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

 1 public final synchronized void join(long millis)
 2     throws InterruptedException {
 3         long base = System.currentTimeMillis();
 4         long now = 0;
 5
 6         if (millis < 0) {
 7             throw new IllegalArgumentException("timeout value is negative");
 8         }
 9
10         if (millis == 0) {
11             while (isAlive()) {
12                 wait(0);
13             }
14         } else {
15             while (isAlive()) {
16                 long delay = millis - now;
17                 if (delay <= 0) {
18                     break;
19                 }
20                 wait(delay);
21                 now = System.currentTimeMillis() - base;
22             }
23         }
24     }