多线程：head first Thread.join()-优快云博客

不使用Thread.join() 测试线程

先上代码：

/**
 * Created by Zero on 2017/8/23.
 */
public class TestJoin implements Runnable { public static int a = 0; @Override public void run() { for (int i = 0; i < 5; i++) { a = a + 1; } } public static void main(String[] args) throws InterruptedException { TestJoin j = new TestJoin(); Thread thread = new Thread(j); thread.start(); System.out.println(a); } }

以上示例会输出5吗？可能性不大，有可能永远输出为0，之前在线程池原理的那篇就提到过，线程的启动和销毁都需要时间，此处因为thread还没启动好，或者正在为它分配资源准备运行，就已经执行完输出了。

怎样才能确保每次都能输出5呢？现在有请我们的主角join方法闪亮登场，代码如下：

/**
 * Created by apple on 2017/8/23.
 */
public class TestJoin implements Runnable { public static int a = 0; @Override public void run() { for (int i = 0; i < 5; i++) { a = a + 1; } } public static void main(String[] args) throws InterruptedException { TestJoin j = new TestJoin(); Thread thread = new Thread(j); thread.start(); /** * 测试join方法的作用，与下面的threadAgain线程作对比。 */ thread.join(); System.out.println(a); a = 0; Thread threadAgain = new Thread(j); threadAgain.start(); System.out.println(a); } }

输出的结果将是5和0。

Thread.join()作用

Thread.join()，之前看资料的时候，有些人说可以理解成“将两个线程合并成一个线程”，我是觉得这样说是很不科学的，虽然这样通俗易懂，但这确实是两个不同的线程，只是在调用Thread.join()后，会先执行完Thread线程后再去执行当前线程，即上述的在主线程中执行到thread.join();后，先去执行thread，直到thread执行完后再去执行主线程。

测试Thread.join(long millis)

/**
 * Created by apple on 2017/8/23.
 */
public class TestJoin implements Runnable { public static int a = 0; @Override public void run() { for (int i = 0; i < 5; i++) { a = a + 1; try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } } public static void main(String[] args) throws InterruptedException { TestJoin j = new TestJoin(); Thread thread = new Thread(j); thread.start(); /** * 测试join方法的作用 */ thread.join(3000); System.out.println("thread线程结果为："+a); a = 0; Thread threadAgain = new Thread(j); threadAgain.start(); System.out.println("threadAgain线程结果为："+a); } }

输出：

thread线程结果为：3
threadAgain线程结果为：0

先上一段源码再来分析：

/**
     * 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 < 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; } } }