Java多线程中join方法的理解
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:
- /**
- * 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:
- 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。
- 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。
- 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)
- 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();
- }
- }
- }
- 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
- 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:
- 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:
- 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 。
- 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 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");
- }
- }
- }
- 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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