9 Java并发ReadWriteLock接口

java.util.concurrent.locks.ReadWriteLock接口允许一次读取多个线程，但一次只能写入一个线程。

• 读锁 - 如果没有线程锁定ReadWriteLock进行写入，则多线程可以访问读锁。
• 写锁 - 如果没有线程正在读或写，那么一个线程可以访问写锁。

锁方法

以下是Lock类中可用的重要方法的列表。

编号	方法	描述
1	public Lock readLock()	返回用于读的锁。
2	public Lock writeLock()	返回用于写的锁。

示例

以下ReadWriteLockTest程序演示了ReadWriteLock接口的这些方法。这里我们使用readlock()获取读锁定和writeLock()来获取写锁定。

package ThreadMethod;

import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ReadWriteLockTest {

   private static final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(true);

   private static String message = "a";

   public static void main(String[] args) throws InterruptedException{
      Thread t1 = new Thread(new WriterA());
      t1.setName("Writer A");
      Thread t2 = new Thread(new WriterB());
      t2.setName("Writer B");
      Thread t3 = new Thread(new Reader());
      t3.setName("Reader");
      t1.start();
      t2.start();
      t3.start();
      t1.join();
      t2.join();
      t3.join();
   }

   static class Reader implements Runnable {

      public void run() {
         if(lock.isWriteLocked()) {
            System.out.println("Write Lock Present.");
         }   
         lock.readLock().lock();
         try {
            Long duration = (long) (Math.random() * 10000);
            System.out.println(Thread.currentThread().getName() 
               + "  Time Taken " + (duration / 1000) + " seconds.");
            Thread.sleep(duration);
         } catch (InterruptedException e) {
            e.printStackTrace();
         } finally {
            System.out.println(Thread.currentThread().getName() +": "+ message );
            lock.readLock().unlock();
         }
      }
   }

   static class WriterA implements Runnable {

      public void run() {
         lock.writeLock().lock();
         try {
            Long duration = (long) (Math.random() * 10000);
            System.out.println(Thread.currentThread().getName() 
               + "  Time Taken " + (duration / 1000) + " seconds.");
            Thread.sleep(duration);
         } catch (InterruptedException e) {
            e.printStackTrace();
         } finally {
            message = message.concat("a");
            lock.writeLock().unlock();
         }
      }
   }

   static class WriterB implements Runnable {

      public void run() {
         lock.writeLock().lock();
         try {
            Long duration = (long) (Math.random() * 10000);
            System.out.println(Thread.currentThread().getName() 
               + "  Time Taken " + (duration / 1000) + " seconds.");
            Thread.sleep(duration);
         } catch (InterruptedException e) {
            e.printStackTrace();
         } finally {
            message = message.concat("b");
            lock.writeLock().unlock();
         }
      }
   }
}

运行

Write Lock Present.
Writer A  Time Taken 4 seconds.
Writer B  Time Taken 2 seconds.
Reader  Time Taken 6 seconds.
Reader: aab