所谓加锁，即对一些公用的资源操作的时候进行锁定，操作完之后释放，写个小例子：

package com.tycyjd.action;

import java.util.HashMap;
import java.util.Map;

public class MyTest {
	private static Map<String, Object> map = new HashMap<String, Object>();
	private static Map<String, Object> preMap = new HashMap<String, Object>();

	// 加锁机制
	public static synchronized boolean setCartMap(Map<String, Object> checkMap) {
		preMap = map;
		System.out.println("oldMap:" + map.toString());
		for (Map.Entry<String, Object> entry : checkMap.entrySet()) {
			if (null != map.get(entry.getKey())) {
				map = preMap;
				System.out.println("newMap:" + map.toString());
				return false;
			} else {
				map.put(entry.getKey(), entry.getValue());
			}
		}
		System.out.println("newMap:" + map.toString());
		// map.clear();
		return true;
	}
}