Registration. Unlike the case for other barriers, the * number of parties 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.) * *
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: * *
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. * *
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. * *
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. * *
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. * *
Memory consistency effects: Actions prior to any form of arrive * method * happen-before a corresponding phase advance and * onAdvance actions (if present), which in turn happen-before * actions following the phase advance. * *
Sample usages: * *
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 all the actions, then deregister, as in: * *
{@code
* void runTasks(List tasks) {
* Phaser startingGate = new Phaser(1); // "1" to register self
* // create and start threads
* for (Runnable task : tasks) {
* startingGate.register();
* new Thread(() -> {
* startingGate.arriveAndAwaitAdvance();
* task.run();
* }).start();
* }
*
* // deregister self to allow threads to proceed
* startingGate.arriveAndDeregister();
* }}
*
* One way to cause a set of threads to repeatedly perform actions * for a given number of iterations is to override {@code onAdvance}: * *
{@code
* void startTasks(List tasks, int iterations) {
* Phaser phaser = new Phaser() {
* protected boolean onAdvance(int phase, int registeredParties) {
* return phase >= iterations - 1 || registeredParties == 0;
* }
* };
* phaser.register();
* for (Runnable task : tasks) {
* phaser.register();
* new Thread(() -> {
* do {
* task.run();
* phaser.arriveAndAwaitAdvance();
* } while (!phaser.isTerminated());
* }).start();
* }
* // allow threads to proceed; don't wait for them
* phaser.arriveAndDeregister();
* }}
*
* If the main task must later await termination, it
* may re-register and then execute a similar loop:
* {@code
* // ...
* phaser.register();
* while (!phaser.isTerminated())
* phaser.arriveAndAwaitAdvance();}
*
* 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: * *
{@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();
* }}
*
* 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: * *
{@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()
* }
* }}
*
* 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.
*
* 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 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.
*
* 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())}.
*
* 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 (STATE.compareAndSet(this, 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 (!STATE.compareAndSet(this, 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 (STATE.compareAndSet(root, 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.
*
* 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.
*
* 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.
*
* 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}:
*
* {@code
* Phaser phaser = new Phaser() {
* protected boolean onAdvance(int phase, int parties) { return false; }
* };}
*
* @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