ReadWriteLock（ReadWriteLock）

java.util.concurrent.locks.ReadWriteLock接口允许多个线程一次读取，但一次只能写入一个线程。

• Read Lock - 如果没有线程锁定ReadWriteLock进行写入，则多个线程可以访问读锁定。

• Write Lock - 如果没有线程在读或写，则一个线程可以访问写锁定。

锁定方法

以下是Lock类中可用的重要方法列表。

例子 (Example)

import java.util.concurrent.locks.ReentrantReadWriteLock;
public class TestThread {
   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();
         }
      }
   }
}

这将产生以下结果。

输出 (Output)

Writer A  Time Taken 6 seconds.
Write Lock Present.
Writer B  Time Taken 2 seconds.
Reader  Time Taken 0 seconds.
Reader: aab