Java 线程间通信不传递消息对象

Question

为了在线程之间进行通信，我遵循了易于编译的 Oracle Guarded Blocks example 和运行s。我的架构略有不同，因为我的消费者产生了生产者任务，尽管我在示例中尝试了这种变体并且它运行完美。

我的主程序中的相关代码；

public static void main(String[] args) {
...
    FrameMsg frameMsg = new FrameMsg();
    AwarenessAnalytics awarenessAnalytic = new AwarenessAnalytics(frameMsg);
    awarenessAnalytic.start();

来自消费者线程的相关代码；

public class AwarenessAnalytics extends Thread implements MotionEventListener{
    FrameMsg frameMsg;
    FrameWithMotionDetection frameWithMotionDetection;

      public AwarenessAnalytics(FrameMsg frameMsg) {
        this.frameMsg = frameMsg;
        System.out.println("AwarenessAnalytic frameMsg = " + this.frameMsg.hashCode());
        }
 AdvancedVideoAnalytics tempIntermediateVA;
 tempIntermediateVA = new AdvancedVideoAnalytics(frameMsg);

public void run() {

    tempIntermediateVA.start();

    while (true) {
        // TODO: create loop to process frames from each video stream
        frameWithMotionDetection = new FrameWithMotionDetection();
        // interthread message from AdvancedAnalytic
        System.out.println("Waiting for FrameMsg");
        frameWithMotionDetection = frameMsg.take();
        System.out.println("FrameMsg received");
}

来自生产者任务的相关代码；

public class AdvancedVideoAnalytics extends Thread {
  FrameMsg frameMsg;
  FrameWithMotionDetection frameWithMotionDetection;

public AdvancedVideoAnalytics (FrameMsg frameMsg) {
    this.frameMsg = frameMsg;
    System.out.println("AdvancedVideoAnalytic frameMsg = " + this.frameMsg.hashCode());
 }

// the run method includes;

// Send frame and any clusters detected
// as frameMsg
frameWithMotionDetection = new FrameWithMotionDetection();

frameWithMotionDetection.setMotionData(contourAnalysisResults);

frameWithMotionDetection.setCurrentFrame(frameToExamine);
System.out.println("Preparing to send message to AwarenessAnalytics thread");
frameMsg.put(frameWithMotionDetection);

FrameMsg class;

public class FrameMsg {
// Message sent from video stream monitors to analytic fusion engine

private FrameWithMotionDetection frameWithMotionData;

//private String message;
// True if consumer should wait
// for producer to send message,
// false if producer should wait for
// consumer to retrieve message.
private boolean empty = true;

public synchronized FrameWithMotionDetection take() {
    // Wait until message is
    // available.
    System.out.println("Getting ready to take frameWithMotionData");
    while (empty) {
        try {
            wait(10);
            System.out.println("Waiting to take frameWithMotionData because empty = true");
        } catch (InterruptedException e) {}
    }
    // Toggle status.
    empty = true;
    System.out.println("Successfully took frameWithMotionData, empty = " + empty);
    // Notify producer that
    // status has changed.
    notifyAll();
    return frameWithMotionData;
}

public synchronized void put(FrameWithMotionDetection frameWithMotionData) {
    // Wait until message has
    // been retrieved.
    System.out.println("Getting ready to put frameWithMotionData");
    while (!empty) {
        try { 
            System.out.println("Waiting to put frameWithMotionData because empty = false");
            wait();
        } catch (InterruptedException e) {}
    }
    // Toggle status.
    empty = false;
    // Store message.
    this.frameWithMotionData = frameWithMotionData;
    System.out.println("Successfully put frameWithMotionData, empty = " + empty);
    // Notify consumer that status
    // has changed.
    notifyAll();
}

}

有趣的是，所有的 frameMsg 对象 ID 都是相同的，我能够 'put' 一个 frameMsg 并将生产者的空设置为 false。但是消费者看到的frameMsg对象总是returns'true'为空

输出摘录如下；

VideoAnalyticsUnitTest frameMsg = 1704856573
AwarenessAnalytic frameMsg = 1704856573
AdvancedVideoAnalytic frameMsg = 1704856573

Waiting to take frameWithMotionData because empty = true
Waiting to take frameWithMotionData because empty = true
(many of these)...
Preparing to send message to AwarenessAnalytics thread
Getting ready to put frameWithMotionData
Successfully put frameWithMotionData, empty = false
Waiting to take frameWithMotionData because empty = true
Preparing to send message to AwarenessAnalytics thread
Getting ready to put frameWithMotionData
Waiting to put frameWithMotionData because empty = false
Waiting to take frameWithMotionData because empty = true
Waiting to take frameWithMotionData because empty = true
Waiting to take frameWithMotionData because empty = true

它像最后三行一样继续，直到我终止程序。

我很困惑，因为； 1.我按照这个例子 2.对象ID匹配

然而，消费者永远不会看到非空的 frameMsg（这是一个复杂的对象）。

我是不是漏掉了一些明显的东西？

我最初使用侦听器发送消息，但我不想让一个庞大的应用程序占用侦听器 space。现在阅读更多评论，似乎我可以使用侦听器并将消息传递给具有阻塞队列的消费者的运行部分。

如果是您，您会采用上述通信方法，还是恢复为具有阻塞队列的侦听器？

Answer 1

如果要更新 empty 参数，请不要使用 synchronized 方法，而是使用 synchronized块

synchronized(this){
//work from here for core logic 

}
//empty =true logic or empty = false

块确实比方法有优势，最重要的是灵活性，因为您可以使用其他对象作为锁，而同步方法将锁定整个 class。

比较：

// locks the whole object
... 
private synchronized void someInputRelatedWork() {
... 
}
private synchronized void someOutputRelatedWork() {
... 
}

比

// Using specific locks
Object inputLock = new Object();
Object outputLock = new Object();

private void someInputRelatedWork() {
synchronize(inputLock) { 
    ... 
} 
}
private void someOutputRelatedWork() {
synchronize(outputLock) { 
    ... 
}
}

此外，如果方法增长，您仍然可以保持同步部分分离：

 private void method() {
 ... code here
 ... code here
 ... code here
 synchronized( lock ) { 
    ...very few lines of code here
 }
 ... code here
 ... code here
 ... code here
 ... code here
}

最后的建议是使用 LinkedBlockingQueue 因为它有很好的性能和好的方法 put and take

Answer 2

正如@Bhargav Modi 指出的那样，代码运行涉及编写多线程应用程序的更精细问题（同步块与 - 方法，在关键变量上使用 volatile 声明）。这些问题在测试过程中经常被遗漏，因为需要一些偶然因素才能使问题出现（最臭名昭著的问题之一是 double checked locking）。

这是使用 Java concurrent classes: there is less chance of writing code that is not thread-safe or has multi-threading issues. In your case, the SynchronousQueue 看起来不错的替代品的充分理由。使用 SynchronousQueue，无需使用 empty 变量、this.frameWithMotionData 变量或 wait/notifyAll 机制。

Java 线程间通信不传递消息对象

Java Interthread Communication not passing message object

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communication

listener