Java多线程 --- join

 

join 可以安排先后顺序了.如果要先完成t1, 再运行t2, 最近运行t3,就可以这样了

t1.start();

t1.join();

t2.start();

t2.join();

t3.start()

 

JDK是这样说的：

Waits at most millis milliseconds for this thread to die. A timeout of 0 means to wait forever.

等待一段时间直到这个线程死亡

 

public class ThreadTest implements Runnable {public static int a = 0;public void run() {for (int k = 0; k < 5; k++) {a = a + 1;}}public static void main(String[] args) throws Exception {Runnable r = new ThreadTest();Thread t = new Thread(r);t.start();System.out.println(a);}} 

 

请问程序的输出结果是5吗？答案是：有可能。这个时候main线程和t线程是并列运行的。

怎样才能让输出结果为5！其实很简单，join() 方法提供了这种功能。join() 方法，它能够使调用该方法的线程在此之前执行完毕。

 

public class ThreadTest implements Runnable {public static int a = 0;public void run() {for (int k = 0; k < 5; k++) {a = a + 1;}}public static void main(String[] args) throws Exception {Runnable r = new ThreadTest();Thread t = new Thread(r);t.start();t.join();System.out.println(a);}} 

 

这个时候，程序输入结果始终为5。

 

public final void join() throws InterruptedException {join(0);} 

 

就是说如果是t.join() = t.join(0) 0　JDK这样说的 A timeout of 0 means to wait forever 字面意思是永远等待,其实是等到t结束后。
这个是怎么实现的吗？ 看JDK代码：

 

/*** Waits at most <code>millis</code> milliseconds for this thread to * die. A timeout of <code>0</code> means to wait forever. ** @param millis the time to wait in milliseconds.* @exception 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 < 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;}}} 

 

 其实Join方法实现是通过wait。

wait()方法

在其他线程调用对象的wait方法后，导致当前线程等待。线程会释放掉它所占有的“锁标志”，从而使别的线程有机会抢占该锁。

 

当main线程调用t.join时候，main线程会失去线程对象t的锁,进入等待，直到该对象唤醒main线程，比如退出后。