--- jsr166/src/jsr166y/ForkJoinTask.java 2009/01/06 14:30:31 1.1
+++ jsr166/src/jsr166y/ForkJoinTask.java 2011/02/22 10:50:51 1.75
@@ -5,848 +5,1126 @@
*/
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.*;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import java.util.RandomAccess;
+import java.util.Map;
+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.Executor;
+import java.util.concurrent.ExecutorService;
+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 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.
- *
- *
ForkJoinTasks are forms of Futures supporting a
- * limited range of use. The "lightness" of ForkJoinTasks is due to 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 supported for ForkJoinTasks are
- * fork, that arranges asynchronous execution, and
- * join, that doesn't proceed until the task's result has
- * been computed. (Cancellation is also supported). The computation
- * defined in the compute method should avoid
- * synchronized methods or blocks, and should minimize
- * 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 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 ForkJoinTask class is not usually directly
- * subclassed. Instead, you subclass one of the abstract classes that
- * support different styles 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, 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. The
- * only way to invoke a "main" driver task is to submit it to a
- * ForkJoinPool. Once started, this will usually in turn start other
- * subtasks.
- *
- *
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. To support such extensions,
- * instances of ForkJoinTasks maintain an atomically updated
- * short representing user-defined control state. Control
- * state is guaranteed initially to be zero, and to be negative upon
- * completion, but may otherwise be used for any other control
- * purposes, such as maintaining join counts. The {@link
- * ForkJoinWorkerThread} class supports additional inspection and
- * tuning methods that can be useful when developing extensions.
+ * Abstract base class for tasks that run within a {@link ForkJoinPool}.
+ * A {@code ForkJoinTask} is a thread-like entity that is much
+ * lighter weight than a normal thread. Huge numbers of tasks and
+ * subtasks may be hosted by a small number of actual threads in a
+ * ForkJoinPool, at the price of some usage limitations.
+ *
+ *
A "main" {@code ForkJoinTask} begins execution when 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 {@code ForkJoinTask} employ only methods
+ * {@link #fork} and {@link #join}, or derivatives such as {@link
+ * #invokeAll(ForkJoinTask...) invokeAll}. However, this class also
+ * provides a number of other methods that can come into play in
+ * advanced usages, as well as extension mechanics that allow
+ * support of new forms of fork/join processing.
+ *
+ *
A {@code ForkJoinTask} is a lightweight form of {@link Future}.
+ * The efficiency of {@code ForkJoinTask}s stems from a set of
+ * restrictions (that are only partially statically enforceable)
+ * reflecting their 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 {@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. 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 {@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.
+ *
+ *
The primary method for awaiting completion and extracting
+ * results of a task is {@link #join}, but there are several variants:
+ * The {@link Future#get} methods support interruptible and/or timed
+ * waits for completion and report results using {@code Future}
+ * conventions. Method {@link #invoke} is semantically
+ * equivalent to {@code fork(); join()} but always attempts to begin
+ * execution in the current thread. The "quiet" forms of
+ * these methods do not extract results or report exceptions. These
+ * may be useful when a set of tasks are being executed, and you need
+ * to delay processing of results or exceptions until all complete.
+ * Method {@code invokeAll} (available in multiple versions)
+ * performs the most common form of parallel invocation: forking a set
+ * of tasks and joining them all.
+ *
+ *
The execution status of tasks may be queried at several levels
+ * of detail: {@link #isDone} is true if a task completed in any way
+ * (including the case where a task was cancelled without executing);
+ * {@link #isCompletedNormally} is true if a task completed without
+ * cancellation or encountering an exception; {@link #isCancelled} is
+ * true if the task was cancelled (in which case {@link #getException}
+ * returns a {@link java.util.concurrent.CancellationException}); and
+ * {@link #isCompletedAbnormally} is true if a task was either
+ * cancelled or encountered an exception, in which case {@link
+ * #getException} will return either the encountered exception or
+ * {@link java.util.concurrent.CancellationException}.
+ *
+ *
The ForkJoinTask class is not usually directly subclassed.
+ * Instead, you subclass one of the abstract classes that support a
+ * particular style of fork/join processing, typically {@link
+ * RecursiveAction} for computations that do not return results, or
+ * {@link RecursiveTask} for those that do. 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. While these methods have {@code public} access (to allow
+ * instances of different task subclasses to call each other's
+ * methods), some of them may only be called from within other
+ * ForkJoinTasks (as may be determined using method {@link
+ * #inForkJoinPool}). Attempts to invoke them in other contexts
+ * result in exceptions or errors, possibly including
+ * {@code ClassCastException}.
+ *
+ *
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.
+ *
+ *
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
- * 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 Serializable, which enables them to
- * be used in extensions such as remote execution frameworks. However,
- * it is in general safe to serialize tasks only before or after, but
- * not during execution. Serialization is not relied on during
- * execution itself.
+ * computation. Large tasks should be split into smaller subtasks,
+ * usually via recursive decomposition. As a very rough rule of thumb,
+ * a task should perform more than 100 and less than 10000 basic
+ * computational steps, and should avoid indefinite looping. If tasks
+ * are too big, then parallelism cannot improve throughput. If too
+ * small, then memory and internal task maintenance overhead may
+ * overwhelm processing.
+ *
+ *
This class provides {@code adapt} methods for {@link Runnable}
+ * and {@link Callable}, that may be of use when mixing execution of
+ * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are
+ * of this form, consider using a pool constructed in asyncMode.
+ *
+ *
ForkJoinTasks are {@code Serializable}, which enables them to be
+ * used in extensions such as remote execution frameworks. It is
+ * sensible to serialize tasks only before or after, but not during,
+ * execution. Serialization is not relied on during execution itself.
+ *
+ * @since 1.7
+ * @author Doug Lea
*/
public abstract class ForkJoinTask implements Future, Serializable {
- /**
- * Status field holding all run status. We pack this into a single
- * int both to minimize footprint overhead and to ensure atomicity
- * (updates are via CAS).
- *
- * Status is initially zero, and takes on nonnegative values until
- * completed, upon which status holds COMPLETED. CANCELLED, or
- * EXCEPTIONAL, which use the top 3 bits. Tasks undergoing
- * blocking waits by other threads have SIGNAL_MASK bits set --
- * bit 15 for external (nonFJ) waits, and the rest a count of
- * waiting FJ threads. (This representation relies on
- * ForkJoinPool max thread limits). Completion of a stolen task
- * with SIGNAL_MASK bits set awakens waiter 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. Note that bits 16-28 are
- * currently unused. Also value 0x80000000 is available as spare
- * completion value.
- */
- volatile int status; // accessed directy by pool and workers
-
- static final int COMPLETION_MASK = 0xe0000000;
- static final int NORMAL = 0xe0000000; // == mask
- static final int CANCELLED = 0xc0000000;
- static final int EXCEPTIONAL = 0xa0000000;
- static final int SIGNAL_MASK = 0x0000ffff;
- static final int INTERNAL_SIGNAL_MASK = 0x00007fff;
- static final int EXTERNAL_SIGNAL = 0x00008000; // top bit of low word
-
- /**
- * 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 us a weak ref table. Note
- * that cancellation exceptions don't appear in the table, but are
- * instead recorded as status values.
- * Todo: Use ConcurrentReferenceHashMap
- */
- static final Map, Throwable> exceptionMap =
- Collections.synchronizedMap
- (new WeakHashMap, Throwable>());
-
- // within-package utilities
-
- /**
- * Get current worker thread, or null if not a worker thread
- */
- static ForkJoinWorkerThread getWorker() {
- Thread t = Thread.currentThread();
- return ((t instanceof ForkJoinWorkerThread)?
- (ForkJoinWorkerThread)t : null);
- }
-
- /**
- * Get pool of current worker thread, or null if not a worker thread
- */
- static ForkJoinPool getWorkerPool() {
- Thread t = Thread.currentThread();
- return ((t instanceof ForkJoinWorkerThread)?
- ((ForkJoinWorkerThread)t).pool : null);
- }
-
- final boolean casStatus(int cmp, int val) {
- return _unsafe.compareAndSwapInt(this, statusOffset, cmp, val);
- }
-
- /**
- * Workaround for not being able to rethrow unchecked exceptions.
- */
- static void rethrowException(Throwable ex) {
- if (ex != null)
- _unsafe.throwException(ex);
- }
- // Setting completion status
+ /*
+ * 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 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.
+ */
+
+ /** The run status of this task */
+ volatile int status; // accessed directly by pool and workers
+ private static final int NORMAL = -1;
+ private static final int CANCELLED = -2;
+ private static final int EXCEPTIONAL = -3;
+ private static final int SIGNAL = 1;
/**
- * Mark completion and wake up threads waiting to join this task.
+ * Marks completion and wakes up threads waiting to join this task,
+ * also clearing signal request bits.
+ *
* @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
+ * @return completion status on exit
*/
- final void setCompletion(int completion) {
- ForkJoinPool pool = getWorkerPool();
- if (pool != null) {
- int s; // Clear signal bits while setting completion status
- do;while ((s = status) >= 0 && !casStatus(s, completion));
-
- if ((s & SIGNAL_MASK) != 0) {
- if ((s &= INTERNAL_SIGNAL_MASK) != 0)
- pool.updateRunningCount(s);
- synchronized(this) { notifyAll(); }
+ private int setCompletion(int completion) {
+ for (int s;;) {
+ if ((s = status) < 0)
+ return s;
+ if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {
+ if (s != 0)
+ synchronized (this) { notifyAll(); }
+ return completion;
}
}
- else
- externallySetCompletion(completion);
}
/**
- * Version of setCompletion for non-FJ threads. Leaves signal
- * bits for unblocked threads to adjust, and always notifies.
+ * Tries to block a worker thread until completed or timed out.
+ * Uses Object.wait time argument conventions.
+ * May fail on contention or interrupt.
+ *
+ * @param millis if > 0, wait time.
*/
- private void externallySetCompletion(int completion) {
+ final void tryAwaitDone(long millis) {
int s;
- do;while ((s = status) >= 0 &&
- !casStatus(s, (s & SIGNAL_MASK) | completion));
- synchronized(this) { notifyAll(); }
- }
-
- /**
- * Sets status to indicate normal completion
- */
- final void setNormalCompletion() {
- // Try typical fast case -- single CAS, no signal, not already done.
- // Manually expand casStatus to improve chances of inlining it
- if (!_unsafe.compareAndSwapInt(this, statusOffset, 0, NORMAL))
- setCompletion(NORMAL);
- }
-
- // internal waiting and notification
-
- /**
- * Performs the actual monitor wait for awaitDone
- */
- private void doAwaitDone() {
- // Minimize lock bias and in/de-flation effects by maximizing
- // chances of waiting inside sync
try {
- while (status >= 0)
- synchronized(this) { if (status >= 0) wait(); }
- } catch (InterruptedException ie) {
- onInterruptedWait();
- }
- }
-
- /**
- * Performs the actual monitor wait for awaitDone
- */
- private void doAwaitDone(long startTime, long nanos) {
- synchronized(this) {
- try {
- while (status >= 0) {
- long nt = nanos - System.nanoTime() - startTime;
- if (nt <= 0)
- break;
- wait(nt / 1000000, (int)(nt % 1000000));
+ if (((s = status) > 0 ||
+ (s == 0 &&
+ UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL))) &&
+ status > 0) {
+ synchronized (this) {
+ if (status > 0)
+ wait(millis);
}
- } catch (InterruptedException ie) {
- onInterruptedWait();
}
+ } catch (InterruptedException ie) {
+ // caller must check termination
}
}
- // Awaiting completion
-
/**
- * Sets status to indicate there is joiner, then waits for join,
- * surrounded with pool notifications.
- * @return status upon exit
+ * Blocks a non-worker-thread until completion.
+ * @return status upon completion
*/
- final int awaitDone(ForkJoinWorkerThread w, boolean maintainParallelism) {
- ForkJoinPool pool = w == null? null : w.pool;
+ private int externalAwaitDone() {
int s;
- while ((s = status) >= 0) {
- if (casStatus(s, pool == null? s|EXTERNAL_SIGNAL : s+1)) {
- if (pool == null || !pool.preJoin(this, maintainParallelism))
- doAwaitDone();
- if (((s = status) & INTERNAL_SIGNAL_MASK) != 0)
- adjustPoolCountsOnUnblock(pool);
- break;
+ if ((s = status) >= 0) {
+ boolean interrupted = false;
+ synchronized (this) {
+ while ((s = status) >= 0) {
+ if (s == 0)
+ UNSAFE.compareAndSwapInt(this, statusOffset,
+ 0, SIGNAL);
+ else {
+ try {
+ wait();
+ } catch (InterruptedException ie) {
+ interrupted = true;
+ }
+ }
+ }
}
+ if (interrupted)
+ Thread.currentThread().interrupt();
}
return s;
}
/**
- * Timed version of awaitDone
- * @return status upon exit
+ * Blocks a non-worker-thread until completion or interruption or timeout.
*/
- final int awaitDone(ForkJoinWorkerThread w, long nanos) {
- ForkJoinPool pool = w == null? null : w.pool;
+ private int externalInterruptibleAwaitDone(long millis)
+ throws InterruptedException {
int s;
- while ((s = status) >= 0) {
- if (casStatus(s, pool == null? s|EXTERNAL_SIGNAL : s+1)) {
- long startTime = System.nanoTime();
- if (pool == null || !pool.preJoin(this, false))
- doAwaitDone(startTime, nanos);
- if ((s = status) >= 0) {
- adjustPoolCountsOnCancelledWait(pool);
- s = status;
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ if ((s = status) >= 0) {
+ synchronized (this) {
+ while ((s = status) >= 0) {
+ if (s == 0)
+ UNSAFE.compareAndSwapInt(this, statusOffset,
+ 0, SIGNAL);
+ else {
+ wait(millis);
+ if (millis > 0L)
+ break;
+ }
}
- if (s < 0 && (s & INTERNAL_SIGNAL_MASK) != 0)
- adjustPoolCountsOnUnblock(pool);
- break;
}
}
return s;
}
/**
- * Notify pool that thread is unblocked. Called by signalled
- * threads when woken by non-FJ threads (which is atypical).
+ * Primary execution method for stolen tasks. Unless done, calls
+ * exec and records status if completed, but doesn't wait for
+ * completion otherwise.
*/
- private void adjustPoolCountsOnUnblock(ForkJoinPool pool) {
- int s;
- do;while ((s = status) < 0 && !casStatus(s, s & COMPLETION_MASK));
- if (pool != null && (s &= INTERNAL_SIGNAL_MASK) != 0)
- pool.updateRunningCount(s);
+ final void doExec() {
+ if (status >= 0) {
+ boolean completed;
+ try {
+ completed = exec();
+ } catch (Throwable rex) {
+ setExceptionalCompletion(rex);
+ return;
+ }
+ if (completed)
+ setCompletion(NORMAL); // must be outside try block
+ }
}
/**
- * Notify pool to adjust counts on cancelled or timed out wait
+ * Primary mechanics for join, get, quietlyJoin.
+ * @return status upon completion
*/
- private void adjustPoolCountsOnCancelledWait(ForkJoinPool pool) {
- if (pool != null) {
- int s;
- while ((s = status) >= 0 && (s & INTERNAL_SIGNAL_MASK) != 0) {
- if (casStatus(s, s - 1)) {
- pool.updateRunningCount(1);
- break;
+ private int doJoin() {
+ Thread t; ForkJoinWorkerThread w; int s; boolean completed;
+ if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
+ if ((s = status) < 0)
+ return s;
+ if ((w = (ForkJoinWorkerThread)t).unpushTask(this)) {
+ try {
+ completed = exec();
+ } catch (Throwable rex) {
+ return setExceptionalCompletion(rex);
}
+ if (completed)
+ return setCompletion(NORMAL);
}
+ return w.joinTask(this);
}
+ else
+ return externalAwaitDone();
}
- private void onInterruptedWait() {
- Thread t = Thread.currentThread();
- if (t instanceof ForkJoinWorkerThread) {
- ForkJoinWorkerThread w = (ForkJoinWorkerThread)t;
- if (w.isTerminating())
- cancelIgnoreExceptions();
- }
- else { // re-interrupt
- try {
- t.interrupt();
- } catch (SecurityException ignore) {
- }
+ /**
+ * Primary mechanics for invoke, quietlyInvoke.
+ * @return status upon completion
+ */
+ private int doInvoke() {
+ int s; boolean completed;
+ if ((s = status) < 0)
+ return s;
+ try {
+ completed = exec();
+ } catch (Throwable rex) {
+ return setExceptionalCompletion(rex);
}
+ if (completed)
+ return setCompletion(NORMAL);
+ else
+ return doJoin();
}
- // Recording and reporting exceptions
+ // Exception table support
- private void setDoneExceptionally(Throwable rex) {
- exceptionMap.put(this, rex);
- setCompletion(EXCEPTIONAL);
- }
+ /**
+ * 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