摘要：攔截器注冊配置攔截器注冊配置通過可以發(fā)現(xiàn)中的的清理工作沒有得到執(zhí)行。只會在出現(xiàn)異常，返回或正常執(zhí)行結(jié)束才會從索引依次往前執(zhí)行。

最近在進行壓測發(fā)現(xiàn)，有一些接口時好時壞，通過sentry日志平臺及sky walking平臺跟蹤發(fā)現(xiàn)，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(jù)請求上下文獲取token, 然后恢復(fù)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(diào)用UserContext.set恢復(fù)用戶登錄上下文，并在請求結(jié)束時調(diào)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(fā)現(xiàn)UserContext中的ThreadLocal的清理工作沒有得到執(zhí)行。導(dǎo)致請求進來時，有可能ThreadLocal已存在了，就不會再根據(jù)請求上下文恢復(fù)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(fā)，然后執(zhí)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(dāng)執(zhí)行到preHandle返回false時，它就會從上一個返回true的handler依次往前執(zhí)行afterCompletion，它自己的afterCompletion得不到執(zhí)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(xiàn)異常，（2）preHandle返回false 或（3）正常執(zhí)行結(jié)束才會從索引interceptorIndex依次往前執(zhí)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(zhí)行的。所以一些必要的清理工作得不到執(zhí)行，會出現(xiàn)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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