Java example source code file (Phaser.java)
The Phaser.java Java example source code/*
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* 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.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;
/**
* A reusable synchronization barrier, similar in functionality to
* {@link java.util.concurrent.CyclicBarrier CyclicBarrier} and
* {@link java.util.concurrent.CountDownLatch CountDownLatch}
* but supporting more flexible usage.
*
* <p>Registration. Unlike the case for other barriers, the
* number of parties <em>registered to synchronize on a phaser
* may vary over time. Tasks may be registered at any time (using
* methods {@link #register}, {@link #bulkRegister}, or forms of
* constructors establishing initial numbers of parties), and
* optionally deregistered upon any arrival (using {@link
* #arriveAndDeregister}). As is the case with most basic
* synchronization constructs, registration and deregistration affect
* only internal counts; they do not establish any further internal
* bookkeeping, so tasks cannot query whether they are registered.
* (However, you can introduce such bookkeeping by subclassing this
* class.)
*
* <p>Synchronization. Like a {@code CyclicBarrier}, a {@code
* Phaser} may be repeatedly awaited. Method {@link
* #arriveAndAwaitAdvance} has effect analogous to {@link
* java.util.concurrent.CyclicBarrier#await CyclicBarrier.await}. Each
* generation of a phaser has an associated phase number. The phase
* number starts at zero, and advances when all parties arrive at the
* phaser, wrapping around to zero after reaching {@code
* Integer.MAX_VALUE}. The use of phase numbers enables independent
* control of actions upon arrival at a phaser and upon awaiting
* others, via two kinds of methods that may be invoked by any
* registered party:
*
* <ul>
*
* <li> Arrival. Methods {@link #arrive} and
* {@link #arriveAndDeregister} record arrival. These methods
* do not block, but return an associated <em>arrival phase
* number</em>; that is, the phase number of the phaser to which
* the arrival applied. When the final party for a given phase
* arrives, an optional action is performed and the phase
* advances. These actions are performed by the party
* triggering a phase advance, and are arranged by overriding
* method {@link #onAdvance(int, int)}, which also controls
* termination. Overriding this method is similar to, but more
* flexible than, providing a barrier action to a {@code
* CyclicBarrier}.
*
* <li> Waiting. Method {@link #awaitAdvance} requires an
* argument indicating an arrival phase number, and returns when
* the phaser advances to (or is already at) a different phase.
* Unlike similar constructions using {@code CyclicBarrier},
* method {@code awaitAdvance} continues to wait even if the
* waiting thread is interrupted. Interruptible and timeout
* versions are also available, but exceptions encountered while
* tasks wait interruptibly or with timeout do not change the
* state of the phaser. If necessary, you can perform any
* associated recovery within handlers of those exceptions,
* often after invoking {@code forceTermination}. Phasers may
* also be used by tasks executing in a {@link ForkJoinPool},
* which will ensure sufficient parallelism to execute tasks
* when others are blocked waiting for a phase to advance.
*
* </ul>
*
* <p>Termination. A phaser may enter a termination
* state, that may be checked using method {@link #isTerminated}. Upon
* termination, all synchronization methods immediately return without
* waiting for advance, as indicated by a negative return value.
* Similarly, attempts to register upon termination have no effect.
* Termination is triggered when an invocation of {@code onAdvance}
* returns {@code true}. The default implementation returns {@code
* true} if a deregistration has caused the number of registered
* parties to become zero. As illustrated below, when phasers control
* actions with a fixed number of iterations, it is often convenient
* to override this method to cause termination when the current phase
* number reaches a threshold. Method {@link #forceTermination} is
* also available to abruptly release waiting threads and allow them
* to terminate.
*
* <p>Tiering. Phasers may be tiered (i.e.,
* constructed in tree structures) to reduce contention. Phasers with
* large numbers of parties that would otherwise experience heavy
* synchronization contention costs may instead be set up so that
* groups of sub-phasers share a common parent. This may greatly
* increase throughput even though it incurs greater per-operation
* overhead.
*
* <p>In a tree of tiered phasers, registration and deregistration of
* child phasers with their parent are managed automatically.
* Whenever the number of registered parties of a child phaser becomes
* non-zero (as established in the {@link #Phaser(Phaser,int)}
* constructor, {@link #register}, or {@link #bulkRegister}), the
* child phaser is registered with its parent. Whenever the number of
* registered parties becomes zero as the result of an invocation of
* {@link #arriveAndDeregister}, the child phaser is deregistered
* from its parent.
*
* <p>Monitoring. While synchronization methods may be invoked
* only by registered parties, the current state of a phaser may be
* monitored by any caller. At any given moment there are {@link
* #getRegisteredParties} parties in total, of which {@link
* #getArrivedParties} have arrived at the current phase ({@link
* #getPhase}). When the remaining ({@link #getUnarrivedParties})
* parties arrive, the phase advances. The values returned by these
* methods may reflect transient states and so are not in general
* useful for synchronization control. Method {@link #toString}
* returns snapshots of these state queries in a form convenient for
* informal monitoring.
*
* <p>Sample usages:
*
* <p>A {@code Phaser} may be used instead of a {@code CountDownLatch}
* to control a one-shot action serving a variable number of parties.
* The typical idiom is for the method setting this up to first
* register, then start the actions, then deregister, as in:
*
* <pre> {@code
* void runTasks(List<Runnable> tasks) {
* final Phaser phaser = new Phaser(1); // "1" to register self
* // create and start threads
* for (final Runnable task : tasks) {
* phaser.register();
* new Thread() {
* public void run() {
* phaser.arriveAndAwaitAdvance(); // await all creation
* task.run();
* }
* }.start();
* }
*
* // allow threads to start and deregister self
* phaser.arriveAndDeregister();
* }}</pre>
*
* <p>One way to cause a set of threads to repeatedly perform actions
* for a given number of iterations is to override {@code onAdvance}:
*
* <pre> {@code
* void startTasks(List<Runnable> tasks, final int iterations) {
* final Phaser phaser = new Phaser() {
* protected boolean onAdvance(int phase, int registeredParties) {
* return phase >= iterations || registeredParties == 0;
* }
* };
* phaser.register();
* for (final Runnable task : tasks) {
* phaser.register();
* new Thread() {
* public void run() {
* do {
* task.run();
* phaser.arriveAndAwaitAdvance();
* } while (!phaser.isTerminated());
* }
* }.start();
* }
* phaser.arriveAndDeregister(); // deregister self, don't wait
* }}</pre>
*
* If the main task must later await termination, it
* may re-register and then execute a similar loop:
* <pre> {@code
* // ...
* phaser.register();
* while (!phaser.isTerminated())
* phaser.arriveAndAwaitAdvance();}</pre>
*
* <p>Related constructions may be used to await particular phase numbers
* in contexts where you are sure that the phase will never wrap around
* {@code Integer.MAX_VALUE}. For example:
*
* <pre> {@code
* void awaitPhase(Phaser phaser, int phase) {
* int p = phaser.register(); // assumes caller not already registered
* while (p < phase) {
* if (phaser.isTerminated())
* // ... deal with unexpected termination
* else
* p = phaser.arriveAndAwaitAdvance();
* }
* phaser.arriveAndDeregister();
* }}</pre>
*
*
* <p>To create a set of {@code n} tasks using a tree of phasers, you
* could use code of the following form, assuming a Task class with a
* constructor accepting a {@code Phaser} that it registers with upon
* construction. After invocation of {@code build(new Task[n], 0, n,
* new Phaser())}, these tasks could then be started, for example by
* submitting to a pool:
*
* <pre> {@code
* void build(Task[] tasks, int lo, int hi, Phaser ph) {
* if (hi - lo > TASKS_PER_PHASER) {
* for (int i = lo; i < hi; i += TASKS_PER_PHASER) {
* int j = Math.min(i + TASKS_PER_PHASER, hi);
* build(tasks, i, j, new Phaser(ph));
* }
* } else {
* for (int i = lo; i < hi; ++i)
* tasks[i] = new Task(ph);
* // assumes new Task(ph) performs ph.register()
* }
* }}</pre>
*
* The best value of {@code TASKS_PER_PHASER} depends mainly on
* expected synchronization rates. A value as low as four may
* be appropriate for extremely small per-phase task bodies (thus
* high rates), or up to hundreds for extremely large ones.
*
* <p>Implementation notes: This implementation restricts the
* maximum number of parties to 65535. Attempts to register additional
* parties result in {@code IllegalStateException}. However, you can and
* should create tiered phasers to accommodate arbitrarily large sets
* of participants.
*
* @since 1.7
* @author Doug Lea
*/
public class Phaser {
/*
* This class implements an extension of X10 "clocks". Thanks to
* Vijay Saraswat for the idea, and to Vivek Sarkar for
* enhancements to extend functionality.
*/
/**
* Primary state representation, holding four bit-fields:
*
* unarrived -- the number of parties yet to hit barrier (bits 0-15)
* parties -- the number of parties to wait (bits 16-31)
* phase -- the generation of the barrier (bits 32-62)
* terminated -- set if barrier is terminated (bit 63 / sign)
*
* Except that a phaser with no registered parties is
* distinguished by the otherwise illegal state of having zero
* parties and one unarrived parties (encoded as EMPTY below).
*
* To efficiently maintain atomicity, these values are packed into
* a single (atomic) long. Good performance relies on keeping
* state decoding and encoding simple, and keeping race windows
* short.
*
* All state updates are performed via CAS except initial
* registration of a sub-phaser (i.e., one with a non-null
* parent). In this (relatively rare) case, we use built-in
* synchronization to lock while first registering with its
* parent.
*
* The phase of a subphaser is allowed to lag that of its
* ancestors until it is actually accessed -- see method
* reconcileState.
*/
private volatile long state;
private static final int MAX_PARTIES = 0xffff;
private static final int MAX_PHASE = Integer.MAX_VALUE;
private static final int PARTIES_SHIFT = 16;
private static final int PHASE_SHIFT = 32;
private static final int UNARRIVED_MASK = 0xffff; // to mask ints
private static final long PARTIES_MASK = 0xffff0000L; // to mask longs
private static final long COUNTS_MASK = 0xffffffffL;
private static final long TERMINATION_BIT = 1L << 63;
// some special values
private static final int ONE_ARRIVAL = 1;
private static final int ONE_PARTY = 1 << PARTIES_SHIFT;
private static final int ONE_DEREGISTER = ONE_ARRIVAL|ONE_PARTY;
private static final int EMPTY = 1;
// The following unpacking methods are usually manually inlined
private static int unarrivedOf(long s) {
int counts = (int)s;
return (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);
}
private static int partiesOf(long s) {
return (int)s >>> PARTIES_SHIFT;
}
private static int phaseOf(long s) {
return (int)(s >>> PHASE_SHIFT);
}
private static int arrivedOf(long s) {
int counts = (int)s;
return (counts == EMPTY) ? 0 :
(counts >>> PARTIES_SHIFT) - (counts & UNARRIVED_MASK);
}
/**
* The parent of this phaser, or null if none
*/
private final Phaser parent;
/**
* The root of phaser tree. Equals this if not in a tree.
*/
private final Phaser root;
/**
* Heads of Treiber stacks for waiting threads. To eliminate
* contention when releasing some threads while adding others, we
* use two of them, alternating across even and odd phases.
* Subphasers share queues with root to speed up releases.
*/
private final AtomicReference<QNode> evenQ;
private final AtomicReference<QNode> oddQ;
private AtomicReference<QNode> queueFor(int phase) {
return ((phase & 1) == 0) ? evenQ : oddQ;
}
/**
* Returns message string for bounds exceptions on arrival.
*/
private String badArrive(long s) {
return "Attempted arrival of unregistered party for " +
stateToString(s);
}
/**
* Returns message string for bounds exceptions on registration.
*/
private String badRegister(long s) {
return "Attempt to register more than " +
MAX_PARTIES + " parties for " + stateToString(s);
}
/**
* Main implementation for methods arrive and arriveAndDeregister.
* Manually tuned to speed up and minimize race windows for the
* common case of just decrementing unarrived field.
*
* @param adjust value to subtract from state;
* ONE_ARRIVAL for arrive,
* ONE_DEREGISTER for arriveAndDeregister
*/
private int doArrive(int adjust) {
final Phaser root = this.root;
for (;;) {
long s = (root == this) ? state : reconcileState();
int phase = (int)(s >>> PHASE_SHIFT);
if (phase < 0)
return phase;
int counts = (int)s;
int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);
if (unarrived <= 0)
throw new IllegalStateException(badArrive(s));
if (UNSAFE.compareAndSwapLong(this, stateOffset, s, s-=adjust)) {
if (unarrived == 1) {
long n = s & PARTIES_MASK; // base of next state
int nextUnarrived = (int)n >>> PARTIES_SHIFT;
if (root == this) {
if (onAdvance(phase, nextUnarrived))
n |= TERMINATION_BIT;
else if (nextUnarrived == 0)
n |= EMPTY;
else
n |= nextUnarrived;
int nextPhase = (phase + 1) & MAX_PHASE;
n |= (long)nextPhase << PHASE_SHIFT;
UNSAFE.compareAndSwapLong(this, stateOffset, s, n);
releaseWaiters(phase);
}
else if (nextUnarrived == 0) { // propagate deregistration
phase = parent.doArrive(ONE_DEREGISTER);
UNSAFE.compareAndSwapLong(this, stateOffset,
s, s | EMPTY);
}
else
phase = parent.doArrive(ONE_ARRIVAL);
}
return phase;
}
}
}
/**
* Implementation of register, bulkRegister
*
* @param registrations number to add to both parties and
* unarrived fields. Must be greater than zero.
*/
private int doRegister(int registrations) {
// adjustment to state
long adjust = ((long)registrations << PARTIES_SHIFT) | registrations;
final Phaser parent = this.parent;
int phase;
for (;;) {
long s = (parent == null) ? state : reconcileState();
int counts = (int)s;
int parties = counts >>> PARTIES_SHIFT;
int unarrived = counts & UNARRIVED_MASK;
if (registrations > MAX_PARTIES - parties)
throw new IllegalStateException(badRegister(s));
phase = (int)(s >>> PHASE_SHIFT);
if (phase < 0)
break;
if (counts != EMPTY) { // not 1st registration
if (parent == null || reconcileState() == s) {
if (unarrived == 0) // wait out advance
root.internalAwaitAdvance(phase, null);
else if (UNSAFE.compareAndSwapLong(this, stateOffset,
s, s + adjust))
break;
}
}
else if (parent == null) { // 1st root registration
long next = ((long)phase << PHASE_SHIFT) | adjust;
if (UNSAFE.compareAndSwapLong(this, stateOffset, s, next))
break;
}
else {
synchronized (this) { // 1st sub registration
if (state == s) { // recheck under lock
phase = parent.doRegister(1);
if (phase < 0)
break;
// finish registration whenever parent registration
// succeeded, even when racing with termination,
// since these are part of the same "transaction".
while (!UNSAFE.compareAndSwapLong
(this, stateOffset, s,
((long)phase << PHASE_SHIFT) | adjust)) {
s = state;
phase = (int)(root.state >>> PHASE_SHIFT);
// assert (int)s == EMPTY;
}
break;
}
}
}
}
return phase;
}
/**
* Resolves lagged phase propagation from root if necessary.
* Reconciliation normally occurs when root has advanced but
* subphasers have not yet done so, in which case they must finish
* their own advance by setting unarrived to parties (or if
* parties is zero, resetting to unregistered EMPTY state).
*
* @return reconciled state
*/
private long reconcileState() {
final Phaser root = this.root;
long s = state;
if (root != this) {
int phase, p;
// CAS to root phase with current parties, tripping unarrived
while ((phase = (int)(root.state >>> PHASE_SHIFT)) !=
(int)(s >>> PHASE_SHIFT) &&
!UNSAFE.compareAndSwapLong
(this, stateOffset, s,
s = (((long)phase << PHASE_SHIFT) |
((phase < 0) ? (s & COUNTS_MASK) :
(((p = (int)s >>> PARTIES_SHIFT) == 0) ? EMPTY :
((s & PARTIES_MASK) | p))))))
s = state;
}
return s;
}
/**
* Creates a new phaser with no initially registered parties, no
* parent, and initial phase number 0. Any thread using this
* phaser will need to first register for it.
*/
public Phaser() {
this(null, 0);
}
/**
* Creates a new phaser with the given number of registered
* unarrived parties, no parent, and initial phase number 0.
*
* @param parties the number of parties required to advance to the
* next phase
* @throws IllegalArgumentException if parties less than zero
* or greater than the maximum number of parties supported
*/
public Phaser(int parties) {
this(null, parties);
}
/**
* Equivalent to {@link #Phaser(Phaser, int) Phaser(parent, 0)}.
*
* @param parent the parent phaser
*/
public Phaser(Phaser parent) {
this(parent, 0);
}
/**
* Creates a new phaser with the given parent and number of
* registered unarrived parties. When the given parent is non-null
* and the given number of parties is greater than zero, this
* child phaser is registered with its parent.
*
* @param parent the parent phaser
* @param parties the number of parties required to advance to the
* next phase
* @throws IllegalArgumentException if parties less than zero
* or greater than the maximum number of parties supported
*/
public Phaser(Phaser parent, int parties) {
if (parties >>> PARTIES_SHIFT != 0)
throw new IllegalArgumentException("Illegal number of parties");
int phase = 0;
this.parent = parent;
if (parent != null) {
final Phaser root = parent.root;
this.root = root;
this.evenQ = root.evenQ;
this.oddQ = root.oddQ;
if (parties != 0)
phase = parent.doRegister(1);
}
else {
this.root = this;
this.evenQ = new AtomicReference<QNode>();
this.oddQ = new AtomicReference<QNode>();
}
this.state = (parties == 0) ? (long)EMPTY :
((long)phase << PHASE_SHIFT) |
((long)parties << PARTIES_SHIFT) |
((long)parties);
}
/**
* Adds a new unarrived party to this phaser. If an ongoing
* invocation of {@link #onAdvance} is in progress, this method
* may await its completion before returning. If this phaser has
* a parent, and this phaser previously had no registered parties,
* this child phaser is also registered with its parent. If
* this phaser is terminated, the attempt to register has
* no effect, and a negative value is returned.
*
* @return the arrival phase number to which this registration
* applied. If this value is negative, then this phaser has
* terminated, in which case registration has no effect.
* @throws IllegalStateException if attempting to register more
* than the maximum supported number of parties
*/
public int register() {
return doRegister(1);
}
/**
* Adds the given number of new unarrived parties to this phaser.
* If an ongoing invocation of {@link #onAdvance} is in progress,
* this method may await its completion before returning. If this
* phaser has a parent, and the given number of parties is greater
* than zero, and this phaser previously had no registered
* parties, this child phaser is also registered with its parent.
* If this phaser is terminated, the attempt to register has no
* effect, and a negative value is returned.
*
* @param parties the number of additional parties required to
* advance to the next phase
* @return the arrival phase number to which this registration
* applied. If this value is negative, then this phaser has
* terminated, in which case registration has no effect.
* @throws IllegalStateException if attempting to register more
* than the maximum supported number of parties
* @throws IllegalArgumentException if {@code parties < 0}
*/
public int bulkRegister(int parties) {
if (parties < 0)
throw new IllegalArgumentException();
if (parties == 0)
return getPhase();
return doRegister(parties);
}
/**
* Arrives at this phaser, without waiting for others to arrive.
*
* <p>It is a usage error for an unregistered party to invoke this
* method. However, this error may result in an {@code
* IllegalStateException} only upon some subsequent operation on
* this phaser, if ever.
*
* @return the arrival phase number, or a negative value if terminated
* @throws IllegalStateException if not terminated and the number
* of unarrived parties would become negative
*/
public int arrive() {
return doArrive(ONE_ARRIVAL);
}
/**
* Arrives at this phaser and deregisters from it without waiting
* for others to arrive. Deregistration reduces the number of
* parties required to advance in future phases. If this phaser
* has a parent, and deregistration causes this phaser to have
* zero parties, this phaser is also deregistered from its parent.
*
* <p>It is a usage error for an unregistered party to invoke this
* method. However, this error may result in an {@code
* IllegalStateException} only upon some subsequent operation on
* this phaser, if ever.
*
* @return the arrival phase number, or a negative value if terminated
* @throws IllegalStateException if not terminated and the number
* of registered or unarrived parties would become negative
*/
public int arriveAndDeregister() {
return doArrive(ONE_DEREGISTER);
}
/**
* Arrives at this phaser and awaits others. Equivalent in effect
* to {@code awaitAdvance(arrive())}. If you need to await with
* interruption or timeout, you can arrange this with an analogous
* construction using one of the other forms of the {@code
* awaitAdvance} method. If instead you need to deregister upon
* arrival, use {@code awaitAdvance(arriveAndDeregister())}.
*
* <p>It is a usage error for an unregistered party to invoke this
* method. However, this error may result in an {@code
* IllegalStateException} only upon some subsequent operation on
* this phaser, if ever.
*
* @return the arrival phase number, or the (negative)
* {@linkplain #getPhase() current phase} if terminated
* @throws IllegalStateException if not terminated and the number
* of unarrived parties would become negative
*/
public int arriveAndAwaitAdvance() {
// Specialization of doArrive+awaitAdvance eliminating some reads/paths
final Phaser root = this.root;
for (;;) {
long s = (root == this) ? state : reconcileState();
int phase = (int)(s >>> PHASE_SHIFT);
if (phase < 0)
return phase;
int counts = (int)s;
int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK);
if (unarrived <= 0)
throw new IllegalStateException(badArrive(s));
if (UNSAFE.compareAndSwapLong(this, stateOffset, s,
s -= ONE_ARRIVAL)) {
if (unarrived > 1)
return root.internalAwaitAdvance(phase, null);
if (root != this)
return parent.arriveAndAwaitAdvance();
long n = s & PARTIES_MASK; // base of next state
int nextUnarrived = (int)n >>> PARTIES_SHIFT;
if (onAdvance(phase, nextUnarrived))
n |= TERMINATION_BIT;
else if (nextUnarrived == 0)
n |= EMPTY;
else
n |= nextUnarrived;
int nextPhase = (phase + 1) & MAX_PHASE;
n |= (long)nextPhase << PHASE_SHIFT;
if (!UNSAFE.compareAndSwapLong(this, stateOffset, s, n))
return (int)(state >>> PHASE_SHIFT); // terminated
releaseWaiters(phase);
return nextPhase;
}
}
}
/**
* Awaits the phase of this phaser to advance from the given phase
* value, returning immediately if the current phase is not equal
* to the given phase value or this phaser is terminated.
*
* @param phase an arrival phase number, or negative value if
* terminated; this argument is normally the value returned by a
* previous call to {@code arrive} or {@code arriveAndDeregister}.
* @return the next arrival phase number, or the argument if it is
* negative, or the (negative) {@linkplain #getPhase() current phase}
* if terminated
*/
public int awaitAdvance(int phase) {
final Phaser root = this.root;
long s = (root == this) ? state : reconcileState();
int p = (int)(s >>> PHASE_SHIFT);
if (phase < 0)
return phase;
if (p == phase)
return root.internalAwaitAdvance(phase, null);
return p;
}
/**
* Awaits the phase of this phaser to advance from the given phase
* value, throwing {@code InterruptedException} if interrupted
* while waiting, or returning immediately if the current phase is
* not equal to the given phase value or this phaser is
* terminated.
*
* @param phase an arrival phase number, or negative value if
* terminated; this argument is normally the value returned by a
* previous call to {@code arrive} or {@code arriveAndDeregister}.
* @return the next arrival phase number, or the argument if it is
* negative, or the (negative) {@linkplain #getPhase() current phase}
* if terminated
* @throws InterruptedException if thread interrupted while waiting
*/
public int awaitAdvanceInterruptibly(int phase)
throws InterruptedException {
final Phaser root = this.root;
long s = (root == this) ? state : reconcileState();
int p = (int)(s >>> PHASE_SHIFT);
if (phase < 0)
return phase;
if (p == phase) {
QNode node = new QNode(this, phase, true, false, 0L);
p = root.internalAwaitAdvance(phase, node);
if (node.wasInterrupted)
throw new InterruptedException();
}
return p;
}
/**
* Awaits the phase of this phaser to advance from the given phase
* value or the given timeout to elapse, throwing {@code
* InterruptedException} if interrupted while waiting, or
* returning immediately if the current phase is not equal to the
* given phase value or this phaser is terminated.
*
* @param phase an arrival phase number, or negative value if
* terminated; this argument is normally the value returned by a
* previous call to {@code arrive} or {@code arriveAndDeregister}.
* @param timeout how long to wait before giving up, in units of
* {@code unit}
* @param unit a {@code TimeUnit} determining how to interpret the
* {@code timeout} parameter
* @return the next arrival phase number, or the argument if it is
* negative, or the (negative) {@linkplain #getPhase() current phase}
* if terminated
* @throws InterruptedException if thread interrupted while waiting
* @throws TimeoutException if timed out while waiting
*/
public int awaitAdvanceInterruptibly(int phase,
long timeout, TimeUnit unit)
throws InterruptedException, TimeoutException {
long nanos = unit.toNanos(timeout);
final Phaser root = this.root;
long s = (root == this) ? state : reconcileState();
int p = (int)(s >>> PHASE_SHIFT);
if (phase < 0)
return phase;
if (p == phase) {
QNode node = new QNode(this, phase, true, true, nanos);
p = root.internalAwaitAdvance(phase, node);
if (node.wasInterrupted)
throw new InterruptedException();
else if (p == phase)
throw new TimeoutException();
}
return p;
}
/**
* Forces this phaser to enter termination state. Counts of
* registered parties are unaffected. If this phaser is a member
* of a tiered set of phasers, then all of the phasers in the set
* are terminated. If this phaser is already terminated, this
* method has no effect. This method may be useful for
* coordinating recovery after one or more tasks encounter
* unexpected exceptions.
*/
public void forceTermination() {
// Only need to change root state
final Phaser root = this.root;
long s;
while ((s = root.state) >= 0) {
if (UNSAFE.compareAndSwapLong(root, stateOffset,
s, s | TERMINATION_BIT)) {
// signal all threads
releaseWaiters(0); // Waiters on evenQ
releaseWaiters(1); // Waiters on oddQ
return;
}
}
}
/**
* Returns the current phase number. The maximum phase number is
* {@code Integer.MAX_VALUE}, after which it restarts at
* zero. Upon termination, the phase number is negative,
* in which case the prevailing phase prior to termination
* may be obtained via {@code getPhase() + Integer.MIN_VALUE}.
*
* @return the phase number, or a negative value if terminated
*/
public final int getPhase() {
return (int)(root.state >>> PHASE_SHIFT);
}
/**
* Returns the number of parties registered at this phaser.
*
* @return the number of parties
*/
public int getRegisteredParties() {
return partiesOf(state);
}
/**
* Returns the number of registered parties that have arrived at
* the current phase of this phaser. If this phaser has terminated,
* the returned value is meaningless and arbitrary.
*
* @return the number of arrived parties
*/
public int getArrivedParties() {
return arrivedOf(reconcileState());
}
/**
* Returns the number of registered parties that have not yet
* arrived at the current phase of this phaser. If this phaser has
* terminated, the returned value is meaningless and arbitrary.
*
* @return the number of unarrived parties
*/
public int getUnarrivedParties() {
return unarrivedOf(reconcileState());
}
/**
* Returns the parent of this phaser, or {@code null} if none.
*
* @return the parent of this phaser, or {@code null} if none
*/
public Phaser getParent() {
return parent;
}
/**
* Returns the root ancestor of this phaser, which is the same as
* this phaser if it has no parent.
*
* @return the root ancestor of this phaser
*/
public Phaser getRoot() {
return root;
}
/**
* Returns {@code true} if this phaser has been terminated.
*
* @return {@code true} if this phaser has been terminated
*/
public boolean isTerminated() {
return root.state < 0L;
}
/**
* Overridable method to perform an action upon impending phase
* advance, and to control termination. This method is invoked
* upon arrival of the party advancing this phaser (when all other
* waiting parties are dormant). If this method returns {@code
* true}, this phaser will be set to a final termination state
* upon advance, and subsequent calls to {@link #isTerminated}
* will return true. Any (unchecked) Exception or Error thrown by
* an invocation of this method is propagated to the party
* attempting to advance this phaser, in which case no advance
* occurs.
*
* <p>The arguments to this method provide the state of the phaser
* prevailing for the current transition. The effects of invoking
* arrival, registration, and waiting methods on this phaser from
* within {@code onAdvance} are unspecified and should not be
* relied on.
*
* <p>If this phaser is a member of a tiered set of phasers, then
* {@code onAdvance} is invoked only for its root phaser on each
* advance.
*
* <p>To support the most common use cases, the default
* implementation of this method returns {@code true} when the
* number of registered parties has become zero as the result of a
* party invoking {@code arriveAndDeregister}. You can disable
* this behavior, thus enabling continuation upon future
* registrations, by overriding this method to always return
* {@code false}:
*
* <pre> {@code
* Phaser phaser = new Phaser() {
* protected boolean onAdvance(int phase, int parties) { return false; }
* }}</pre>
*
* @param phase the current phase number on entry to this method,
* before this phaser is advanced
* @param registeredParties the current number of registered parties
* @return {@code true} if this phaser should terminate
*/
protected boolean onAdvance(int phase, int registeredParties) {
return registeredParties == 0;
}
/**
* Returns a string identifying this phaser, as well as its
* state. The state, in brackets, includes the String {@code
* "phase = "} followed by the phase number, {@code "parties = "}
* followed by the number of registered parties, and {@code
* "arrived = "} followed by the number of arrived parties.
*
* @return a string identifying this phaser, as well as its state
*/
public String toString() {
return stateToString(reconcileState());
}
/**
* Implementation of toString and string-based error messages
*/
private String stateToString(long s) {
return super.toString() +
"[phase = " + phaseOf(s) +
" parties = " + partiesOf(s) +
" arrived = " + arrivedOf(s) + "]";
}
// Waiting mechanics
/**
* Removes and signals threads from queue for phase.
*/
private void releaseWaiters(int phase) {
QNode q; // first element of queue
Thread t; // its thread
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ;
while ((q = head.get()) != null &&
q.phase != (int)(root.state >>> PHASE_SHIFT)) {
if (head.compareAndSet(q, q.next) &&
(t = q.thread) != null) {
q.thread = null;
LockSupport.unpark(t);
}
}
}
/**
* Variant of releaseWaiters that additionally tries to remove any
* nodes no longer waiting for advance due to timeout or
* interrupt. Currently, nodes are removed only if they are at
* head of queue, which suffices to reduce memory footprint in
* most usages.
*
* @return current phase on exit
*/
private int abortWait(int phase) {
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ;
for (;;) {
Thread t;
QNode q = head.get();
int p = (int)(root.state >>> PHASE_SHIFT);
if (q == null || ((t = q.thread) != null && q.phase == p))
return p;
if (head.compareAndSet(q, q.next) && t != null) {
q.thread = null;
LockSupport.unpark(t);
}
}
}
/** The number of CPUs, for spin control */
private static final int NCPU = Runtime.getRuntime().availableProcessors();
/**
* The number of times to spin before blocking while waiting for
* advance, per arrival while waiting. On multiprocessors, fully
* blocking and waking up a large number of threads all at once is
* usually a very slow process, so we use rechargeable spins to
* avoid it when threads regularly arrive: When a thread in
* internalAwaitAdvance notices another arrival before blocking,
* and there appear to be enough CPUs available, it spins
* SPINS_PER_ARRIVAL more times before blocking. The value trades
* off good-citizenship vs big unnecessary slowdowns.
*/
static final int SPINS_PER_ARRIVAL = (NCPU < 2) ? 1 : 1 << 8;
/**
* Possibly blocks and waits for phase to advance unless aborted.
* Call only on root phaser.
*
* @param phase current phase
* @param node if non-null, the wait node to track interrupt and timeout;
* if null, denotes noninterruptible wait
* @return current phase
*/
private int internalAwaitAdvance(int phase, QNode node) {
// assert root == this;
releaseWaiters(phase-1); // ensure old queue clean
boolean queued = false; // true when node is enqueued
int lastUnarrived = 0; // to increase spins upon change
int spins = SPINS_PER_ARRIVAL;
long s;
int p;
while ((p = (int)((s = state) >>> PHASE_SHIFT)) == phase) {
if (node == null) { // spinning in noninterruptible mode
int unarrived = (int)s & UNARRIVED_MASK;
if (unarrived != lastUnarrived &&
(lastUnarrived = unarrived) < NCPU)
spins += SPINS_PER_ARRIVAL;
boolean interrupted = Thread.interrupted();
if (interrupted || --spins < 0) { // need node to record intr
node = new QNode(this, phase, false, false, 0L);
node.wasInterrupted = interrupted;
}
}
else if (node.isReleasable()) // done or aborted
break;
else if (!queued) { // push onto queue
AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ;
QNode q = node.next = head.get();
if ((q == null || q.phase == phase) &&
(int)(state >>> PHASE_SHIFT) == phase) // avoid stale enq
queued = head.compareAndSet(q, node);
}
else {
try {
ForkJoinPool.managedBlock(node);
} catch (InterruptedException ie) {
node.wasInterrupted = true;
}
}
}
if (node != null) {
if (node.thread != null)
node.thread = null; // avoid need for unpark()
if (node.wasInterrupted && !node.interruptible)
Thread.currentThread().interrupt();
if (p == phase && (p = (int)(state >>> PHASE_SHIFT)) == phase)
return abortWait(phase); // possibly clean up on abort
}
releaseWaiters(phase);
return p;
}
/**
* Wait nodes for Treiber stack representing wait queue
*/
static final class QNode implements ForkJoinPool.ManagedBlocker {
final Phaser phaser;
final int phase;
final boolean interruptible;
final boolean timed;
boolean wasInterrupted;
long nanos;
final long deadline;
volatile Thread thread; // nulled to cancel wait
QNode next;
QNode(Phaser phaser, int phase, boolean interruptible,
boolean timed, long nanos) {
this.phaser = phaser;
this.phase = phase;
this.interruptible = interruptible;
this.nanos = nanos;
this.timed = timed;
this.deadline = timed ? System.nanoTime() + nanos : 0L;
thread = Thread.currentThread();
}
public boolean isReleasable() {
if (thread == null)
return true;
if (phaser.getPhase() != phase) {
thread = null;
return true;
}
if (Thread.interrupted())
wasInterrupted = true;
if (wasInterrupted && interruptible) {
thread = null;
return true;
}
if (timed) {
if (nanos > 0L) {
nanos = deadline - System.nanoTime();
}
if (nanos <= 0L) {
thread = null;
return true;
}
}
return false;
}
public boolean block() {
if (isReleasable())
return true;
else if (!timed)
LockSupport.park(this);
else if (nanos > 0L)
LockSupport.parkNanos(this, nanos);
return isReleasable();
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long stateOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> k = Phaser.class;
stateOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("state"));
} catch (Exception e) {
throw new Error(e);
}
}
}
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