java.util.concurrent.FJTask Maven / Gradle / Ivy
/*
* 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/
*/
/*
* Any changes or additions made by the maintainers of the
* streamsupport (https://github.com/stefan-zobel/streamsupport)
* or retrostreams (https://github.com/retrostreams) libraries are
* also released to the public domain, as explained at
* https://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.lang.ref.WeakReference;
import java.io.Serializable;
import java.lang.ref.ReferenceQueue;
import java.lang.reflect.Constructor;
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 FJPool}.
* A {@code FJTask} 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
* FJPool, at the price of some usage limitations.
*
* A "main" {@code FJTask} begins execution when it is
* explicitly submitted to a {@link FJPool}, or, if not already
* engaged in a ForkJoin computation, commenced in the {@link
* FJPool#commonPool()} via {@code fork()}, 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 FJTask} employ only method {@code fork()}.
* 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 FJTask} is a lightweight form of {@link Future}.
* The efficiency of {@code FJTask}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 mechanism is {@code fork()}, that arranges
* asynchronous execution. 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 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
* FJPool.ManagedBlocker} API is used, or the number of possibly
* blocked tasks is known to be less than the pool's level, the
* pool cannot guarantee that enough threads will be available
* to ensure progress or good performance.
*
*
The {@link Future#get} methods support interruptible and/or timed
* waits for completion and report results using {@code Future}
* conventions. 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 #isCancelled} is true if the task was cancelled (in which
* case {@link #getException} returns a {@link java.util.concurrent.CancellationException}).
*
*
The FJTask 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. Normally, a concrete FJTask 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 {@code 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} ) that may be of use in
* constructing custom subclasses for problems that are not statically
* structured as DAGs.
*
*
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
*/
abstract class FJTask implements Future, Serializable {
// CVS rev. 1.117
/*
* See the internal documentation of class FJPool for a
* general implementation overview. ForkJoinTasks are mainly
* responsible for maintaining their "status" field amidst relays
* to methods in FJWorkerThread and FJPool.
*
* 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.
*/
/** The run status of this task */
volatile int status; // accessed directly by pool and workers
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
/**
* 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 = FJPool.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 = (FJPool.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 FJPool.awaitJoin.
*
* @return status upon completion
*/
private int doJoin() {
int s; Thread t; FJWorkerThread wt; FJPool.WorkQueue w;
return (s = status) < 0 ? s :
((t = Thread.currentThread()) instanceof FJWorkerThread) ?
(w = (wt = (FJWorkerThread)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; FJWorkerThread wt;
return (s = doExec()) < 0 ? s :
((t = Thread.currentThread()) instanceof FJWorkerThread) ?
(wt = (FJWorkerThread)t).pool.
awaitJoin(wt.workQueue, this, 0L) :
externalAwaitDone();
}
// Exception table support
/**
* 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<>();
/**
* 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 FJPool will call helpExpungeStaleExceptions when its
* pool becomes isQuiescent.
*/
static final class ExceptionNode extends WeakReference> {
final Throwable ex;
ExceptionNode next;
final long thrower; // use id not ref to avoid weak cycles
final int hashCode; // store task hashCode before weak ref disappears
ExceptionNode(FJTask 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);
}
}
/**
* 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) {
return recordExceptionalCompletion(ex);
}
/**
* 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(FJTask t) {
if (t != null && t.status >= 0) {
try {
t.cancel(false);
} catch (Throwable ignore) {
}
}
}
/**
* 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;
}
/**
* 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 FJPool 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) {
FJTask.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");
}
/**
* 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());
}
// public methods
/**
* 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
*/
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 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
*/
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));
}
/**
* 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 FJPool and was interrupted while waiting
*/
public final V get() throws InterruptedException, ExecutionException {
int s = (Thread.currentThread() instanceof FJWorkerThread) ?
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 FJWorkerThread) {
FJWorkerThread wt = (FJWorkerThread)t;
s = wt.pool.awaitJoin(wt.workQueue, this, deadline);
}
else if ((s = (FJPool.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.
*/
final void quietlyJoin() {
doJoin();
}
/**
* Returns {@code true} if the current thread is a {@link
* FJWorkerThread} executing as a FJPool computation.
*
* @return {@code true} if the current thread is a {@link
* FJWorkerThread} executing as a FJPool computation,
* or {@code false} otherwise
*/
static boolean inForkJoinPool() {
return Thread.currentThread() instanceof FJWorkerThread;
}
// Extension methods
/**
* 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. 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();
// tag operations
/**
* 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
*/
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;
}
}
/**
* Adapter for Runnables. This implements RunnableFuture
* to be compliant with AbstractExecutorService constraints
* when used in FJPool.
*/
static final class AdaptedRunnable extends FJTask
implements RunnableFuture {
final Runnable runnable;
T result;
AdaptedRunnable(Runnable runnable, T result) {
this.runnable = Utils.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 + "]";
}
private static final long serialVersionUID = 5232453952276885070L;
}
/**
* Adapter for Runnables without results.
*/
static final class AdaptedRunnableAction extends FJTask
implements RunnableFuture {
final Runnable runnable;
AdaptedRunnableAction(Runnable runnable) {
this.runnable = Utils.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 + "]";
}
private static final long serialVersionUID = 5232453952276885070L;
}
/**
* Adapter for Runnables in which failure forces worker exception.
*/
static final class RunnableExecuteAction extends FJTask {
final Runnable runnable;
RunnableExecuteAction(Runnable runnable) {
this.runnable = Utils.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.
}
private static final long serialVersionUID = 5232453952276885070L;
}
/**
* Adapter for Callables.
*/
static final class AdaptedCallable extends FJTask
implements RunnableFuture {
final Callable callable;
T result;
AdaptedCallable(Callable callable) {
this.callable = Utils.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 + "]";
}
private static final long serialVersionUID = 2838392045355241008L;
}
// Serialization support
private static final long serialVersionUID = -7721805057305804111L;
/**
* 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());
}
/**
* 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);
}
// Unsafe mechanics
private static final sun.misc.Unsafe U = UnsafeAccess.unsafe;
private static final long STATUS;
static {
try {
STATUS = U.objectFieldOffset(FJTask.class
.getDeclaredField("status"));
} catch (Exception e) {
throw new ExceptionInInitializerError(e);
}
}
}