/*
* 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/licenses/publicdomain
*/
package jsr166y;
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.
*
*
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 {@code Phaser} has an associated phase number. The
* phase number starts at zero, and advances when all parties arrive
* at the barrier, wrapping around to zero after reaching {@code
* Integer.MAX_VALUE}. The use of phase numbers enables independent
* control of actions upon arrival at a barrier and upon awaiting
* others, via two kinds of methods that may be invoked by any
* registered party:
*
*
*
* - Arrival. Methods {@link #arrive} and
* {@link #arriveAndDeregister} record arrival at a
* barrier. These methods do not block, but return an associated
* arrival phase number; that is, the phase number of
* the barrier to which the arrival applied. When the final
* party for a given phase arrives, an optional barrier action
* is performed and the phase advances. Barrier actions,
* performed by the party triggering a phase advance, 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}.
*
*
- Waiting. Method {@link #awaitAdvance} requires an
* argument indicating an arrival phase number, and returns when
* the barrier 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 barrier. 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.
*
*
*
* Termination. A {@code Phaser} may enter a
* termination state in which all synchronization methods
* immediately return without updating phaser state or waiting for
* advance, and indicating (via a negative phase value) that execution
* is complete. Termination is triggered when an invocation of {@code
* onAdvance} returns {@code true}. 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., arranged
* 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.
*
*
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.
*
*
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 the actions, then deregister, as in:
*
*
{@code
* void runTasks(List tasks) {
* final Phaser phaser = new Phaser(1); // "1" to register self
* // create and start threads
* for (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();
* }}
*
* 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, 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
* }}
*
* 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 tasks using a tree of phasers,
* you could use code of the following form, assuming a
* Task class with a constructor accepting a phaser that
* it registers with upon construction:
*
*
{@code
* void build(Task[] actions, 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(actions, i, j, new Phaser(ph));
* }
* } else {
* for (int i = lo; i < hi; ++i)
* actions[i] = new Task(ph);
* // assumes new Task(ph) performs ph.register()
* }
* }
* // .. initially called, for n tasks via
* build(new Task[n], 0, n, new Phaser());}
*
* The best value of {@code TASKS_PER_PHASER} depends mainly on
* expected barrier synchronization rates. A value as low as four may
* be appropriate for extremely small per-barrier 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.
*/
/**
* Barrier state representation. Conceptually, a barrier contains
* four values:
*
* * 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)
*
* However, to efficiently maintain atomicity, these values are
* packed into a single (atomic) long. Termination uses the sign
* bit of 32 bit representation of phase, so phase is set to -1 on
* termination. Good performance relies on keeping state decoding
* and encoding simple, and keeping race windows short.
*/
private volatile long state;
private static final int MAX_COUNT = 0xffff;
private static final int MAX_PHASE = 0x7fffffff;
private static final int PARTIES_SHIFT = 16;
private static final int PHASE_SHIFT = 32;
private static final long UNARRIVED_MASK = 0xffffL;
private static final long PARTIES_MASK = 0xffff0000L;
private static final long ONE_ARRIVAL = 1L;
private static final long ONE_PARTY = 1L << PARTIES_SHIFT;
private static final long TERMINATION_PHASE = -1L << PHASE_SHIFT;
// The following unpacking methods are usually manually inlined
private static int unarrivedOf(long s) {
return (int) (s & UNARRIVED_MASK);
}
private static int partiesOf(long s) {
return ((int) (s & PARTIES_MASK)) >>> PARTIES_SHIFT;
}
private static int phaseOf(long s) {
return (int) (s >>> PHASE_SHIFT);
}
private static int arrivedOf(long s) {
return partiesOf(s) - unarrivedOf(s);
}
/**
* 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. Used to
* support faster state push-down.
*/
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 evenQ;
private final AtomicReference oddQ;
private AtomicReference queueFor(int phase) {
return ((phase & 1) == 0) ? evenQ : oddQ;
}
/**
* 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 adj - adjustment to apply to state -- either
* ONE_ARRIVAL (for arrive) or
* ONE_ARRIVAL|ONE_PARTY (for arriveAndDeregister)
*/
private int doArrive(long adj) {
for (;;) {
long s;
int phase, unarrived;
if ((phase = (int)((s = state) >>> PHASE_SHIFT)) < 0)
return phase;
else if ((unarrived = (int)(s & UNARRIVED_MASK)) == 0)
checkBadArrive(s);
else if (UNSAFE.compareAndSwapLong(this, stateOffset, s, s -= adj)){
if (unarrived == 1) {
Phaser par;
long p = s & PARTIES_MASK; // unshifted parties field
long lu = p >>> PARTIES_SHIFT;
int u = (int)lu;
int nextPhase = (phase + 1) & MAX_PHASE;
long next = ((long)nextPhase << PHASE_SHIFT) | p | lu;
if ((par = parent) == null) {
UNSAFE.compareAndSwapLong
(this, stateOffset, s, onAdvance(phase, u)?
next | TERMINATION_PHASE : next);
releaseWaiters(phase);
}
else {
par.doArrive(u == 0?
ONE_ARRIVAL|ONE_PARTY : ONE_ARRIVAL);
if ((int)(par.state >>> PHASE_SHIFT) != nextPhase ||
((int)(state >>> PHASE_SHIFT) != nextPhase &&
!UNSAFE.compareAndSwapLong(this, stateOffset,
s, next)))
reconcileState();
}
}
return phase;
}
}
}
/**
* Rechecks state and throws bounds exceptions on arrival -- called
* only if unarrived is apparently zero.
*/
private void checkBadArrive(long s) {
if (reconcileState() == s)
throw new IllegalStateException
("Attempted arrival of unregistered party for " +
stateToString(s));
}
/**
* Implementation of register, bulkRegister
*
* @param registrations number to add to both parties and unarrived fields
*/
private int doRegister(int registrations) {
long adj = (long)registrations; // adjustment to state
adj |= adj << PARTIES_SHIFT;
Phaser par = parent;
for (;;) {
int phase, parties;
long s = par == null? state : reconcileState();
if ((phase = (int)(s >>> PHASE_SHIFT)) < 0)
return phase;
if ((parties = ((int)(s & PARTIES_MASK)) >>> PARTIES_SHIFT) != 0 &&
(s & UNARRIVED_MASK) == 0)
internalAwaitAdvance(phase, null); // wait for onAdvance
else if (parties + registrations > MAX_COUNT)
throw new IllegalStateException(badRegister(s));
else if (UNSAFE.compareAndSwapLong(this, stateOffset, s, s + adj))
return phase;
}
}
/**
* Returns message string for bounds exceptions on registration
*/
private String badRegister(long s) {
return "Attempt to register more than " +
MAX_COUNT + " parties for " + stateToString(s);
}
/**
* Recursively resolves lagged phase propagation from root if
* necessary.
*/
private long reconcileState() {
Phaser par = parent;
if (par == null)
return state;
Phaser rt = root;
for (;;) {
long s, u;
int phase, rPhase, pPhase;
if ((phase = (int)((s = state)>>> PHASE_SHIFT)) < 0 ||
(rPhase = (int)(rt.state >>> PHASE_SHIFT)) == phase)
return s;
long pState = par.parent == null? par.state : par.reconcileState();
if (state == s) {
if ((rPhase < 0 || (s & UNARRIVED_MASK) == 0) &&
((pPhase = (int)(pState >>> PHASE_SHIFT)) < 0 ||
pPhase == ((phase + 1) & MAX_PHASE)))
UNSAFE.compareAndSwapLong
(this, stateOffset, s,
(((long) pPhase) << PHASE_SHIFT) |
(u = s & PARTIES_MASK) |
(u >>> PARTIES_SHIFT)); // reset unarrived to parties
else
releaseWaiters(phase); // help release others
}
}
}
/**
* Creates a new phaser without any initially registered parties,
* initial phase number 0, and no parent. 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, initial phase number 0, and no parent.
*
* @param parties the number of parties required to trip barrier
* @throws IllegalArgumentException if parties less than zero
* or greater than the maximum number of parties supported
*/
public Phaser(int parties) {
this(null, parties);
}
/**
* Creates a new phaser with the given parent, without any
* initially registered parties. If parent is non-null this phaser
* is registered with the parent and its initial phase number is
* the same as that of parent phaser.
*
* @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. If parent is non-null, this phaser
* is registered with the parent and its initial phase number is
* the same as that of parent phaser.
*
* @param parent the parent phaser
* @param parties the number of parties required to trip barrier
* @throws IllegalArgumentException if parties less than zero
* or greater than the maximum number of parties supported
*/
public Phaser(Phaser parent, int parties) {
if (parties < 0 || parties > MAX_COUNT)
throw new IllegalArgumentException("Illegal number of parties");
int phase;
this.parent = parent;
if (parent != null) {
Phaser r = parent.root;
this.root = r;
this.evenQ = r.evenQ;
this.oddQ = r.oddQ;
phase = parent.register();
}
else {
this.root = this;
this.evenQ = new AtomicReference();
this.oddQ = new AtomicReference();
phase = 0;
}
long p = (long)parties;
this.state = (((long) phase) << PHASE_SHIFT) | p | (p << PARTIES_SHIFT);
}
/**
* Adds a new unarrived party to this phaser.
* If an ongoing invocation of {@link #onAdvance} is in progress,
* this method may wait until its completion before registering.
*
* @return the arrival phase number to which this registration applied
* @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 wait until its completion before registering.
*
* @param parties the number of additional parties required to trip barrier
* @return the arrival phase number to which this registration applied
* @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 > MAX_COUNT)
throw new IllegalStateException(badRegister(state));
if (parties == 0)
return getPhase();
return doRegister(parties);
}
/**
* Arrives at the barrier, but does not wait for others. (You can
* in turn wait for others via {@link #awaitAdvance}). It is an
* unenforced usage error for an unregistered party to invoke this
* method.
*
* @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 the barrier and deregisters from it without waiting
* for others. Deregistration reduces the number of parties
* required to trip the barrier in future phases. If this phaser
* has a parent, and deregistration causes this phaser to have
* zero parties, this phaser also arrives at and is deregistered
* from its parent. It is an unenforced usage error for an
* unregistered party to invoke this method.
*
* @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_ARRIVAL|ONE_PARTY);
}
/**
* Arrives at the barrier 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 {@link #arriveAndDeregister}. It is an unenforced
* usage error for an unregistered party to invoke this method.
*
* @return the arrival phase number, or a negative number if terminated
* @throws IllegalStateException if not terminated and the number
* of unarrived parties would become negative
*/
public int arriveAndAwaitAdvance() {
return awaitAdvance(arrive());
}
/**
* Awaits the phase of the barrier to advance from the given phase
* value, returning immediately if the current phase of the
* barrier is not equal to the given phase value or this barrier
* 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 its variants
* @return the next arrival phase number, or a negative value
* if terminated or argument is negative
*/
public int awaitAdvance(int phase) {
if (phase < 0)
return phase;
int p = (int)((parent==null? state : reconcileState()) >>> PHASE_SHIFT);
if (p != phase)
return p;
return internalAwaitAdvance(phase, null);
}
/**
* Awaits the phase of the barrier to advance from the given phase
* value, throwing {@code InterruptedException} if interrupted
* while waiting, or returning immediately if the current phase of
* the barrier is not equal to the given phase value or this
* barrier 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 its variants
* @return the next arrival phase number, or a negative value
* if terminated or argument is negative
* @throws InterruptedException if thread interrupted while waiting
*/
public int awaitAdvanceInterruptibly(int phase)
throws InterruptedException {
if (phase < 0)
return phase;
int p = (int)((parent==null? state : reconcileState()) >>> PHASE_SHIFT);
if (p != phase)
return p;
QNode node = new QNode(this, phase, true, false, 0L);
p = internalAwaitAdvance(phase, node);
if (node.wasInterrupted)
throw new InterruptedException();
else
return p;
}
/**
* Awaits the phase of the barrier 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 of the barrier is
* not equal to the given phase value or this barrier 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 its variants
* @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 a negative value
* if terminated or argument is negative
* @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);
if (phase < 0)
return phase;
int p = (int)((parent==null? state : reconcileState()) >>> PHASE_SHIFT);
if (p != phase)
return p;
QNode node = new QNode(this, phase, true, true, nanos);
p = internalAwaitAdvance(phase, node);
if (node.wasInterrupted)
throw new InterruptedException();
else if (p == phase)
throw new TimeoutException();
else
return p;
}
/**
* Forces this barrier to enter termination state. Counts of
* arrived and registered parties are unaffected. If this phaser
* has a parent, it too is terminated. This method may be useful
* for coordinating recovery after one or more tasks encounter
* unexpected exceptions.
*/
public void forceTermination() {
Phaser r = root; // force at root then reconcile
long s;
while ((s = r.state) >= 0)
UNSAFE.compareAndSwapLong(r, stateOffset, s, s | TERMINATION_PHASE);
reconcileState();
releaseWaiters(0); // signal all threads
releaseWaiters(1);
}
/**
* 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.
*
* @return the phase number, or a negative value if terminated
*/
public final int getPhase() {
return (int)((parent==null? state : reconcileState()) >>> PHASE_SHIFT);
}
/**
* Returns the number of parties registered at this barrier.
*
* @return the number of parties
*/
public int getRegisteredParties() {
return partiesOf(parent==null? state : reconcileState());
}
/**
* Returns the number of registered parties that have arrived at
* the current phase of this barrier.
*
* @return the number of arrived parties
*/
public int getArrivedParties() {
return arrivedOf(parent==null? state : reconcileState());
}
/**
* Returns the number of registered parties that have not yet
* arrived at the current phase of this barrier.
*
* @return the number of unarrived parties
*/
public int getUnarrivedParties() {
return unarrivedOf(parent==null? state : 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 barrier has been terminated.
*
* @return {@code true} if this barrier has been terminated
*/
public boolean isTerminated() {
return (parent == null? state : reconcileState()) < 0;
}
/**
* Overridable method to perform an action upon impending phase
* advance, and to control termination. This method is invoked
* upon arrival of the party tripping the barrier (when all other
* waiting parties are dormant). If this method returns {@code
* true}, then, rather than advance the phase number, this barrier
* will be set to a final termination state, 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 trip the barrier, 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.
*
*
The default version returns {@code true} when the number of
* registered parties is zero. Normally, overrides that arrange
* termination for other reasons should also preserve this
* property.
*
* @param phase the phase number on entering the barrier
* @param registeredParties the current number of registered parties
* @return {@code true} if this barrier 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 barrier, 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) {
AtomicReference head = queueFor(phase);
QNode q;
int p;
while ((q = head.get()) != null &&
((p = q.phase) == phase ||
(int)(root.state >>> PHASE_SHIFT) != p)) {
if (head.compareAndSet(q, q.next))
q.signal();
}
}
/**
* Tries to enqueue given node in the appropriate wait queue.
*
* @return true if successful
*/
private boolean tryEnqueue(int phase, QNode node) {
releaseWaiters(phase-1); // ensure old queue clean
AtomicReference head = queueFor(phase);
QNode q = head.get();
return ((q == null || q.phase == phase) &&
(int)(root.state >>> PHASE_SHIFT) == phase &&
head.compareAndSet(node.next = q, node));
}
/** 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 continuing to try to
* block. 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.
*
* @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) {
Phaser current = this; // to eventually wait at root if tiered
boolean queued = false; // true when node is enqueued
int lastUnarrived = -1; // to increase spins upon change
int spins = SPINS_PER_ARRIVAL;
for (;;) {
int p, unarrived;
Phaser par;
long s = current.state;
if ((p = (int)(s >>> PHASE_SHIFT)) != phase) {
if (node != null)
node.onRelease();
releaseWaiters(phase);
return p;
}
else if ((unarrived = (int)(s & UNARRIVED_MASK)) != lastUnarrived) {
if ((lastUnarrived = unarrived) < NCPU)
spins += SPINS_PER_ARRIVAL;
}
else if (unarrived == 0 && (par = current.parent) != null) {
current = par; // if all arrived, use parent
par = par.parent;
lastUnarrived = -1;
}
else if (spins > 0)
--spins;
else if (node == null) // must be noninterruptible
node = new QNode(this, phase, false, false, 0L);
else if (node.isReleasable()) {
if ((int)(reconcileState() >>> PHASE_SHIFT) == phase)
return phase; // aborted
}
else if (!queued)
queued = tryEnqueue(phase, node);
else {
try {
ForkJoinPool.managedBlock(node);
} catch (InterruptedException ie) {
node.wasInterrupted = true;
}
}
}
}
/**
* 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;
long lastTime;
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.lastTime = timed? System.nanoTime() : 0L;
thread = Thread.currentThread();
}
public boolean isReleasable() {
Thread t = thread;
if (t != null) {
if (phaser.getPhase() != phase)
t = null;
else {
if (Thread.interrupted())
wasInterrupted = true;
if (interruptible && wasInterrupted)
t = null;
else if (timed) {
if (nanos > 0) {
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
}
if (nanos <= 0)
t = null;
}
}
if (t != null)
return false;
thread = null;
}
return true;
}
public boolean block() {
if (isReleasable())
return true;
else if (!timed)
LockSupport.park(this);
else if (nanos > 0)
LockSupport.parkNanos(this, nanos);
return isReleasable();
}
void signal() {
Thread t = thread;
if (t != null) {
thread = null;
LockSupport.unpark(t);
}
}
void onRelease() { // actions upon return from internalAwaitAdvance
if (!interruptible && wasInterrupted)
Thread.currentThread().interrupt();
if (thread != null)
thread = null;
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = getUnsafe();
private static final long stateOffset =
objectFieldOffset("state", Phaser.class);
private static long objectFieldOffset(String field, Class> klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
}
/**
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
* Replace with a simple call to Unsafe.getUnsafe when integrating
* into a jdk.
*
* @return a sun.misc.Unsafe
*/
private static sun.misc.Unsafe getUnsafe() {
try {
return sun.misc.Unsafe.getUnsafe();
} catch (SecurityException se) {
try {
return java.security.AccessController.doPrivileged
(new java.security
.PrivilegedExceptionAction() {
public sun.misc.Unsafe run() throws Exception {
java.lang.reflect.Field f = sun.misc
.Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
return (sun.misc.Unsafe) f.get(null);
}});
} catch (java.security.PrivilegedActionException e) {
throw new RuntimeException("Could not initialize intrinsics",
e.getCause());
}
}
}
}