Java.util.concurrent.Phaser class in Java with Examples

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");        }    }}
Starting
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");        }    }}
Starting
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 executedclass 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 phaserclass 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");    }}
Starting
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 
ramandeepm007
Java - util package
Java-concurrent-package
Java-Functions
Java-Phaser
Java
Java
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