Spring MVC系列（9）-HttpMessageConverter报文转换流程源码解析

短命女 2023-10-09 13:27 8阅读 0赞

### HttpMessageConverters类 ###

HttpMessageConverters是org.springframework.boot.autoconfigure.http下的一个类，是  
Spring Boot提供的管理应用程序中使用HttpMessageConverter的Bean。

提供一种方便的方法来添加和配置额外的HttpMessageConverter到 Web 应用程序。如果需要，可以使用特定的HttpMessageConverters注册此 bean 的实例，否则将使用默认转换器。

HttpMessageConverters维护了一个HttpMessageConverter集合。

private final List<HttpMessageConverter<?>> converters;

### 注入HttpMessageConverters ###

在HttpMessageConvertersAutoConfiguration类中，注入了一个默认的HttpMessageConverters。

@Bean
        @ConditionalOnMissingBean
        public HttpMessageConverters messageConverters(ObjectProvider<HttpMessageConverter<?>> converters) {
        
            return new HttpMessageConverters((Collection)converters.orderedStream().collect(Collectors.toList()));
        }

调用了HttpMessageConverters的构造方法。

public HttpMessageConverters(boolean addDefaultConverters, Collection<HttpMessageConverter<?>> converters) {
        
            List<HttpMessageConverter<?>> combined = this.getCombinedConverters(converters, addDefaultConverters ? this.getDefaultConverters() : Collections.emptyList());
            combined = this.postProcessConverters(combined);
            this.converters = Collections.unmodifiableList(combined);
        }

HttpMessageConverters构造方法最终调用到WebMvcConfigurationSupport的addDefaultHttpMessageConverters添加转换器。

protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
        
        	// 1. 添加ByteArrayHttpMessageConverter、StringHttpMessageConverter、ResourceHttpMessageConverter、ResourceRegionHttpMessageConverter
            messageConverters.add(new ByteArrayHttpMessageConverter());
            messageConverters.add(new StringHttpMessageConverter());
            messageConverters.add(new ResourceHttpMessageConverter());
            messageConverters.add(new ResourceRegionHttpMessageConverter());
            // 2. 没有忽略XML，添加SourceHttpMessageConverter。
            if (!shouldIgnoreXml) {
        
                try {
        
                    messageConverters.add(new SourceHttpMessageConverter());
                } catch (Throwable var3) {
        
                }
            }
    		
            messageConverters.add(new AllEncompassingFormHttpMessageConverter());
            // 3. 是否存在com.rometools.rome.feed.WireFeed这个类，有则添加AtomFeedHttpMessageConverter、RssChannelHttpMessageConverter
            if (romePresent) {
        
                messageConverters.add(new AtomFeedHttpMessageConverter());
                messageConverters.add(new RssChannelHttpMessageConverter());
            }
    		// 4. 没有配置忽略XML
            Jackson2ObjectMapperBuilder builder;
            if (!shouldIgnoreXml) {
        
                if (jackson2XmlPresent) {
        
                    builder = Jackson2ObjectMapperBuilder.xml();
                    if (this.applicationContext != null) {
        
                        builder.applicationContext(this.applicationContext);
                    }
    				// 5. 存在com.fasterxml.jackson.dataformat.xml.XmlMapper这个类，添加MappingJackson2XmlHttpMessageConverter解析器。
                    messageConverters.add(new MappingJackson2XmlHttpMessageConverter(builder.build()));
                } else if (jaxb2Present) {
        
                	// 6. 存在javax.xml.bind.Binder类，添加Jaxb2RootElementHttpMessageConverter
                    messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
                }
            }
    		// 7. 存在kotlinx.serialization.json.Json，添加KotlinSerializationJsonHttpMessageConverter
            if (kotlinSerializationJsonPresent) {
        
                messageConverters.add(new KotlinSerializationJsonHttpMessageConverter());
            }
    		// 8. 存在javax.xml.bind.Binder类，添加Jaxb2RootElementHttpMessageConverter
            if (jackson2Present) {
        
                builder = Jackson2ObjectMapperBuilder.json();
                if (this.applicationContext != null) {
        
                    builder.applicationContext(this.applicationContext);
                }
    			
            }
    		// ......others
        }

可以看到在addDefaultHttpMessageConverters方法中，会去classLoader中查询相关的类，有最添加相关的解析器，比如我们引入了om.google.gson.Gson包，就会添加GsonHttpMessageConverter转换器。

可以看到默认加载了10个转换器。  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16]

### 报文转换源码分析 ###

HttpMessageConverter执行报文转换的基本流程如下。![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16 1]  
接下来使用下面这个使用了@ResponseBody、@RequestBody注解的控制器方法，是如何进行报文转换的。

@PostMapping("/testRequestBody")
        @ResponseBody
        public Object testRequestBody(@RequestBody User user) {
        
            return user;
        }

#### 1. 初始化处理器适配器 ####

在之前的文档中，分析了HttpMessageConverters的初始化，了解到了有哪些HttpMessageConverter注入了IOC容器中的。

在DispatcherServlet流程分析中，我们没分析到HttpMessageConverter是如何转换的，实际是因为HttpMessageConverter不直接参与DispatcherServlet的执行流程，而是由处理器适配器HandlerAdapter来调用执行的。

在WebMvcConfigurationSupport类中，我们可以看到注入RequestMappingHandlerAdapter适配器时，加载了很多HttpMessageConverter，上面HttpMessageConverters加载也是调用的此方法。

@Bean
        public RequestMappingHandlerAdapter requestMappingHandlerAdapter(@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager, @Qualifier("mvcConversionService") FormattingConversionService conversionService, @Qualifier("mvcValidator") Validator validator) {
        
            RequestMappingHandlerAdapter adapter = this.createRequestMappingHandlerAdapter();
            adapter.setContentNegotiationManager(contentNegotiationManager);
            adapter.setMessageConverters(this.getMessageConverters());
        }

最终，适配器，获取到了这些Converters，关于适配器组件的相关分析，后续会补上。  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16 2]

#### 2. 转换请求报文 ####

发送请求，会进入到DispatcherServlet的doDispatch方法，适配器进行执行处理，调用的是doDispatch的如下代码：

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

handle方法，调用的是RequestMappingHandlerAdapter父类AbstractHandlerMethodAdapter的handle方法。

@Nullable
        public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        
            return this.handleInternal(request, response, (HandlerMethod)handler);
        }

接下来调转到RequestMappingHandlerAdapter的handleInternal方法。

protected ModelAndView handleInternal(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
        
        	// 1. 检查请求
            this.checkRequest(request);
            ModelAndView mav;
            // 2. 设置了Session同步，则会加锁执行
            if (this.synchronizeOnSession) {
        
                HttpSession session = request.getSession(false);
                if (session != null) {
        
                    Object mutex = WebUtils.getSessionMutex(session);
                    synchronized(mutex) {
        
                        mav = this.invokeHandlerMethod(request, response, handlerMethod);
                    }
                } else {
        
                    mav = this.invokeHandlerMethod(request, response, handlerMethod);
                }
            } else {
        
            	// 3. 调用处理程序方法
                mav = this.invokeHandlerMethod(request, response, handlerMethod);
            }
    
            if (!response.containsHeader("Cache-Control")) {
        
                if (this.getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
        
                    this.applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
                } else {
        
                    this.prepareResponse(response);
                }
            }
            return mav;
        }

handleInternal接着进入到invokeHandlerMethod，开始执行处理器方法，invokeHandlerMethod会设置一些解析器、处理器。  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16 3]  
invokeHandlerMethod方法会进入invokeAndHandle调用和处理请求。其中调用的invokeForRequest就是处理请求了。

Object returnValue = this.invokeForRequest(webRequest, mavContainer, providedArgs);

invokeForRequest方法会首先获取控制器参数值getMethodArgumentValues。

@Nullable
        public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
        
            Object[] args = this.getMethodArgumentValues(request, mavContainer, providedArgs);
            if (logger.isTraceEnabled()) {
        
                logger.trace("Arguments: " + Arrays.toString(args));
            }
    
            return this.doInvoke(args);
        }

InvocableHandlerMethod.getMethodArgumentValues方法如下：

protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
        
        	// 1. 获取方法的参数对象
            MethodParameter[] parameters = this.getMethodParameters();
            if (ObjectUtils.isEmpty(parameters)) {
        
                return EMPTY_ARGS;
            } else {
        
                Object[] args = new Object[parameters.length];
    			// 2. 循环参数
                for(int i = 0; i < parameters.length; ++i) {
        
                    MethodParameter parameter = parameters[i];
                    parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
                    args[i] = findProvidedArgument(parameter, providedArgs);
                    if (args[i] == null) {
        
                    	// 3. 根据参数获取参数解析器
                        if (!this.resolvers.supportsParameter(parameter)) {
        
                            throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
                        }
    
                        try {
        
                        	// 4. 调用解析器解析参数
                            args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
                        } catch (Exception var10) {
        
                  // .... others
        }

resolveArgument方法，最终调用到RequestResponseBodyMethodProcessor。它就是ResponseBody、RequestBody注解标识方法的参数解析器。

![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16 4]

RequestResponseBodyMethodProcessor的resolveArgument源码如下：

public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
        
            parameter = parameter.nestedIfOptional();
            // 1. 消息转换
            Object arg = this.readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
            // 2. 获取参数名
            String name = Conventions.getVariableNameForParameter(parameter);
            if (binderFactory != null) {
        
                WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name);
            // .... others
        }

在resolveArgument方法中，终于看到了HttpMessageConverter了，其readWithMessageConverters方法如下：

![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16 5]

接着调用的父类readWithMessageConverters方法，读取请求消息，转换为Java对象

@Nullable
        protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter, Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
        
            boolean noContentType = false;
    
            MediaType contentType;
            try {
        
            	// 1. 获取contentType  application/json;charset=UTF-8
                contentType = inputMessage.getHeaders().getContentType();
            } catch (InvalidMediaTypeException var16) {
        
                throw new HttpMediaTypeNotSupportedException(var16.getMessage());
            }
    
            if (contentType == null) {
        
                noContentType = true;
                contentType = MediaType.APPLICATION_OCTET_STREAM;
            }
    		// 2. 获取class org.pearl.springboot.minio.controller.MvcController
            Class<?> contextClass = parameter.getContainingClass();
            // 3. 获取参数class class org.pearl.springboot.minio.pojo.query.User
            Class<T> targetClass = targetType instanceof Class ? (Class)targetType : null;
            if (targetClass == null) {
        
                ResolvableType resolvableType = ResolvableType.forMethodParameter(parameter);
                targetClass = resolvableType.resolve();
            }
    
            HttpMethod httpMethod = inputMessage instanceof HttpRequest ? ((HttpRequest)inputMessage).getMethod() : null;
            Object body = NO_VALUE;
    
            AbstractMessageConverterMethodArgumentResolver.EmptyBodyCheckingHttpInputMessage message;
            try {
        
                label98: {
        
                	// 4. 获取到请求报文、请求头
                    message = new AbstractMessageConverterMethodArgumentResolver.EmptyBodyCheckingHttpInputMessage(inputMessage);
                    // 5. 获取到消息转换器集合
                    Iterator var11 = this.messageConverters.iterator();
    
                    HttpMessageConverter converter;
                    Class converterType;
                    GenericHttpMessageConverter genericConverter;
                    // 循环
                    while(true) {
        
                        if (!var11.hasNext()) {
        
                            break label98;
                        }
    					// 消息转换器
                        converter = (HttpMessageConverter)var11.next();
                        // 消息转换器 Class名
                        converterType = converter.getClass();
                        // 6. 是不是通用 Http 消息转换器GenericHttpMessageConverter
                        genericConverter = converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter)converter : null;
                        if (genericConverter != null) {
        
                            if (genericConverter.canRead(targetType, contextClass, contentType)) {
        
                                break;
                            }
                        } else if (targetClass != null && converter.canRead(targetClass, contentType)) {
        
                            break;
                        }
                    }
    				// 7. 处理请求报文
                    if (message.hasBody()) {
        
                        HttpInputMessage msgToUse = this.getAdvice().beforeBodyRead(message, parameter, targetType, converterType);		// 8. 调用消息转换器MappingJackson2HttpMessageConverter， 转换数据为Java对象
                        body = genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) : converter.read(targetClass, msgToUse);
                        body = this.getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);
                    } else {
        
                        body = this.getAdvice().handleEmptyBody((Object)null, message, parameter, targetType, converterType);
                    }
                }
         // .... others
        }

可以看到解析参数时，这里使用的是MappingJackson2HttpMessageConverter转换器，使用 objectMapper.readValue方法，将请求报文转为JAVA对象，这个读取请求报文，转为JAVA对象的流程就结束了。

![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16 6]

#### 3. 转换响应报文 ####

在InvocableHandlerMethod.invokeForRequest方法处理完参数之后，就开始执行执行器方法了，也就是我们写的controller方法。

controller方法执行完成之后，重新回到ServletInvocableHandlerMethod.invokeAndHandle方法，并获取到了方法返回值。

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
        
            Object returnValue = this.invokeForRequest(webRequest, mavContainer, providedArgs);
             // .... others
            mavContainer.setRequestHandled(false);
            try {
        
            	// 1. 处理响应值
                this.returnValueHandlers.handleReturnValue(returnValue, this.getReturnValueType(returnValue), mavContainer, webRequest);
            } 
        }

handleReturnValue方法会进入到HandlerMethodReturnValueHandlerComposite类中，会调用处理器RequestResponseBodyMethodProcessor。

public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
        
            HandlerMethodReturnValueHandler handler = this.selectHandler(returnValue, returnType);
            if (handler == null) {
        
                throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
            } else {
        
                handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
            }
        }

handler.handleReturnValue会根据返回值，创建ServletServerHttpResponse对象，

public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
        
            mavContainer.setRequestHandled(true);
            ServletServerHttpRequest inputMessage = this.createInputMessage(webRequest);
            ServletServerHttpResponse outputMessage = this.createOutputMessage(webRequest);
            this.writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
        }

最懂调用writeWithMessageConverters，也是匹配一个转换器将返回值对象写入到响应体中，然后经过DispathServlet的其他处理，浏览器就收到响应的JAVA对象了。  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5LqR54Of5oiQ6ZuoY3Nkbg_size_20_color_FFFFFF_t_70_g_se_x_16 7]

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