Java.util.concurrent.Phaser class in Java with Examples
Last Updated :
19 Feb, 2021
Phaser's primary purpose is to enable synchronization of threads that represent one or more phases of activity. It lets us define a synchronization object that waits until a specific phase has been completed. It then advances to the next phase until that phase concludes. It can also be used to synchronize a single phase, and in that regard, it acts much like a CyclicBarrier.
Class Hierarchy
java.lang.Object
? java.util.concurrent
? Class Phaser Syntax
public class Phaser
extends Object
Constructors:
- Phaser() - This creates a phaser with initially zero registered parties. A thread can only use this phaser after registering for it.
public Phaser()
- Phaser(int parties) - This creates a phaser that requires parties number of threads to advance to the next phase.
public Phaser(int parties)
throws IllegalArgumentException
- Phaser(Phaser parent) - This specifies a parent phaser for the new object. The number of registered parties is set to zero.
public Phaser(Phaser parent)
- Phaser(Phaser parent, int parties) - This specifies a parent phaser for the newly created object and the number of parties required to advance to the next phase.
public Phaser(Phaser parent, int parties)
throws IllegalArgumentException
Example1:
Note: Output may vary with each run.
Java
// Java program to show Phaser Class
import java.util.concurrent.Phaser;
// A thread of execution that uses a phaser.
class MyThread implements Runnable {
Phaser phaser;
String title;
public MyThread(Phaser phaser, String title)
{
this.phaser = phaser;
this.title = title;
phaser.register();
new Thread(this).start();
}
@Override public void run()
{
System.out.println("Thread: " + title
+ " Phase Zero Started");
phaser.arriveAndAwaitAdvance();
// Stop execution to prevent jumbled output
try {
Thread.sleep(10);
}
catch (InterruptedException e) {
System.out.println(e);
}
System.out.println("Thread: " + title
+ " Phase One Started");
phaser.arriveAndAwaitAdvance();
// Stop execution to prevent jumbled output
try {
Thread.sleep(10);
}
catch (InterruptedException e) {
System.out.println(e);
}
System.out.println("Thread: " + title
+ " Phase Two Started");
phaser.arriveAndDeregister();
}
}
public class PhaserExample {
public static void main(String[] args)
{
Phaser phaser = new Phaser();
phaser.register();
int currentPhase;
System.out.println("Starting");
new MyThread(phaser, "A");
new MyThread(phaser, "B");
new MyThread(phaser, "C");
// Wait for all threads to complete phase Zero.
currentPhase = phaser.getPhase();
phaser.arriveAndAwaitAdvance();
System.out.println("Phase " + currentPhase
+ " Complete");
System.out.println("Phase Zero Ended");
// Wait for all threads to complete phase One.
currentPhase = phaser.getPhase();
phaser.arriveAndAwaitAdvance();
System.out.println("Phase " + currentPhase
+ " Complete");
System.out.println("Phase One Ended");
currentPhase = phaser.getPhase();
phaser.arriveAndAwaitAdvance();
System.out.println("Phase " + currentPhase
+ " Complete");
System.out.println("Phase Two Ended");
// Deregister the main thread.
phaser.arriveAndDeregister();
if (phaser.isTerminated()) {
System.out.println("Phaser is terminated");
}
}
}
OutputStarting
Thread: B Phase Zero Started
Thread: A Phase Zero Started
Thread: C Phase Zero Started
Thread: A Phase One Started
Thread: B Phase One Started
Thread: C Phase One Started
Phase 0 Complete
Phase Zero Ended
Phase 1 Complete
Phase One Ended
Thread: C Phase Two Started
Thread: A Phase Two Started
Thread: B Phase Two Started
Phase 2 Complete
Phase Two Ended
Phaser is terminated
Example2:
Java
// Java program to show Phaser Class
import java.util.concurrent.Phaser;
// A thread of execution that uses a phaser.
class MyThread implements Runnable {
Phaser phaser;
String title;
public MyThread(Phaser phaser, String title)
{
this.phaser = phaser;
this.title = title;
phaser.register();
new Thread(this).start();
}
@Override public void run()
{
System.out.println("Thread: " + title
+ " Phase Zero Started");
phaser.arriveAndAwaitAdvance();
// Stop execution to prevent jumbled output
try {
Thread.sleep(10);
}
catch (InterruptedException e) {
System.out.println(e);
}
System.out.println("Thread: " + title
+ " Phase One Started");
phaser.arriveAndAwaitAdvance();
// Stop execution to prevent jumbled output
try {
Thread.sleep(10);
}
catch (InterruptedException e) {
System.out.println(e);
}
System.out.println("Thread: " + title
+ " Phase Two Started");
phaser.arriveAndDeregister();
}
}
public class PhaserExample {
public static void main(String[] args)
{
Phaser phaser = new Phaser();
phaser.register();
int currentPhase;
System.out.println("Starting");
new MyThread(phaser, "A");
new MyThread(phaser, "B");
new MyThread(phaser, "C");
// Wait for all threads to complete phase Zero.
currentPhase = phaser.getPhase();
phaser.arriveAndAwaitAdvance();
System.out.println("Phase " + currentPhase
+ " Complete");
System.out.println("Phase Zero Ended");
// Wait for all threads to complete phase One.
currentPhase = phaser.getPhase();
phaser.arriveAndAwaitAdvance();
System.out.println("Phase " + currentPhase
+ " Complete");
System.out.println("Phase One Ended");
currentPhase = phaser.getPhase();
phaser.arriveAndAwaitAdvance();
System.out.println("Phase " + currentPhase
+ " Complete");
System.out.println("Phase Two Ended");
// Deregister the main thread.
phaser.arriveAndDeregister();
if (phaser.isTerminated()) {
System.out.println("Phaser is terminated");
}
}
}
OutputStarting
Thread: C Phase Zero Started
Thread: A Phase Zero Started
Thread: B Phase Zero Started
Thread: B Phase One Started
Thread: C Phase One Started
Thread: A Phase One Started
Phase 0 Complete
Phase Zero Ended
Phase 1 Complete
Phase One Ended
Thread: A Phase Two Started
Thread: C Phase Two Started
Thread: B Phase Two Started
Phase 2 Complete
Phase Two Ended
Phaser is terminated
Methods:
- int register() - This method is used to register parties after a phaser has been constructed. It returns the phase number of the phase to which it is registered.
public int register()
throws IllegalArgumentException
- int arrive() - This method signals that a thread has completed some portion of the task. It does not suspend the execution of the calling thread. It returns the current phase number or a negative value if the phaser has been terminated.
public int arrive()
throws IllegalStateException
- int arriveAndDeregister() - This method enables a thread to arrive at a phase and deregister itself, without waiting for other threads to arrive. It returns the current phase number or a negative value if the phaser has been terminated.
public int arriveAndDeregister()
throws IllegalStateException
- int arriveAndAwaitAdvance() - This method suspends the execution of the thread at a phase, to wait for other threads. It returns the current phase number or a negative value if the phaser has been terminated.
public int arriveAndAwaitAdvance()
throws IllegalStateException
- final int getPhase() - This method returns the current phase number. A negative value is returned if the invoking phasers terminated.
public final int getPhase()
- boolean onAdvance(int phase, int parties) - This method helps in defining how a phase advancement should occur. To do this, the user must override this method. To terminate the phaser, onAdvance() method returns true, otherwise, it returns false;
protected boolean onAdvance(int phase, int parties)
Example to demonstrate the methods of Phaser class - where the method is overridden so that the phaser executes only a specified number of phases.
Java
// Java program to demonstrate
// the methods of Phaser class
import java.util.concurrent.Phaser;
// Extend MyPhaser and override onAdvance()
// so that only specific number of phases
// are executed
class MyPhaser extends Phaser {
int numPhases;
MyPhaser(int parties, int phaseCount)
{
super(parties);
numPhases = phaseCount - 1;
}
@Override
protected boolean onAdvance(int phase,
int registeredParties)
{
System.out.println("Phase " + phase
+ " completed.\n");
// If all phases have completed, return true.
if (phase == numPhases || registeredParties == 0) {
return true;
}
// otherwise, return false
return false;
}
}
// A thread of execution that uses a phaser
class ModifiedThread implements Runnable {
Phaser phsr;
String name;
ModifiedThread(Phaser p, String n)
{
phsr = p;
name = n;
phsr.register();
new Thread(this).start();
}
@Override public void run()
{
while (!phsr.isTerminated()) {
System.out.println("Thread " + name
+ " Beginning Phase "
+ phsr.getPhase());
phsr.arriveAndAwaitAdvance();
try {
Thread.sleep(10);
}
catch (InterruptedException e) {
System.out.println(e);
}
}
}
}
public class PhaserExample2 {
public static void main(String[] args)
{
MyPhaser phsr = new MyPhaser(1, 4);
System.out.println("Starting");
new ModifiedThread(phsr, "A");
new ModifiedThread(phsr, "B");
new ModifiedThread(phsr, "C");
while (!phsr.isTerminated()) {
phsr.arriveAndAwaitAdvance();
}
System.out.println("The phaser is terminated\n");
}
}
OutputStarting
Thread B Beginning Phase 0
Thread C Beginning Phase 0
Thread A Beginning Phase 0
Phase 0 completed.
Thread A Beginning Phase 1
Thread B Beginning Phase 1
Thread C Beginning Phase 1
Phase 1 completed.
Thread C Beginning Phase 2
Thread A Beginning Phase 2
Thread B Beginning Phase 2
Phase 2 completed.
Thread A Beginning Phase 3
Thread B Beginning Phase 3
Thread C Beginning Phase 3
Phase 3 completed.
The phaser is terminated
Reference: https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/Phaser.html
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