Java线程编程中isAlive()和join()的使用详解

一个线程如何知道另一线程已经结束？thread类提供了回答此问题的方法。

有两种方法可以判定一个线程是否结束。第一，可以在线程中调用isalive()。这种方法由thread定义，它的通常形式如下：

  final boolean isalive( )

如果所调用线程仍在运行，isalive()方法返回true，如果不是则返回false。但isalive（）很少用到，等待线程结束的更常用的方法是调用join()，描述如下：

  final void join( ) throws interruptedexception

该方法等待所调用线程结束。该名字来自于要求线程等待直到指定线程参与的概念。join()的附加形式允许给等待指定线程结束定义一个最大时间。下面是前面例子的改进版本。运用join()以确保主线程最后结束。同样，它也演示了isalive()方法。

// using join() to wait for threads to finish.
class newthread implements runnable {
  string name; // name of thread
  thread t;
  newthread(string threadname) {
    name = threadname;
    t = new thread(this, name);
    system.out.println("new thread: " + t);
    t.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(name + ": " + i);
        thread.sleep(1000);
      }
    } catch (interruptedexception e) {
      system.out.println(name + " interrupted.");
    }
    system.out.println(name + " exiting.");
  }
}

class demojoin {
  public static void main(string args[]) {
    newthread ob1 = new newthread("one");
    newthread ob2 = new newthread("two");
    newthread ob3 = new newthread("three");
    system.out.println("thread one is alive: "+ ob1.t.isalive());
    system.out.println("thread two is alive: "+ ob2.t.isalive());
    system.out.println("thread three is alive: "+ ob3.t.isalive());
    // wait for threads to finish
    try {
      system.out.println("waiting for threads to finish.");
      ob1.t.join();
      ob2.t.join();
      ob3.t.join();
    } catch (interruptedexception e) {
      system.out.println("main thread interrupted");
    }
    system.out.println("thread one is alive: "+ ob1.t.isalive());
    system.out.println("thread two is alive: "+ ob2.t.isalive());
    system.out.println("thread three is alive: "+ ob3.t.isalive());
    system.out.println("main thread exiting.");
  }
}

程序输出如下所示：

new thread: thread[one,5,main]
new thread: thread[two,5,main]
new thread: thread[three,5,main]
thread one is alive: true
thread two is alive: true
thread three is alive: true
waiting for threads to finish.
one: 5
two: 5
three: 5
one: 4
two: 4
three: 4
one: 3
two: 3
three: 3
one: 2
two: 2
three: 2
one: 1
two: 1
three: 1
two exiting.
three exiting.
one exiting.
thread one is alive: false
thread two is alive: false
thread three is alive: false
main thread exiting.

如你所见，调用join()后返回，线程终止执行。