threadlocal用法及原理

最新推荐文章于 2025-07-11 18:39:42 发布

gokeiryou263

最新推荐文章于 2025-07-11 18:39:42 发布

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分类专栏： javaWeb技术

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本文介绍了一个JavaWeb应用中如何使用ThreadLocal实现跨请求的会话管理。通过UserSessionFilter过滤器，文章详细展示了如何在HTTP会话(HttpSession)与线程局部变量(ThreadLocal)之间传递用户会话数据。

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用一个简单的java web实例来说明：

UserSessionFilter.java 过滤器

HttpSession httpsession = ((HttpServletRequest)req).getSession();

private static ThreadLocal<HttpSession> session = new ThreadLocal<HttpSession>();

UserSessionFilter.session.set(httpsession); >>> 本类中的session初始值为HttpSession

Object userSession = httpsession.getAttribute("user"); >>> 从HttpSession中取得"user"的值

ThreadLocalUserSession.set(userSession); >>> 将从HttpSession中取得"user"的值设置到ThreadLocal中

Map<String, Object> map = (Map<String, Object>)session.getAttribute("vars"); 取HttpSession中的"vars"并设置到ThreadLocalUserSession类中
ThreadLocalUserSession.setVars(map);

UserSessionUtil.java 工具类

设置session：

UserSessionFilter.getHttpSession().setAttribute("user", userSession); >>> 本质是往HttpSession中设置

ThreadLocalUserSession.set(userSession); >>> 本质是往ThreadLocal中设置

UserSessionFilter.getHttpSession().setAttribute("vars", vars); >>> 本质是往HttpSession中设置
ThreadLocalUserSession.setVars(vars); >>> 本质是往ThreadLocal中设置

取值session：

ThreadLocal.get(); >>> 本质是从ThreadLocal中取值

====================================

web.xml:

<filter>
       <filter-name>UserSessionFilter</filter-name>
       <filter-class>com.licaike.framework.server.filter.UserSessionFilter</filter-class>
</filter>
<filter-mapping>
      <filter-name>UserSessionFilter</filter-name>
      <url-pattern>/*</url-pattern>
</filter-mapping>

UserSessionFilter.java:

public class UserSessionFilter implements Filter{
   private static ThreadLocal<HttpSession> session = new ThreadLocal<HttpSession>();

   public void destroy() {
   }

   public static HttpSession getHttpSession(){
       return UserSessionFilter.session.get();
   }

   public void doFilter(ServletRequest req, ServletResponse res, FilterChain filterChain) throws IOException, ServletException {
       // 仅仅是记录下WEB session
       HttpSession session = ((HttpServletRequest)req).getSession();
       UserSessionFilter.session.set(session);

        //设置用户身份信息
       Object userSession = session.getAttribute("user");
       ThreadLocalUserSession.set(userSession);

       // 设置session变量
       Map<String, Object> map = (Map<String, Object>)session.getAttribute("vars");
       if(map == null){
           map = new HashMap<String,Object>();
       }
       ThreadLocalUserSession.setVars(map);
       filterChain.doFilter(req, res);
   }

   public void init(FilterConfig arg0) throws ServletException {
   }

}

UserSessionUtil.java:

public class UserSessionUtil {
   public static UserSession get(){
       return (UserSession)ThreadLocalUserSession.get();
   }

   //调用此方法真正写入到session
   public static void set(UserSession userSession){
       UserSessionFilter.getHttpSession().setAttribute("user", userSession); //过滤器里设到本地线程变量里
       ThreadLocalUserSession.set(userSession); //此处设到本地线程变量里
   }

   public static Map<String,Object> getVars(){
       return ThreadLocalUserSession.getVars();
   }

   public static void setVars(Map<String,Object> vars){
       UserSessionFilter.getHttpSession().setAttribute("vars", vars); //过滤器里设到本地线程变量里
       ThreadLocalUserSession.setVars(vars); //此处设到本地线程变量里
   }

   public static void initSession(){
       //初始化userSession
       if(get()==null) set(new UserSession());
       //初始化其他
       if(getVars()==null) setVars(new HashMap<String,Object>());
   }
}

ThreadLocalUserSession.java:

public class ThreadLocalUserSession implements Serializable{
   private static ThreadLocal<Object> threadLocalUserSession = new ThreadLocal<Object>() ;
   private static ThreadLocal<Map<String,Object>> vars = new ThreadLocal<Map<String,Object>>() ;

   public static void set(Object object){
       threadLocalUserSession.set(object);
   }
   public static Object get(){
       Object object = threadLocalUserSession.get();
       return object;
   }

   public static void setVars(Map<String,Object> vars){
       ThreadLocalUserSession.vars.set(vars);
   }
   public static Map<String,Object> getVars(){
       Map<String,Object> map = vars.get();
       return map;
   }

   private static final long serialVersionUID = 4343534534354L;
}

====================================

关于Theadlocal的说明：

进入Theadlocal代码，可以发现如下的片段：

public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }

这说明ThreadLocal确实只有一个变量，但是它内部包含一个map，针对每个thread保留一个entry。