从生产者/消费者模式角度思考线程间通信

多线程中等待/通知模式最经典的案例就是“生产者/消费者”模式。但此模式在使用上有几种“变形”，还有一些小的注意事项，但原理是基于wait/notify的。该模式可以通过操作值来实现。

一生产与一消费

代码清单1

//生产者
public class P {
    private String lock;

    public P(String lock) {
        super();
        this.lock = lock;
    }

    public void setValue() {
        try {
            synchronized (lock) {
                if (!ValueObject.value.equals("")) {
                    lock.wait();
                }
                String value = System.currentTimeMillis() + "_" + System.nanoTime();
                System.out.println("set的值是" + value);
                ValueObject.value = value;
                lock.notify();
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

//消费者
public class C {

    private String lock;

    public C(String lock) {
        super();
        this.lock = lock;
    }

    public void getValue() {
        try {
            synchronized (lock) {
                if (ValueObject.value.equals("")) {
                    lock.wait();
                }
                System.out.println("get的值是" + ValueObject.value);
                ValueObject.value = "";
                lock.notify();
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

//存储值的对象
public class ValueObject {
    public static String value = "";
}

//生产者线程
public class ThreadP extends Thread {

    private P p;

    public ThreadP(P p) {
        super();
        this.p = p;
    }

    @Override
    public void run() {
        while (true) {
            p.setValue();
        }
    }
}

//消费者线程
public class ThreadC extends Thread {

    private C r;

    public ThreadC(C r) {
        super();
        this.r = r;
    }

    @Override
    public void run() {
        while (true) {
            r.getValue();
        }
    }
}

//运行类
public class Run {

    public static void main(String[] args) {

        String lock = new String("");
        P p = new P(lock);
        C r = new C(lock);

        ThreadP pThread = new ThreadP(p);
        ThreadC rThread = new ThreadC(r);

        pThread.start();
        rThread.start();
    }
}

运行结果

set的值是1514222349229_7168218698116
get的值是1514222349229_7168218698116
set的值是1514222349231_7168221053134
get的值是1514222349231_7168221053134
...

本示例是一个生产者和一个消费者进行数据的交互，在控制台中打印的日志set与get是交替运行的。

多生产与多消费

代码清单2

//生产者
public class P {

    private String lock;

    public P(String lock) {
        super();
        this.lock = lock;
    }

    public void setValue() {
        try {
            synchronized (lock) {
                while (!ValueObject.value.equals("")) {
                    System.out.println("生产者 "
                            + Thread.currentThread().getName() + " WAITING了★");
                    lock.wait();
                }
                System.out.println("生产者 " + Thread.currentThread().getName()
                        + " RUNNABLE了");
                String value = System.currentTimeMillis() + "_"
                        + System.nanoTime();
                ValueObject.value = value;
                lock.notifyAll();
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
//消费者
public class C {

    private String lock;

    public C(String lock) {
        super();
        this.lock = lock;
    }

    public void getValue() {
        try {
            synchronized (lock) {
                while (ValueObject.value.equals("")) {
                    System.out.println("消费者"
                            + Thread.currentThread().getName() + " WAITING了☆");
                    lock.wait();
                }
                System.out.println("消费者" + Thread.currentThread().getName()
                        + " RUNNABLE了");
                ValueObject.value = "";
                lock.notifyAll();
            }

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
//生产者线程
public class ThreadP extends Thread {

    private P p;

    public ThreadP(P p) {
        super();
        this.p = p;
    }

    @Override
    public void run() {
        while (true) {
            p.setValue();
        }
    }
}
//消费者x
public class ThreadC extends Thread {

    private C r;

    public ThreadC(C r) {
        super();
        this.r = r;
    }

    @Override
    public void run() {
        while (true) {
            r.getValue();
        }
    }
}

public class ValueObject {

    public static String value = "";
}

public class Run {

    public static void main(String[] args) throws InterruptedException {

        String lock = new String("");
        P p = new P(lock);
        C r = new C(lock);

        ThreadP[] pThread = new ThreadP[2];
        ThreadC[] rThread = new ThreadC[2];

        for (int i = 0; i < 2; i++) {
            pThread[i] = new ThreadP(p);
            pThread[i].setName("生产者" + (i + 1));

            rThread[i] = new ThreadC(r);
            rThread[i].setName("消费者" + (i + 1));

            pThread[i].start();
            rThread[i].start();
        }

        Thread.sleep(5000);
        Thread[] threadArray = new Thread[Thread.currentThread()
                .getThreadGroup().activeCount()];
        Thread.currentThread().getThreadGroup().enumerate(threadArray);

        for (int i = 0; i < threadArray.length; i++) {
            System.out.println(threadArray[i].getName() + " "
                    + threadArray[i].getState());
        }
    }
}

运行结果

生产者 生产者1 RUNNABLE了
生产者 生产者1 WAITING了★
消费者消费者1 RUNNABLE了
消费者消费者1 WAITING了☆
消费者消费者2 WAITING了☆
生产者 生产者2 RUNNABLE了
生产者 生产者2 WAITING了★
消费者消费者2 RUNNABLE了
消费者消费者2 WAITING了☆
消费者消费者1 WAITING了☆
生产者 生产者1 RUNNABLE了
生产者 生产者1 WAITING了★
...

注意事项

比较上面两种模式，主要改动了两处。

• if()判断改为while()循环，重新判断条件，避免逻辑混乱;
• notify()改为notifyAll()避免进入“假死”状态。

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