多线程：控制多线程按顺序执行

示例代码

static Thread t1 = new Thread(new Runnable() {
public void run() {
System.out.println("Thread1");
}
});
static Thread t2 = new Thread(new Runnable() {
public void run() {
System.out.println("Thread2");
}
});
static Thread t3 = new Thread(new Runnable() {
public void run() {
System.out.println("Thread3");
}
});
1.使用join
public static void main(String[] args) throws InterruptedException {
t1.start();
t1.join();
t2.start();
t2.join();
t3.start();
}
join源码：
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;
            }
        }
    }
join让主线程等待，至子线程执行结束
2.使用ExecutorService
static ExecutorService es = Executors.newSingleThreadExecutor();
public static void main(String[] args) throws InterruptedException {
es.submit(t1);
es.submit(t2);
es.submit(t3);
es.shutdown();
}
ExecutorService 提供了4种线程池，分别是：
newCachedThreadPool;缓存线程池，可灵活回收、新建
newFixedThreadPool;定长线程池，可控制最大并发数，超出在队列中等待
newScheduledThreadPool;定长线程池,可定时、周期执行
newSingleThreadExecutor;单一线程池，按顺序执行（FIFO,LIFO,优先级）