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Java多线程中join方法的理解

2014-10-10 11:29 645 查看

thread.Join把指定的线程加入到当前线程，可以将两个交替执行的线程合并为顺序执行的线程。比如在线程B中调用了线程A的Join()方法，直到线程A执行完毕后，才会继续执行线程B。

t.join(); //使调用线程 t 在此之前执行完毕。

t.join(1000); //等待 t 线程，等待时间是1000毫秒

先上一段JDK中代码：

Java代码

/**
     *  Waits at most <code>millis</code> milliseconds for this thread to
     * die. A timeout of <code>0</code> means to wait forever.
     */
    //此处A timeout of 0 means to wait forever 字面意思是永远等待，其实是等到t结束后。
    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;
            }
        }
    }

/**
*  Waits at most <code>millis</code> milliseconds for this thread to
* die. A timeout of <code>0</code> means to wait forever.
*/
//此处A timeout of 0 means to wait forever 字面意思是永远等待，其实是等到t结束后。
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;
}
}
}

从代码上看，如果线程被生成了，但还未被起动，调用它的
join() 方法是没有作用的，将直接继续向下执行

Join方法实现是通过wait（小提示：Object 提供的方法）。
当main线程调用t.join时候，main线程会获得线程对象t的锁（wait 意味着拿到该对象的锁),调用该对象的wait(等待时间)，直到该对象唤醒main线程，比如退出后。这就意味着main 线程调用t.join时，必须能够拿到线程t对象的锁

Example1：

Java代码

public class JoinTest implements Runnable{

    public static int a = 0;

    public void run() {
        for (int k = 0; k < 5; k++) {
            a = a + 1;
        }
    }

    public static void main(String[] args) throws Exception {
        Runnable r = new JoinTest();
        Thread t = new Thread(r);
        t.start();
        System.out.println(a);
    }
}

public class JoinTest implements Runnable{

public static int a = 0;

public void run() {
for (int k = 0; k < 5; k++) {
a = a + 1;
}
}

public static void main(String[] args) throws Exception {
Runnable r = new JoinTest();
Thread t = new Thread(r);
t.start();
System.out.println(a);
}
}

请问程序的输出结果是5吗？答案是：有可能。其实你很难遇到输出5的时候，通常情况下都不是5。当然这也和机器有严重的关系。为什么呢？我的解释是当主线程 main方法执行System.out.println(a);这条语句时，线程还没有真正开始运行，或许正在为它分配资源准备运行。因为为线程分配资源需要时间，而main方法执行完t.start()方法后继续往下执行System.out.println(a);,这个时候得到的结果是a还没有被
改变的值0 。怎样才能让输出结果为5！其实很简单，join() 方法提供了这种功能。join() 方法，它能够使调用该方法的线程在此之前执行完毕。

Java代码

public static void main(String[] args) throws Exception {
        Runnable r = new JoinTest();
        Thread t = new Thread(r);
        t.start();
        t.join(); //加入join()
        System.out.println(a);
    }

public static void main(String[] args) throws Exception {
Runnable r = new JoinTest();
Thread t = new Thread(r);
t.start();
t.join(); //加入join()
System.out.println(a);
}

这个时候，程序输入结果始终为5。

为了证明如果不使用t.join()方法，主线程main方法的System.out.println(a);语句将抢先执行，我们可以在main方法中加入一个循环，这个循环用来延长main方法执行的时间，循环次数将严重取决于机器性能。如果循环次数得当，我们也可以看到a的输出结果是5。

Java代码

public static void main(String[] args) throws Exception {
        Runnable r = new JoinTest();
        Thread t = new Thread(r);
        t.start();
        //t.join(); //加入join()
            /*
             注意循环体内一定要有实际执行语句，否则编译器或JVM可能优化掉你的这段代码，视这段代
             码为无效。
            */
            for (int i=0; i<300; i++) {
                System.out.print(i);
            }
            System.out.println();
        System.out.println(a);
    }

public static void main(String[] args) throws Exception {
Runnable r = new JoinTest();
Thread t = new Thread(r);
t.start();
//t.join(); //加入join()
/*
注意循环体内一定要有实际执行语句，否则编译器或JVM可能优化掉你的这段代码，视这段代
码为无效。
*/
for (int i=0; i<300; i++) {
System.out.print(i);
}
System.out.println();
System.out.println(a);
}

经自己测试，最后a一直是5.

本例参考：http://agio.iteye.com/blog/210600

Example2：join(n)

Java代码

class RunnableImpl implements Runnable {

    public void run() {
        try {
            System.out.println("Begin sleep");
            Thread.sleep(1000);
           System.out.println("End sleep");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
}

class RunnableImpl implements Runnable {

public void run() {
try {
System.out.println("Begin sleep");
Thread.sleep(1000);
System.out.println("End sleep");
} catch (InterruptedException e) {
e.printStackTrace();
}

}
}

Java代码

public class JoinTest{

    public static void main(String[] args) {
        Thread t = new Thread(new RunnableImpl());
        t.start();
        try {
            t.join(1000);
            System.out.println("joinFinish");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class JoinTest{

public static void main(String[] args) {
Thread t = new Thread(new RunnableImpl());
t.start();
try {
t.join(1000);
System.out.println("joinFinish");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

结果是：

Begin sleep

End sleep

joinFinish

明白了吧，当main线程调用t.join时，main线程等待t线程，等待时间是1000，如果t线程Sleep 2000呢

Java代码

class RunnableImpl implements Runnable {

    public void run() {
        try {
            System.out.println("Begin sleep");
            Thread.sleep(2000); //原来为1000
           System.out.println("End sleep");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
}

class RunnableImpl implements Runnable {

public void run() {
try {
System.out.println("Begin sleep");
Thread.sleep(2000); //原来为1000
System.out.println("End sleep");
} catch (InterruptedException e) {
e.printStackTrace();
}

}
}

结果是：

Begin sleep

joinFinish

End sleep

也就是说main线程只等1000毫秒，不管T什么时候结束.

参考：http://blog.csdn.net/FG2006/archive/2011/05/04/6393768.aspx

Example3：

Java代码

class CustomThread1 extends Thread {

    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start.");
        try {
            for (int i = 0; i < 5; i++) {
                System.out.println(threadName + " loop at " + i);
                Thread.sleep(1000);
            }
            System.out.println(threadName + " end.");
        } catch (Exception e) {
            System.out.println("Exception from " + threadName + ".run");
        }
    }
}

class CustomThread extends Thread {
    CustomThread1 t1;
    public CustomThread(CustomThread1 t1) {
        this.t1 = t1;
    }
    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start.");
        try {
            t1.join();
            System.out.println(threadName + " end.");
        } catch (Exception e) {
            System.out.println("Exception from " + threadName + ".run");
        }
    }
}

public class JoinTestDemo {

    public static void main(String[] args) {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start.");
        CustomThread1 t1 = new CustomThread1();
        CustomThread t = new CustomThread(t1);
        try {
            t1.start();
            Thread.sleep(2000);
            t.start();
            t.join(); //在代碼2里，將此處注釋掉
        } catch (Exception e) {
            System.out.println("Exception from main");
        }
        System.out.println(threadName + " end!");
    }
}

class CustomThread1 extends Thread {

public void run() {
String threadName = Thread.currentThread().getName();
System.out.println(threadName + " start.");
try {
for (int i = 0; i < 5; i++) {
System.out.println(threadName + " loop at " + i);
Thread.sleep(1000);
}
System.out.println(threadName + " end.");
} catch (Exception e) {
System.out.println("Exception from " + threadName + ".run");
}
}
}

class CustomThread extends Thread {
CustomThread1 t1;
public CustomThread(CustomThread1 t1) {
this.t1 = t1;
}
public void run() {
String threadName = Thread.currentThread().getName();
System.out.println(threadName + " start.");
try {
t1.join();
System.out.println(threadName + " end.");
} catch (Exception e) {
System.out.println("Exception from " + threadName + ".run");
}
}
}

public class JoinTestDemo {

public static void main(String[] args) {
String threadName = Thread.currentThread().getName();
System.out.println(threadName + " start.");
CustomThread1 t1 = new CustomThread1();
CustomThread t = new CustomThread(t1);
try {
t1.start();
Thread.sleep(2000);
t.start();
t.join(); //在代碼2里，將此處注釋掉
} catch (Exception e) {
System.out.println("Exception from main");
}
System.out.println(threadName + " end!");
}
}

结果：

main start.    //main方法所在的线程起动，但没有马上结束，因为调用t.join();，所以要等到t结束了，此线程才能向下执行。

[CustomThread1] Thread start.     //线程CustomThread1起动

[CustomThread1] Thread loop at 0 //线程CustomThread1执行

[CustomThread1] Thread loop at 1 //线程CustomThread1执行

[CustomThread] Thread start.      //线程CustomThread起动，但没有马上结束，因为调用t1.join();，所以要等到t1结束了，此线程才能向下执行。

[CustomThread1] Thread loop at 2 //线程CustomThread1继续执行

[CustomThread1] Thread loop at 3 //线程CustomThread1继续执行

[CustomThread1] Thread loop at 4 //线程CustomThread1继续执行

[CustomThread1] Thread end.       //线程CustomThread1结束了

[CustomThread] Thread end.        // 线程CustomThread在t1.join();阻塞处起动，向下继续执行的结果

main end!      //线程CustomThread结束，此线程在t.join();阻塞处起动，向下继续执行的结果。

将上例中的join注释掉：

Java代码

public class JoinTestDemo {
    public static void main(String[] args) {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start.");
        CustomThread1 t1 = new CustomThread1();
        CustomThread t = new CustomThread(t1);
        try {
            t1.start();
            Thread.sleep(2000);
            t.start();
           //t.join();
        } catch (Exception e) {
            System.out.println("Exception from main");
        }
        System.out.println(threadName + " end!");
    }
}

public class JoinTestDemo {
public static void main(String[] args) {
String threadName = Thread.currentThread().getName();
System.out.println(threadName + " start.");
CustomThread1 t1 = new CustomThread1();
CustomThread t = new CustomThread(t1);
try {
t1.start();
Thread.sleep(2000);
t.start();
//t.join();
} catch (Exception e) {
System.out.println("Exception from main");
}
System.out.println(threadName + " end!");
}
}

结果：

main start. // main方法所在的线程起动，但没有马上结束，这里并不是因为join方法，而是因为Thread.sleep(2000);

[CustomThread1] Thread start.      //线程CustomThread1起动

[CustomThread1] Thread loop at 0   //线程CustomThread1执行

[CustomThread1] Thread loop at 1   //线程CustomThread1执行

main end!   // Thread.sleep(2000);结束，虽然在线程CustomThread执行了t1.join();，但这并不会影响到其他线程(这里main方法所在的线程)。

[CustomThread] Thread start.       //线程CustomThread起动，但没有马上结束，因为调用t1.join();，所以要等到t1结束了，此线程才能向下执行。

[CustomThread1] Thread loop at 2   //线程CustomThread1继续执行

[CustomThread1] Thread loop at 3   //线程CustomThread1继续执行

[CustomThread1] Thread loop at 4   //线程CustomThread1继续执行

[CustomThread1] Thread end.       //线程CustomThread1结束了

[CustomThread] Thread end.        // 线程CustomThread在t1.join();阻塞处起动，向下继续执行的结果

本例参考：http://blog.csdn.net/bzwm/archive/2009/02/12/3881392.aspx

Example4 ：

main 线程调用t.join时，必须能够拿到线程t对象的锁，如果拿不到它是无法wait的，刚开的例子t.join(1000)不是说明了main线程等待1 秒，如果在它等待之前，其他线程获取了t对象的锁，它等待时间可不就是1毫秒了。

Java代码

class RunnableImpl implements Runnable {

    public void run() {
        try {
            System.out.println("Begin sleep");
            Thread.sleep(2000);
            System.out.println("End sleep");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

class RunnableImpl implements Runnable {

public void run() {
try {
System.out.println("Begin sleep");
Thread.sleep(2000);
System.out.println("End sleep");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

Java代码

class ThreadTest extends Thread {

    Thread thread;

    public ThreadTest(Thread thread) {
        this.thread = thread;
    }

    @Override
    public void run() {
        synchronized (thread) {
            System.out.println("getObjectLock");
            try {
                Thread.sleep(9000);
            } catch (InterruptedException ex) {
             ex.printStackTrace();
            }
            System.out.println("ReleaseObjectLock");
        }
    }
}

class ThreadTest extends Thread {

Thread thread;

public ThreadTest(Thread thread) {
this.thread = thread;
}

@Override
public void run() {
synchronized (thread) {
System.out.println("getObjectLock");
try {
Thread.sleep(9000);
} catch (InterruptedException ex) {
ex.printStackTrace();
}
System.out.println("ReleaseObjectLock");
}
}
}

Java代码

public class JoinTest {
        public static void main(String[] args) {
            Thread t = new Thread(new RunnableImpl());
           new ThreadTest(t).start();
            t.start();
            try {
                t.join();
                System.out.println("joinFinish");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
}

public class JoinTest {
public static void main(String[] args) {
Thread t = new Thread(new RunnableImpl());
new ThreadTest(t).start();
t.start();
try {
t.join();
System.out.println("joinFinish");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

结果：
getObjectLock

Begin sleep

End sleep

ReleaseObjectLock

joinFinish

在main方法中通过new
ThreadTest(t).start()实例化 ThreadTest 线程对象，它
通过 synchronized (thread) ，获取线程对象t的锁，并Sleep（9000）后释放，这就意味着，即使main方法t.join(1000)等待一秒钟，它必须等待ThreadTest
线程释放t锁后才能进入wait方法中，它实际等待时间是9000+1000ms。

例子参考Example2来源.

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标签： Java 多线程 join

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