SpringMVC @RequestBody和@ResponseBody原理解析

逃离我推掉我的手 2023-09-30 13:31 17阅读 0赞

## SpringMVC @RequestBody和@ResponseBody原理解析 ##

## 前言 ##

@RequestBody作用是将http请求解析为对应的对象。例如：  
http请求的参数（application/json格式）：

{
        
      "accountId": 10,
      "adGroupId": "12345678",
      "campaignId": "12345678",
      "dataType": 0,
      "sign": "abcdefg",
      "site": "us",
      "timeStamp": 1453250,
      "userId": 10
    }

通过@RequestBody可以解析为ProductSyncNegativeDto对象（如下代码所示）

public CustResponse syncNegative(@RequestBody ProductSyncNegativeDto productSyncNegativeDto)

**那@RequestBody注解是如何实现http请求报文转对象的呢？**  
@ResponseBody作用是将返回的对象转为json字符串，例如我们返回一个CustResponse对象，那postman中的结果会是啥？

{
        
        "code": 100,
        "msg": "",
        "details": [
            10,
            10,
            "us",
            "12345678",
            "12345678",
            0
        ]
    }

我们可以发现，结果是一个json字符串，那@ResponseBody注解到底是如何将对象转为json字符串返回的呢？  
接下来老师会带童鞋们一些来揭秘,@RequestBody、@ResponseBody的底层实现原理。

## 概述 ##

### @Controller注解 ###

在开始之前，我们先来介绍一下@Controller，做过ssm/ssh项目的同学肯定都接触过springMVC，那必然会用到@Controller注解。Controller方法被封装成ServletInvocableHandlerMethod类，并且由invokeAndHandle方法完成请求处理。

### HttpMessageConverter ###

SpringMVC处理请求和响应时，支持多种类型的请求参数和返回类型，而此种功能的实现就需要对HTTP消息体和参数及返回值进行转换，为此SpringMVC提供了大量的转换类，所有转换类都实现了HttpMessageConverter接口。

public interface HttpMessageConverter<T> {
        
    
        // 当前转换器是否能将HTTP报文转换为对象类型
        boolean canRead(Class<?> clazz, MediaType mediaType);
    
        // 当前转换器是否能将对象类型转换为HTTP报文
        boolean canWrite(Class<?> clazz, MediaType mediaType);
    
        // 转换器能支持的HTTP媒体类型
        List<MediaType> getSupportedMediaTypes();
    
        // 转换HTTP报文为特定类型
        T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
                throws IOException, HttpMessageNotReadableException;
    
        // 将特定类型对象转换为HTTP报文
        void write(T t, MediaType contentType, HttpOutputMessage outputMessage)
                throws IOException, HttpMessageNotWritableException;
    
    }

HttpMessageConverter接口定义了5个方法，用于将HTTP请求报文转换为java对象，以及将java对象转换为HTTP响应报文。

### HandlerMethodArgumentResolver与HandlerMethodReturnValueHandler ###

对应到SpringMVC的Controller方法，read方法即是读取HTTP请求转换为参数对象，write方法即是将返回值对象转换为HTTP响应报文。SpringMVC定义了两个接口来操作这两个过程：参数解析器HandlerMethodArgumentResolver和返回值处理器HandlerMethodReturnValueHandler。

// 参数解析器接口
    public interface HandlerMethodArgumentResolver {
        
    
        // 解析器是否支持方法参数
        boolean supportsParameter(MethodParameter parameter);
    
        // 解析HTTP报文中对应的方法参数
        Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
                NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception;
    
    }
    
    // 返回值处理器接口
    public interface HandlerMethodReturnValueHandler {
        
    
        // 处理器是否支持返回值类型
        boolean supportsReturnType(MethodParameter returnType);
    
        // 将返回值解析为HTTP响应报文
        void handleReturnValue(Object returnValue, MethodParameter returnType,
                ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;
    
    }

参数解析器和返回值处理器在底层处理时，都是通过HttpMessageConverter进行转换。流程如下：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1X3poYW5nX3c_size_16_color_FFFFFF_t_70]

## @RequestBody解析过程 ##

所有的http请求都会进入ServletInvocableHandlerMethod类（继承InvocableHandlerMethod，所有的参数解析器都会在在这里面进行初始化）的invokeAndHandle方法中，我们来具体看看invokeAndHandle方法是干什么的。

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
    			Object... providedArgs) throws Exception {
        
        // 执行http请求
    	Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
    		setResponseStatus(webRequest);
    	if (returnValue == null) {
        
    		if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
        
    			disableContentCachingIfNecessary(webRequest);
    			mavContainer.setRequestHandled(true);
    			return;
    		}
    	}
    	else if (StringUtils.hasText(getResponseStatusReason())) {
        
    		mavContainer.setRequestHandled(true);
    		return;
    	}
    
    	mavContainer.setRequestHandled(false);
    	Assert.state(this.returnValueHandlers != null, "No return value handlers");
    	// 返回值处理
    	try {
        
    		this.returnValueHandlers.handleReturnValue(
    					returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
    	}catch (Exception ex) {
        
    		if (logger.isTraceEnabled()){
        
    		logger.trace(formatErrorForReturnValue(returnValue), ex);
    		}
    		throw ex;
    	}
    }

我们可以看到invokeAndHandle方法都会进入invokeForRequest方法中，invokeForRequest方法就是实现@RequestBody注解的功能，将http请求报文解析为我们设置的对象。我们进入该方法看看，里面具体做了哪些事情。

public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
    			Object... providedArgs) throws Exception {
        
        // http报文解析为对象数组
    	Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
    	if (logger.isTraceEnabled()) {
        
    		logger.trace("Arguments: " + Arrays.toString(args));
    	}
    	//执行@PostMapping、@GetMapping等接口
    	return doInvoke(args);
    }

我们可以看到invokeForRequest中主要做了两件事情，一个是通过getMethodArgumentValues方法返回http解析后的对象数组，然后通过doInvoke方法执行接口的具体业务逻辑代码。  
我们接着进入getMethodArgumentValues方法，细看一下@RequestBody的具体解析过程。

protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
    			Object... providedArgs) throws Exception {
        
        
        // 获取http请求参数
    	MethodParameter[] parameters = getMethodParameters();
    	if (ObjectUtils.isEmpty(parameters)) {
        
    		return EMPTY_ARGS;
    	}
    	Object[] args = new Object[parameters.length];
    	// 遍历所有参数，挨个解析
    	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) {
        
    			continue;
    		}
    		if (!this.resolvers.supportsParameter(parameter)) {
        
    			throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
    		}
    		try {
        
    		    // 参数解析器解析HTTP报文
    			args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
    		}catch (Exception ex) {
        
    			// Leave stack trace for later, exception may actually be resolved and handled...
    			if (logger.isDebugEnabled()) {
        
    				String exMsg = ex.getMessage();
    					if (exMsg != null && !exMsg.contains(parameter.getExecutable().toGenericString())) {
        
    					logger.debug(formatArgumentError(parameter, exMsg));
    				}
    			}
    			throw ex;
    		}
    	}
    	return args;
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1X3poYW5nX3c_size_16_color_FFFFFF_t_70 1]

其中this.resolvers.supportsParameter(parameter)用来判断请求参数是否合法，`this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory)`方法最终实现`@RequestBody` 解析操作。我们来看看 `this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory)` 中做了什么。

@Override
    @Nullable
    public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
    			NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
        
        // 获取对应的解析器
    	HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
    	if (resolver == null) {
        
    		throw new IllegalArgumentException(
    					"Unsupported parameter type [" + parameter.getParameterType().getName() + "]." +
    							" supportsParameter should be called first.");
    	}
    	// 通过HandlerMethodArgumentResolver 解析器解析http报文
    	return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
    }

`getArgumentResolver`方法来获取对应的`HandlerMethodArgumentResolver`参数解析器，参数解析器最终通过`RequestResponseBodyMethodProcessor`类来具体执行解析过程，我们接着来看看`RequestResponseBodyMethodProcessor`中`resolveArgument`方法又是怎样的一个处理过程。

不同的resolvers（HandlerMethodArgumentResolver接口）会对应不同的参数解析器，例如`public String testDemo(String name)`，解析器就会变成`ServletRequestMethodArgumentResolver`，如果是`@RequestBody`，参数解析器就是`RequestResponseBodyMethodProcessor`

@Override
    public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
    			NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
        
    
    	parameter = parameter.nestedIfOptional();
    	// 通过HttpMessageConverter来解析http报文为Object对象
    	Object arg = readWithMessageConverters(webRequest,parameter,parameter.getNestedGenericParameterType());
    	String name = Conventions.getVariableNameForParameter(parameter);
    
    	if (binderFactory != null) {
        
    		WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name);
    		if (arg != null) {
        
    			validateIfApplicable(binder, parameter);
    			if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) {
        
    					throw new MethodArgumentNotValidException(parameter, binder.getBindingResult());
    			}
    		}
    		if (mavContainer != null) {
        
    			mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
    		}
    	}
    	return adaptArgumentIfNecessary(arg, parameter);
    }

`readWithMessageConverters`方法中，`HttpMessageConverter`（接口对应实现类）的`read`方法实现了http报文解析，我们来看看最终http参数解析部分的代码。

protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
    			Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
        
    	try {
        
    		message = new EmptyBodyCheckingHttpInputMessage(inputMessage);
    
    		for (HttpMessageConverter<?> converter : this.messageConverters) {
        
    			Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
    			GenericHttpMessageConverter<?> genericConverter =
    						(converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
    			// 判断转换器是否支持参数类型
    			if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
    						(targetClass != null && converter.canRead(targetClass, contentType))) {
        
    				if (message.hasBody()) {
        
    					HttpInputMessage msgToUse =
    								getAdvice().beforeBodyRead(message, parameter, targetType, converterType);
    					// read方法执行HTTP报文到参数的转换
    					body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :
    								((HttpMessageConverter<T>) converter).read(targetClass, msgToUse));
    					body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);
    				}else {
        
    					body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);
    				}
    				break;
    			}
    		}
    	}catch (IOException ex) {
        
    		throw new HttpMessageNotReadableException("I/O error while reading input message", ex, inputMessage);
    	}
    }

代码部分省略了，关键部分即是遍历所有的`HttpMessageConverter`，然后通过canRead方法判断解析器是否支持，最后执行`AbstractJackson2HttpMessageConverter`对象（`HttpMessageConverter`实现类）的read方法完成最后的参数解析。  
`AbstractJackson2HttpMessageConverter`对象的read方法，核心是利用了jackson工具，将http报文的json字符串转换为object对象并返回。

## @ResponseBody返回值序列化过程 ##

执行完doInvoke逻辑代码之后，通过`ServletInvocableHandlerMethod`对象的`invokeAndHandle`方法，利用返回值处理器对返回值进行序列化输出。

`this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);`  
`returnValueHandlers`为`HandlerMethodReturnValueHandlerComposite`对象，该对象实现了`HandlerMethodReturnValueHandler`接口，我们接着来看看`handleReturnValue`方法的具体实现。

@Override
    	public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
    			ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
        
            // 选择合适的HandlerMethodReturnValueHandler返回值处理器
    		HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
    		if (handler == null) {
        
    			throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
    		}
    		// 执行返回值处理
    		handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
    	}

selectHandler方法会提供合适的HandlerMethodReturnValueHandler，用来处理返回值。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1X3poYW5nX3c_size_16_color_FFFFFF_t_70 2]  
我们看到的`HandlerMethodReturnValueHandler`处理器最终也是由`RequestResponseBodyMethodProcessor`实现的，我们具体来看看`handleReturnValue`方法。

`handler（HandlerMethodReturnValueHandler`）接口会根据不同类型选择不同的返回值处理器，例如页面跳转类型的处理器就是`ViewNameMethodReturnValueHandler`。

@Override
    public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
    		ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
    			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
        
    
    		mavContainer.setRequestHandled(true);
    		ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
    		ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
    
    		// 调用HttpMessageConverter执行
    		writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
    	}
    	
    protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
    			ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
    			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
        
    	for (HttpMessageConverter<?> converter : this.messageConverters) {
        
    		GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
    				(GenericHttpMessageConverter<?>) converter : null);
    		// 判断是否支持返回值类型，返回值类型很有可能不同，如String，Map，List等
    		if (genericConverter != null ?
    				((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :
    				converter.canWrite(valueType, selectedMediaType)) {
        
    			body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType,
    					(Class<? extends HttpMessageConverter<?>>) converter.getClass(),
    					inputMessage, outputMessage);
    			if (body != null) {
        
    				Object theBody = body;
    				LogFormatUtils.traceDebug(logger, traceOn ->
    						"Writing [" + LogFormatUtils.formatValue(theBody, !traceOn) + "]");
    				addContentDispositionHeader(inputMessage, outputMessage);
    				if (genericConverter != null) {
        
    				    // 执行返回值转换
    					genericConverter.write(body, targetType, selectedMediaType, outputMessage);
    				}
    				else {
        
    				    // 执行返回值转换
    					((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);
    				}
    			}
    			else {
        
    				if (logger.isDebugEnabled()) {
        
    					logger.debug("Nothing to write: null body");
    				}
    			}
    			return;
    		}
    	}
    }

我们看到最终还是由`HttpMessageConverter`（`AbstractGenericHttpMessageConverter`实现类）的write方法来进行对象的序列化输出。

大家都知道`@ResponseBody`需要通过io流来读取，也就`HttpMessageConverter`最终的`write`会写入到io输出流中，上面的`createOutputMessage(webRequest)`方法就是创建一个输出流，我们来具体看看它的实现。

protected ServletServerHttpResponse createOutputMessage(NativeWebRequest webRequest) {
        
    	HttpServletResponse response = webRequest.getNativeResponse(HttpServletResponse.class);
    		Assert.state(response != null, "No HttpServletResponse");
    		return new ServletServerHttpResponse(response);
    }
    	
    
    public class ServletServerHttpResponse implements ServerHttpResponse {
        
    
    	private final HttpServletResponse servletResponse;
    	private final HttpHeaders headers;
    	private boolean headersWritten = false;
    	private boolean bodyUsed = false;
    
    	/**
    	 * Construct a new instance of the ServletServerHttpResponse based on the given {@link HttpServletResponse}.
    	 * @param servletResponse the servlet response
    	 */
    	public ServletServerHttpResponse(HttpServletResponse servletResponse) {
        
    		Assert.notNull(servletResponse, "HttpServletResponse must not be null");
    		this.servletResponse = servletResponse;
    		this.headers = new ServletResponseHttpHeaders();
    	}
    }
    
    public interface ServletResponse {
        
        String getCharacterEncoding();
    
        String getContentType();
    
        ServletOutputStream getOutputStream() throws IOException;
    
        PrintWriter getWriter() throws IOException;
    
        void setCharacterEncoding(String var1);
    
        void setContentLength(int var1);
    
        void setContentLengthLong(long var1);
    
        void setContentType(String var1);
    
        void setBufferSize(int var1);
    
        int getBufferSize();
    
        void flushBuffer() throws IOException;
    
        void resetBuffer();
    
        boolean isCommitted();
    
        void reset();
    
        void setLocale(Locale var1);
    
        Locale getLocale();
    }

`createOutputMessage`方法中创建了`ServletServerHttpResponse` ，然后通过 `((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage)`方法写入到输出流中。write方法的核心也是通过Jackson工具将对象解析为json字符串。我们最后来看看write的核心处理方法`writeInternal`。

protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessage outputMessage)
    			throws IOException, HttpMessageNotWritableException {
        
    
    		MediaType contentType = outputMessage.getHeaders().getContentType();
    		JsonEncoding encoding = getJsonEncoding(contentType);
    		JsonGenerator generator = this.objectMapper.getFactory().createGenerator(outputMessage.getBody(), encoding);
    		try {
        
    			writePrefix(generator, object);
    
    			Object value = object;
    			Class<?> serializationView = null;
    			FilterProvider filters = null;
    			JavaType javaType = null;
    
    			if (object instanceof MappingJacksonValue) {
        
    				MappingJacksonValue container = (MappingJacksonValue) object;
    				value = container.getValue();
    				serializationView = container.getSerializationView();
    				filters = container.getFilters();
    			}
    			if (type != null && TypeUtils.isAssignable(type, value.getClass())) {
        
    				javaType = getJavaType(type, null);
    			}
    
    			ObjectWriter objectWriter = (serializationView != null ?
    					this.objectMapper.writerWithView(serializationView) : this.objectMapper.writer());
    			if (filters != null) {
        
    				objectWriter = objectWriter.with(filters);
    			}
    			if (javaType != null && javaType.isContainerType()) {
        
    				objectWriter = objectWriter.forType(javaType);
    			}
    			SerializationConfig config = objectWriter.getConfig();
    			if (contentType != null && contentType.isCompatibleWith(MediaType.TEXT_EVENT_STREAM) &&
    					config.isEnabled(SerializationFeature.INDENT_OUTPUT)) {
        
    				objectWriter = objectWriter.with(this.ssePrettyPrinter);
    			}
    			objectWriter.writeValue(generator, value);
    
    			writeSuffix(generator, object);
    			generator.flush();
    		}
    		catch (InvalidDefinitionException ex) {
        
    			throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);
    		}
    		catch (JsonProcessingException ex) {
        
    			throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getOriginalMessage(), ex);
    		}
    	}
    	
    	@Override
    	public OutputStream getBody() throws IOException {
        
    		this.bodyUsed = true;
    		writeHeaders();
    		return this.servletResponse.getOutputStream();
    	}

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2d1X3poYW5nX3c_size_16_color_FFFFFF_t_70 3]

`objectWriter.writeValue(generator, value)`方法中将value对象通过serialize序列化方法，将对象转为`json`字符串，然后设置到io流中。我们最后看看Jackson最终的序列化是怎么样的？

@Override
        public final void serialize(Object bean, JsonGenerator gen, SerializerProvider provider)
            throws IOException
        {
        
            if (_objectIdWriter != null) {
        
                gen.setCurrentValue(bean); // [databind#631]
                _serializeWithObjectId(bean, gen, provider, true);
                return;
            }
            // 设置json的开始符号（"{"）
            gen.writeStartObject(bean);
            if (_propertyFilterId != null) {
        
               // 循环将对象设置为json字符串 serializeFieldsFiltered(bean, gen, provider);
            } else {
        
                serializeFields(bean, gen, provider);
            }
            // 设置json的结束符号（"}"）
            gen.writeEndObject();
        }

在serialize方法中通过JsonGenerator将要返回的对象转为json格式的字符串。

## springMVC初始化 ##

至此我们就基本走完了一个HTTP请求和响应的过程。现在你可能有个疑惑，SpringMVC我们都是开箱即用，这些参数解析器和返回值处理器在哪里定义的呢？在核心的HandlerAdapter实现类RequestMappingHandlerAdapter的初始化方法中定义的。

而在`RequestMappingHandlerAdapter`构造时，也同时初始化了众多的`HttpMessageConverter`，以支持多样的转换需求。

WebMvcConfigurationSupport.java
    
    protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
        
        StringHttpMessageConverter stringConverter = new StringHttpMessageConverter();
        stringConverter.setWriteAcceptCharset(false);
    
        messageConverters.add(new ByteArrayHttpMessageConverter());
        messageConverters.add(stringConverter);
        messageConverters.add(new ResourceHttpMessageConverter());
        messageConverters.add(new SourceHttpMessageConverter<Source>());
        messageConverters.add(new AllEncompassingFormHttpMessageConverter());
    
        if (romePresent) {
        
            messageConverters.add(new AtomFeedHttpMessageConverter());
            messageConverters.add(new RssChannelHttpMessageConverter());
        }
    
        if (jackson2XmlPresent) {
        
            ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.xml().applicationContext(this.applicationContext).build();
            messageConverters.add(new MappingJackson2XmlHttpMessageConverter(objectMapper));
        }
        else if (jaxb2Present) {
        
            messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
        }
    
        if (jackson2Present) {
        
            ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.json().applicationContext(this.applicationContext).build();
            messageConverters.add(new MappingJackson2HttpMessageConverter(objectMapper));
        }
        else if (gsonPresent) {
        
            messageConverters.add(new GsonHttpMessageConverter());
        }
    }

五、相关依赖  
大家可能会发现springboot项目都没有jackson相关的依赖，那为什么可以进行jackson的序列化呢，那是因为在spring-boot-starter-web依赖中其实已经包含了jackson相关的依赖。

<dependency>
          <groupId>com.fasterxml.jackson.core</groupId>
          <artifactId>jackson-databind</artifactId>
          <version>2.9.8</version>
          <scope>compile</scope>
          <optional>true</optional>
        </dependency>
        <dependency>
          <groupId>com.fasterxml.jackson.dataformat</groupId>
          <artifactId>jackson-dataformat-cbor</artifactId>
          <version>2.9.8</version>
          <scope>compile</scope>
          <optional>true</optional>
        </dependency>
        <dependency>
          <groupId>com.fasterxml.jackson.dataformat</groupId>
          <artifactId>jackson-dataformat-smile</artifactId>
          <version>2.9.8</version>
          <scope>compile</scope>
          <optional>true</optional>
        </dependency>
        <dependency>
          <groupId>com.fasterxml.jackson.dataformat</groupId>
          <artifactId>jackson-dataformat-xml</artifactId>
          <version>2.9.8</version>
          <scope>compile</scope>
          <optional>true</optional>
        </dependency>

## 总结 ##

看似简简单单的`@RequestBody`和`@ResponseBody`两个注解，其实内部做了大量的准备工作。现在童鞋们明白这整个过程的实现原理吧。

--------------------

#### 举例使用 ####

@RequestBody是把整个HttpServletRequest的输入(request.getInputStream())，转换成一个对象，常用的转换是采用json方式，在spring中是RequestResponseBodyMethodProcessor利用HttpMessageConventer做的。

##### 前端代码必须采用POST请求(GET请求的输入是空): #####

<!DOCTYPE html>
    <html>
    <head>
        <title></title>
        <script type="text/javascript" src="http://www.w3school.com.cn/jquery/jquery-1.11.1.min.js"></script>
        <script type="text/javascript">
        $(document).ready(function(){
            
            var saveDataAry=[];  
            var data1={
          "userName":"test","address":"gz"};  
            var data2={
          "userName":"ququ","address":"gr"};  
            saveDataAry.push(data1);  
            saveDataAry.push(data2);         
            $.ajax({
           
                type:"POST",
                url:"http://localhost:8080/test/mvc/saveUser", 
                dataType:"json",      
                contentType:"application/json",               
                data:JSON.stringify(saveDataAry), 
                success:function(data){
           
                        alert("succes" +data);                   
                },
                error:function(argument) {
          
                    alert("error" + argument);
                }
             }); 
        });  
        </script>
    </head>
    <body>
       test
    </body>
    </html>

##### 后端代码 #####

@Controller
    public class UserController {
        
    
        @RequestMapping(value = "saveUser")
        @ResponseBody
        public String saveUser(@RequestBody List<User> users) {
        
    
            System.out.println("UserController.saveUser users size:"+users.size());
            return "ok";
        }
    }

#### 整体springmvc 调用链路 ####

![这里写图片描述][20160909142843819]

#### @RequestBody ####

在处理参数时候各种HandlerMethodArgumentResolver来解决参数实际需要传递参数，其中默认的有24中；而处理@RequestBody的是RequestResponseBodyMethodProcessor。相对的，处理@CookieValue注释的则是ServletCookieValueMethodArgumentResolver。

#### @ResponseBody ####

而responseBody同样是RequestResponseBodyMethodProcessor作为HandlerMethodReturnValueHandler，处理实际执行的controller HandlerMethod函数的return value.

public void handleReturnValue(Object returnValue, MethodParameter returnType,
                ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
                throws IOException, HttpMediaTypeNotAcceptableException {
        
    
            mavContainer.setRequestHandled(true);  //最重要一步，设置请求已经处理过
            if (returnValue != null) {
        
                writeWithMessageConverters(returnValue, returnType, webRequest);
            }
        }

RequestResponseBodyMethodProcessor 的handleReturnValue（）函数处理时，首先ModelAndViewContainer.requestHandled设置成功，表明整个http请求已经处理，后续就不会执行veiw渲染了（跳过render view）.

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[20160909142843819]: https://img-blog.csdn.net/20160909142843819