Java並行ReadWriteLock介面

java.util.concurrent.locks.ReadWriteLock介面允許一次讀取多個執行緒，但一次只能寫入一個執行緒。

• 讀鎖 - 如果沒有執行緒鎖定ReadWriteLock進行寫入，則多執行緒可以存取讀鎖。
• 寫鎖 - 如果沒有執行緒正在讀或寫，那麼一個執行緒可以存取寫鎖。

鎖方法

以下是Lock類中可用的重要方法的列表。

範例

以下TestThread程式演示了ReadWriteLock介面的這些方法。這裡我們使用readlock()獲取讀鎖定和writeLock()來獲取寫鎖定。

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();
         }
      }
   }
}

這將產生以下結果,如下所示 -

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