SpringMVC HandlerInterceptor诡异问题排查

布满荆棘的人生 2022-01-20 03:25 423阅读 0赞

### 发现问题 ###

最近在进行压测发现，有一些接口时好时坏，通过sentry日志平台及sky walking平台跟踪发现，用户张三获取到的用户上下文确是李四。

#### 代码走读 ####

用户登录下上文

/**
     * 用户登录下上文
     *
     * @author : jamesfu
     * @date : 22/5/2019
     * @time : 9:18 AM
     */
    @Data
    public class UserContext {
        private final static ThreadLocal<UserContext> threadLocal = new ThreadLocal<>();
    
        private Long id;
    
        private String loginName;
    
        public static UserContext get() {
            UserContext context = threadLocal.get();
            if (context == null) {
                // TODO(james.h.fu):根据请求上下文获取token, 然后恢复用户登录下上文
                context = new UserContext() {
        {
                    setId(1L);
                    setLoginName("james.h.fu1");
                }};
                threadLocal.set(context);
            }
    
            return context;
        }
    
        public static void clear() {
            threadLocal.remove();
        }
    
        public static void set(UserContext context) {
            if (context != null) {
                threadLocal.set(context);
            }
        }
    }
    复制代码

在拦截器中有调用UserContext.set恢复用户登录上下文，并在请求结束时调用UserContext.clear清理用户登录上下文。

拦截器注册配置

/**
     * 拦截器注册配置
     *
     * @author : jamesfu
     * @date : 22/5/2019
     * @time : 9:15 AM
     */
    @Configuration
    public class FilterConfig implements WebMvcConfigurer {
        @Autowired
        private JsonRpcInterceptor jsonRpcInterceptor;
    
        @Override
        public void addInterceptors(InterceptorRegistry registry) {
            registry.addInterceptor(jsonRpcInterceptor)
                    .addPathPatterns("/json.rpc");
        }
    }
    复制代码

通过debug可以发现UserContext中的ThreadLocal的清理工作没有得到执行。导致请求进来时，有可能ThreadLocal已存在了，就不会再根据请求上下文恢复了。

#### springmvc 源码走读 ####

tomcat 在收到http请求后，最终会交由spring mvc的`DispatcherServlet`处理。 这里可以从doDispatch按图索骥，顺藤摸瓜地往下看起走。

#### 源码走读：DispatcherServlet ####

/**
    	 * Process the actual dispatching to the handler.
    	 * <p>The handler will be obtained by applying the servlet's HandlerMappings in order. * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
    	 * to find the first that supports the handler class.
    	 * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers * themselves to decide which methods are acceptable. * @param request current HTTP request * @param response current HTTP response * @throws Exception in case of any kind of processing failure */ protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception 复制代码

请求会得到分发，然后执行各个已注册Handler的preHandle-->postHandle-->afterCompletion。

### 源码走读：HandlerExecutionChain ###

##### applyPreHandle #####

/**
    	 * Apply preHandle methods of registered interceptors.
    	 * @return {@code true} if the execution chain should proceed with the
    	 * next interceptor or the handler itself. Else, DispatcherServlet assumes
    	 * that this interceptor has already dealt with the response itself.
    	 */
    	boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
    		HandlerInterceptor[] interceptors = getInterceptors();
    		if (!ObjectUtils.isEmpty(interceptors)) {
    			for (int i = 0; i < interceptors.length; i++) {
    				HandlerInterceptor interceptor = interceptors[i];
    				if (!interceptor.preHandle(request, response, this.handler)) {
    					triggerAfterCompletion(request, response, null);
    					return false;
    				}
    				this.interceptorIndex = i;
    			}
    		}
    		return true;
    	}
    复制代码

当执行到preHandle返回false时，它就会从上一个返回true的handler依次往前执行afterCompletion，它自己的afterCompletion得不到执行。

##### triggerAfterCompletion #####

/**
    	 * Trigger afterCompletion callbacks on the mapped HandlerInterceptors.
    	 * Will just invoke afterCompletion for all interceptors whose preHandle invocation
    	 * has successfully completed and returned true.
    	 */
    	void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex)
    			throws Exception {
    
    		HandlerInterceptor[] interceptors = getInterceptors();
    		if (!ObjectUtils.isEmpty(interceptors)) {
    			for (int i = this.interceptorIndex; i >= 0; i--) {
    				HandlerInterceptor interceptor = interceptors[i];
    				try {
    					interceptor.afterCompletion(request, response, this.handler, ex);
    				}
    				catch (Throwable ex2) {
    					logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
    				}
    			}
    		}
    	}
    复制代码

triggerAfterCompletion只会在（1）出现异常，（2）preHandle返回false 或（3）正常执行结束才会从索引interceptorIndex依次往前执行。

所以基于以上源码可以得知，在写拦截器时preHandle返回false时，afterCompletion是不会执行的。所以一些必要的清理工作得不到执行，会出现类似我们遇到的帐号串的问题。

#### 参考资料 ####

*  [Spring 拦截器——HandlerInterceptor][Spring _HandlerInterceptor]
 *  [SpringMVC HandlerInterceptor诡异问题排查][SpringMVC HandlerInterceptor]

#### 关注公众号交流学习 ####

[Spring _HandlerInterceptor]: https://link.juejin.im?target=https%3A%2F%2Fwww.jianshu.com%2Fp%2Fdc5cc2e25ab2
[SpringMVC HandlerInterceptor]: https://link.juejin.im?target=https%3A%2F%2Fwww.jianshu.com%2Fp%2Fccad5a5fcbaa