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;
}
}
}源码爸爸说了，孩子，我给你millis这么长的时间，能不能完成任务那是你的事情了，能提前完成，咱就提前走下去，不能完成，过期不候，自己看着办吧。
默认情况下，Thread.join()即Thread.join(0)，当为0的时候，那才叫真爱呢，线程会一直等下去，知道执行结束为止，才会继续朝下执行。
isAlive()：用来测试线程是否处于活动状态，相当于 run 是否还在执行。