A {@code ForkJoinPool} differs from other kinds of {@link * ExecutorService} mainly by virtue of employing * work-stealing: all threads in the pool attempt to find and * execute subtasks created by other active tasks (eventually blocking * waiting for work if none exist). This enables efficient processing * when most tasks spawn other subtasks (as do most {@code * ForkJoinTask}s). When setting asyncMode to true in * constructors, {@code ForkJoinPool}s may also be appropriate for use * with event-style tasks that are never joined. * *
A {@code ForkJoinPool} is constructed with a given target * parallelism level; by default, equal to the number of available * processors. The pool attempts to maintain enough active (or * available) threads by dynamically adding, suspending, or resuming * internal worker threads, even if some tasks are stalled waiting to * join others. However, no such adjustments are guaranteed in the * face of blocked IO or other unmanaged synchronization. The nested * {@link ManagedBlocker} interface enables extension of the kinds of * synchronization accommodated. * *
In addition to execution and lifecycle control methods, this * class provides status check methods (for example * {@link #getStealCount}) that are intended to aid in developing, * tuning, and monitoring fork/join applications. Also, method * {@link #toString} returns indications of pool state in a * convenient form for informal monitoring. * *
As is the case with other ExecutorServices, there are three * main task execution methods summarized in the following * table. These are designed to be used by clients not already engaged * in fork/join computations in the current pool. The main forms of * these methods accept instances of {@code ForkJoinTask}, but * overloaded forms also allow mixed execution of plain {@code * Runnable}- or {@code Callable}- based activities as well. However, * tasks that are already executing in a pool should normally * NOT use these pool execution methods, but instead use the * within-computation forms listed in the table. * *
| * | Call from non-fork/join clients | *Call from within fork/join computations | *
| Arrange async execution | *{@link #execute(ForkJoinTask)} | *{@link ForkJoinTask#fork} | *
| Await and obtain result | *{@link #invoke(ForkJoinTask)} | *{@link ForkJoinTask#invoke} | *
| Arrange exec and obtain Future | *{@link #submit(ForkJoinTask)} | *{@link ForkJoinTask#fork} (ForkJoinTasks are Futures) | *
Sample Usage. Normally a single {@code ForkJoinPool} is * used for all parallel task execution in a program or subsystem. * Otherwise, use would not usually outweigh the construction and * bookkeeping overhead of creating a large set of threads. For * example, a common pool could be used for the {@code SortTasks} * illustrated in {@link RecursiveAction}. Because {@code * ForkJoinPool} uses threads in {@linkplain java.lang.Thread#isDaemon * daemon} mode, there is typically no need to explicitly {@link * #shutdown} such a pool upon program exit. * *
* static final ForkJoinPool mainPool = new ForkJoinPool();
* ...
* public void sort(long[] array) {
* mainPool.invoke(new SortTask(array, 0, array.length));
* }
*
*
* Implementation notes: This implementation restricts the * maximum number of running threads to 32767. Attempts to create * pools with greater than the maximum number result in * {@code IllegalArgumentException}. * *
This implementation rejects submitted tasks (that is, by throwing
* {@link RejectedExecutionException}) only when the pool is shut down
* or internal resources have been exhausted.
*
* @since 1.7
* @author Doug Lea
*/
public class ForkJoinPool extends AbstractExecutorService {
/*
* Implementation Overview
*
* This class provides the central bookkeeping and control for a
* set of worker threads: Submissions from non-FJ threads enter
* into a submission queue. Workers take these tasks and typically
* split them into subtasks that may be stolen by other workers.
* Preference rules give first priority to processing tasks from
* their own queues (LIFO or FIFO, depending on mode), then to
* randomized FIFO steals of tasks in other worker queues, and
* lastly to new submissions.
*
* The main throughput advantages of work-stealing stem from
* decentralized control -- workers mostly take tasks from
* themselves or each other. We cannot negate this in the
* implementation of other management responsibilities. The main
* tactic for avoiding bottlenecks is packing nearly all
* essentially atomic control state into a single 64bit volatile
* variable ("ctl"). This variable is read on the order of 10-100
* times as often as it is modified (always via CAS). (There is
* some additional control state, for example variable "shutdown"
* for which we can cope with uncoordinated updates.) This
* streamlines synchronization and control at the expense of messy
* constructions needed to repack status bits upon updates.
* Updates tend not to contend with each other except during
* bursts while submitted tasks begin or end. In some cases when
* they do contend, threads can instead do something else
* (usually, scan for tasks) until contention subsides.
*
* To enable packing, we restrict maximum parallelism to (1<<15)-1
* (which is far in excess of normal operating range) to allow
* ids, counts, and their negations (used for thresholding) to fit
* into 16bit fields.
*
* Recording Workers. Workers are recorded in the "workers" array
* that is created upon pool construction and expanded if (rarely)
* necessary. This is an array as opposed to some other data
* structure to support index-based random steals by workers.
* Updates to the array recording new workers and unrecording
* terminated ones are protected from each other by a seqLock
* (scanGuard) but the array is otherwise concurrently readable,
* and accessed directly by workers. To simplify index-based
* operations, the array size is always a power of two, and all
* readers must tolerate null slots. To avoid flailing during
* start-up, the array is presized to hold twice #parallelism
* workers (which is unlikely to need further resizing during
* execution). But to avoid dealing with so many null slots,
* variable scanGuard includes a mask for the nearest power of two
* that contains all current workers. All worker thread creation
* is on-demand, triggered by task submissions, replacement of
* terminated workers, and/or compensation for blocked
* workers. However, all other support code is set up to work with
* other policies. To ensure that we do not hold on to worker
* references that would prevent GC, ALL accesses to workers are
* via indices into the workers array (which is one source of some
* of the messy code constructions here). In essence, the workers
* array serves as a weak reference mechanism. Thus for example
* the wait queue field of ctl stores worker indices, not worker
* references. Access to the workers in associated methods (for
* example signalWork) must both index-check and null-check the
* IDs. All such accesses ignore bad IDs by returning out early
* from what they are doing, since this can only be associated
* with termination, in which case it is OK to give up.
*
* All uses of the workers array, as well as queue arrays, check
* that the array is non-null (even if previously non-null). This
* allows nulling during termination, which is currently not
* necessary, but remains an option for resource-revocation-based
* shutdown schemes.
*
* Wait Queuing. Unlike HPC work-stealing frameworks, we cannot
* let workers spin indefinitely scanning for tasks when none can
* be found immediately, and we cannot start/resume workers unless
* there appear to be tasks available. On the other hand, we must
* quickly prod them into action when new tasks are submitted or
* generated. We park/unpark workers after placing in an event
* wait queue when they cannot find work. This "queue" is actually
* a simple Treiber stack, headed by the "id" field of ctl, plus a
* 15bit counter value to both wake up waiters (by advancing their
* count) and avoid ABA effects. Successors are held in worker
* field "nextWait". Queuing deals with several intrinsic races,
* mainly that a task-producing thread can miss seeing (and
* signalling) another thread that gave up looking for work but
* has not yet entered the wait queue. We solve this by requiring
* a full sweep of all workers both before (in scan()) and after
* (in tryAwaitWork()) a newly waiting worker is added to the wait
* queue. During a rescan, the worker might release some other
* queued worker rather than itself, which has the same net
* effect. Because enqueued workers may actually be rescanning
* rather than waiting, we set and clear the "parked" field of
* ForkJoinWorkerThread to reduce unnecessary calls to unpark.
* (Use of the parked field requires a secondary recheck to avoid
* missed signals.)
*
* Signalling. We create or wake up workers only when there
* appears to be at least one task they might be able to find and
* execute. When a submission is added or another worker adds a
* task to a queue that previously had two or fewer tasks, they
* signal waiting workers (or trigger creation of new ones if
* fewer than the given parallelism level -- see signalWork).
* These primary signals are buttressed by signals during rescans
* as well as those performed when a worker steals a task and
* notices that there are more tasks too; together these cover the
* signals needed in cases when more than two tasks are pushed
* but untaken.
*
* Trimming workers. To release resources after periods of lack of
* use, a worker starting to wait when the pool is quiescent will
* time out and terminate if the pool has remained quiescent for
* SHRINK_RATE nanosecs. This will slowly propagate, eventually
* terminating all workers after long periods of non-use.
*
* Submissions. External submissions are maintained in an
* array-based queue that is structured identically to
* ForkJoinWorkerThread queues except for the use of
* submissionLock in method addSubmission. Unlike the case for
* worker queues, multiple external threads can add new
* submissions, so adding requires a lock.
*
* Compensation. Beyond work-stealing support and lifecycle
* control, the main responsibility of this framework is to take
* actions when one worker is waiting to join a task stolen (or
* always held by) another. Because we are multiplexing many
* tasks on to a pool of workers, we can't just let them block (as
* in Thread.join). We also cannot just reassign the joiner's
* run-time stack with another and replace it later, which would
* be a form of "continuation", that even if possible is not
* necessarily a good idea since we sometimes need both an
* unblocked task and its continuation to progress. Instead we
* combine two tactics:
*
* Helping: Arranging for the joiner to execute some task that it
* would be running if the steal had not occurred. Method
* ForkJoinWorkerThread.joinTask tracks joining->stealing
* links to try to find such a task.
*
* Compensating: Unless there are already enough live threads,
* method tryPreBlock() may create or re-activate a spare
* thread to compensate for blocked joiners until they
* unblock.
*
* The ManagedBlocker extension API can't use helping so relies
* only on compensation in method awaitBlocker.
*
* It is impossible to keep exactly the target parallelism number
* of threads running at any given time. Determining the
* existence of conservatively safe helping targets, the
* availability of already-created spares, and the apparent need
* to create new spares are all racy and require heuristic
* guidance, so we rely on multiple retries of each. Currently,
* in keeping with on-demand signalling policy, we compensate only
* if blocking would leave less than one active (non-waiting,
* non-blocked) worker. Additionally, to avoid some false alarms
* due to GC, lagging counters, system activity, etc, compensated
* blocking for joins is only attempted after rechecks stabilize
* (retries are interspersed with Thread.yield, for good
* citizenship). The variable blockedCount, incremented before
* blocking and decremented after, is sometimes needed to
* distinguish cases of waiting for work vs blocking on joins or
* other managed sync. Both cases are equivalent for most pool
* control, so we can update non-atomically. (Additionally,
* contention on blockedCount alleviates some contention on ctl).
*
* Shutdown and Termination. A call to shutdownNow atomically sets
* the ctl stop bit and then (non-atomically) sets each workers
* "terminate" status, cancels all unprocessed tasks, and wakes up
* all waiting workers. Detecting whether termination should
* commence after a non-abrupt shutdown() call requires more work
* and bookkeeping. We need consensus about quiesence (i.e., that
* there is no more work) which is reflected in active counts so
* long as there are no current blockers, as well as possible
* re-evaluations during independent changes in blocking or
* quiescing workers.
*
* Style notes: There is a lot of representation-level coupling
* among classes ForkJoinPool, ForkJoinWorkerThread, and
* ForkJoinTask. Most fields of ForkJoinWorkerThread maintain
* data structures managed by ForkJoinPool, so are directly
* accessed. Conversely we allow access to "workers" array by
* workers, and direct access to ForkJoinTask.status by both
* ForkJoinPool and ForkJoinWorkerThread. There is little point
* trying to reduce this, since any associated future changes in
* representations will need to be accompanied by algorithmic
* changes anyway. All together, these low-level implementation
* choices produce as much as a factor of 4 performance
* improvement compared to naive implementations, and enable the
* processing of billions of tasks per second, at the expense of
* some ugliness.
*
* Methods signalWork() and scan() are the main bottlenecks so are
* especially heavily micro-optimized/mangled. There are lots of
* inline assignments (of form "while ((local = field) != 0)")
* which are usually the simplest way to ensure the required read
* orderings (which are sometimes critical). This leads to a
* "C"-like style of listing declarations of these locals at the
* heads of methods or blocks. There are several occurrences of
* the unusual "do {} while (!cas...)" which is the simplest way
* to force an update of a CAS'ed variable. There are also other
* coding oddities that help some methods perform reasonably even
* when interpreted (not compiled).
*
* The order of declarations in this file is: (1) declarations of
* statics (2) fields (along with constants used when unpacking
* some of them), listed in an order that tends to reduce
* contention among them a bit under most JVMs. (3) internal
* control methods (4) callbacks and other support for
* ForkJoinTask and ForkJoinWorkerThread classes, (5) exported
* methods (plus a few little helpers). (6) static block
* initializing all statics in a minimally dependent order.
*/
/**
* Factory for creating new {@link ForkJoinWorkerThread}s.
* A {@code ForkJoinWorkerThreadFactory} must be defined and used
* for {@code ForkJoinWorkerThread} subclasses that extend base
* functionality or initialize threads with different contexts.
*/
public static interface ForkJoinWorkerThreadFactory {
/**
* Returns a new worker thread operating in the given pool.
*
* @param pool the pool this thread works in
* @throws NullPointerException if the pool is null
*/
public ForkJoinWorkerThread newThread(ForkJoinPool pool);
}
/**
* Default ForkJoinWorkerThreadFactory implementation; creates a
* new ForkJoinWorkerThread.
*/
static class DefaultForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
return new ForkJoinWorkerThread(pool);
}
}
/**
* Creates a new ForkJoinWorkerThread. This factory is used unless
* overridden in ForkJoinPool constructors.
*/
public static final ForkJoinWorkerThreadFactory
defaultForkJoinWorkerThreadFactory;
/**
* Permission required for callers of methods that may start or
* kill threads.
*/
private static final RuntimePermission modifyThreadPermission;
/**
* If there is a security manager, makes sure caller has
* permission to modify threads.
*/
private static void checkPermission() {
SecurityManager security = System.getSecurityManager();
if (security != null)
security.checkPermission(modifyThreadPermission);
}
/**
* Generator for assigning sequence numbers as pool names.
*/
private static final AtomicInteger poolNumberGenerator;
/**
* Generator for initial random seeds for worker victim
* selection. This is used only to create initial seeds. Random
* steals use a cheaper xorshift generator per steal attempt. We
* don't expect much contention on seedGenerator, so just use a
* plain Random.
*/
static final Random workerSeedGenerator;
/**
* Array holding all worker threads in the pool. Initialized upon
* construction. Array size must be a power of two. Updates and
* replacements are protected by scanGuard, but the array is
* always kept in a consistent enough state to be randomly
* accessed without locking by workers performing work-stealing,
* as well as other traversal-based methods in this class, so long
* as reads memory-acquire by first reading ctl. All readers must
* tolerate that some array slots may be null.
*/
ForkJoinWorkerThread[] workers;
/**
* Initial size for submission queue array. Must be a power of
* two. In many applications, these always stay small so we use a
* small initial cap.
*/
private static final int INITIAL_QUEUE_CAPACITY = 8;
/**
* Maximum size for submission queue array. Must be a power of two
* less than or equal to 1 << (31 - width of array entry) to
* ensure lack of index wraparound, but is capped at a lower
* value to help users trap runaway computations.
*/
private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M
/**
* Array serving as submission queue. Initialized upon construction.
*/
private ForkJoinTask[] submissionQueue;
/**
* Lock protecting submissions array for addSubmission
*/
private final ReentrantLock submissionLock;
/**
* Condition for awaitTermination, using submissionLock for
* convenience.
*/
private final Condition termination;
/**
* Creation factory for worker threads.
*/
private final ForkJoinWorkerThreadFactory factory;
/**
* The uncaught exception handler used when any worker abruptly
* terminates.
*/
final Thread.UncaughtExceptionHandler ueh;
/**
* Prefix for assigning names to worker threads
*/
private final String workerNamePrefix;
/**
* Sum of per-thread steal counts, updated only when threads are
* idle or terminating.
*/
private volatile long stealCount;
/**
* Main pool control -- a long packed with:
* AC: Number of active running workers minus target parallelism (16 bits)
* TC: Number of total workers minus target parallelism (16bits)
* ST: true if pool is terminating (1 bit)
* EC: the wait count of top waiting thread (15 bits)
* ID: ~poolIndex of top of Treiber stack of waiting threads (16 bits)
*
* When convenient, we can extract the upper 32 bits of counts and
* the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e =
* (int)ctl. The ec field is never accessed alone, but always
* together with id and st. The offsets of counts by the target
* parallelism and the positionings of fields makes it possible to
* perform the most common checks via sign tests of fields: When
* ac is negative, there are not enough active workers, when tc is
* negative, there are not enough total workers, when id is
* negative, there is at least one waiting worker, and when e is
* negative, the pool is terminating. To deal with these possibly
* negative fields, we use casts in and out of "short" and/or
* signed shifts to maintain signedness.
*/
volatile long ctl;
// bit positions/shifts for fields
private static final int AC_SHIFT = 48;
private static final int TC_SHIFT = 32;
private static final int ST_SHIFT = 31;
private static final int EC_SHIFT = 16;
// bounds
private static final int MAX_ID = 0x7fff; // max poolIndex
private static final int SMASK = 0xffff; // mask short bits
private static final int SHORT_SIGN = 1 << 15;
private static final int INT_SIGN = 1 << 31;
// masks
private static final long STOP_BIT = 0x0001L << ST_SHIFT;
private static final long AC_MASK = ((long)SMASK) << AC_SHIFT;
private static final long TC_MASK = ((long)SMASK) << TC_SHIFT;
// units for incrementing and decrementing
private static final long TC_UNIT = 1L << TC_SHIFT;
private static final long AC_UNIT = 1L << AC_SHIFT;
// masks and units for dealing with u = (int)(ctl >>> 32)
private static final int UAC_SHIFT = AC_SHIFT - 32;
private static final int UTC_SHIFT = TC_SHIFT - 32;
private static final int UAC_MASK = SMASK << UAC_SHIFT;
private static final int UTC_MASK = SMASK << UTC_SHIFT;
private static final int UAC_UNIT = 1 << UAC_SHIFT;
private static final int UTC_UNIT = 1 << UTC_SHIFT;
// masks and units for dealing with e = (int)ctl
private static final int E_MASK = 0x7fffffff; // no STOP_BIT
private static final int EC_UNIT = 1 << EC_SHIFT;
/**
* The target parallelism level.
*/
final int parallelism;
/**
* Index (mod submission queue length) of next element to take
* from submission queue. Usage is identical to that for
* per-worker queues -- see ForkJoinWorkerThread internal
* documentation.
*/
volatile int queueBase;
/**
* Index (mod submission queue length) of next element to add
* in submission queue. Usage is identical to that for
* per-worker queues -- see ForkJoinWorkerThread internal
* documentation.
*/
int queueTop;
/**
* True when shutdown() has been called.
*/
volatile boolean shutdown;
/**
* True if use local fifo, not default lifo, for local polling
* Read by, and replicated by ForkJoinWorkerThreads
*/
final boolean locallyFifo;
/**
* The number of threads in ForkJoinWorkerThreads.helpQuiescePool.
* When non-zero, suppresses automatic shutdown when active
* counts become zero.
*/
volatile int quiescerCount;
/**
* The number of threads blocked in join.
*/
volatile int blockedCount;
/**
* Counter for worker Thread names (unrelated to their poolIndex)
*/
private volatile int nextWorkerNumber;
/**
* The index for the next created worker. Accessed under scanGuard.
*/
private int nextWorkerIndex;
/**
* SeqLock and index masking for updates to workers array. Locked
* when SG_UNIT is set. Unlocking clears bit by adding
* SG_UNIT. Staleness of read-only operations can be checked by
* comparing scanGuard to value before the reads. The low 16 bits
* (i.e, anding with SMASK) hold (the smallest power of two
* covering all worker indices, minus one, and is used to avoid
* dealing with large numbers of null slots when the workers array
* is overallocated.
*/
volatile int scanGuard;
private static final int SG_UNIT = 1 << 16;
/**
* The wakeup interval (in nanoseconds) for a worker waiting for a
* task when the pool is quiescent to instead try to shrink the
* number of workers. The exact value does not matter too
* much. It must be short enough to release resources during
* sustained periods of idleness, but not so short that threads
* are continually re-created.
*/
private static final long SHRINK_RATE =
4L * 1000L * 1000L * 1000L; // 4 seconds
/**
* Top-level loop for worker threads: On each step: if the
* previous step swept through all queues and found no tasks, or
* there are excess threads, then possibly blocks. Otherwise,
* scans for and, if found, executes a task. Returns when pool
* and/or worker terminate.
*
* @param w the worker
*/
final void work(ForkJoinWorkerThread w) {
boolean swept = false; // true on empty scans
long c;
while (!w.terminate && (int)(c = ctl) >= 0) {
int a; // active count
if (!swept && (a = (int)(c >> AC_SHIFT)) <= 0)
swept = scan(w, a);
else if (tryAwaitWork(w, c))
swept = false;
}
}
// Signalling
/**
* Wakes up or creates a worker.
*/
final void signalWork() {
/*
* The while condition is true if: (there is are too few total
* workers OR there is at least one waiter) AND (there are too
* few active workers OR the pool is terminating). The value
* of e distinguishes the remaining cases: zero (no waiters)
* for create, negative if terminating (in which case do
* nothing), else release a waiter. The secondary checks for
* release (non-null array etc) can fail if the pool begins
* terminating after the test, and don't impose any added cost
* because JVMs must perform null and bounds checks anyway.
*/
long c; int e, u;
while ((((e = (int)(c = ctl)) | (u = (int)(c >>> 32))) &
(INT_SIGN|SHORT_SIGN)) == (INT_SIGN|SHORT_SIGN) && e >= 0) {
if (e > 0) { // release a waiting worker
int i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
if ((ws = workers) == null ||
(i = ~e & SMASK) >= ws.length ||
(w = ws[i]) == null)
break;
long nc = (((long)(w.nextWait & E_MASK)) |
((long)(u + UAC_UNIT) << 32));
if (w.eventCount == e &&
UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
w.eventCount = (e + EC_UNIT) & E_MASK;
if (w.parked)
UNSAFE.unpark(w);
break;
}
}
else if (UNSAFE.compareAndSwapLong
(this, ctlOffset, c,
(long)(((u + UTC_UNIT) & UTC_MASK) |
((u + UAC_UNIT) & UAC_MASK)) << 32)) {
addWorker();
break;
}
}
}
/**
* Variant of signalWork to help release waiters on rescans.
* Tries once to release a waiter if active count < 0.
*
* @return false if failed due to contention, else true
*/
private boolean tryReleaseWaiter() {
long c; int e, i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
if ((e = (int)(c = ctl)) > 0 &&
(int)(c >> AC_SHIFT) < 0 &&
(ws = workers) != null &&
(i = ~e & SMASK) < ws.length &&
(w = ws[i]) != null) {
long nc = ((long)(w.nextWait & E_MASK) |
((c + AC_UNIT) & (AC_MASK|TC_MASK)));
if (w.eventCount != e ||
!UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
return false;
w.eventCount = (e + EC_UNIT) & E_MASK;
if (w.parked)
UNSAFE.unpark(w);
}
return true;
}
// Scanning for tasks
/**
* Scans for and, if found, executes one task. Scans start at a
* random index of workers array, and randomly select the first
* (2*#workers)-1 probes, and then, if all empty, resort to 2
* circular sweeps, which is necessary to check quiescence. and
* taking a submission only if no stealable tasks were found. The
* steal code inside the loop is a specialized form of
* ForkJoinWorkerThread.deqTask, followed bookkeeping to support
* helpJoinTask and signal propagation. The code for submission
* queues is almost identical. On each steal, the worker completes
* not only the task, but also all local tasks that this task may
* have generated. On detecting staleness or contention when
* trying to take a task, this method returns without finishing
* sweep, which allows global state rechecks before retry.
*
* @param w the worker
* @param a the number of active workers
* @return true if swept all queues without finding a task
*/
private boolean scan(ForkJoinWorkerThread w, int a) {
int g = scanGuard; // mask 0 avoids useless scans if only one active
int m = (parallelism == 1 - a && blockedCount == 0) ? 0 : g & SMASK;
ForkJoinWorkerThread[] ws = workers;
if (ws == null || ws.length <= m) // staleness check
return false;
for (int r = w.seed, k = r, j = -(m + m); j <= m + m; ++j) {
ForkJoinTask t; ForkJoinTask[] q; int b, i;
ForkJoinWorkerThread v = ws[k & m];
if (v != null && (b = v.queueBase) != v.queueTop &&
(q = v.queue) != null && (i = (q.length - 1) & b) >= 0) {
long u = (i << ASHIFT) + ABASE;
if ((t = q[i]) != null && v.queueBase == b &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
int d = (v.queueBase = b + 1) - v.queueTop;
v.stealHint = w.poolIndex;
if (d != 0)
signalWork(); // propagate if nonempty
w.execTask(t);
}
r ^= r << 13; r ^= r >>> 17; w.seed = r ^ (r << 5);
return false; // store next seed
}
else if (j < 0) { // xorshift
r ^= r << 13; r ^= r >>> 17; k = r ^= r << 5;
}
else
++k;
}
if (scanGuard != g) // staleness check
return false;
else { // try to take submission
ForkJoinTask t; ForkJoinTask[] q; int b, i;
if ((b = queueBase) != queueTop &&
(q = submissionQueue) != null &&
(i = (q.length - 1) & b) >= 0) {
long u = (i << ASHIFT) + ABASE;
if ((t = q[i]) != null && queueBase == b &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
queueBase = b + 1;
w.execTask(t);
}
return false;
}
return true; // all queues empty
}
}
/**
* Tries to enqueue worker w in wait queue and await change in
* worker's eventCount. If the pool is quiescent and there is
* more than one worker, possibly terminates worker upon exit.
* Otherwise, before blocking, rescans queues to avoid missed
* signals. Upon finding work, releases at least one worker
* (which may be the current worker). Rescans restart upon
* detected staleness or failure to release due to
* contention. Note the unusual conventions about Thread.interrupt
* here and elsewhere: Because interrupts are used solely to alert
* threads to check termination, which is checked here anyway, we
* clear status (using Thread.interrupted) before any call to
* park, so that park does not immediately return due to status
* being set via some other unrelated call to interrupt in user
* code.
*
* @param w the calling worker
* @param c the ctl value on entry
* @return true if waited or another thread was released upon enq
*/
private boolean tryAwaitWork(ForkJoinWorkerThread w, long c) {
int v = w.eventCount;
w.nextWait = (int)c; // w's successor record
long nc = (long)(v & E_MASK) | ((c - AC_UNIT) & (AC_MASK|TC_MASK));
if (ctl != c || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
long d = ctl; // return true if lost to a deq, to force scan
return (int)d != (int)c && ((d - c) & AC_MASK) >= 0L;
}
for (int sc = w.stealCount; sc != 0;) { // accumulate stealCount
long s = stealCount;
if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s + sc))
sc = w.stealCount = 0;
else if (w.eventCount != v)
return true; // update next time
}
if ((!shutdown || !tryTerminate(false)) &&
(int)c != 0 && parallelism + (int)(nc >> AC_SHIFT) == 0 &&
blockedCount == 0 && quiescerCount == 0)
idleAwaitWork(w, nc, c, v); // quiescent
for (boolean rescanned = false;;) {
if (w.eventCount != v)
return true;
if (!rescanned) {
int g = scanGuard, m = g & SMASK;
ForkJoinWorkerThread[] ws = workers;
if (ws != null && m < ws.length) {
rescanned = true;
for (int i = 0; i <= m; ++i) {
ForkJoinWorkerThread u = ws[i];
if (u != null) {
if (u.queueBase != u.queueTop &&
!tryReleaseWaiter())
rescanned = false; // contended
if (w.eventCount != v)
return true;
}
}
}
if (scanGuard != g || // stale
(queueBase != queueTop && !tryReleaseWaiter()))
rescanned = false;
if (!rescanned)
Thread.yield(); // reduce contention
else
Thread.interrupted(); // clear before park
}
else {
w.parked = true; // must recheck
if (w.eventCount != v) {
w.parked = false;
return true;
}
LockSupport.park(this);
rescanned = w.parked = false;
}
}
}
/**
* If inactivating worker w has caused pool to become
* quiescent, check for pool termination, and wait for event
* for up to SHRINK_RATE nanosecs (rescans are unnecessary in
* this case because quiescence reflects consensus about lack
* of work). On timeout, if ctl has not changed, terminate the
* worker. Upon its termination (see deregisterWorker), it may
* wake up another worker to possibly repeat this process.
*
* @param w the calling worker
* @param currentCtl the ctl value after enqueuing w
* @param prevCtl the ctl value if w terminated
* @param v the eventCount w awaits change
*/
private void idleAwaitWork(ForkJoinWorkerThread w, long currentCtl,
long prevCtl, int v) {
if (w.eventCount == v) {
if (shutdown)
tryTerminate(false);
ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs
while (ctl == currentCtl) {
long startTime = System.nanoTime();
w.parked = true;
if (w.eventCount == v) // must recheck
LockSupport.parkNanos(this, SHRINK_RATE);
w.parked = false;
if (w.eventCount != v)
break;
else if (System.nanoTime() - startTime <
SHRINK_RATE - (SHRINK_RATE / 10)) // timing slop
Thread.interrupted(); // spurious wakeup
else if (UNSAFE.compareAndSwapLong(this, ctlOffset,
currentCtl, prevCtl)) {
w.terminate = true; // restore previous
w.eventCount = ((int)currentCtl + EC_UNIT) & E_MASK;
break;
}
}
}
}
// Submissions
/**
* Enqueues the given task in the submissionQueue. Same idea as
* ForkJoinWorkerThread.pushTask except for use of submissionLock.
*
* @param t the task
*/
private void addSubmission(ForkJoinTask t) {
final ReentrantLock lock = this.submissionLock;
lock.lock();
try {
ForkJoinTask[] q; int s, m;
if ((q = submissionQueue) != null) { // ignore if queue removed
long u = (((s = queueTop) & (m = q.length-1)) << ASHIFT)+ABASE;
UNSAFE.putOrderedObject(q, u, t);
queueTop = s + 1;
if (s - queueBase == m)
growSubmissionQueue();
}
} finally {
lock.unlock();
}
signalWork();
}
// (pollSubmission is defined below with exported methods)
/**
* Creates or doubles submissionQueue array.
* Basically identical to ForkJoinWorkerThread version.
*/
private void growSubmissionQueue() {
ForkJoinTask[] oldQ = submissionQueue;
int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY;
if (size > MAXIMUM_QUEUE_CAPACITY)
throw new RejectedExecutionException("Queue capacity exceeded");
if (size < INITIAL_QUEUE_CAPACITY)
size = INITIAL_QUEUE_CAPACITY;
ForkJoinTask[] q = submissionQueue = new ForkJoinTask[size];
int mask = size - 1;
int top = queueTop;
int oldMask;
if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) {
for (int b = queueBase; b != top; ++b) {
long u = ((b & oldMask) << ASHIFT) + ABASE;
Object x = UNSAFE.getObjectVolatile(oldQ, u);
if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null))
UNSAFE.putObjectVolatile
(q, ((b & mask) << ASHIFT) + ABASE, x);
}
}
}
// Blocking support
/**
* Tries to increment blockedCount, decrement active count
* (sometimes implicitly) and possibly release or create a
* compensating worker in preparation for blocking. Fails
* on contention or termination.
*
* @return true if the caller can block, else should recheck and retry
*/
private boolean tryPreBlock() {
int b = blockedCount;
if (UNSAFE.compareAndSwapInt(this, blockedCountOffset, b, b + 1)) {
int pc = parallelism;
do {
ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w;
int e, ac, tc, rc, i;
long c = ctl;
int u = (int)(c >>> 32);
if ((e = (int)c) < 0) {
// skip -- terminating
}
else if ((ac = (u >> UAC_SHIFT)) <= 0 && e != 0 &&
(ws = workers) != null &&
(i = ~e & SMASK) < ws.length &&
(w = ws[i]) != null) {
long nc = ((long)(w.nextWait & E_MASK) |
(c & (AC_MASK|TC_MASK)));
if (w.eventCount == e &&
UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
w.eventCount = (e + EC_UNIT) & E_MASK;
if (w.parked)
UNSAFE.unpark(w);
return true; // release an idle worker
}
}
else if ((tc = (short)(u >>> UTC_SHIFT)) >= 0 && ac + pc > 1) {
long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
return true; // no compensation needed
}
else if (tc + pc < MAX_ID) {
long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
addWorker();
return true; // create a replacement
}
}
// try to back out on any failure and let caller retry
} while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
b = blockedCount, b - 1));
}
return false;
}
/**
* Decrements blockedCount and increments active count
*/
private void postBlock() {
long c;
do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, // no mask
c = ctl, c + AC_UNIT));
int b;
do {} while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
b = blockedCount, b - 1));
}
/**
* Possibly blocks waiting for the given task to complete, or
* cancels the task if terminating. Fails to wait if contended.
*
* @param joinMe the task
*/
final void tryAwaitJoin(ForkJoinTask joinMe) {
int s;
Thread.interrupted(); // clear interrupts before checking termination
if (joinMe.status >= 0) {
if (tryPreBlock()) {
joinMe.tryAwaitDone(0L);
postBlock();
}
else if ((ctl & STOP_BIT) != 0L)
joinMe.cancelIgnoringExceptions();
}
}
/**
* Possibly blocks the given worker waiting for joinMe to
* complete or timeout
*
* @param joinMe the task
* @param millis the wait time for underlying Object.wait
*/
final void timedAwaitJoin(ForkJoinTask joinMe, long nanos) {
while (joinMe.status >= 0) {
Thread.interrupted();
if ((ctl & STOP_BIT) != 0L) {
joinMe.cancelIgnoringExceptions();
break;
}
if (tryPreBlock()) {
long last = System.nanoTime();
while (joinMe.status >= 0) {
long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
if (millis <= 0)
break;
joinMe.tryAwaitDone(millis);
if (joinMe.status < 0)
break;
if ((ctl & STOP_BIT) != 0L) {
joinMe.cancelIgnoringExceptions();
break;
}
long now = System.nanoTime();
nanos -= now - last;
last = now;
}
postBlock();
break;
}
}
}
/**
* If necessary, compensates for blocker, and blocks
*/
private void awaitBlocker(ManagedBlocker blocker)
throws InterruptedException {
while (!blocker.isReleasable()) {
if (tryPreBlock()) {
try {
do {} while (!blocker.isReleasable() && !blocker.block());
} finally {
postBlock();
}
break;
}
}
}
// Creating, registering and deregistring workers
/**
* Tries to create and start a worker; minimally rolls back counts
* on failure.
*/
private void addWorker() {
Throwable ex = null;
ForkJoinWorkerThread t = null;
try {
t = factory.newThread(this);
} catch (Throwable e) {
ex = e;
}
if (t == null) { // null or exceptional factory return
long c; // adjust counts
do {} while (!UNSAFE.compareAndSwapLong
(this, ctlOffset, c = ctl,
(((c - AC_UNIT) & AC_MASK) |
((c - TC_UNIT) & TC_MASK) |
(c & ~(AC_MASK|TC_MASK)))));
// Propagate exception if originating from an external caller
if (!tryTerminate(false) && ex != null &&
!(Thread.currentThread() instanceof ForkJoinWorkerThread))
UNSAFE.throwException(ex);
}
else
t.start();
}
/**
* Callback from ForkJoinWorkerThread constructor to assign a
* public name
*/
final String nextWorkerName() {
for (int n;;) {
if (UNSAFE.compareAndSwapInt(this, nextWorkerNumberOffset,
n = nextWorkerNumber, ++n))
return workerNamePrefix + n;
}
}
/**
* Callback from ForkJoinWorkerThread constructor to
* determine its poolIndex and record in workers array.
*
* @param w the worker
* @return the worker's pool index
*/
final int registerWorker(ForkJoinWorkerThread w) {
/*
* In the typical case, a new worker acquires the lock, uses
* next available index and returns quickly. Since we should
* not block callers (ultimately from signalWork or
* tryPreBlock) waiting for the lock needed to do this, we
* instead help release other workers while waiting for the
* lock.
*/
for (int g;;) {
ForkJoinWorkerThread[] ws;
if (((g = scanGuard) & SG_UNIT) == 0 &&
UNSAFE.compareAndSwapInt(this, scanGuardOffset,
g, g | SG_UNIT)) {
int k = nextWorkerIndex;
try {
if ((ws = workers) != null) { // ignore on shutdown
int n = ws.length;
if (k < 0 || k >= n || ws[k] != null) {
for (k = 0; k < n && ws[k] != null; ++k)
;
if (k == n)
ws = workers = Arrays.copyOf(ws, n << 1);
}
ws[k] = w;
nextWorkerIndex = k + 1;
int m = g & SMASK;
g = (k > m) ? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1);
}
} finally {
scanGuard = g;
}
return k;
}
else if ((ws = workers) != null) { // help release others
for (ForkJoinWorkerThread u : ws) {
if (u != null && u.queueBase != u.queueTop) {
if (tryReleaseWaiter())
break;
}
}
}
}
}
/**
* Final callback from terminating worker. Removes record of
* worker from array, and adjusts counts. If pool is shutting
* down, tries to complete termination.
*
* @param w the worker
*/
final void deregisterWorker(ForkJoinWorkerThread w, Throwable ex) {
int idx = w.poolIndex;
int sc = w.stealCount;
int steps = 0;
// Remove from array, adjust worker counts and collect steal count.
// We can intermix failed removes or adjusts with steal updates
do {
long s, c;
int g;
if (steps == 0 && ((g = scanGuard) & SG_UNIT) == 0 &&
UNSAFE.compareAndSwapInt(this, scanGuardOffset,
g, g |= SG_UNIT)) {
ForkJoinWorkerThread[] ws = workers;
if (ws != null && idx >= 0 &&
idx < ws.length && ws[idx] == w)
ws[idx] = null; // verify
nextWorkerIndex = idx;
scanGuard = g + SG_UNIT;
steps = 1;
}
if (steps == 1 &&
UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
(((c - AC_UNIT) & AC_MASK) |
((c - TC_UNIT) & TC_MASK) |
(c & ~(AC_MASK|TC_MASK)))))
steps = 2;
if (sc != 0 &&
UNSAFE.compareAndSwapLong(this, stealCountOffset,
s = stealCount, s + sc))
sc = 0;
} while (steps != 2 || sc != 0);
if (!tryTerminate(false)) {
if (ex != null) // possibly replace if died abnormally
signalWork();
else
tryReleaseWaiter();
}
}
// Shutdown and termination
/**
* Possibly initiates and/or completes termination.
*
* @param now if true, unconditionally terminate, else only
* if shutdown and empty queue and no active workers
* @return true if now terminating or terminated
*/
private boolean tryTerminate(boolean now) {
long c;
while (((c = ctl) & STOP_BIT) == 0) {
if (!now) {
if ((int)(c >> AC_SHIFT) != -parallelism)
return false;
if (!shutdown || blockedCount != 0 || quiescerCount != 0 ||
queueBase != queueTop) {
if (ctl == c) // staleness check
return false;
continue;
}
}
if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, c | STOP_BIT))
startTerminating();
}
if ((short)(c >>> TC_SHIFT) == -parallelism) { // signal when 0 workers
final ReentrantLock lock = this.submissionLock;
lock.lock();
try {
termination.signalAll();
} finally {
lock.unlock();
}
}
return true;
}
/**
* Runs up to three passes through workers: (0) Setting
* termination status for each worker, followed by wakeups up to
* queued workers; (1) helping cancel tasks; (2) interrupting
* lagging threads (likely in external tasks, but possibly also
* blocked in joins). Each pass repeats previous steps because of
* potential lagging thread creation.
*/
private void startTerminating() {
cancelSubmissions();
for (int pass = 0; pass < 3; ++pass) {
ForkJoinWorkerThread[] ws = workers;
if (ws != null) {
for (ForkJoinWorkerThread w : ws) {
if (w != null) {
w.terminate = true;
if (pass > 0) {
w.cancelTasks();
if (pass > 1 && !w.isInterrupted()) {
try {
w.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
}
terminateWaiters();
}
}
}
/**
* Polls and cancels all submissions. Called only during termination.
*/
private void cancelSubmissions() {
while (queueBase != queueTop) {
ForkJoinTask task = pollSubmission();
if (task != null) {
try {
task.cancel(false);
} catch (Throwable ignore) {
}
}
}
}
/**
* Tries to set the termination status of waiting workers, and
* then wakes them up (after which they will terminate).
*/
private void terminateWaiters() {
ForkJoinWorkerThread[] ws = workers;
if (ws != null) {
ForkJoinWorkerThread w; long c; int i, e;
int n = ws.length;
while ((i = ~(e = (int)(c = ctl)) & SMASK) < n &&
(w = ws[i]) != null && w.eventCount == (e & E_MASK)) {
if (UNSAFE.compareAndSwapLong(this, ctlOffset, c,
(long)(w.nextWait & E_MASK) |
((c + AC_UNIT) & AC_MASK) |
(c & (TC_MASK|STOP_BIT)))) {
w.terminate = true;
w.eventCount = e + EC_UNIT;
if (w.parked)
UNSAFE.unpark(w);
}
}
}
}
// misc ForkJoinWorkerThread support
/**
* Increment or decrement quiescerCount. Needed only to prevent
* triggering shutdown if a worker is transiently inactive while
* checking quiescence.
*
* @param delta 1 for increment, -1 for decrement
*/
final void addQuiescerCount(int delta) {
int c;
do {} while (!UNSAFE.compareAndSwapInt(this, quiescerCountOffset,
c = quiescerCount, c + delta));
}
/**
* Directly increment or decrement active count without
* queuing. This method is used to transiently assert inactivation
* while checking quiescence.
*
* @param delta 1 for increment, -1 for decrement
*/
final void addActiveCount(int delta) {
long d = delta < 0 ? -AC_UNIT : AC_UNIT;
long c;
do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
((c + d) & AC_MASK) |
(c & ~AC_MASK)));
}
/**
* Returns the approximate (non-atomic) number of idle threads per
* active thread.
*/
final int idlePerActive() {
// Approximate at powers of two for small values, saturate past 4
int p = parallelism;
int a = p + (int)(ctl >> AC_SHIFT);
return (a > (p >>>= 1) ? 0 :
a > (p >>>= 1) ? 1 :
a > (p >>>= 1) ? 2 :
a > (p >>>= 1) ? 4 :
8);
}
// Exported methods
// Constructors
/**
* Creates a {@code ForkJoinPool} with parallelism equal to {@link
* java.lang.Runtime#availableProcessors}, using the {@linkplain
* #defaultForkJoinWorkerThreadFactory default thread factory},
* no UncaughtExceptionHandler, and non-async LIFO processing mode.
*
* @throws SecurityException if a security manager exists and
* the caller is not permitted to modify threads
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool() {
this(Runtime.getRuntime().availableProcessors(),
defaultForkJoinWorkerThreadFactory, null, false);
}
/**
* Creates a {@code ForkJoinPool} with the indicated parallelism
* level, the {@linkplain
* #defaultForkJoinWorkerThreadFactory default thread factory},
* no UncaughtExceptionHandler, and non-async LIFO processing mode.
*
* @param parallelism the parallelism level
* @throws IllegalArgumentException if parallelism less than or
* equal to zero, or greater than implementation limit
* @throws SecurityException if a security manager exists and
* the caller is not permitted to modify threads
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool(int parallelism) {
this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
}
/**
* Creates a {@code ForkJoinPool} with the given parameters.
*
* @param parallelism the parallelism level. For default value,
* use {@link java.lang.Runtime#availableProcessors}.
* @param factory the factory for creating new threads. For default value,
* use {@link #defaultForkJoinWorkerThreadFactory}.
* @param handler the handler for internal worker threads that
* terminate due to unrecoverable errors encountered while executing
* tasks. For default value, use {@code null}.
* @param asyncMode if true,
* establishes local first-in-first-out scheduling mode for forked
* tasks that are never joined. This mode may be more appropriate
* than default locally stack-based mode in applications in which
* worker threads only process event-style asynchronous tasks.
* For default value, use {@code false}.
* @throws IllegalArgumentException if parallelism less than or
* equal to zero, or greater than implementation limit
* @throws NullPointerException if the factory is null
* @throws SecurityException if a security manager exists and
* the caller is not permitted to modify threads
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public ForkJoinPool(int parallelism,
ForkJoinWorkerThreadFactory factory,
Thread.UncaughtExceptionHandler handler,
boolean asyncMode) {
checkPermission();
if (factory == null)
throw new NullPointerException();
if (parallelism <= 0 || parallelism > MAX_ID)
throw new IllegalArgumentException();
this.parallelism = parallelism;
this.factory = factory;
this.ueh = handler;
this.locallyFifo = asyncMode;
long np = (long)(-parallelism); // offset ctl counts
this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
this.submissionQueue = new ForkJoinTask[INITIAL_QUEUE_CAPACITY];
// initialize workers array with room for 2*parallelism if possible
int n = parallelism << 1;
if (n >= MAX_ID)
n = MAX_ID;
else { // See Hackers Delight, sec 3.2, where n < (1 << 16)
n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8;
}
workers = new ForkJoinWorkerThread[n + 1];
this.submissionLock = new ReentrantLock();
this.termination = submissionLock.newCondition();
StringBuilder sb = new StringBuilder("ForkJoinPool-");
sb.append(poolNumberGenerator.incrementAndGet());
sb.append("-worker-");
this.workerNamePrefix = sb.toString();
}
// Execution methods
/**
* Performs the given task, returning its result upon completion.
* If the computation encounters an unchecked Exception or Error,
* it is rethrown as the outcome of this invocation. 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 current thread
* as well as the thread actually encountering the exception;
* minimally only the latter.
*
* @param task the task
* @return the task's result
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public A {@code ManagedBlocker} provides two methods. Method
* {@code isReleasable} must return {@code true} if blocking is
* not necessary. Method {@code block} blocks the current thread
* if necessary (perhaps internally invoking {@code isReleasable}
* before actually blocking). These actions are performed by any
* thread invoking {@link ForkJoinPool#managedBlock}. The
* unusual methods in this API accommodate synchronizers that may,
* but don't usually, block for long periods. Similarly, they
* allow more efficient internal handling of cases in which
* additional workers may be, but usually are not, needed to
* ensure sufficient parallelism. Toward this end,
* implementations of method {@code isReleasable} must be amenable
* to repeated invocation.
*
* For example, here is a ManagedBlocker based on a
* ReentrantLock:
* Here is a class that possibly blocks waiting for an
* item on a given queue:
* If the caller is not a {@link ForkJoinTask}, this method is
* behaviorally equivalent to
* {@code
* class ManagedLocker implements ManagedBlocker {
* final ReentrantLock lock;
* boolean hasLock = false;
* ManagedLocker(ReentrantLock lock) { this.lock = lock; }
* public boolean block() {
* if (!hasLock)
* lock.lock();
* return true;
* }
* public boolean isReleasable() {
* return hasLock || (hasLock = lock.tryLock());
* }
* }}
*
* {@code
* class QueueTaker
*/
public static interface ManagedBlocker {
/**
* Possibly blocks the current thread, for example waiting for
* a lock or condition.
*
* @return {@code true} if no additional blocking is necessary
* (i.e., if isReleasable would return true)
* @throws InterruptedException if interrupted while waiting
* (the method is not required to do so, but is allowed to)
*/
boolean block() throws InterruptedException;
/**
* Returns {@code true} if blocking is unnecessary.
*/
boolean isReleasable();
}
/**
* Blocks in accord with the given blocker. If the current thread
* is a {@link ForkJoinWorkerThread}, this method possibly
* arranges for a spare thread to be activated if necessary to
* ensure sufficient parallelism while the current thread is blocked.
*
* {@code
* while (!blocker.isReleasable())
* if (blocker.block())
* return;
* }
*
* If the caller is a {@code ForkJoinTask}, then the pool may
* first be expanded to ensure parallelism, and later adjusted.
*
* @param blocker the blocker
* @throws InterruptedException if blocker.block did so
*/
public static void managedBlock(ManagedBlocker blocker)
throws InterruptedException {
Thread t = Thread.currentThread();
if (t instanceof ForkJoinWorkerThread) {
ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
w.pool.awaitBlocker(blocker);
}
else {
do {} while (!blocker.isReleasable() && !blocker.block());
}
}
// AbstractExecutorService overrides. These rely on undocumented
// fact that ForkJoinTask.adapt returns ForkJoinTasks that also
// implement RunnableFuture.
protected