多線程（三）Join

Join

join方法允許一個線程等待另一個線程運行完畢再繼續。

thread.join();

如果一個線程中包含代碼：

t.join();//導致當前線程暫停，等待線程t執行完畢。可以理解，join也會拋interruptedException。

看完以上的內容，我們就可以看一個包含了上述概念的實際的多線程例子：

 http://download.oracle.com/javase/tutorial/essential/concurrency/simple.html

 看上去很長其實很簡單，要耐心看~

 public class SimpleThreads {

    //Display a message, preceded by the name of the current thread
    static void threadMessage(String message) {
        String threadName = Thread.currentThread().getName();
        System.out.format("%s: %s%n", threadName, message);
    }

    private static class MessageLoop implements Runnable {
        public void run() {
            String importantInfo[] = {
                "Mares eat oats",
                "Does eat oats",
                "Little lambs eat ivy",
                "A kid will eat ivy too"
            };
            try {
                for (int i = 0; i < importantInfo.length; i++) {
                    //Pause for 4 seconds
                    Thread.sleep(4000);
                    //Print a message
                    threadMessage(importantInfo[i]);
                }
            } catch (InterruptedException e) {
                threadMessage("I wasn't done!");
            }
        }
    }

    public static void main(String args[]) throws InterruptedException {

        //Delay, in milliseconds before we interrupt MessageLoop
        //thread (default one hour).
        long patience = 1000 * 60 * 60;

        //If command line argument present, gives patience in seconds.
        if (args.length > 0) {
            try {
                patience = Long.parseLong(args[0]) * 1000;
            } catch (NumberFormatException e) {
                System.err.println("Argument must be an integer.");
                System.exit(1);
            }

        }

        threadMessage("Starting MessageLoop thread");
        long startTime = System.currentTimeMillis();
        Thread t = new Thread(new MessageLoop());//MessageLoop()必須是runnable同時有run方法。
        t.start(); //子線程啟動，開始迴圈列印String數組裡的內容。

        threadMessage("Waiting for MessageLoop thread to finish");
        //loop until MessageLoop thread exits
        while (t.isAlive()) {
            threadMessage("Still waiting..."); //在超過時限之前會一直列印這句話
            //Wait maximum of 1 second for MessageLoop thread to
            //finish.
            t.join(1000); //這句的效果和sleep(1000)一樣
            if (((System.currentTimeMillis() - startTime) > patience) &&
                    t.isAlive()) {
                threadMessage("Tired of waiting!");
                t.interrupt(); //interrupt方法會中斷線程。此時子線程會提示"I wasn't done!"，然後被停止。
                //Shouldn't be long now -- wait indefinitely
                t.join(); //join方法的功能是，當前主線程會等待子線程t結束。不過應該很快。
            }

        }
        threadMessage("Finally!"); //最後程式結束。
    }
}

 

當主線程運行完所有語句，但是有非守護子線程還在運行時，主線程就會一直等待。

(有一點例外。。。如果主線程中最後一句是return。。。那主線程就會立即結束了，從而程式會立即結束)

如果只有守護子線程運行，守護子線程就會結束，之後主線程就運行完畢了。

運行結果：

傳入參數為2時

main: Starting MessageLoop thread
main: Waiting for MessageLoop thread to finish
main: Still waiting...
main: Still waiting...
main: Tired of waiting!
Thread-0: I wasn't done!
main: Finally!