concurrency - ReadWriteLock

ReadWriteLockTest.java

import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

import junit.framework.TestCase;

public class BasicConcurrentTest extends TestCase {
  public void testReadWriteLock() throws Exception {
    final Counter c = new Counter();

    Thread write1 = new Thread() {
      @Override
      public void run() {
        c.increment();
      }
    };
    
    Thread write2 = new Thread() {
      @Override
      public void run() {
        c.increment();
      }
    };
    
    Thread read1 = new Thread() {
      @Override
      public void run() {
        int current = c.current();
        System.out.println(current);
      }
    };
    Thread read2 = new Thread() {
      @Override
      public void run() {
        int current = c.current();
        System.out.println(current);
      }
    };

    
    write1.start();
    write2.start();
    read1.start();
    read2.start();
    
    System.out.println("");
  }
  
  class Counter {
    private final ReadWriteLock lock = new ReentrantReadWriteLock();
    private int value =0;
    
    /**
     * WriteLock은
     * 1) ReadLock이 걸린 경우 대기한다
     * 2) 다른 WriteLock이 걸린 경우에도 대기한다.
     * @return
     */
    public int increment() {
      lock.writeLock().lock();
      try {
        ++value;
        return value;
      } finally {
        lock.writeLock().unlock();
      }
    }
    
    /**
     * ReadLock은 
     * 1) WriteLock이 걸린 경우 대기한다.
     * 2) 다른 ReadLock에는 영향을 받지 않는다.
     * 
     * @return
     */
    public int current() {
      lock.readLock().lock();
      try{
        return value;
      } finally {
        lock.readLock().unlock();
      }
    }
  }
}