在多线程中线程的执行是CPU随机调度的，无法指定线程的执行顺序，比如我们要让多线程按顺序执行输出
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| Thread thread1 = new Thread(() -> System.out.println(Thread.currentThread().getName()), "thread1");
Thread thread2 = new Thread(() -> System.out.println(Thread.currentThread().getName()), "thread2");
Thread thread3 = new Thread(() -> System.out.println(Thread.currentThread().getName()), "thread3");
thread1.start();
thread2.start();
thread3.start();
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这段代码的输出顺序是未知的，可能是123，可能是213也可能是其它情况，我们无法保证哪一个线程会最先执行
这种情况下要保证顺序输出就要求Thread2在Thread1结束后执行，Thread3在Thread2完成后执行
使用Thread的join()方法
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| public static void main(String[] args) throws InterruptedException {
Thread thread1 = new Thread(() -> System.out.println(Thread.currentThread().getName()), "thread1");
Thread thread2 = new Thread(() -> System.out.println(Thread.currentThread().getName()), "thread2");
Thread thread3 = new Thread(() -> System.out.println(Thread.currentThread().getName()), "thread3");
thread1.start();
thread1.join();
thread2.start();
thread2.join();
thread3.start();
}
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join方法会阻塞直到该线程执行结束
来看一下Thread类内部的join()方法
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| public final void join() throws InterruptedException {
join(0);
}
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;
}
}
}
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当调用join()方法时，millis参数为0，表示当当前线程未结束时，会一直执行wait()方法阻塞主线程，wait()是native方法由其它语言实现
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| public final native void wait(long timeout) throws InterruptedException;
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当timeout参数为0时会无限期等待
使用单线程的线程池
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| public static void main(String[] args) throws InterruptedException {
Thread thread1 = new Thread(() -> System.out.println("1"));
Thread thread2 = new Thread(() -> System.out.println("2"));
Thread thread3 = new Thread(() -> System.out.println("3"));
ExecutorService executorService = Executors.newSingleThreadExecutor();
executorService.submit(thread1);
executorService.submit(thread2);
executorService.submit(thread3);
executorService.shutdown();
}
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