线程通信问题

黄玉昆

本帖最后由て淡莣了陌生于 2013-4-28 08:36 编辑

class Demo1_Notify {

      public static void main(String[] args) {
            final Printer p = new Printer();

            new Thread(){
                     public void run() {
                              while(true)
                                    try {
                                             p.print1();
                                    } catch(Exception e) {
                                             e.printStackTrace();
                                    }
                     }
            }.start();

            new Thread(){
                     public void run() {
                              for(;;)
                                    try {
                                             p.print2();
                                    } catch(Exception e) {
                                             e.printStackTrace();
                                    }
                     }
            }.start();

            new Thread(){
                     public void run() {
                              for(;;)
                                    try {
                                             p.print3();
                                    } catch(Exception e) {
                                             e.printStackTrace();
                                    }
                     }
            }.start();
      }

}

class Printer {
      private int flag = 1;

      public synchronized void print1() throws Exception {
            if (flag != 1)
                     wait();
            System.out.print("好");
            System.out.print("好");
            System.out.print("学");
            System.out.print("习");
            System.out.print("\r\n");
            flag = 2;
            notifyAll();
      }

      public synchronized void print2() throws Exception {
            if (flag != 2)
                     wait();
            System.out.print("天");
            System.out.print("天");
            System.out.print("向");
            System.out.print("上");
            System.out.print("\r\n");
            flag = 3;
            notifyAll();
      }

      public synchronized void print3() throws Exception {
            if (flag != 3)
                     wait();
            System.out.print("A");
            System.out.print("l");
            System.out.print("r");
            System.out.print("e");
            System.out.print("a");
            System.out.print("d");
            System.out.print("\r\n");
            flag = 1;
            notifyAll();
      }

}

这段代码执行后为什么没有按照顺序来打印呢,研究了大半天了都没找出哪里出了问题,哪位大神能指点一下迷津啊,谢谢了啊!!!

java冬冬 · java冬冬

本帖最后由 java冬冬于 2013-4-27 18:21 编辑

代码修改如下：{:soso_e152:}

class Demo1_Notify {
public static void main(String[] args) {
final Printer p = new Printer();
new Thread(){
public void run() {
while(true)
try {
p.print1();
} catch(Exception e) {
e.printStackTrace();
}
}
}.start();
new Thread(){
public void run() {
for(;;)
try {
p.print2();
} catch(Exception e) {
e.printStackTrace();
}
}
}.start();
new Thread(){
public void run() {
for(;;)
try {
p.print3();
} catch(Exception e) {
e.printStackTrace();
}
}
}.start();
}
}
class Printer {
private int flag = 1;
public synchronized void print1() throws Exception {
while (flag != 1)
wait();
System.out.print("好");
System.out.print("好");
System.out.print("学");
System.out.print("习");
System.out.print("\r\n");
flag = 2;
notifyAll();
}
public synchronized void print2() throws Exception {
while (flag != 2)
wait();
System.out.print("天");
System.out.print("天");
System.out.print("向");
System.out.print("上");
System.out.print("\r\n");
flag = 3;
notifyAll();
}
public synchronized void print3() throws Exception {
while(flag != 3)
wait();
System.out.print("A");
System.out.print("l");
System.out.print("r");
System.out.print("e");
System.out.print("a");
System.out.print("d");
System.out.print("\r\n");
flag = 1;
notifyAll();
}
}

复制代码

运行结果：

杨同旺 · 杨同旺

本帖最后由杨同旺于 2013-4-27 17:55 编辑

把以上代码中的if,全部改为while,便能解决问题,

原因是由于,先打印print1,此时print3等待,

当print1打印完成后,print3恰好被唤醒,此时print3将不会返回去再判断if,而是不再判断直接就打印了,

所以,需要加上while,print3在打印之前,将会回去判断一下while(flag!=3),结果就正确了.

Miss小强 · Miss小强

本帖最后由黄玉昆于 2013-4-27 19:50 编辑

楼上的说的对，补充点：
为什么要用while，那时因为会出现假唤醒的现象，本来没有被notify，
被唤醒了之后没有判断标记，就容易出问题；
给你一个我写的吧：

package cn.itcast.concurrent;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Business {
/* *
* 需求：
* 有三个线程：一个主线程，两个子线程A,B；
* 主线程循环20次，A线程循环30次，B循环40次；
* 如此交替50次；即主---》A---》B；
* 思路：
* 可以定义一个锁，在一个锁上获得三个condition分别代表主，A，B；
* 定义一个int变量：作为一个标记，来标识三个线程是否可以执行；
* 1---》主 2---》A 3---》B；
*/
private Lock lock = new ReentrantLock();
private int flag = 1;
private Condition main_condition = lock.newCondition();
private Condition a_condition = lock.newCondition();
private Condition b_condition = lock.newCondition();
public void main(int i) {
lock.lock();
try {
while (flag != 1) {
main_condition.await();
}
for (int j = 0; j < 20; j++) {
System.out.println(Thread.currentThread().getName() + "main loop" + j + "of" + i);
}
flag=2;
a_condition.signal();
} catch (InterruptedException e) {
e.printStackTrace();
}
finally {
lock.unlock();
}
}
public void subA(int i) {
lock.lock();
try{
while(flag!=2){
a_condition.await();
}
for (int j = 0; j < 20; j++) {
System.out.println(Thread.currentThread().getName() + "A loop" + j + "of" + i);
}
flag=3;
b_condition.signal();
} catch (InterruptedException e) {
e.printStackTrace();
}
finally{
lock.unlock();
}
}
public void subB(int i) {
lock.lock();
try{
while(flag!=3){
b_condition.await();
}
for (int j = 0; j < 20; j++) {
System.out.println(Thread.currentThread().getName() + "B loop" + j + "of" + i);
}
flag=1;
main_condition.signal();
} catch (InterruptedException e) {
e.printStackTrace();
}
finally{
lock.unlock();
}
}
}

复制代码

Miss小强 · Miss小强

本帖最后由黄玉昆于 2013-4-27 19:52 编辑

package cn.itcast.concurrent;
public class ThreeThreadcommunicati {
public static void main(String[] args) {
final Business business = new Business();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 50; i++) {
business.subA(i);
}
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 50; i++) {
business.subB(i);
}
}
}).start();
for (int i = 0; i < 50; i++) {
business.main(i);
}
}
}

复制代码

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线程通信问题

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