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分享关于多线程中wait使用的问题。
wait应该是只有wait所同步的对象调用notify或者notifyAll才会继续吗？
为什么下面代码我没有调用notify，主线程也继续向下执行了？
PS:还有两个小问题：
1.如果b.wait只能是b.notify才能唤醒吗？那么也必须是b.notifyAll吗？
为什么下面代码我没有调用notify，主线程也继续向下执行了？
public class WaitTest {
public static void main(String[] args) {
ThreadB b = new ThreadB();
b.start();
synchronized (b) {
try {
System.out.println("Waiting for b to complete...");
b.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Total is: " + b.total);
}
}
}
class ThreadB extends Thread {
int total;
public void run() {
// synchronized (this) {
System.out.println("ThreadB");
for (int i = 0; i < 50; i++) {
total += i;
try {
Thread.sleep(100);
System.out.print(i);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// notify();
System.out.println();
}
// }
}
PS:还有两个小问题：
1.如果b.wait只能是b.notify才能唤醒吗？那么也必须是b.notifyAll吗？
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godwanglong 2013-03-11
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“This implementation uses a loop of this.wait calls conditioned on this.isAlive. As a thread terminates the this.notifyAll method is invoked. It is recommended that applications not use wait, notify, or notifyAll on Thread instances”这句话在java 7的文档上有的，而在java 6的文档里面没有。不过java 6中也的确存在这个现象
chaofengwang09 2013-03-08
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最近在搞udp 这些方法都没怎么用了 学习了
iwome 2013-03-07
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你去搜索下
WaitForSingleObject
WaitForMultipleObject
的java实现版本，那个东西比较通用，控制多线程之间的协调很方便。
-迷糊- 2013-03-06
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试了2楼的方法，果然是不加notify就一直等待了！
wait同步的对象是线程的话，那么同步锁对象(线程)执行完毕后会调用notifyAll？
第一次听说，学习了...
有没有相关资料 orz
小菜的粉丝 2013-03-06
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又学到新知识了，不错！
浪漫小和 2013-03-06
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赞一个，研究官方的doc得出结论，这才是学习之道
yongger520 2013-03-06
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赞一个，研究官方的doc得出结论，这才是学习之道
而我开始只是猜测
dracularking 2013-03-06
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我找到的这个（method exit from class Thread）：
/**
* This method is called by the system to give a Thread
* a chance to clean up before it actually exits.
*/
private void exit() {
if (group != null) {
group.remove(this);
group = null;
}
/* Aggressively null out all reference fields: see bug 4006245 */
target = null;
/* Speed the release of some of these resources */
threadLocals = null;
inheritableThreadLocals = null;
inheritedAccessControlContext = null;
blocker = null;
uncaughtExceptionHandler = null;
}
/**
* Removes the specified Thread from this group.
* @param t the Thread to be removed
* @return if the Thread has already been destroyed.
*/
void remove(Thread t) {
synchronized (this) {
if (destroyed) {
return;
}
for (int i = 0 ; i < nthreads ; i++) {
if (threads[i] == t) {
System.arraycopy(threads, i + 1, threads, i, --nthreads - i);
// Zap dangling reference to the dead thread so that
// the garbage collector will collect it.
threads[nthreads] = null;
break;
}
}
if (nthreads == 0) {
notifyAll();
}
if (daemon && (nthreads == 0) &&
(nUnstartedThreads == 0) && (ngroups == 0))
{
destroy();
}
}
}
龙四 2013-03-06
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http://hg.openjdk.java.net/jdk6/jdk6/hotspot/file/20dbc199874e/src/share/vm/runtime/thread.cpp
中1552行void JavaThread::exit(bool destroy_vm, ExitType exit_type)方法中的第1677行有ensure_join(this);调用
static void ensure_join(JavaThread* thread)方法在1529行，notifyAll操作在1544行，且方法开头就注释了：// We do not need to grap the Threads_lock, since we are operating on ourself.
龙四 2013-03-06
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dracularking的说法是正确的。
see here：http://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#join%28long%29
As a thread terminates the this.notifyAll method is invoked. It is recommended that applications not use wait, notify, or notifyAll on Thread instances.
dracularking 2013-03-05
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不wait是因为使用的同步锁对象的关系，b对象是一个Thread对象，该thread执行完毕后，会调用notifyAll方法，唤醒等待在该线程对象上的所有线程。
yanxing2012 2013-03-05
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package com.love.yan.xing;
class ThreadB extends Thread {
int total;
public void run() {
// synchronized (this) {
System.out.println("ThreadB");
for (int i = 0; i < 50; i++) {
total += i;
try {
Thread.sleep(100);
System.out.print(i);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized(new WaitTest().yan()){
new WaitTest().yan().notify();
}
System.out.println();
}
// }
}
package com.love.yan.xing;
public class WaitTest {
static ThreadB b = new ThreadB();
public static void main(String[] args) {
ThreadB a = new ThreadB();
a.start();
synchronized (b) {
try {
System.out.println("Waiting for b to complete...");
b.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Total is: " + b.total);
}
}
public Thread yan(){
return b;
}
}
-迷糊- 2013-03-05
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这三条应该都没有满足吧1.没有设置wait的超时时间。2.没调用interrupt。3.没调用notify
-迷糊- 2013-03-05
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哦，这样啊。
那就是说就不用notify的话，wait也会唤醒吗？
那么唤醒的时机又在什么时候呢？ orz
dracularking 2013-03-05
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推荐的wait使用方式是这样的：
synchronized (obj) {
while (<condition does not hold>)
obj.wait();
... // Perform action appropriate to condition
}
如果不加while，貌似就会得到楼主所观察到的现象
yanxing2012 2013-03-05
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唤醒三种方式：1.时间到期。即线程在指定一段时间过去后会醒过来。2.线程被中断。其他线程调用此线程的interrupt（）方法。3.对象得到通知。
yongger520 2013-03-05
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public class WaitTest {
public static Object o = new Object();
public static void main(String[] args) {
ThreadB b = new ThreadB();
b.start();
synchronized (o) {
try {
System.out.println("Waiting for b to complete...");
o.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Total is: " + b.total);
}
}
}
class ThreadB extends Thread {
int total;
public void run() {
synchronized (WaitTest.o) {
System.out.println("ThreadB");
for (int i = 0; i < 50; i++) {
total += i;
try {
Thread.sleep(100);
System.out.print(i);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
WaitTest.o.notify();
System.out.println();
}
}
}