springboot系列--web相关知识探索二

映射

指的是与请求处理方法关联的URL路径，通过在Spring MVC的控制器类（使用@RestController注解修饰的类）上使用注解（如 @RequestMapping、@GetMapping）来指定请求映射路径，可以将不同的HTTP请求映射到相应的处理方法上。说白了就是将具体的请求映射到具体的接口当中。

二、springboot对Rest风格的支持

springboot默认情况下是支持Rest风格，但是唯独对表单类型的请求例外。表单情况下提交请求，无论你的method是put还是delete都会变成get请求。如果想要在表单类型下使用rest风格请求方式，需要把表单method属性设置为post，隐藏域设置为_method=put,这个时候请求才会进入到put类型的接口当中。

隐藏域：<input type="hidden" name="_method" value="delete">

原理：

表单提交以后，会被springboot中的OrderedHiddenHttpMethodFilter组件拦截，当然前提是开启了这个组件，不然表单也无法使用rest风格请求。OrderedHiddenHttpMethodFilter是继承HiddenHttpMethodFilter，请求会打到这个类的doFilterInternal方法上。该方法原理如下：

spring:

mvc:

hiddenmethod:

filter:

enabled: true #开启页面表单的Rest功能

java 复制代码

    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        HttpServletRequest requestToUse = request;
        // 只有post请求才会打到这里，所以表单请求必须是post
        if ("POST".equals(request.getMethod()) && request.getAttribute("javax.servlet.error.exception") == null) {
            // 请求需要带this.methodParam对应的是_method,paramValue 对应的是具体的请求类型
            String paramValue = request.getParameter(this.methodParam);
            // 不为空的时候才进去处理
            if (StringUtils.hasLength(paramValue)) {
                String method = paramValue.toUpperCase(Locale.ENGLISH);
                // ALLOWED_METHODS 包含delete、put、patch等类型,也就是说隐藏域所带的值必须是这几个
                if (ALLOWED_METHODS.contains(method)) {
                    // 这里就是对原始请求进行包装，将隐藏域的值作为新的请求类型。包装了原生请求的HttpMethodRequestWrapper类重写了getMethod方法，返回的是传入的method
                    requestToUse = new HttpMethodRequestWrapper(request, method);
                }
            }
        }

        filterChain.doFilter((ServletRequest)requestToUse, response);
    }




static {
        ALLOWED_METHODS = Collections.unmodifiableList(Arrays.asList(HttpMethod.PUT.name(), HttpMethod.DELETE.name(), HttpMethod.PATCH.name()));
    }

java 复制代码

// 包装了原生请求的类    
private static class HttpMethodRequestWrapper extends HttpServletRequestWrapper {
        private final String method;

        // 将外部的method重新赋值进来
        public HttpMethodRequestWrapper(HttpServletRequest request, String method) {
            super(request);
            this.method = method;
        }
        
        // 重写了getMethod，返回的是外部赋予的值
        public String getMethod() {
            return this.method;
        }
    }

表单提交指定了隐藏域需要带_method为key，值为put、patch、delete等参数，才能修改成符合rest风格请求。这是由于key="_method",是在底层源码写死了的。

java 复制代码

public class HiddenHttpMethodFilter extends OncePerRequestFilter {
    private static final List<String> ALLOWED_METHODS;
    public static final String DEFAULT_METHOD_PARAM = "_method";
    // 此处写死了隐藏域key必须为_method
    private String methodParam = "_method";

    public HiddenHttpMethodFilter() {
    }

    public void setMethodParam(String methodParam) {
        Assert.hasText(methodParam, "'methodParam' must not be empty");
        this.methodParam = methodParam;
    }

    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        HttpServletRequest requestToUse = request;
        if ("POST".equals(request.getMethod()) && request.getAttribute("javax.servlet.error.exception") == null) {
            String paramValue = request.getParameter(this.methodParam);
            if (StringUtils.hasLength(paramValue)) {
                String method = paramValue.toUpperCase(Locale.ENGLISH);
                if (ALLOWED_METHODS.contains(method)) {
                    requestToUse = new HttpMethodRequestWrapper(request, method);
                }
            }
        }

        filterChain.doFilter((ServletRequest)requestToUse, response);
    }

    static {
        ALLOWED_METHODS = Collections.unmodifiableList(Arrays.asList(HttpMethod.PUT.name(), HttpMethod.DELETE.name(), HttpMethod.PATCH.name()));
    }

    private static class HttpMethodRequestWrapper extends HttpServletRequestWrapper {
        private final String method;

        public HttpMethodRequestWrapper(HttpServletRequest request, String method) {
            super(request);
            this.method = method;
        }

        public String getMethod() {
            return this.method;
        }
    }
}

如果想要修改，可以重写这个组件。

java 复制代码

//自定义filter
    @Bean
    public HiddenHttpMethodFilter hiddenHttpMethodFilter(){
        HiddenHttpMethodFilter methodFilter = new HiddenHttpMethodFilter();
        // 这个时候，key就必须等于_test了
        methodFilter.setMethodParam("_test");
        return methodFilter;
    }

二、请求映射原理

springmvc中所有的请求都是会先经过DispatcherServlet（前端控制器）这个类，这个类的其中一个父类HttpServlet，这个类有几个方法，doGet、doPost、doPut、doDelete，就是最原始处理get、put等请求的方法，HttpServlet是一个抽象类，这些方法是在FrameworkServlet中实现的。

里面每一个doGet、doPost等方法都调用了processRequest这个方法。

FrameworkServlet类中的这个方法是一个抽象方法，
复制代码
protected abstract void doService(HttpServletRequest var1, HttpServletResponse var2) throws Exception;
所以需要看它的子类DispatcherServlet里面的doService方法。这个方法调用了doDispatch这个方法，每个请求进来都是调用到它。

java 复制代码

    protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        this.logRequest(request);
        Map<String, Object> attributesSnapshot = null;
        if (WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap();
            Enumeration<?> attrNames = request.getAttributeNames();

            label104:
            while(true) {
                String attrName;
                do {
                    if (!attrNames.hasMoreElements()) {
                        break label104;
                    }

                    attrName = (String)attrNames.nextElement();
                } while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet"));

                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }

        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.getWebApplicationContext());
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.getThemeSource());
        if (this.flashMapManager != null) {
            FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
            if (inputFlashMap != null) {
                request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
            }

            request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
            request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
        }

        RequestPath previousRequestPath = null;
        if (this.parseRequestPath) {
            previousRequestPath = (RequestPath)request.getAttribute(ServletRequestPathUtils.PATH_ATTRIBUTE);
            ServletRequestPathUtils.parseAndCache(request);
        }

        try {
            // 这行代码之前，其实大部分逻辑都是初始化设置各种值，doDispatch就是做各种转发
            this.doDispatch(request, response);
        } finally {
            if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) {
                this.restoreAttributesAfterInclude(request, attributesSnapshot);
            }

            ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request);
        }

    }

下面是doDispatch源码以及一些与原理

复制代码

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;
		boolean multipartRequestParsed = false;

		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

		try {
			ModelAndView mv = null;
			Exception dispatchException = null;

			try {
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);

				// 找到当前请求使用哪个Handler（Controller的方法）处理
				mappedHandler = getHandler(processedRequest);
                
                //HandlerMapping：处理器映射。

 @Nullable
    protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        // 循环遍历每个处理器映射器
        if (this.handlerMappings != null) {
            Iterator var2 = this.handlerMappings.iterator();

            while(var2.hasNext()) {
                // 处理请求的处理器映射器RequestMappingHandlerMapping
                HandlerMapping mapping = (HandlerMapping)var2.next();
                // 通过url获取到具体的处理器映射器
                HandlerExecutionChain handler = mapping.getHandler(request);
                if (handler != null) {
                    return handler;
                }
            }
        }

        return null;
    }

1、this.handlerMappings != null中handlerMappings就是springboot所有的处理器映射器。

2、其中RequestMappingHandlerMapping：保存了所有@RequestMapping 和handler的映射规则。其实就是保存了@RequestMapping中配置的各种url和对应的controller

3、所有的请求映射都在HandlerMapping中。然后通过HandlerMapping找到请求url对应的具体的controller。

4、SpringBoot自动配置欢迎页的 WelcomePageHandlerMapping 。访问 /能访问到index.html；

springboot底层通过url获取到具体的controller原理,需要从

HandlerExecutionChain handler = mapping.getHandler(request);

这个方法进入

复制代码

    protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
         // 获取@GetMapping("/test1")上的请求路径/test1
        String lookupPath = this.initLookupPath(request);
        //mappingRegistry保存了所有的请求路径对应的controller信息，加锁是防止并发获取
        this.mappingRegistry.acquireReadLock();

        HandlerMethod var4;
        try {
            // 根据url和请求信息，找到controller中对应能够处理的方法，其实就是具体的那个接口的信息
            HandlerMethod handlerMethod = this.lookupHandlerMethod(lookupPath, request);
            var4 = handlerMethod != null ? handlerMethod.createWithResolvedBean() : null;
        } finally {
            this.mappingRegistry.releaseReadLock();
        }

        return var4;
    }





    // 这个方法主要是获取url请求路径
    protected String initLookupPath(HttpServletRequest request) {
        if (this.usesPathPatterns()) {
            request.removeAttribute(UrlPathHelper.PATH_ATTRIBUTE);
            RequestPath requestPath = ServletRequestPathUtils.getParsedRequestPath(request);
            String lookupPath = requestPath.pathWithinApplication().value();
            return UrlPathHelper.defaultInstance.removeSemicolonContent(lookupPath);
        } else {
            // 请求进来是在这里处理的
            return this.getUrlPathHelper().resolveAndCacheLookupPath(request);
        }
    }






    /**
    
    String lookupPath:注解上的请求路径
    HttpServletRequest request：具体请求
    **/
    @Nullable
    protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {

        List<AbstractHandlerMethodMapping<T>.Match> matches = new ArrayList();
        // 通过url找到多个个能够处理的接口或者说方法。比如说/test1可能会有put请求，也有get请求。post请求等等
        List<T> directPathMatches = this.mappingRegistry.getMappingsByDirectPath(lookupPath);
        if (directPathMatches != null) {
        // 找到了多个接口，就是添加到这个方法里，这个方法会根据你请求进来的方式（post、get或put等等）匹配到最优的处理方法，也就是精确找到你这个进来的请求方式所对应的处理接口。并且把匹配到的放到第一位且一般只有一个，具体不展开叙述
            this.addMatchingMappings(directPathMatches, matches, request);
        }

        if (matches.isEmpty()) {
            this.addMatchingMappings(this.mappingRegistry.getRegistrations().keySet(), matches, request);
        }

        if (matches.isEmpty()) {
            return this.handleNoMatch(this.mappingRegistry.getRegistrations().keySet(), lookupPath, request);
        } else {
               // 这里就是从上面保存符合条件的集合中获取第一个，一般只有一个，如果有两个，例如/test1 get请求，写了两个方法且请求方式，路径都一样，就matches.size() > 1就为true，然后会进入里面的处理逻辑并报错
            AbstractHandlerMethodMapping<T>.Match bestMatch = (Match)matches.get(0);
           
            // 就是这里
             if (matches.size() > 1) {
                Comparator<AbstractHandlerMethodMapping<T>.Match> comparator = new MatchComparator(this.getMappingComparator(request));
                matches.sort(comparator);
                bestMatch = (Match)matches.get(0);
                if (this.logger.isTraceEnabled()) {
                    this.logger.trace(matches.size() + " matching mappings: " + matches);
                }

                if (CorsUtils.isPreFlightRequest(request)) {
                    Iterator var7 = matches.iterator();

                    while(var7.hasNext()) {
                        AbstractHandlerMethodMapping<T>.Match match = (Match)var7.next();
                        if (match.hasCorsConfig()) {
                            return PREFLIGHT_AMBIGUOUS_MATCH;
                        }
                    }
                } else {
                    AbstractHandlerMethodMapping<T>.Match secondBestMatch = (Match)matches.get(1);
                    if (comparator.compare(bestMatch, secondBestMatch) == 0) {
                        Method m1 = bestMatch.getHandlerMethod().getMethod();
                        Method m2 = secondBestMatch.getHandlerMethod().getMethod();
                        String uri = request.getRequestURI();
                        // 有两个相同的url且请求方式一致的处理方法
                        throw new IllegalStateException("Ambiguous handler methods mapped for '" + uri + "': {" + m1 + ", " + m2 + "}");
                    }
                }
            }

            request.setAttribute(BEST_MATCHING_HANDLER_ATTRIBUTE, bestMatch.getHandlerMethod());
            this.handleMatch(bestMatch.mapping, lookupPath, request);
            // 这里就是获取到具体的处理方法，也就是接口对应的方法，例如/test1 对应的是com.example.estest.controller.MvcTestController#test()这个接口，会返回com.example.estest.controller.MvcTestController#test()这个东西的具体信息
            return bestMatch.getHandlerMethod();
        }
    }

1、另外SpringBoot自动配置了默认配置了一个处理器映射器就是 RequestMappingHandlerMapping用于处理@GetMapping、@PusMapping、@RequestMapping等注解的映射

2、如果需要一些自定义的映射处理，我们也可以自己给容器中放HandlerMapping 。自定义 HandlerMapping。可以参考WebMvcAutoConfiguration类下的requestMappingHandlerMapping组件定义。

复制代码

        @Bean
        @Primary
        public RequestMappingHandlerMapping requestMappingHandlerMapping(@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager, @Qualifier("mvcConversionService") FormattingConversionService conversionService, @Qualifier("mvcResourceUrlProvider") ResourceUrlProvider resourceUrlProvider) {
            return super.requestMappingHandlerMapping(contentNegotiationManager, conversionService, resourceUrlProvider);
        }