Producer Consumer模块只输出null

Question

我尝试制作一个生产者消费者 Java 程序，但它似乎没有工作：它只是没有终止，终端就这样保持：我希望代码在新行上打印 Text 1 然后 Text 2 然后 Text 3。我希望写入线程写入要打印的文本，然后读取线程实际打印文本。我的代码在下面。我的另一个问题是什么时候会使用这种模型，目前作为初学者 Java 程序员是否值得学习。谢谢。代码如下：

public class Test {
    public static void main(String[] args) {
        Model model = new Model();
        new Thread(new ModelThreadWriter(model)).start();
        new Thread(new ModelThreadReader(model)).start();
        
    }
}

class Model {
    private String data;
    private boolean isEmpty = true;

    public synchronized void writeData(String data) {
        while (!isEmpty) {
            try {
                wait();
            } catch (InterruptedException e) {

            }
            this.data = data;
            isEmpty = false;
            notifyAll();
        }
    }

    public synchronized String readData() {
        while (isEmpty) {
            try {
                wait();
            } catch (InterruptedException e) {

            }
        }
        isEmpty = true;
        notifyAll();
        return data;
    }
}

class ModelThreadWriter implements Runnable {
    private Model model;

    public ModelThreadWriter(Model model) {
        this.model = model;
    }
    
    @Override
    public void run() {
        String[] text = {
            "Text 1",
            "Text 2",
            "Text 3"
        };

        for (int i=0; i<text.length; i++) {
            model.writeData(text[i]);

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {

            }
        }
        model.writeData("done");

        
    }
}

class ModelThreadReader implements Runnable {
    private Model model;

    public ModelThreadReader(Model model) {
        this.model = model;
    }

    @Override
    public void run() {
        for (String msg = model.readData(); !(msg == "done"); model.readData()) {
            System.out.println(msg);

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                
            }
        }
    }
}

Answer 1

除了上面评论中提到的其他问题，您的 ModelThreadReader#run() 方法中的 for 循环永远不会终止。

for (String msg = model.readData(); !(msg == "done"); model.readData()) {
    ...
}

String msg变量会在循环体第一次执行前被设置为调用model.readData()的结果，之后就不会再设置了。除非第一次调用发生在 return "done" 上，否则循环不会终止。

也许你是故意的？

for (String msg=model.readData(); !(msg=="done"); msg=model.readData())...

Answer 2

您的代码中有几个错误。

1.In writeData 方法，this.data = data;isEmpty = false;notifyAll(); 应该移出 while 循环。

2.In ModelThreadReader, == => equals.

3.In ModelThreadReader, for (String msg = model.readData(); !(msg == "done"); model.readData()) => !(msg.equals("done")); msg = model.readData(),否则每次都会打印出Text1

class Model {
    private String data;
    private boolean isEmpty = true;

    public synchronized void writeData(String data) {
        while (!isEmpty) {
            try {
                wait();
            } catch (InterruptedException e) {

            }
        }
        this.data = data;
        isEmpty = false;
        notifyAll();
    }

    public synchronized String readData() {
        while (isEmpty) {
            try {
                wait();
            } catch (InterruptedException e) {

            }
        }
        isEmpty = true;
        notifyAll();
        return this.data;
    }
}

class ModelThreadWriter implements Runnable {
    private Model model;

    public ModelThreadWriter(Model model) {
        this.model = model;
    }

    @Override
    public void run() {
        String[] text = {
            "Text 1",
            "Text 2",
            "Text 3"
        };
        for (int i=0; i<text.length; i++) {
            model.writeData(text[i]);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {

            }
        }
        model.writeData("done");
    }
}

class ModelThreadReader implements Runnable {
    private Model model;

    public ModelThreadReader(Model model) {
        this.model = model;
    }

    @Override
    public void run() {
        String msg = model.readData();
        for (; !(msg.equals("done")); msg = model.readData()) {
            System.out.println(msg);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {

            }
        }
    }
}

Producer Consumer模块只输出null

Producer Consumer module just outputting null

java

concurrency

producer-consumer