有一个笔试题，A、B、C三个线程，怎么保证A执行完后再执行B，B执行完后再执行C。
最简单的就是join 了吧。直接上代码：
public static void main(String[] args) throws InterruptedException {
PrintThread threadA = new PrintThread("Thread A");
PrintThread threadB = new PrintThread("Thread B");
PrintThread threadC = new PrintThread("Thread C");
threadA.start();
threadA.join();
threadB.start();
threadB.join();
threadC.start();
}
static class PrintThread extends Thread {
PrintThread(String name) {
super(name);
}
@Override
public void run() {
System.out.println(getName() + " start");
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(getName() + " end ...");
}
}
执行结果：
Thread A start
Thread A end ...
Thread B start
Thread B end ...
Thread C start
Thread C end ...
那么join是怎么实现的呢？看下源码：
/**
* Waits for this thread to die.
*
* <p> An invocation of this method behaves in exactly the same
* way as the invocation
*
* <blockquote>
* {@linkplain #join(long) join}{@code (0)}
* </blockquote>
*
* @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 void join() throws InterruptedException {
join(0);
}
注意下注释：等待此线程执行完。
join()方法实际上调用了join(long millis),参数是0，再往下看：
/**
* 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;
}
}
}
翻译一下注释：等待millis 毫秒终止线程，假如这段时间内该线程还没执行完，那么结束等待。如果是0，意味着一直等待，知道线程终止。
很明显，有一个while循环使该线程一直停留在阻塞状态，再看一下isAlive()方法：
/**
* Tests if this thread is alive. A thread is alive if it has
* been started and has not yet died.
*
* @return <code>true</code> if this thread is alive;
* <code>false</code> otherwise.
*/
public final native boolean isAlive();
是一个native方法，但是看一下注释：判断该线程是否是正在执行状态，正在执行状态是指已经start了，但是还没结束。
通过不停判断该线程的状态，来决定是不是要wait，线程结束，或者达到指定的延迟时间后，终止代码。
