Thread 的 join() 源码解读

join()使用场景

当程序希望各个线程执行完后，将他们的计算结果最终合并运算时，换句话说要等待多个线程将子任务执行完后，才能进行合并结果操作，这时候就可以使用join()。

源码

 /**
     * Waits at most {@code millis} milliseconds for this thread to
     * die. A timeout of {@code 0} means to wait forever.
     *
     * <p> This implementation uses a loop of {@code this.wait} calls
     * conditioned on {@code this.isAlive}. As a thread terminates the
     * {@code this.notifyAll} method is invoked. It is recommended that
     * applications not use {@code wait}, {@code notify}, or
     * {@code notifyAll} on {@code Thread} instances.
     *
     * @param  millis
     *         the time to wait in milliseconds
     *
     * @throws  IllegalArgumentException
     *          if the value of {@code millis} is negative
     *
     * @throws  InterruptedException
     *          if any thread has interrupted the current thread. The
     *          <i>interrupted status</i> of the current thread is
     *          cleared when this exception is thrown.
     */
    public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;
    if (millis &lt; 0) {
        throw new IllegalArgumentException("timeout value is negative");
    }

    if (millis == 0) {
        while (isAlive()) {
            wait(0);
        }
    } else {
        while (isAlive()) {
            long delay = millis - now;
            if (delay &lt;= 0) {
                break;
            }
            wait(delay);
            now = System.currentTimeMillis() - base;
        }
    }
}<br /><br /></pre>
​
有同学疑问，调用join后到底是哪个线程阻塞了？其实很好理解,就是当前执行的线程阻塞，而不是调用join的线程！
public static void main(String[] args) throws InterruptedException {
		final Thread t = new Thread(new Runnable() {
			public void run() {
				try {
					System.out.println("t线程开始"+System.currentTimeMillis());
					Thread.sleep(2000);
					System.out.println("t线程结束"+System.currentTimeMillis());
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
		});
	Thread t2 = new Thread(new Runnable() {
		public void run() {
			try {
				t.start();
				t.join();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			System.out.println("t2线程结束"+System.currentTimeMillis());
		}
	});
	t2.start();</pre>
​
                  Thread.currentThread().sleep(1000);
                  System.out.println(t2.getState());
		System.out.println("t2线程开始"+System.currentTimeMillis());
}</pre>
​

将上面的测试代码里的join方法，用join的源码替换就一目了然了如下
public static void main(String[] args) throws InterruptedException {
		final Thread t = new Thread(new Runnable() {
			public void run() {
				try {
					System.out.println("t线程开始"+System.currentTimeMillis());
					Thread.sleep(2000);
					System.out.println("t线程结束"+System.currentTimeMillis());
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
		});
	Thread t2 = new Thread(new Runnable() {
		public void run() {
			try {
				t.start();
				//t.join();
				//join源码开始
				long millis = 0;
				long base = System.currentTimeMillis();
		        long now = 0;
		        if (millis &lt; 0) {
		            throw new IllegalArgumentException("timeout value is negative");
		        }

		        if (millis == 0) {
		            while (t.isAlive()) {
		            	//wait就是对t2线程的操作
		                t.wait(0);
		            }
		        } else {
		            while (t.isAlive()) {
		                long delay = millis - now;
		                if (delay &lt;= 0) {
		                    break;
		                }
		                t.wait(delay);
		                now = System.currentTimeMillis() - base;
		            }
		        }
			} catch (InterruptedException e) {
				e.printStackTrace();
			}//join源码结束
			System.out.println("t2线程结束"+System.currentTimeMillis());
		}
	});
	t2.start();
	System.out.println("t2线程开始"+System.currentTimeMillis());
	
	
}</pre>
​