單生產者與單消費者

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();
    }

    /**
     * 生產者
     */
    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();
        }
      }
    }

    /**
     * 消費者
     */
    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 = "";
    }
}

執行結果如下：

多生產者與多消費者

這種模式下，容易出現“假死”，也就是全部執行緒都進入了 WAITNG 狀態，程式不在執行任何業務功能了，整個專案呈停止狀態。

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();
        }
      },"生產者" + (i + 1));

      cThread[i] = new Thread(() -> {
        while (true) {
          consume.getValue();
        }
      },"消費者" + (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("生產者 " + Thread.currentThread().getName() + " WAITING了⭐");
            lock.wait();
          }
          System.out.println("生產者 " + 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("消費者 " + Thread.currentThread().getName() + " WAITING了⭐");
            lock.wait();
          }
          System.out.println("消費者 " + Thread.currentThread().getName() + "RUNNABLE了");
          ValueObject.value = "";
          lock.notify();
        }
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }
  }

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

執行結果如圖：

分析：

雖然程式碼中通過 wait/notify 進行通訊了，但是不能保證 notify 喚醒的一定是異類，也可能是同類，比如“生產者”喚醒了“生產者”這樣的情況。

解決方案：

假死出現的主要原因是有可能連續喚醒了同類。所以解決方案很簡單，就是把 notify() 改為 notifyAll() 即可。

以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支援我們。