Java线程的join方法可用于暂停当前线程的执行直至目标线程死亡。Thread中一共有三个join的重载方法。
public final void join():该方法将当前线程放入等待队列中,直至被它调用的线程死亡为止。如果该线程被中断,则会抛出InterruptedException异常。
public final synchronized void join(long millis):该方法用于让当前线程进入等待状态,直至被它调用的线程死亡或是经过millis毫秒。由于线程的执行依赖于操作系统,所以该方法无法保证当前线程等待的时间和指定时间一致。
public final synchronized void join(long millis, int nanos):该方法用于让线程暂停millis毫秒nanos纳秒。
下面的例子演示了join方法的使用。该段代码的目的是确保main线程最后结束,并且仅当第一个线程死亡才能启动第三个线程。
ThreadJoinExample.java
package com.journaldev.threads; public class ThreadJoinExample { Thread t1 = new Thread(new MyRunnable(), "t1"); Thread t2 = new Thread(new MyRunnable(), "t2"); Thread t3 = new Thread(new MyRunnable(), "t3"); t1.start(); //start second thread after waiting for 2 seconds or if it's dead try { t1.join(2000); } catch (InterruptedException e) { e.printStackTrace(); } t2.start(); //start third thread only when first thread is dead try { t1.join(); } catch (InterruptedException e) { e.printStackTrace(); } t3.start(); //let all threads finish execution before finishing main thread try { t1.join(); t2.join(); t3.join(); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("All threads are dead, exiting main thread"); } class MyRunnable implements Runnable { @Override public void run() { System.out.println("Thread started:::" + Thread.currentThread.getName()); try { Thread.sleep(4000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Thread ended:::" + Thread.currentThread.getName()); } }
上述程序的输出结果为:
Thread started:::t1 Thread started:::t2 Thread ended:::t1 Thread started:::t3 Thread ended:::t2 Thread ended:::t3 All threads are dead, exiting main thread