Java - Thread Stopping, Suspending, and Resuming

Introduction

You can use a flag to control if to stop a thread.

If the flag is set, the thread should stop executing.

The following code simulates the stop(), suspend(), and resume() methods of the Thread class in your thread.

Demo

public class Main {
  public static void main(String[] args) throws Exception {
    MyThread t = new MyThread();
    t.start();//  w  ww . ja  v a 2  s  . c  o m

    Thread.sleep(2000);// Sleep for 2 seconds
    t.suspendThread();

    // Sleep for 2 seconds
    Thread.sleep(2000);
    // Resume the thread
    t.resumeThread();

    Thread.sleep(2000);

    t.stopThread();
  }
}

class MyThread extends Thread {
  private volatile boolean keepRunning = true;
  private boolean suspended = false;

  public synchronized void stopThread() {
    this.keepRunning = false;

    this.notify();
  }

  public synchronized void suspendThread() {
    this.suspended = true;
  }

  public synchronized void resumeThread() {
    this.suspended = false;
    this.notify();
  }

  public void run() {
    System.out.println("Thread started...");
    while (keepRunning) {
      try {
        System.out.println("Going to sleep...");
        Thread.sleep(1000);
        synchronized (this) {
          while (suspended) {
            System.out.println("Suspended...");
            this.wait();
            System.out.println("Resumed...");
          }
        }
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
    System.out.println("Thread stopped...");
  }
}

Result

Note

The suspended instance variable is not declared volatile.

It is not necessary to declare it volatile because it is always accessed inside a synchronized method/block.

The following code in the run() method is used to implement the suspend and resume features:

synchronized (this) {
        while (suspended) {
                System.out.println("Suspended...");
                this.wait();
                System.out.println("Resumed...");
        }
}

When the suspended instance variable is set to true, the thread calls the wait() method on itself to wait.

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