现在有T1、T2、T3三个线程，你怎样保证T2在T1执行完后执行，T3在T2执行完后执行？

Thread t1 = new Thread(new T1()); 
Thread t2 = new Thread(new T2()); 
Thread t3 = new Thread(new T3());

t1.start(); 
t1.join();

t2.start(); 
t2.join();

t3.start(); 
t3.join();

主要是使用了join()方法

看看jdk的join方法是如何实现的吧

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;
            }
        }
    }