/* * 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/licenses/publicdomain */ package jsr166y; import java.io.Serializable; import java.util.*; import java.util.concurrent.*; import java.util.concurrent.atomic.*; import sun.misc.Unsafe; import java.lang.reflect.*; /** * Abstract base class for tasks that run within a {@link * ForkJoinPool}. A 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" ForkJoinTask begins execution when submitted to a
* {@link ForkJoinPool}. Once started, it will usually in turn start
* other subtasks. As indicated by the name of this class, many
* programs using ForkJoinTasks employ only methods fork
* and join
, or derivatives such as
* 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 ForkJoinTask is a lightweight form of {@link Future}. The
* efficiency of ForkJoinTasks stems from a set of restrictions (that
* are only partially statically enforceable) reflecting their
* intended 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 avoid 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. Tasks should also not perform blocking IO,
* and should ideally access variables that are completely independent
* of those accessed by other running tasks. Minor breaches of these
* restrictions, for example using shared output streams, may be
* tolerable in practice, but frequent use may result in poor
* performance, and the potential to indefinitely stall if the number
* of threads not waiting for IO or other external synchronization
* becomes exhausted. This usage restriction is in part enforced by
* not permitting checked exceptions such as IOExceptions
* to be thrown. However, computations may still encounter unchecked
* exceptions, that are rethrown to callers attempting join
* them. These exceptions may additionally include
* RejectedExecutionExceptions stemming from internal resource
* exhaustion such as failure to allocate internal task queues.
*
*
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 Future
* conventions. Method {@link #helpJoin} enables callers to actively
* execute other tasks while awaiting joins, which is sometimes more
* efficient but only applies when all subtasks are known to be
* strictly tree-structured. Method {@link #invoke} is semantically
* equivalent to 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 invokeAll
(available in multiple versions)
* performs the most common form of parallel invocation: forking a set
* of tasks and joining them all.
*
*
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. Normally, a concrete
* ForkJoinTask subclass declares fields comprising its parameters,
* established in a constructor, and then defines a compute
* method that somehow uses the control methods supplied by this base
* class. While these methods have public
access (to allow
* instances of different task subclasses to call each others
* methods), some of them may only be called from within other
* ForkJoinTasks. Attempts to invoke them in other contexts result in
* exceptions or errors including ClassCastException.
*
*
Most base support methods are final
because their
* implementations are intrinsically tied to the underlying
* lightweight task scheduling framework, and so cannot be overridden.
* Developers creating new basic styles of fork/join processing should
* minimally implement protected
methods
* exec
, setRawResult
, and
* getRawResult
, while also introducing an abstract
* computational method that can be implemented in its subclasses,
* possibly relying on other protected
methods provided
* by this class.
*
*
ForkJoinTasks should perform relatively small amounts of * computations, othewise splitting into smaller tasks. As a very * rough rule of thumb, a task should perform more than 100 and less * than 10000 basic computational steps. If tasks are too big, then * parellelism cannot improve throughput. If too small, then memory * and internal task maintenance overhead may overwhelm processing. * *
ForkJoinTasks are This method may be overridden in subclasses, but if so, must
* still ensure that these minimal properties hold. In particular,
* the 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 Serializable
, which enables them
* to be used in extensions such as remote execution frameworks. It is
* in general sensible to serialize tasks only before or after, but
* not during execution. Serialization is not relied on during
* execution itself.
*/
public abstract class ForkJoinTaskget
in that abnormal
* completion results in RuntimeExceptions or Errors, not
* ExecutionExceptions.
*
* @return the computed result
*/
public final V join() {
ForkJoinWorkerThread w = getWorker();
if (w == null || status < 0 || !w.unpushTask(this) || !tryExec())
reportException(awaitDone(w, true));
return getRawResult();
}
public final V get() throws InterruptedException, ExecutionException {
ForkJoinWorkerThread w = getWorker();
if (w == null || status < 0 || !w.unpushTask(this) || !tryQuietlyInvoke())
awaitDone(w, true);
return reportFutureResult();
}
public final V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
ForkJoinWorkerThread w = getWorker();
if (w == null || status < 0 || !w.unpushTask(this) || !tryQuietlyInvoke())
awaitDone(w, unit.toNanos(timeout));
return reportTimedFutureResult();
}
/**
* Commences performing this task, awaits its completion if
* necessary, and return its result.
* @throws Throwable (a RuntimeException, Error, or unchecked
* exception) if the underlying computation did so.
* @return the computed result
*/
public final V invoke() {
if (status >= 0 && tryExec())
return getRawResult();
else
return join();
}
/**
* Forks both tasks, returning when isDone
holds for
* both of them or an exception is encountered. This method may be
* invoked only from within ForkJoinTask computations. Attempts to
* invoke in other contexts result in exceptions or errors
* including ClassCastException.
* @param t1 one task
* @param t2 the other task
* @throws NullPointerException if t1 or t2 are null
* @throws RuntimeException or Error if either task did so.
*/
public static void invokeAll(ForkJoinTask>t1, ForkJoinTask> t2) {
t2.fork();
t1.invoke();
t2.join();
}
/**
* Forks the given tasks, returning when isDone
holds
* for all of them. If any task encounters an exception, others
* may be cancelled. This method may be invoked only from within
* ForkJoinTask computations. Attempts to invoke in other contexts
* result in exceptions or errors including ClassCastException.
* @param tasks the array of tasks
* @throws NullPointerException if tasks or any element are null.
* @throws RuntimeException or Error if any task did so.
*/
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 {
t.quietlyInvoke();
if (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 {
t.quietlyJoin();
if (ex == null)
ex = t.getException();
}
}
}
if (ex != null)
rethrowException(ex);
}
/**
* Forks all tasks in the collection, returning when
* isDone
holds for all of them. If any task
* encounters an exception, others may be cancelled. This method
* may be invoked only from within ForkJoinTask
* computations. Attempts to invoke in other contexts resul!t in
* exceptions or errors including ClassCastException.
* @param tasks the collection of tasks
* @throws NullPointerException if tasks or any element are null.
* @throws RuntimeException or Error if any task did so.
*/
public static void invokeAll(Collection extends ForkJoinTask>> tasks) {
if (!(tasks instanceof List)) {
invokeAll(tasks.toArray(new ForkJoinTask[tasks.size()]));
return;
}
List extends ForkJoinTask>> ts =
(List extends ForkJoinTask>>)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 {
t.quietlyInvoke();
if (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 {
t.quietlyJoin();
if (ex == null)
ex = t.getException();
}
}
}
if (ex != null)
rethrowException(ex);
}
/**
* Returns true if the computation performed by this task has
* completed (or has been cancelled).
* @return true if this computation has completed
*/
public final boolean isDone() {
return status < 0;
}
/**
* Returns true if this task was cancelled.
* @return true if this task was cancelled
*/
public final boolean isCancelled() {
return (status & COMPLETION_MASK) == CANCELLED;
}
/**
* Returns true if this task threw an exception or was cancelled
* @return true if this task threw an exception or was cancelled
*/
public final boolean isCompletedAbnormally() {
return (status & COMPLETION_MASK) < NORMAL;
}
/**
* Returns the exception thrown by the base computation, or a
* CancellationException if cancelled, or null if none or if the
* method has not yet completed.
* @return the exception, or null if none
*/
public final Throwable getException() {
int s = status & COMPLETION_MASK;
if (s >= NORMAL)
return null;
if (s == CANCELLED)
return new CancellationException();
return exceptionMap.get(this);
}
/**
* Asserts that the results of this task's computation will not be
* used. If a cancellation occurs before atempting to execute this
* task, then execution will be suppressed, isCancelled
* will report true, and join
will result in a
* CancellationException
being thrown. Otherwise, when
* cancellation races with completion, there are no guarantees
* about whether isCancelled
will report true, whether
* join
will return normally or via an exception, or
* whether these behaviors will remain consistent upon repeated
* invocation.
*
* completeExceptionally
.
*
* @param mayInterruptIfRunning this value is ignored in the
* default implementation because tasks are not in general
* cancelled via interruption.
*
* @return true if this task is now cancelled
*/
public boolean cancel(boolean mayInterruptIfRunning) {
setCompletion(CANCELLED);
return (status & COMPLETION_MASK) == CANCELLED;
}
/**
* Completes this task abnormally, and if not already aborted or
* cancelled, causes it to throw the given exception upon
* 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 likely to be wrong. This method is
* overridable, but overridden versions must invoke super
* implementation to maintain guarantees.
*
* @param ex the exception to throw. If this exception is
* not a RuntimeException or Error, the actual exception thrown
* will be a RuntimeException with cause ex.
*/
public void completeExceptionally(Throwable ex) {
setDoneExceptionally((ex instanceof RuntimeException) ||
(ex instanceof Error)? ex :
new RuntimeException(ex));
}
/**
* Completes this task, and if not already aborted or cancelled,
* returning a null
result upon 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 likely to be wrong. This method is
* overridable, but overridden versions must invoke super
* implementation to maintain guarantees.
*
* @param value the result value for this task.
*/
public void complete(V value) {
try {
setRawResult(value);
} catch(Throwable rex) {
setDoneExceptionally(rex);
return;
}
setNormalCompletion();
}
/**
* Possibly executes other tasks until this task is ready, then
* returns the result of the computation. This method may be more
* efficient than join
, but is only applicable when
* there are no potemtial dependencies between continuation of the
* current task and that of any other task that might be executed
* while helping. (This usually holds for pure divide-and-conquer
* tasks). This method may be invoked only from within
* ForkJoinTask computations. Attempts to invoke in other contexts
* resul!t in exceptions or errors including ClassCastException.
* @return the computed result
*/
public final V helpJoin() {
ForkJoinWorkerThread w = (ForkJoinWorkerThread)(Thread.currentThread());
if (status < 0 || !w.unpushTask(this) || !tryExec())
reportException(w.helpJoinTask(this));
return getRawResult();
}
/**
* Possibly executes other tasks until this task is ready. This
* method may be invoked only from within ForkJoinTask
* computations. Attempts to invoke in other contexts resul!t in
* exceptions or errors including ClassCastException.
*/
public final void quietlyHelpJoin() {
if (status >= 0) {
ForkJoinWorkerThread w =
(ForkJoinWorkerThread)(Thread.currentThread());
if (!w.unpushTask(this) || !tryQuietlyInvoke())
w.helpJoinTask(this);
}
}
/**
* Joins this task, without returning its result or throwing an
* 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() {
if (status >= 0) {
ForkJoinWorkerThread w = getWorker();
if (w == null || !w.unpushTask(this) || !tryQuietlyInvoke())
awaitDone(w, true);
}
}
/**
* Commences performing this task and awaits its completion if
* necessary, without returning its result or throwing an
* 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 quietlyInvoke() {
if (status >= 0 && !tryQuietlyInvoke())
quietlyJoin();
}
/**
* Resets the internal bookkeeping state of this task, allowing a
* subsequent 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, and are
* almost surely wrong. This method may be useful when executing
* pre-constructed trees of subtasks in loops.
*/
public void reinitialize() {
if ((status & COMPLETION_MASK) == EXCEPTIONAL)
exceptionMap.remove(this);
status = 0;
}
/**
* Returns the pool hosting the current task execution, or null
* if this task is executing outside of any pool.
* @return the pool, or null if none.
*/
public static ForkJoinPool getPool() {
Thread t = Thread.currentThread();
return ((t instanceof ForkJoinWorkerThread)?
((ForkJoinWorkerThread)t).pool : null);
}
/**
* Tries to unschedule this task for execution. This method will
* typically 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. This method may be invoked only from within
* ForkJoinTask computations. Attempts to invoke in other contexts
* result in exceptions or errors including ClassCastException.
* @return true if unforked
*/
public boolean tryUnfork() {
return ((ForkJoinWorkerThread)(Thread.currentThread())).unpushTask(this);
}
/**
* Possibly executes tasks until the pool hosting the current task
* {@link ForkJoinPool#isQuiescent}. 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() {
((ForkJoinWorkerThread)(Thread.currentThread())).
helpQuiescePool();
}
/**
* 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() {
return ((ForkJoinWorkerThread)(Thread.currentThread())).
getQueueSize();
}
/**
* Returns a 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. 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 ((ForkJoinWorkerThread)(Thread.currentThread()))
.getEstimatedSurplusTaskCount();
}
// Extension methods
/**
* Returns the result that would be returned by join
,
* even if this task completed abnormally, or 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 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. This method
* is designed to support extensions, and should not in general be
* called otherwise. The return value controls whether this task
* is considered to be done normally. It may return false in
* asynchronous actions that require explicit invocations of
* complete
to become joinable. It may throw exceptions
* to indicate abnormal exit.
* @return true if completed normally
* @throws Error or RuntimeException if encountered during computation
*/
protected abstract boolean exec();
/**
* Returns, but does not unschedule or execute, the task most
* recently forked by the current thread but not yet executed, if
* one is available. There is no guarantee that this task will
* actually be polled or executed next.
* This method is designed primarily to support extensions,
* and is unlikely to be useful otherwise.
*
* @return the next task, or null if none are available
*/
protected static ForkJoinTask> peekNextLocalTask() {
return ((ForkJoinWorkerThread)(Thread.currentThread())).peekTask();
}
/**
* Unschedules and returns, without executing, the task most
* recently forked by the current thread but not yet executed.
* This method is designed primarily to support extensions,
* and is unlikely to be useful otherwise.
*
* @return the next task, or null if none are available
*/
protected static ForkJoinTask> pollNextLocalTask() {
return ((ForkJoinWorkerThread)(Thread.currentThread())).popTask();
}
/**
* Unschedules and returns, without executing, the task most
* recently forked 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 null
result does not necessarily
* imply quiecence 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 null if none are available
*/
protected static ForkJoinTask> pollTask() {
return ((ForkJoinWorkerThread)(Thread.currentThread())).
getLocalOrStolenTask();
}
// Serialization support
private static final long serialVersionUID = -7721805057305804111L;
/**
* Save the state to a stream.
*
* @serialData the current run status and the exception thrown
* during execution, or null if none.
* @param s the stream
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
s.defaultWriteObject();
s.writeObject(getException());
}
/**
* Reconstitute the instance from a stream.
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
status &= ~INTERNAL_SIGNAL_MASK; // clear internal signal counts
status |= EXTERNAL_SIGNAL; // conservatively set external signal
Object ex = s.readObject();
if (ex != null)
setDoneExceptionally((Throwable)ex);
}
// Temporary Unsafe mechanics for preliminary release
static final Unsafe _unsafe;
static final long statusOffset;
static {
try {
if (ForkJoinTask.class.getClassLoader() != null) {
Field f = Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
_unsafe = (Unsafe)f.get(null);
}
else
_unsafe = Unsafe.getUnsafe();
statusOffset = _unsafe.objectFieldOffset
(ForkJoinTask.class.getDeclaredField("status"));
} catch (Exception ex) { throw new Error(ex); }
}
}