ThreadLocal原理与运用

ThreadLocal原理与应用
    最近在维护一个老（很老）系统，本想重构，但是目前没这个时间，基于struts1+dao，有些还是jsp+dao的方式，用户需要在敏感的操作上加比较详细的日志，日志的功能在dao根据需求手动写入，主要的问题是原有的接口没有传入request的相关信息，比如用户信息，我想到了ThreadLocal，因为每个servlet是基于多线程，但是每个request的执行是再一个独立的线程中完成，ThreadLocal正适合了我这种需求，可以用来完成我对这个系统的业务扩展。
    在比如struts2的ServletActionContext中对request的获取，还有hibernate中对sessin的管理就是基于ThreadLocal来实现。
    看了下ThreadLocal的源码，相对简单，主要的思路是在每个Thread中维护一个ThreadLocalMap,调用ThreadLocal的set(T value)的时候，以当前的ThreadLocal实例为key保存到当前线程的ThreadLocalMap中。

关键源码如下：
public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }
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();
    }

理解了上面的以后，使用起来就方便了。我的地体实现如在
1.在web.xml中配置系统每个请求都经过的过滤器
<filter>
<filter-name>AppRequestFilter</filter-name>
<filter-class>com.clifford.restaurant.AppRequestFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>AppRequestFilter</filter-name>
    <url-pattern>*.jsp</url-pattern>
</filter-mapping>
<filter-mapping>
    <filter-name>AppRequestFilter</filter-name>
    <url-pattern>*.do</url-pattern>
</filter-mapping>

2.过滤器实现
public class AppRequestFilter implements Filter{
public void destroy() {
}
public void doFilter(ServletRequest arg0, ServletResponse response,
FilterChain filterChain) throws IOException, ServletException {
HttpServletRequest request = ((HttpServletRequest)arg0);
RequestThreadLocal.setRequestThreadLocal(request);
filterChain.doFilter(request, response);
}
}



3.ThreadLocal相关实现
public class RequestThreadLocal {
private static ThreadLocal<HttpServletRequest> requestThreadLocal = new ThreadLocal<HttpServletRequest>();
public static void setRequestThreadLocal(HttpServletRequest request) {
requestThreadLocal.set(request);
}
public static User getLoginUser() {
HttpServletRequest request = requestThreadLocal.get();
Session session = request==null?null:request.getSession(false);
return session==null?null:(User)session.getAttribute(IConstants.USER_KEY);
}
public static String getRequestIp() {
HttpServletRequest request = requestThreadLocal.get();
return request==null?"":request.getRemoteHost();
}

}