synchronized 用法，实例讲解

package com.asiainfolinkage.ems.web.controller.base;

import java.math.BigInteger;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;

/**
 * 附带token的控制器
 * @version 1.0 Copyright 2013年7月29日 下午3:38:30
 */
public abstract class TokenBaseController extends BaseController {

	private static Map<String, String> springmvc_token = new HashMap<String, String>();

	/** 生成一个唯一值的token */
	public String generateGUID(HttpSession session) {
		String token = "";
		Date date = new Date();
		synchronized (springmvc_token) {
			try {
				Object obj = session.getAttribute(Constants.SPRING_MVC_TOKENNAME);
				if (obj != null)
					springmvc_token = (Map<String, String>) session.getAttribute(Constants.SPRING_MVC_TOKENNAME);
				token = new BigInteger(165, new Random()).toString(36).toUpperCase();
				springmvc_token.put(Constants.DEFAULT_TOKEN_NAME + "." + token, token);
				session.setAttribute(Constants.SPRING_MVC_TOKENNAME, springmvc_token);
				Constants.TOKEN_VALUE = token;
			} catch (IllegalStateException e) {
				_log.error("generateGUID() mothod find bug,by token session...");
			}
		}
		return token;
	}

	/** 验证表单token值和session中的token值是否一致 */
	public boolean validToken(HttpServletRequest request) {
		String inputToken = getInputToken(request);

		if (inputToken == null) {
			_log.warn("token is not valid!inputToken is NULL");
			return false;
		}
		HttpSession session = request.getSession();
		Map<String, String> tokenMap = (Map<String, String>) session.getAttribute(Constants.SPRING_MVC_TOKENNAME);
		if (tokenMap == null || tokenMap.size() < 1) {
			_log.warn("token is not valid!sessionToken is NULL");
			return false;
		}
		String sessionToken = tokenMap.get(Constants.DEFAULT_TOKEN_NAME + "." + inputToken);
		if (!inputToken.equals(sessionToken)) {
			_log.warn("token is not valid!inputToken='" + inputToken + "',sessionToken = '" + sessionToken + "'");
			return false;
		}
		tokenMap.remove(Constants.DEFAULT_TOKEN_NAME + "." + inputToken);
		session.setAttribute(Constants.SPRING_MVC_TOKENNAME, tokenMap);

		return true;
	}

	/** 获取表单中token值 */
	private String getInputToken(HttpServletRequest request) {
		Map params = request.getParameterMap();
		if (!params.containsKey(Constants.DEFAULT_TOKEN_NAME)) {
			_log.warn("Could not find token name in params.");
			return null;
		}
		String[] tokens = (String[]) (String[]) params.get(Constants.DEFAULT_TOKEN_NAME);
		if ((tokens == null) || (tokens.length < 1)) {
			_log.warn("Got a null or empty token name.");
			return null;
		}
		return tokens[0];
	}
}