Java併發 - 讀寫鎖介面

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java.util.concurrent.locks.ReadWriteLock 介面允許多個執行緒同時讀取，但一次只能有一個執行緒寫入。

• 讀鎖 - 如果沒有執行緒鎖定 ReadWriteLock 用於寫入，則多個執行緒可以訪問讀鎖。

• 寫鎖 - 如果沒有執行緒正在讀取或寫入，則一個執行緒可以訪問寫鎖。

鎖方法

以下是 Lock 類中可用的一些重要方法。

示例

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

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。

輸出

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