java简单实现缓存的三种方法

package com.tb.test;

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

public class CachedTest {
	// 第一种方法 用hashmap存 然后加锁get方法

	// private HashMap<String, Object> cached = new HashMap<String, Object>();
	//
	// public synchronized Object get(String user){
	//	Object value=cached.get(user);
	// if( value== null){
	//
	//  value ="sss"; //实际上是从数据库中读取
	// cached.put(user, value);
	// }
	//
	// return value;
	// }

	// 第二种方法是用ConcurrentHashMap
	// private ConcurrentHashMap<String, Object> cached= new
	// ConcurrentHashMap<String, Object>();
	//
	// public Object get(String user){
	//	Object value=cached.get(user);
	// if(value == null){
	//
	//  value ="sss"; //实际上是从数据库中读取
	// cached.put(user, value);
	// }
	//
	// return value;
	//
	//
	// }

	// 第三种 用hashmap 加读写锁
	private HashMap<String, Object> cached = new HashMap<String, Object>();

	private ReadWriteLock readWriteLock = new ReentrantReadWriteLock();

	public Object get(String user) {
		
		readWriteLock.readLock().lock();
		Object value = null;
		try {
			value = cached.get(user);
			if ( value == null) {
				readWriteLock.readLock().unlock();
				readWriteLock.writeLock().lock();
				try {
					if(value ==  null){
					 value = "aa"; // 实际上从数据库读取
					 cached.put(user, value);
					}
				} finally {
					readWriteLock.writeLock().unlock();
				}
				
				readWriteLock.readLock().lock();
				
			}
			
		} finally {
			readWriteLock.readLock().unlock();
		}
		return value;
	}
}