《Java架构筑基》——详解SpringMvc请求如何获取相关HandlerMapping

在Servlet如何关联spring中，说过当请求来后会被转发到springDispatcherServlet类中的doService方法。

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
   doDispatch(request, response);
}

转发到doDispatch中

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    mappedHandler = getHandler(processedRequest);
}

getHandler获取相关

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    if (this.handlerMappings != null) {
        for (MyHandlerMapping hm : this.handlerMappings) {
            if (logger.isTraceEnabled()) {
                logger.trace(
                        "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
            }
            HandlerExecutionChain handler = hm.getHandler(request);
            if (handler != null) {
                return handler;
            }
        }
    }
    return null;
}

可以看出，循环遍历初始化时加载的handlerMappings。在初始化时加载是BeanNameUrlHandlerMapping和RequestMappingHandlerMapping(不知道的，可以参考上篇文章spingMvc初始HandlerMapping)。 BeanNameUrlHandlerMapping不满条件返回null，而RequestMappingHandlerMapping满足条件，通过getHandler返回我们想要的HandlerExecutionChain。

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    Object handler = getHandlerInternal(request);

    HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
    return executionChain;
}

getHandlerExecutionChain可见是对handler和request的封装，真正的处理还是如何获取到这个handler。具体代码写在了AbstractHandlerMethodMapping中

protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
    String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);

    this.mappingRegistry.acquireReadLock();
    try {
        HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
        }
        return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
    }
    finally {
        this.mappingRegistry.releaseReadLock();
    }
}

在前文分析中，handlerMethod在初始化的时候放在了，mappingRegistry中，由上述代码，可以看出是从mappingRegistry中，匹配请求，并返回handlerMethod。那我们看下lookupHandlerMethod方法吧。

protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
    List<Match> matches = new ArrayList<>();
    List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);
    if (directPathMatches != null) {
        addMatchingMappings(directPathMatches, matches, request);
    }
    if (matches.isEmpty()) {
        // No choice but to go through all mappings...
        addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);
    }

    if (!matches.isEmpty()) {
        Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
        matches.sort(comparator);
        if (logger.isTraceEnabled()) {
            logger.trace("Found " + matches.size() + " matching mapping(s) for [" + lookupPath + "] : " + matches);
        }
        Match bestMatch = matches.get(0);
        if (matches.size() > 1) {
            if (CorsUtils.isPreFlightRequest(request)) {
                return PREFLIGHT_AMBIGUOUS_MATCH;
            }
            Match secondBestMatch = matches.get(1);
            if (comparator.compare(bestMatch, secondBestMatch) == 0) {
                Method m1 = bestMatch.handlerMethod.getMethod();
                Method m2 = secondBestMatch.handlerMethod.getMethod();
                throw new IllegalStateException("Ambiguous handler methods mapped for HTTP path '" +
                        request.getRequestURL() + "': {" + m1 + ", " + m2 + "}");
            }
        }
        handleMatch(bestMatch.mapping, lookupPath, request);
        return bestMatch.handlerMethod;
    }
    else {
        return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);
    }
}

果不其然是对我们的请求路径进行最佳匹配。这样我们就回去到初始化handlerMapping时生成的handlerMethod，为后续handlerAdapter做好参数准备。