Java 併發 - Condition 介面

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java.util.concurrent.locks.Condition 介面提供了一種執行緒能夠掛起其執行的能力，直到給定的條件為真。Condition 物件必須繫結到一個 Lock，並且必須使用 newCondition() 方法獲取。

Condition 方法

以下是 Condition 類中可用的一些重要方法列表。

示例

以下 TestThread 程式演示了 Condition 介面的這些方法。在這裡，我們使用了 signal() 來通知，並使用 await() 來掛起執行緒。

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class TestThread {

   public static void main(String[] args) throws InterruptedException {
      ItemQueue itemQueue = new ItemQueue(10);

      //Create a producer and a consumer.
      Thread producer = new Producer(itemQueue);
      Thread consumer = new Consumer(itemQueue);

      //Start both threads.
      producer.start();
      consumer.start();

      //Wait for both threads to terminate.
      producer.join();
      consumer.join();
   }

   static class ItemQueue {
      private Object[] items = null;
      private int current = 0;
      private int placeIndex = 0;
      private int removeIndex = 0;

      private final Lock lock;
      private final Condition isEmpty;
      private final Condition isFull;

      public ItemQueue(int capacity) {
         this.items = new Object[capacity];
         lock = new ReentrantLock();
         isEmpty = lock.newCondition();
         isFull = lock.newCondition();
      }

      public void add(Object item) throws InterruptedException {
         lock.lock();

         while(current >= items.length)
            isFull.await();

         items[placeIndex] = item;
         placeIndex = (placeIndex + 1) % items.length;
         ++current;

         //Notify the consumer that there is data available.
         isEmpty.signal();
         lock.unlock();
      }

      public Object remove() throws InterruptedException {
         Object item = null;

         lock.lock();

         while(current <= 0) {
            isEmpty.await();
         }
         item = items[removeIndex];
         removeIndex = (removeIndex + 1) % items.length;
         --current;

         //Notify the producer that there is space available.
         isFull.signal();
         lock.unlock();

         return item;
      }

      public boolean isEmpty() {
         return (items.length == 0);
      }
   }

   static class Producer extends Thread {
      private final ItemQueue queue;
      
      public Producer(ItemQueue queue) {
         this.queue = queue;
      }

      @Override
      public void run() {
         String[] numbers =
            {"1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12"};

         try {
            
            for(String number: numbers) {
               System.out.println("[Producer]: " + number);
            }
            queue.add(null);
         } catch (InterruptedException ex) {
            ex.printStackTrace();
         } 
      }
   }

   static class Consumer extends Thread {
      private final ItemQueue queue;
      
      public Consumer(ItemQueue queue) {
         this.queue = queue;
      }

      @Override
      public void run() {
         
         try {
            
            do {
               Object number = queue.remove();
               System.out.println("[Consumer]: " + number);

               if(number == null) {
                  return;
               }
            } while(!queue.isEmpty());
         } catch (InterruptedException ex) {
            ex.printStackTrace();
         }
      }
   }
}

這將產生以下結果。

輸出

[Producer]: 1
[Producer]: 2
[Producer]: 3
[Producer]: 4
[Producer]: 5
[Producer]: 6
[Producer]: 7
[Producer]: 8
[Producer]: 9
[Producer]: 10
[Producer]: 11
[Producer]: 12
[Consumer]: null