背景：我有三个主题。 ThreadA负责将元素写入队列，如果队列已满，则通知ThreadC从队列中读取元素。 ThreadB是另一个条件，如果队列未满，但是时间超过5秒，则Thread通知threadC从队列中获取元素，最后，ThreadC通知ThreadB刷新其时间戳。
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class Main {
private Lock lock = new ReentrantLock();
private Condition conA = lock.newCondition();
private Condition conB = lock.newCondition();
private Condition conC = lock.newCondition();
ArrayBlockingQueue<Integer> readQueueA = new ArrayBlockingQueue<>(3);
public static void main(String[] args) {
Main main1 = new Main();
try {
ThreadA threadWrite = main1.new ThreadA();
Thread threadOut = new Thread(threadWrite);
threadOut.start();
ThreadB threadB = main1.new ThreadB();
Thread threadBB = new Thread(threadB);
threadBB.start();
ThreadC threadRead = main1.new ThreadC();
Thread threadIn = new Thread(threadRead);
threadIn.start();
} catch (Exception ex) {
ex.printStackTrace();
}
}
class ThreadA implements Runnable {
public void addElement(Integer i) {
try {
readQueueA.put(i);
} catch (Exception ex ){
ex.printStackTrace();
}
}
@Override
public void run() {
int i = 0;
while (i < 10) {
lock.lock();
try {
if (readQueueA.size() < 3) {
addElement(i++);
} else {
System.out.println("notice C: " + new Date());
conC.signal();
conA.await();
}
} catch (Exception ex) {
ex.printStackTrace();
} finally {
lock.unlock();
}
}
}
}
class ThreadB implements Runnable {
@Override
public void run() {
reSetStartTime();
while (true) {
lock.lock();
try {
if (!conB.await(5, TimeUnit.SECONDS)) {
System.out.println("Timeout Zzzzzzz: " + new Date());
conC.signal();
} else {
System.out.println("RefreshB。。。。" + new Date());
}
} catch (Exception ex) {
ex.printStackTrace();
} finally {
lock.unlock();
}
}
}
}
class ThreadC implements Runnable {
public void printQueue(ArrayBlockingQueue<Integer> printQueue) {
int len = printQueue.size();
System.out.println("Queen Size :" + printQueue.size());
for (int i = 0; i < len; i++) {
System.out.print(printQueue.poll());
}
System.out.println();
}
@Override
public void run() {
while (true) {
lock.lock();
try {
System.out.println("I'm thread C " + new Date());
conC.await();
System.out.println("I'm thread C, and I wake up " + new Date());
if (readQueueA.size() > 0) {
System.out.print("OUTPUT: ");
printQueue(readQueueA);
conA.signal();
conB.signal();
} else {
System.out.println("No elements");
}
} catch (Exception ex) {
ex.printStackTrace();
} finally {
lock.unlock();
}
}
}
}
}
输出：
"C:\Program Files\Java\jdk1.8.0_181\bin\java.exe"
notice C: Fri Sep 21 10:07:35 CST 2018
I'm thread C Fri Sep 21 10:07:35 CST 2018
TimeOut Zzzzzzz: Fri Sep 21 10:07:40 CST 2018
I'm thread C, and I wake up Fri Sep 21 10:07:40 CST 2018
OUTPUT: Queen Size :3
012
I'm thread C Fri Sep 21 10:07:40 CST 2018
notice C: Fri Sep 21 10:07:40 CST 2018
RefreshB。。。。Fri Sep 21 10:07:40 CST 2018
I'm thread C, and I wake up Fri Sep 21 10:07:40 CST 2018
OUTPUT: Queen Size :3
345
I'm thread C Fri Sep 21 10:07:40 CST 2018
notice C: Fri Sep 21 10:07:40 CST 2018
RefreshB。。。。Fri Sep 21 10:07:40 CST 2018
I'm thread C, and I wake up Fri Sep 21 10:07:40 CST 2018
OUTPUT: Queen Size :3
678
I'm thread C Fri Sep 21 10:07:40 CST 2018
RefreshB。。。。Fri Sep 21 10:07:40 CST 2018
TimeOut Zzzzzzz: Fri Sep 21 10:07:45 CST 2018
I'm thread C, and I wake up Fri Sep 21 10:07:45 CST 2018
OUTPUT: Queen Size :1
9
I'm thread C Fri Sep 21 10:07:45 CST 2018
RefreshB。。。。Fri Sep 21 10:07:45 CST 2018
TimeOut Zzzzzzz: Fri Sep 21 10:07:50 CST 2018
I'm thread C, and I wake up Fri Sep 21 10:07:50 CST 2018
No elements
....
但是输出不是我所期望的，为什么一开始为什么ThreadB outPut TimeOut Zzz？我认为应该不输出该项目，因为ThreadA唤醒了ThreadC，下一步应该执行ThreadC并且Thread可以在短时间内完成运行5s。有人可以帮我解释或修复它吗？感谢你！
预期：
notice C: Fri Sep 21 10:28:31 CST 2018
I'm thread C Fri Sep 21 10:28:**31** CST 2018
I'm thread C, and I wake up Fri Sep 21 10:28:31 CST 2018
OUTPUT: Queen Size :3
012
I'm thread C Fri Sep 21 10:28:31 CST 2018
notice C: Fri Sep 21 10:28:31 CST 2018
RefreshB。。。。Fri Sep 21 10:28:31 CST 2018
...
最佳答案
这是在您发布的特定情况下发生的情况：A开始，运行while循环，直到发出C信号并等待。然后，C启动并等待，从而出现问题。在C开始等待之前发出了一个信号，因此该信号呼叫丢失了，现在A和C都在等待。所以在这一点上，控制台上是
通知C ...
我是线程C ...
现在，B启动，并等待整整5秒钟，因为没有其他线程可发出信号。这样，conB.await(5, TimeUnit.SECONDS)返回false，它打印Timeout Zzzzzzz:，然后向C发出信号。这就是为什么事情看起来混乱的原因，只有当另一个线程已经在等待时，信号才起作用！
要解决此问题，请尝试更改
conC.await();
System.out.println("I'm thread C, and I wake up " + new Date());
至
if (readQueueA.size() < 3) {
conC.await();
System.out.println("I'm thread C, and I wake up " + new Date());
}
这样，如果等待条件已经满足，C就不会等待。
关于java - 三线程通讯与同步，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/52427246/