条件（Condition）

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2023-12-01

java.util.concurrent.locks.Condition接口提供了一个线程暂停执行的能力，直到给定的条件为真。 Condition对象必然绑定到Lock并使用newCondition（）方法获取。

条件方法

以下是Condition类中可用的重要方法列表。

Sr.No.	方法和描述
1	public void await() 导致当前线程等待，直到发出信号或中断为止。
2	public boolean await(long time, TimeUnit unit) 导致当前线程等待，直到发出信号或中断，或者指定的等待时间过去。
3	public long awaitNanos(long nanosTimeout) 导致当前线程等待，直到发出信号或中断，或者指定的等待时间过去。
4	public long awaitUninterruptibly() 导致当前线程等待直到发出信号。
5	public long awaitUntil() 导致当前线程等待，直到发出信号或中断，或者指定的截止时间过去。
6	public void signal() 唤醒一个等待线程。
7	public void signalAll() 唤醒所有等待的线程。

例子 (Example)

以下TestThread程序演示了Condition接口的这些方法。这里我们使用signal（）来通知和await（）来挂起线程。

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();
         }
      }
   }
}

这将产生以下结果。

输出 (Output)

[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