInvoke.
*
* @return status upon completion
*/
private int doInvoke() {
int s;
Thread t;
ForkJoinWorkerThread wt;
return (s = doExec()) < 0 ? s
: ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
? (wt = (ForkJoinWorkerThread) t).pool.awaitJoin(wt.workQueue, this, 0L)
: externalAwaitDone();
}
/**
* Records exception and sets status.
*
* @return status on exit
*/
final int recordExceptionalCompletion(Throwable ex) {
int s;
if ((s = status) >= 0) {
int h = System.identityHashCode(this);
final ReentrantLock lock = exceptionTableLock;
lock.lock();
try {
expungeStaleExceptions();
ExceptionNode[] t = exceptionTable;
int i = h & (t.length - 1);
for (ExceptionNode e = t[i];; e = e.next) {
if (e == null) {
t[i] = new ExceptionNode(this, ex, t[i], exceptionTableRefQueue);
break;
}
if (e.get() == this) // already present
break;
}
} finally {
lock.unlock();
}
s = setCompletion(EXCEPTIONAL);
}
return s;
}
/**
* Records exception and possibly propagates.
*
* @return status on exit
*/
private int setExceptionalCompletion(Throwable ex) {
int s = recordExceptionalCompletion(ex);
if ((s & DONE_MASK) == EXCEPTIONAL)
internalPropagateException(ex);
return s;
}
/**
* Hook for exception propagation support for tasks with completers.
*/
void internalPropagateException(Throwable ex) {}
/**
* Removes exception node and clears status.
*/
private void clearExceptionalCompletion() {
int h = System.identityHashCode(this);
final ReentrantLock lock = exceptionTableLock;
lock.lock();
try {
ExceptionNode[] t = exceptionTable;
int i = h & (t.length - 1);
ExceptionNode e = t[i];
ExceptionNode pred = null;
while (e != null) {
ExceptionNode next = e.next;
if (e.get() == this) {
if (pred == null)
t[i] = next;
else
pred.next = next;
break;
}
pred = e;
e = next;
}
expungeStaleExceptions();
status = 0;
} finally {
lock.unlock();
}
}
/**
* Returns a rethrowable exception for this task, if available. To provide accurate stack traces,
* if the exception was not thrown by the current thread, we try to create a new exception of the
* same type as the one thrown, but with the recorded exception as its cause. If there is no such
* constructor, we instead try to use a no-arg constructor, followed by initCause, to the same
* effect. If none of these apply, or any fail due to other exceptions, we return the recorded
* exception, which is still correct, although it may contain a misleading stack trace.
*
* @return the exception, or null if none
*/
private Throwable getThrowableException() {
int h = System.identityHashCode(this);
ExceptionNode e;
final ReentrantLock lock = exceptionTableLock;
lock.lock();
try {
expungeStaleExceptions();
ExceptionNode[] t = exceptionTable;
e = t[h & (t.length - 1)];
while (e != null && e.get() != this)
e = e.next;
} finally {
lock.unlock();
}
Throwable ex;
if (e == null || (ex = e.ex) == null)
return null;
if (e.thrower != Thread.currentThread().getId()) {
try {
Constructor noArgCtor = null;
// public ctors only
for (Constructor c : ex.getClass().getConstructors()) {
Class[] ps = c.getParameterTypes();
if (ps.length == 0)
noArgCtor = c;
else if (ps.length == 1 && ps[0] == Throwable.class)
return (Throwable) c.newInstance(ex);
}
if (noArgCtor != null) {
Throwable wx = (Throwable) noArgCtor.newInstance();
wx.initCause(ex);
return wx;
}
} catch (Exception ignore) {
}
}
return ex;
}
/**
* Throws exception, if any, associated with the given status.
*/
private void reportException(int s) {
if (s == CANCELLED)
throw new CancellationException();
if (s == EXCEPTIONAL)
rethrow(getThrowableException());
}
/**
* Arranges to asynchronously execute this task in the pool the current task is running in, if
* applicable, or using the {@link ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}.
* While it is not necessarily enforced, it is a usage error to fork a task more than once unless
* it has completed and been reinitialized. Subsequent modifications to the state of this task or
* any data it operates on are not necessarily consistently observable by any thread other than
* the one executing it unless preceded by a call to {@link #join} or related methods, or a call
* to {@link #isDone} returning {@code
* true}.
*
* @return {@code this}, to simplify usage
*/
public final ForkJoinTask fork() {
Thread t;
if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
((ForkJoinWorkerThread) t).workQueue.push(this);
else
ForkJoinPool.common.externalPush(this);
return this;
}
/**
* Returns the result of the computation when it {@linkplain #isDone is done}. This method differs
* from {@link #get()} in that abnormal completion results in {@code RuntimeException} or
* {@code Error}, not {@code ExecutionException}, and that interrupts of the calling thread do
* not cause the method to abruptly return by throwing {@code InterruptedException}.
*
* @return the computed result
*/
public final V join() {
int s;
if ((s = doJoin() & DONE_MASK) != NORMAL)
reportException(s);
return getRawResult();
}
/**
* Commences performing this task, awaits its completion if necessary, and returns its result, or
* throws an (unchecked) {@code RuntimeException} or {@code Error} if the underlying computation
* did so.
*
* @return the computed result
*/
public final V invoke() {
int s;
if ((s = doInvoke() & DONE_MASK) != NORMAL)
reportException(s);
return getRawResult();
}
/**
* Attempts to cancel execution of this task. This attempt will fail if the task has already
* completed or could not be cancelled for some other reason. If successful, and this task has not
* started when {@code cancel} is called, execution of this task is suppressed. After this method
* returns successfully, unless there is an intervening call to {@link #reinitialize}, subsequent
* calls to {@link #isCancelled}, {@link #isDone}, and {@code cancel} will return {@code true} and
* calls to {@link #join} and related methods will result in {@code CancellationException}.
*
*
* This method may be overridden in subclasses, but if so, must still ensure that these properties
* hold. In particular, the {@code cancel} method itself must not throw exceptions.
*
*
* This method is designed to be invoked by other tasks. To terminate the current task,
* you can just return or throw an unchecked exception from its computation method, or invoke
* {@link #completeExceptionally(Throwable)}.
*
* @param mayInterruptIfRunning this value has no effect in the default implementation because
* interrupts are not used to control cancellation.
*
* @return {@code true} if this task is now cancelled
*/
public boolean cancel(boolean mayInterruptIfRunning) {
return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED;
}
public final boolean isDone() {
return status < 0;
}
public final boolean isCancelled() {
return (status & DONE_MASK) == CANCELLED;
}
/**
* Returns {@code true} if this task threw an exception or was cancelled.
*
* @return {@code true} if this task threw an exception or was cancelled
*/
public final boolean isCompletedAbnormally() {
return status < NORMAL;
}
/**
* Returns {@code true} if this task completed without throwing an exception and was not
* cancelled.
*
* @return {@code true} if this task completed without throwing an exception and was not cancelled
*/
public final boolean isCompletedNormally() {
return (status & DONE_MASK) == NORMAL;
}
// Extension methods
/**
* Returns the exception thrown by the base computation, or a {@code CancellationException} if
* cancelled, or {@code null} if none or if the method has not yet completed.
*
* @return the exception, or {@code null} if none
*/
public final Throwable getException() {
int s = status & DONE_MASK;
return ((s >= NORMAL) ? null
: (s == CANCELLED) ? new CancellationException() : getThrowableException());
}
/**
* Completes this task abnormally, and if not already aborted or cancelled, causes it to throw the
* given exception upon {@code join} and related operations. This method may be used to induce
* exceptions in asynchronous tasks, or to force completion of tasks that would not otherwise
* complete. Its use in other situations is discouraged. This method is overridable, but
* overridden versions must invoke {@code super} implementation to maintain guarantees.
*
* @param ex the exception to throw. If this exception is not a {@code RuntimeException} or
* {@code Error}, the actual exception thrown will be a {@code RuntimeException} with cause
* {@code ex}.
*/
public void completeExceptionally(Throwable ex) {
setExceptionalCompletion(
(ex instanceof RuntimeException) || (ex instanceof Error) ? ex : new RuntimeException(ex));
}
/**
* Completes this task, and if not already aborted or cancelled, returning the given value as the
* result of subsequent invocations of {@code join} and related operations. This method may be
* used to provide results for asynchronous tasks, or to provide alternative handling for tasks
* that would not otherwise complete normally. Its use in other situations is discouraged. This
* method is overridable, but overridden versions must invoke {@code super} implementation to
* maintain guarantees.
*
* @param value the result value for this task
*/
public void complete(V value) {
try {
setRawResult(value);
} catch (Throwable rex) {
setExceptionalCompletion(rex);
return;
}
setCompletion(NORMAL);
}
/**
* Completes this task normally without setting a value. The most recent value established by
* {@link #setRawResult} (or {@code
* null} by default) will be returned as the result of subsequent invocations of {@code join} and
* related operations.
*
* @since 1.8
*/
public final void quietlyComplete() {
setCompletion(NORMAL);
}
/**
* Waits if necessary for the computation to complete, and then retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an exception
* @throws InterruptedException if the current thread is not a member of a ForkJoinPool and was
* interrupted while waiting
*/
public final V get() throws InterruptedException, ExecutionException {
int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ? doJoin()
: externalInterruptibleAwaitDone();
if ((s &= DONE_MASK) == CANCELLED)
throw new CancellationException();
if (s == EXCEPTIONAL)
throw new ExecutionException(getThrowableException());
return getRawResult();
}
/**
* Waits if necessary for at most the given time for the computation to complete, and then
* retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an exception
* @throws InterruptedException if the current thread is not a member of a ForkJoinPool and was
* interrupted while waiting
* @throws TimeoutException if the wait timed out
*/
public final V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
int s;
long nanos = unit.toNanos(timeout);
if (Thread.interrupted())
throw new InterruptedException();
if ((s = status) >= 0 && nanos > 0L) {
long d = System.nanoTime() + nanos;
long deadline = (d == 0L) ? 1L : d; // avoid 0
Thread t = Thread.currentThread();
if (t instanceof ForkJoinWorkerThread) {
ForkJoinWorkerThread wt = (ForkJoinWorkerThread) t;
s = wt.pool.awaitJoin(wt.workQueue, this, deadline);
} else if ((s = ((this instanceof CountedCompleter)
? ForkJoinPool.common.externalHelpComplete((CountedCompleter) this, 0)
: ForkJoinPool.common.tryExternalUnpush(this) ? doExec() : 0)) >= 0) {
long ns, ms; // measure in nanosecs, but wait in millisecs
while ((s = status) >= 0 && (ns = deadline - System.nanoTime()) > 0L) {
if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L
&& U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
synchronized (this) {
if (status >= 0)
wait(ms); // OK to throw InterruptedException
else
notifyAll();
}
}
}
}
}
if (s >= 0)
s = status;
if ((s &= DONE_MASK) != NORMAL) {
if (s == CANCELLED)
throw new CancellationException();
if (s != EXCEPTIONAL)
throw new TimeoutException();
throw new ExecutionException(getThrowableException());
}
return getRawResult();
}
/**
* Joins this task, without returning its result or throwing its exception. This method may be
* useful when processing collections of tasks when some have been cancelled or otherwise known to
* have aborted.
*/
public final void quietlyJoin() {
doJoin();
}
// tag operations
/**
* Commences performing this task and awaits its completion if necessary, without returning its
* result or throwing its exception.
*/
public final void quietlyInvoke() {
doInvoke();
}
/**
* Resets the internal bookkeeping state of this task, allowing a subsequent {@code fork}. This
* method allows repeated reuse of this task, but only if reuse occurs when this task has either
* never been forked, or has been forked, then completed and all outstanding joins of this task
* have also completed. Effects under any other usage conditions are not guaranteed. This method
* may be useful when executing pre-constructed trees of subtasks in loops.
*
*
* Upon completion of this method, {@code isDone()} reports {@code false}, and
* {@code getException()} reports {@code
* null}. However, the value returned by {@code getRawResult} is unaffected. To clear this value,
* you can invoke {@code
* setRawResult(null)}.
*/
public void reinitialize() {
if ((status & DONE_MASK) == EXCEPTIONAL)
clearExceptionalCompletion();
else
status = 0;
}
/**
* Tries to unschedule this task for execution. This method will typically (but is not guaranteed
* to) succeed if this task is the most recently forked task by the current thread, and has not
* commenced executing in another thread. This method may be useful when arranging alternative
* local processing of tasks that could have been, but were not, stolen.
*
* @return {@code true} if unforked
*/
public boolean tryUnfork() {
Thread t;
return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
? ((ForkJoinWorkerThread) t).workQueue.tryUnpush(this)
: ForkJoinPool.common.tryExternalUnpush(this));
}
/**
* Returns the result that would be returned by {@link #join}, even if this task completed
* abnormally, or {@code null} if this task is not known to have been completed. This method is
* designed to aid debugging, as well as to support extensions. Its use in any other context is
* discouraged.
*
* @return the result, or {@code null} if not completed
*/
public abstract V getRawResult();
/**
* Forces the given value to be returned as a result. This method is designed to support
* extensions, and should not in general be called otherwise.
*
* @param value the value
*/
protected abstract void setRawResult(V value);
/**
* Immediately performs the base action of this task and returns true if, upon return from this
* method, this task is guaranteed to have completed normally. This method may return false
* otherwise, to indicate that this task is not necessarily complete (or is not known to be
* complete), for example in asynchronous actions that require explicit invocations of completion
* methods. This method may also throw an (unchecked) exception to indicate abnormal exit. This
* method is designed to support extensions, and should not in general be called otherwise.
*
* @return {@code true} if this task is known to have completed normally
*/
protected abstract boolean exec();
/**
* Returns the tag for this task.
*
* @return the tag for this task
* @since 1.8
*/
public final short getForkJoinTaskTag() {
return (short) status;
}
/**
* Atomically sets the tag value for this task and returns the old value.
*
* @param newValue the new tag value
* @return the previous value of the tag
* @since 1.8
*/
public final short setForkJoinTaskTag(short newValue) {
for (int s;;) {
if (U.compareAndSwapInt(this, STATUS, s = status, (s & ~SMASK) | (newValue & SMASK)))
return (short) s;
}
}
/**
* Atomically conditionally sets the tag value for this task. Among other applications, tags can
* be used as visit markers in tasks operating on graphs, as in methods that check: {@code
* if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))} before processing, otherwise
* exiting because the node has already been visited.
*
* @param expect the expected tag value
* @param update the new tag value
* @return {@code true} if successful; i.e., the current value was equal to {@code expect} and was
* changed to {@code update}.
* @since 1.8
*/
public final boolean compareAndSetForkJoinTaskTag(short expect, short update) {
for (int s;;) {
if ((short) (s = status) != expect)
return false;
if (U.compareAndSwapInt(this, STATUS, s, (s & ~SMASK) | (update & SMASK)))
return true;
}
}
/**
* Saves this task to a stream (that is, serializes it).
*
* @param s the stream
* @throws java.io.IOException if an I/O error occurs
* @serialData the current run status and the exception thrown during execution, or {@code null}
* if none
*/
private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
s.defaultWriteObject();
s.writeObject(getException());
}
// Serialization support
/**
* Reconstitutes this task from a stream (that is, deserializes it).
*
* @param s the stream
* @throws ClassNotFoundException if the class of a serialized object could not be found
* @throws java.io.IOException if an I/O error occurs
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
Object ex = s.readObject();
if (ex != null)
setExceptionalCompletion((Throwable) ex);
}
/**
* Key-value nodes for exception table. The chained hash table uses identity comparisons, full
* locking, and weak references for keys. The table has a fixed capacity because it only maintains
* task exceptions long enough for joiners to access them, so should never become very large for
* sustained periods. However, since we do not know when the last joiner completes, we must use
* weak references and expunge them. We do so on each operation (hence full locking). Also, some
* thread in any ForkJoinPool will call helpExpungeStaleExceptions when its pool becomes
* isQuiescent.
*/
static final class ExceptionNode extends WeakReference> {
final Throwable ex;
final long thrower; // use id not ref to avoid weak cycles
final int hashCode; // store task hashCode before weak ref disappears
ExceptionNode next;
ExceptionNode(ForkJoinTask task, Throwable ex, ExceptionNode next,
ReferenceQueue> exceptionTableRefQueue) {
super(task, exceptionTableRefQueue);
this.ex = ex;
this.next = next;
this.thrower = Thread.currentThread().getId();
this.hashCode = System.identityHashCode(task);
}
}
/**
* Adapter for Runnables. This implements RunnableFuture to be compliant with
* AbstractExecutorService constraints when used in ForkJoinPool.
*/
static final class AdaptedRunnable extends ForkJoinTask implements RunnableFuture {
private static final long serialVersionUID = 5232453952276885070L;
final Runnable runnable;
T result;
AdaptedRunnable(Runnable runnable, T result) {
this.runnable = Objects.requireNonNull(runnable);
this.result = result; // OK to set this even before completion
}
public final T getRawResult() {
return result;
}
public final void setRawResult(T v) {
result = v;
}
public final boolean exec() {
runnable.run();
return true;
}
public final void run() {
invoke();
}
public String toString() {
return super.toString() + "[Wrapped task = " + runnable + "]";
}
}
/**
* Adapter for Runnables without results.
*/
static final class AdaptedRunnableAction extends ForkJoinTask
implements RunnableFuture {
private static final long serialVersionUID = 5232453952276885070L;
final Runnable runnable;
AdaptedRunnableAction(Runnable runnable) {
this.runnable = Objects.requireNonNull(runnable);
}
public final Void getRawResult() {
return null;
}
public final void setRawResult(Void v) {}
public final boolean exec() {
runnable.run();
return true;
}
public final void run() {
invoke();
}
public String toString() {
return super.toString() + "[Wrapped task = " + runnable + "]";
}
}
/**
* Adapter for Runnables in which failure forces worker exception.
*/
static final class RunnableExecuteAction extends ForkJoinTask {
private static final long serialVersionUID = 5232453952276885070L;
final Runnable runnable;
RunnableExecuteAction(Runnable runnable) {
this.runnable = Objects.requireNonNull(runnable);
}
public final Void getRawResult() {
return null;
}
public final void setRawResult(Void v) {}
public final boolean exec() {
runnable.run();
return true;
}
void internalPropagateException(Throwable ex) {
rethrow(ex); // rethrow outside exec() catches.
}
}
/**
* Adapter for Callables.
*/
static final class AdaptedCallable extends ForkJoinTask implements RunnableFuture {
private static final long serialVersionUID = 2838392045355241008L;
final Callable callable;
T result;
AdaptedCallable(Callable callable) {
this.callable = Objects.requireNonNull(callable);
}
public final T getRawResult() {
return result;
}
public final void setRawResult(T v) {
result = v;
}
public final boolean exec() {
try {
result = callable.call();
return true;
} catch (RuntimeException rex) {
throw rex;
} catch (Exception ex) {
throw new RuntimeException(ex);
}
}
public final void run() {
invoke();
}
public String toString() {
return super.toString() + "[Wrapped task = " + callable + "]";
}
}
}