Thread類的join方法用來使main線程進入阻塞狀態，進而等待調用join方法的線程執行，join方法有如下三種形式 ~~~ public final void join(long millis) throws InterruptedException public final void join(long millis, int nanos) throws InterruptedException public final void join() throws InterruptedException ~~~ 帶參數的join方法，表示至多等待線程A的時間，而無參數的join方法，則會一直等待線程A執行結束。通過下例來說明。首先我們看不加join方法的版本，兩個線程并行執行，在run方法中睡眠15秒后同時結束： ~~~ public static void main(String[] args) { System.out.println("Main Thread Start..."+new Date()); CreateThreadTest ctt = new CreateThreadTest(); Thread t1 = new Thread(ctt,"t1"); Thread t2 = new Thread(ctt,"t2"); t1.start(); t2.start(); } public class CreateThreadTest implements Runnable { @Override public void run() { System.out.println("線程"+Thread.currentThread().getName()+"：開始運行"+new Date()); try{ Thread.sleep(15000); //doDBProcessing(); }catch(InterruptedException e){ e.printStackTrace(); } System.out.println("線程"+Thread.currentThread().getName()+"：結束運行"+new Date()); } } ~~~ 結果： ~~~ Main Thread Start...Thu Jul 06 14:48:37 CST 2017 線程t1：開始運行Thu Jul 06 14:48:38 CST 2017 線程t2：開始運行Thu Jul 06 14:48:38 CST 2017 線程t2：結束運行Thu Jul 06 14:48:53 CST 2017 線程t1：結束運行Thu Jul 06 14:48:53 CST 2017 ~~~ 如果加入join()方法呢： ~~~ public static void main(String[] args) { System.out.println("Main Thread Start..."+new Date()); CreateThreadTest ctt = new CreateThreadTest(); Thread t1 = new Thread(ctt,"t1"); Thread t2 = new Thread(ctt,"t2"); try{ t1.start(); t1.join(); t2.start(); }catch(Exception e){ e.printStackTrace(); } } ~~~ 原本并行的兩個線程變為了串行執行，也就是說，父線程會等待子線程的執行完畢： ~~~ Main Thread Start...Thu Jul 06 14:53:34 CST 2017 線程t1：開始運行Thu Jul 06 14:53:34 CST 2017 線程t1：結束運行Thu Jul 06 14:53:49 CST 2017 線程t2：開始運行Thu Jul 06 14:53:49 CST 2017 線程t2：結束運行Thu Jul 06 14:54:04 CST 2017 ~~~