/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*/
/*
*
*
*
*
*
* 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 java.util.concurrent;
import java.io.
Serializable;
import java.util.
Collection;
import java.util.
List;
import java.util.
RandomAccess;
import java.lang.ref.
WeakReference;
import java.lang.ref.
ReferenceQueue;
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;
import java.lang.reflect.
Constructor;
/**
* 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.
*
* <p>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.
*
* <p>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.
*
* <p>It is possible to define and use ForkJoinTasks that may block,
* but doing do requires three further considerations: (1) Completion
* of few if any <em>other</em> 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.
*
* <p>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 "<em>quiet</em>" 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.
*
* <p>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}.
*
* <p>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}.
*
* <p>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
* RecursiveAction} for most computations that do not return results,
* {@link RecursiveTask} for those that do, and {@link
* 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.
*
* <p>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 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 <em>tagged</em> 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.)
*
* <p>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.
*
* <p>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.
*
* <p>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 <em>asyncMode</em>.
*
* <p>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<V> implements
Future<V>,
Serializable {
/*
* 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.
*/
/** 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 = ((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();
}
// Exception table support
/**
* 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.
*
* Note: These statics are initialized below in static block.
*/
private static final
ExceptionNode[]
exceptionTable;
private static final
ReentrantLock exceptionTableLock;
private static final
ReferenceQueue<
Object>
exceptionTableRefQueue;
/**
* Fixed capacity for exceptionTable.
*/
private static final int
EXCEPTION_MAP_CAPACITY = 32;
/**
* 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<
ForkJoinTask<?>> {
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(
ForkJoinTask<?>
task,
Throwable ex,
ExceptionNode next) {
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]);
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) {
}
/**
* 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) {
}
}
}
/**
* 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 the given 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() {
if ((
status &
DONE_MASK) !=
EXCEPTIONAL)
return null;
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()) {
Class<? extends
Throwable>
ec =
ex.
getClass();
try {
Constructor<?>
noArgCtor = null;
Constructor<?>[]
cs =
ec.
getConstructors();// public ctors only
for (int
i = 0;
i <
cs.length; ++
i) {
Constructor<?>
c =
cs[
i];
Class<?>[]
ps =
c.
getParameterTypes();
if (
ps.length == 0)
noArgCtor =
c;
else if (
ps.length == 1 &&
ps[0] ==
Throwable.class) {
Throwable wx = (
Throwable)
c.
newInstance(
ex);
return (
wx == null) ?
ex :
wx;
}
}
if (
noArgCtor != null) {
Throwable wx = (
Throwable)(
noArgCtor.
newInstance());
if (
wx != null) {
wx.
initCause(
ex);
return
wx;
}
}
} catch (
Exception ignore) {
}
}
return
ex;
}
/**
* Poll stale refs and remove them. Call only while holding lock.
*/
private static void
expungeStaleExceptions() {
for (
Object x; (
x =
exceptionTableRefQueue.
poll()) != null;) {
if (
x instanceof
ExceptionNode) {
int
hashCode = ((
ExceptionNode)
x).
hashCode;
ExceptionNode[]
t =
exceptionTable;
int
i =
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, poll stale refs and remove 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) {
if (
ex != null)
ForkJoinTask.<
RuntimeException>
uncheckedThrow(
ex);
}
/**
* The sneaky part of sneaky throw, relying on generics
* limitations to evade compiler complaints about rethrowing
* unchecked exceptions
*/
@
SuppressWarnings("unchecked") static <T extends
Throwable>
void
uncheckedThrow(
Throwable t) throws T {
throw (T)
t; // rely on vacuous cast
}
/**
* 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
/**
* 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<V>
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 {@link #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 <em>not</em> 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();
}
/**
* 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 <T> 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 <T extends
ForkJoinTask<?>>
Collection<T>
invokeAll(
Collection<T>
tasks) {
if (!(
tasks instanceof
RandomAccess) || !(
tasks instanceof
List<?>)) {
invokeAll(
tasks.
toArray(new
ForkJoinTask<?>[
tasks.
size()]));
return
tasks;
}
@
SuppressWarnings("unchecked")
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 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;
}
/**
* 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}.
*
* <p>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.
*
* <p>This method is designed to be invoked by <em>other</em>
* 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;
}
/**
* 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();
Throwable ex;
if ((
s &=
DONE_MASK) ==
CANCELLED)
throw new
CancellationException();
if (
s ==
EXCEPTIONAL && (
ex =
getThrowableException()) != null)
throw new
ExecutionException(
ex);
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) {
Throwable ex;
if (
s ==
CANCELLED)
throw new
CancellationException();
if (
s !=
EXCEPTIONAL)
throw new
TimeoutException();
if ((
ex =
getThrowableException()) != null)
throw new
ExecutionException(
ex);
}
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();
}
/**
* Commences performing this task and awaits its completion if
* necessary, without returning its result or throwing its
* exception.
*/
public final void
quietlyInvoke() {
doInvoke();
}
/**
* Possibly executes tasks until the pool hosting the current task
* {@link 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();
}
/**
* 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.
*
* <p>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;
}
/**
* Returns the pool hosting the current task execution, or null
* if this task is executing outside of any ForkJoinPool.
*
* @see #inForkJoinPool
* @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;
}
/**
* 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 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();
}
// 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 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, 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;
}
/**
* 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;
}
// tag operations
/**
* 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.
*
* @param tag the tag value
* @return the previous value of the tag
* @since 1.8
*/
public final short
setForkJoinTaskTag(short
tag) {
for (int
s;;) {
if (
U.
compareAndSwapInt(this,
STATUS,
s =
status,
(
s & ~
SMASK) | (
tag &
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 e the expected tag value
* @param tag the new tag value
* @return {@code true} if successful; i.e., the current value was
* equal to e and is now tag.
* @since 1.8
*/
public final boolean
compareAndSetForkJoinTaskTag(short
e, short
tag) {
for (int
s;;) {
if ((short)(
s =
status) !=
e)
return false;
if (
U.
compareAndSwapInt(this,
STATUS,
s,
(
s & ~
SMASK) | (
tag &
SMASK)))
return true;
}
}
/**
* Adaptor for Runnables. This implements RunnableFuture
* to be compliant with AbstractExecutorService constraints
* when used in ForkJoinPool.
*/
static final class
AdaptedRunnable<T> extends
ForkJoinTask<T>
implements
RunnableFuture<T> {
final
Runnable runnable;
T
result;
AdaptedRunnable(
Runnable runnable, T
result) {
if (
runnable == null) throw new
NullPointerException();
this.
runnable =
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(); }
private static final long
serialVersionUID = 5232453952276885070L;
}
/**
* Adaptor for Runnables without results
*/
static final class
AdaptedRunnableAction extends
ForkJoinTask<
Void>
implements
RunnableFuture<
Void> {
final
Runnable runnable;
AdaptedRunnableAction(
Runnable runnable) {
if (
runnable == null) throw new
NullPointerException();
this.
runnable =
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(); }
private static final long
serialVersionUID = 5232453952276885070L;
}
/**
* Adaptor for Runnables in which failure forces worker exception
*/
static final class
RunnableExecuteAction extends
ForkJoinTask<
Void> {
final
Runnable runnable;
RunnableExecuteAction(
Runnable runnable) {
if (
runnable == null) throw new
NullPointerException();
this.
runnable =
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;
}
/**
* Adaptor for Callables
*/
static final class
AdaptedCallable<T> extends
ForkJoinTask<T>
implements
RunnableFuture<T> {
final
Callable<? extends T>
callable;
T
result;
AdaptedCallable(
Callable<? extends T>
callable) {
if (
callable == null) throw new
NullPointerException();
this.
callable =
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 (
Error err) {
throw
err;
} catch (
RuntimeException rex) {
throw
rex;
} catch (
Exception ex) {
throw new
RuntimeException(
ex);
}
}
public final void
run() {
invoke(); }
private static final long
serialVersionUID = 2838392045355241008L;
}
/**
* 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 <T> the type of the result
* @return the task
*/
public static <T>
ForkJoinTask<T>
adapt(
Runnable runnable, T
result) {
return new
AdaptedRunnable<T>(
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 <T> the type of the callable's result
* @return the task
*/
public static <T>
ForkJoinTask<T>
adapt(
Callable<? extends T>
callable) {
return new
AdaptedCallable<T>(
callable);
}
// 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;
private static final long
STATUS;
static {
exceptionTableLock = new
ReentrantLock();
exceptionTableRefQueue = new
ReferenceQueue<
Object>();
exceptionTable = new
ExceptionNode[
EXCEPTION_MAP_CAPACITY];
try {
U = sun.misc.
Unsafe.
getUnsafe();
Class<?>
k =
ForkJoinTask.class;
STATUS =
U.
objectFieldOffset
(
k.
getDeclaredField("status"));
} catch (
Exception e) {
throw new
Error(
e);
}
}
}