Java多线程生产者消费者模式实现过程解析

单生产者与单消费者

示例：

public class ProduceConsume {
    public static void main(String[] args) {
      String lock = new String("");
      Produce produce = new Produce(lock);
      Consume consume = new Consume(lock);

      new Thread(() -> {
        while (true) {
          produce.setValue();
        }
      }, "ProductThread").start();
      new Thread(() -> {
        while (true) {
          consume.getValue();
        }
      }, "ConsumeThread").start();
    }

    /**
     * 生产者
     */
    static class Produce {
      private String lock;

      public Produce(String lock) {
        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
     */
    static class Consume {
      private String lock;

      public Consume(String lock) {
        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();
        }
      }
    }

    static class ValueObject {
      public static String value = "";
    }
}

执行结果如下：

多生产者与多消费者

这种模式下，容易出现“假死”，也就是全部线程都进入了 WAITNG 状态，程序不在执行任何业务功能了，整个项目呈停止状态。

示例：

public class MultiProduceConsume {
  public static void main(String[] args) throws InterruptedException {
    String lock = new String("");
    Produce produce = new Produce(lock);
    Consume consume = new Consume(lock);
    Thread[] pThread = new Thread[2];
    Thread[] cThread = new Thread[2];
    for (int i = 0; i < 2; i++) {
      pThread[i] = new Thread(() -> {
        while (true) {
          produce.setValue();
        }
      }, "生产者" + (i + 1));

      cThread[i] = new Thread(() -> {
        while (true) {
          consume.getValue();
        }
      }, "消费者" + (i + 1));
      pThread[i].start();
      cThread[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());
    }
  }

  static class Produce {
    private String lock;

    public Produce(String lock) {
      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.notify();
        }
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
  }

  static class Consume {
    private String lock;

    public Consume(String lock) {
      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.notify();
        }
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
  }

  static class ValueObject {
    public static String value = "";
  }
}

运行结果如图：

分析：

虽然代码中通过 wait/notify 进行通信了，但是不能保证 notify 唤醒的一定是异类，也可能是同类，比如“生产者”唤醒了“生产者”这样的情况。

解决方案：

假死出现的主要原因是有可能连续唤醒了同类。所以解决方案很简单，就是把 notify() 改为 notifyAll() 即可。

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