摘要：攔截器注冊配置攔截器注冊配置通過可以發現中的的清理工作沒有得到執行。只會在出現異常，返回或正常執行結束才會從索引依次往前執行。

最近在進行壓測發現，有一些接口時好時壞，通過sentry日志平臺及sky walking平臺跟蹤發現，用戶張三獲取到的用戶上下文確是李四。

代碼走讀

用戶登錄下上文

/**
 * 用戶登錄下上文
 *
 * @author : jamesfu
 * @date : 22/5/2019
 * @time : 9:18 AM
 */
@Data
public class UserContext {
    private final static ThreadLocal threadLocal = new ThreadLocal<>();

    private Long id;

    private String loginName;

    public static UserContext get() {
        UserContext context = threadLocal.get();
        if (context == null) {
            // TODO(james.h.fu):根據請求上下文獲取token, 然后恢復用戶登錄下上文
            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);
        }
    }
}

在攔截器中有調用UserContext.set恢復用戶登錄上下文，并在請求結束時調用UserContext.clear清理用戶登錄上下文。

攔截器注冊配置

/**
 * 攔截器注冊配置
 *
 * @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");
    }
}

通過debug可以發現UserContext中的ThreadLocal的清理工作沒有得到執行。導致請求進來時，有可能ThreadLocal已存在了，就不會再根據請求上下文恢復了。

springmvc 源碼走讀

tomcat 在收到http請求后，最終會交由spring mvc的DispatcherServlet處理。 這里可以從doDispatch按圖索驥，順藤摸瓜地往下看起走。

源碼走讀：DispatcherServlet

/**
	 * Process the actual dispatching to the handler.
	 * 
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.
	 * 
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

請求會得到分發，然后執行各個已注冊Handler的preHandle-->postHandle-->afterCompletion。

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;
	}

當執行到preHandle返回false時，它就會從上一個返回true的handler依次往前執行afterCompletion，它自己的afterCompletion得不到執行。

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);
				}
			}
		}
	}

triggerAfterCompletion只會在（1）出現異常，（2）preHandle返回false 或（3）正常執行結束才會從索引interceptorIndex依次往前執行。

所以基于以上源碼可以得知，在寫攔截器時preHandle返回false時，afterCompletion是不會執行的。所以一些必要的清理工作得不到執行，會出現類似我們遇到的帳號串的問題。

參考資料

Spring 攔截器——HandlerInterceptor

SpringMVC HandlerInterceptor詭異問題排查

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