现有线程threadone、threadtwo和threadthree，想要的运行顺序为threadone->threadtwo->threadthree，应该如何处理？这里需要用到一个简单的线程方法join().
join()方法的说明：join方法挂起当前调用线程，直到被调用线程完成后在继续执行(join() method suspends the execution of the calling thread until the object called finishes its execution).
下面看一个例子：
/** * Thread one for test. */ public class ThreadOne implements Runnable { @Override public void run() { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start..."); System.out.println(threadName + " end."); } }
/** * Thread two for test. */ public class ThreadTwo implements Runnable { @Override public void run() { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start..."); System.out.println(threadName + " end."); } }
/** * Thread three for test. */ public class ThreadThree implements Runnable { @Override public void run() { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start..."); System.out.println(threadName + " end."); } }
public class JoinMainTest { public static void main(String[] args) { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start..."); Thread firstThread = new Thread(new ThreadOne()); Thread secondThread = new Thread(new ThreadTwo()); Thread thirdThread = new Thread(new ThreadThree()); // 1. no order thread run firstThread.start(); secondThread.start(); thirdThread.start(); System.out.println(threadName + " end."); } }
上面得到的结果如下：
/**
*
main start...
Thread-0 start...
main end.
Thread-0 end.
Thread-1
start...
Thread-1 end.
Thread-2 start...
Thread-2 end.
*/
public class JoinMainTest { public static void main(String[] args) { String threadName = Thread.currentThread().getName(); System.out.println(threadName + " start..."); Thread firstThread = new Thread(new ThreadOne()); Thread secondThread = new Thread(new ThreadTwo()); Thread thirdThread = new Thread(new ThreadThree()); // 2. thread run in order try { firstThread.start(); firstThread.join(); secondThread.start(); secondThread.join(); thirdThread.start(); thirdThread.join(); } catch (Exception ex) { System.out.println("thread join exception."); } System.out.println(threadName + " end."); } }
这里得到的结果就是顺序执行的了：
/**
*
main start...
Thread-0 start...
Thread-0 end.
Thread-1 start...
Thread-1
end.
Thread-2 start...
Thread-2 end.
main end.
*/
再来看一下join()的源码，join()调用的就是join(0).可以看到，其实join就是wait，直到线程执行完成。
public final synchronized void join(long millis) throws InterruptedException { long base = System.currentTimeMillis(); long now = 0; if (millis < 0) { throw new IllegalArgumentException("timeout value is negative"); } if (millis == 0) { while (isAlive()) { wait(0); } } else { while (isAlive()) { long delay = millis - now; if (delay <= 0) { break; } wait(delay); now = System.currentTimeMillis() - base; } }
}