Class Phaser
CyclicBarrier
and 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 register()
, bulkRegister(int)
, or forms of
constructors establishing initial numbers of parties), and
optionally deregistered upon any arrival (using 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 CyclicBarrier
, a
Phaser
may be repeatedly awaited. Method arriveAndAwaitAdvance()
has effect analogous to 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
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:
- Arrival. Methods
arrive()
andarriveAndDeregister()
record arrival. These methods do not block, but return an associated arrival phase number; 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 methodonAdvance(int, int)
, which also controls termination. Overriding this method is similar to, but more flexible than, providing a barrier action to aCyclicBarrier
. - Waiting. Method
awaitAdvance(int)
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 usingCyclicBarrier
, methodawaitAdvance
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 invokingforceTermination
. Phasers may also be used by tasks executing in aForkJoinPool
. Progress is ensured if the pool's parallelism level can accommodate the maximum number of simultaneously blocked parties.
Termination. A phaser may enter a termination
state, that may be checked using method 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 onAdvance
returns true
. The default implementation returns
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 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 Phaser(Phaser,int)
constructor, register()
, or bulkRegister(int)
), the
child phaser is registered with its parent. Whenever the number of
registered parties becomes zero as the result of an invocation of
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 getRegisteredParties()
parties in total, of which getArrivedParties()
have arrived at the current phase (getPhase()
). When the remaining (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 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 Phaser
may be used instead of a 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:
void runTasks(List<Runnable> 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 onAdvance
:
void startTasks(List<Runnable> 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:
// ...
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
Integer.MAX_VALUE
. For example:
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 n
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. After invocation of build(new Task[n], 0, n,
new Phaser())
, these tasks could then be started, for example by
submitting to a pool:
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 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 IllegalStateException
. However, you can and
should create tiered phasers to accommodate arbitrarily large sets
of participants.
- Since:
- 1.7
-
Constructor Summary
ConstructorDescriptionPhaser()
Creates a new phaser with no initially registered parties, no parent, and initial phase number 0.Phaser
(int parties) Creates a new phaser with the given number of registered unarrived parties, no parent, and initial phase number 0.Equivalent toPhaser(parent, 0)
.Creates a new phaser with the given parent and number of registered unarrived parties. -
Method Summary
Modifier and TypeMethodDescriptionint
arrive()
Arrives at this phaser, without waiting for others to arrive.int
Arrives at this phaser and awaits others.int
Arrives at this phaser and deregisters from it without waiting for others to arrive.int
awaitAdvance
(int phase) 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.int
awaitAdvanceInterruptibly
(int phase) Awaits the phase of this phaser to advance from the given phase value, throwingInterruptedException
if interrupted while waiting, or returning immediately if the current phase is not equal to the given phase value or this phaser is terminated.int
awaitAdvanceInterruptibly
(int phase, long timeout, TimeUnit unit) Awaits the phase of this phaser to advance from the given phase value or the given timeout to elapse, throwingInterruptedException
if interrupted while waiting, or returning immediately if the current phase is not equal to the given phase value or this phaser is terminated.int
bulkRegister
(int parties) Adds the given number of new unarrived parties to this phaser.void
Forces this phaser to enter termination state.int
Returns the number of registered parties that have arrived at the current phase of this phaser.Returns the parent of this phaser, ornull
if none.final int
getPhase()
Returns the current phase number.int
Returns the number of parties registered at this phaser.getRoot()
Returns the root ancestor of this phaser, which is the same as this phaser if it has no parent.int
Returns the number of registered parties that have not yet arrived at the current phase of this phaser.boolean
Returnstrue
if this phaser has been terminated.protected boolean
onAdvance
(int phase, int registeredParties) Overridable method to perform an action upon impending phase advance, and to control termination.int
register()
Adds a new unarrived party to this phaser.toString()
Returns a string identifying this phaser, as well as its state.
-
Constructor Details
-
Phaser
public Phaser()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. -
Phaser
public Phaser(int parties) Creates a new phaser with the given number of registered unarrived parties, no parent, and initial phase number 0.- Parameters:
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
-
Phaser
Equivalent toPhaser(parent, 0)
.- Parameters:
parent
- the parent phaser
-
Phaser
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.- Parameters:
parent
- the parent phaserparties
- 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
-
-
Method Details
-
register
public int register()Adds a new unarrived party to this phaser. If an ongoing invocation ofonAdvance(int, int)
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.- Returns:
- 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
-
bulkRegister
public int bulkRegister(int parties) Adds the given number of new unarrived parties to this phaser. If an ongoing invocation ofonAdvance(int, int)
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.- Parameters:
parties
- the number of additional parties required to advance to the next phase- Returns:
- 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 partiesIllegalArgumentException
- ifparties < 0
-
arrive
public int arrive()Arrives at this phaser, without waiting for others to arrive.It is a usage error for an unregistered party to invoke this method. However, this error may result in an
IllegalStateException
only upon some subsequent operation on this phaser, if ever.- Returns:
- the arrival phase number, or a negative value if terminated
- Throws:
IllegalStateException
- if not terminated and the number of unarrived parties would become negative
-
arriveAndDeregister
public int arriveAndDeregister()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
IllegalStateException
only upon some subsequent operation on this phaser, if ever.- Returns:
- 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
-
arriveAndAwaitAdvance
public int arriveAndAwaitAdvance()Arrives at this phaser and awaits others. Equivalent in effect toawaitAdvance(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 theawaitAdvance
method. If instead you need to deregister upon arrival, useawaitAdvance(arriveAndDeregister())
.It is a usage error for an unregistered party to invoke this method. However, this error may result in an
IllegalStateException
only upon some subsequent operation on this phaser, if ever.- Returns:
- the arrival phase number, or the (negative) current phase if terminated
- Throws:
IllegalStateException
- if not terminated and the number of unarrived parties would become negative
-
awaitAdvance
public int awaitAdvance(int phase) 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.- Parameters:
phase
- an arrival phase number, or negative value if terminated; this argument is normally the value returned by a previous call toarrive
orarriveAndDeregister
.- Returns:
- the next arrival phase number, or the argument if it is negative, or the (negative) current phase if terminated
-
awaitAdvanceInterruptibly
Awaits the phase of this phaser to advance from the given phase value, throwingInterruptedException
if interrupted while waiting, or returning immediately if the current phase is not equal to the given phase value or this phaser is terminated.- Parameters:
phase
- an arrival phase number, or negative value if terminated; this argument is normally the value returned by a previous call toarrive
orarriveAndDeregister
.- Returns:
- the next arrival phase number, or the argument if it is negative, or the (negative) current phase if terminated
- Throws:
InterruptedException
- if thread interrupted while waiting
-
awaitAdvanceInterruptibly
public int awaitAdvanceInterruptibly(int phase, long timeout, TimeUnit unit) throws InterruptedException, TimeoutException Awaits the phase of this phaser to advance from the given phase value or the given timeout to elapse, throwingInterruptedException
if interrupted while waiting, or returning immediately if the current phase is not equal to the given phase value or this phaser is terminated.- Parameters:
phase
- an arrival phase number, or negative value if terminated; this argument is normally the value returned by a previous call toarrive
orarriveAndDeregister
.timeout
- how long to wait before giving up, in units ofunit
unit
- aTimeUnit
determining how to interpret thetimeout
parameter- Returns:
- the next arrival phase number, or the argument if it is negative, or the (negative) current phase if terminated
- Throws:
InterruptedException
- if thread interrupted while waitingTimeoutException
- if timed out while waiting
-
forceTermination
public void forceTermination()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. -
getPhase
public final int getPhase()Returns the current phase number. The maximum phase number isInteger.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 viagetPhase() + Integer.MIN_VALUE
.- Returns:
- the phase number, or a negative value if terminated
-
getRegisteredParties
public int getRegisteredParties()Returns the number of parties registered at this phaser.- Returns:
- the number of parties
-
getArrivedParties
public int getArrivedParties()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.- Returns:
- the number of arrived parties
-
getUnarrivedParties
public int getUnarrivedParties()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.- Returns:
- the number of unarrived parties
-
getParent
Returns the parent of this phaser, ornull
if none.- Returns:
- the parent of this phaser, or
null
if none
-
getRoot
Returns the root ancestor of this phaser, which is the same as this phaser if it has no parent.- Returns:
- the root ancestor of this phaser
-
isTerminated
public boolean isTerminated()Returnstrue
if this phaser has been terminated.- Returns:
true
if this phaser has been terminated
-
onAdvance
protected boolean onAdvance(int phase, int registeredParties) 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 returnstrue
, this phaser will be set to a final termination state upon advance, and subsequent calls toisTerminated()
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
onAdvance
are unspecified and should not be relied on.If this phaser is a member of a tiered set of phasers, then
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
true
when the number of registered parties has become zero as the result of a party invokingarriveAndDeregister
. You can disable this behavior, thus enabling continuation upon future registrations, by overriding this method to always returnfalse
:Phaser phaser = new Phaser() { protected boolean onAdvance(int phase, int parties) { return false; } };
- Parameters:
phase
- the current phase number on entry to this method, before this phaser is advancedregisteredParties
- the current number of registered parties- Returns:
true
if this phaser should terminate
-
toString
Returns a string identifying this phaser, as well as its state. The state, in brackets, includes the String"phase = "
followed by the phase number,"parties = "
followed by the number of registered parties, and"arrived = "
followed by the number of arrived parties.
-