Read-write lock for multi-threaded concurrent threads

/*
Read-write lock, read-read is not mutually exclusive, read-write is mutually exclusive, write-write is mutually exclusive
 Mutually exclusive in the same class */

import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
 publicclass ReadWriteLockTest {
   publicstaticvoid main(String[] args) {
      final Queue3 q3 = new Queue3();
      for (inti = 0; i <3; i ++)
      {
        new Thread(){
           publicvoid run(){
              while(true){
                 q3.get();            
              }
           }
        }.start();
                      
        new Thread(){
           publicvoid run(){
              while(true){
                 q3.put(new Random().nextInt(10000));
              }
           }          
        }.start();
      }
   }
}
 
class Queue3{
   private Object data = null;//Shared data, only one thread can write the data, but multiple threads can read the data at the same time.
   ReadWriteLock rwl = new ReentrantReadWriteLock();
  
   publicvoid get(){
          rwl.readLock().lock();
      try{
        System.out.println(Thread.currentThread().getName() + " be ready to read data!");
        Thread.sleep((long)(Math.random()*1000));
        System.out.println(Thread.currentThread().getName() + "have read data :" + data);      
      }catch(InterruptedException e){
        e.printStackTrace ();
      }finally{
        rwl.readLock().unlock();
      }
   }
  
   publicvoid put(Object data){
      rwl.writeLock().lock();
      try{
        System.out.println(Thread.currentThread().getName() + " be ready to write data!");            
        Thread.sleep((long)(Math.random()*1000));
        this.data = data;
        System.out.println(Thread.currentThread().getName() + " have write data: " + data);           
      }catch (InterruptedException e){
        e.printStackTrace ();
      }finally{
        rwl.writeLock().unlock();
      }
   }
}



read-write lock cache
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class CacheDemo {
   private Map<String, Object> cache = new HashMap<String, Object>();
   public static void main(String[] args) {
 
   }
   private ReadWriteLock rwl = new ReentrantReadWriteLock();
   public  Object getData(String key){
      rwl.readLock().lock();
      Object value = null;
      try{
        value = cache.get(key);
        if(value == null){
           rwl.readLock( ).unlock();
           rwl.writeLock().lock();
           try{
              if(value==null){
                 value = "aaaa";//Actually lost queryDB();
              }
           }finally{
              rwl.writeLock().unlock();
           }
           rwl.readLock().lock();
        }
      }finally{
        rwl.readLock().unlock();
      }
      return value;
   }
}