threadgroupdemo.java

java2参考大全上的例子的源码和自己的理解.

package threadgroup;

/**
 线程组提供了一种方便的方法，可以将一组线程当做一个单元来管理。这在想挂起或
 恢复一些相关的线程的情况下，是特别有用的。例如假想在一个程序中，有一组线程被用
 来打印文档，另一组线程被用来将该文档显示在屏幕上，同时另一组线程将文档保存为磁
 盘文件。如果打印被异常中止了，想用一种很简单的方法停止所有与打印有关的线程。线
 程组为这种处理提供了方便。下面的程序说明了这种用法，在程序中创建两个线程组，每
 一线程组中有两个线程：
 */
// Demonstrate thread groups.
class NewThread
    extends Thread {
  boolean suspendFlag;

  NewThread(String threadname, ThreadGroup tgOb) {
    super(tgOb, threadname);
    System.out.println("New thread: " + this);
    suspendFlag = false;
    start(); // Start the thread
  }

  // This is the entry point for thread.
  public void run() {
    try {
      for (int i = 5; i > 0; i--) {
        System.out.println(getName() + ": " + i);
        Thread.sleep(1000);
        synchronized (this) {
          while (suspendFlag) {
            wait();
          }
        }
      }
    }
    catch (Exception e) {
      System.out.println("Exception in " + getName());
    }
    System.out.println(getName() + " exiting.");
  }

  void mysuspend() {
    suspendFlag = true;
  }

  synchronized void myresume() {
    suspendFlag = false;
    notify();
  }
}

class ThreadGroupDemo {
  public static void main(String args[]) {
    ThreadGroup groupA = new ThreadGroup("Group A");
    ThreadGroup groupB = new ThreadGroup("Group B");

    NewThread ob1 = new NewThread("One", groupA);
    NewThread ob2 = new NewThread("Two", groupA);
    NewThread ob3 = new NewThread("Three", groupB);
    NewThread ob4 = new NewThread("Four", groupB);

    System.out.println("\nHere is output from list():");
    groupA.list();
    groupB.list();
    System.out.println();

    System.out.println("Suspending Group A");
    Thread tga[] = new Thread[groupA.activeCount()];
    groupA.enumerate(tga); // get threads in group
    for (int i = 0; i < tga.length; i++) {
      ( (NewThread) tga[i]).mysuspend(); // suspend each thread
    }

    try {
      Thread.sleep(4000);
    }
    catch (InterruptedException e) {
      System.out.println("Main thread interrupted.");
    }

    System.out.println("Resuming Group A");
    for (int i = 0; i < tga.length; i++) {
      ( (NewThread) tga[i]).myresume(); // resume threads in group
    }

    // wait for threads to finish

    try {
      System.out.println("Waiting for threads to finish.");
      ob1.join();
      ob2.join();
      ob3.join();
      ob4.join();
    }
    catch (Exception e) {
      System.out.println("Exception in Main thread");
    }

    System.out.println("Main thread exiting.");
  }
}