Spring MVC源码---- @RequestBody和@ResponseBody原理解析

@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。

一、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 {

    // 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 getSupportedMediaTypes();

    // 转换HTTP报文为特定类型
    T read(Class 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：

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二、@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;
}

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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  Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
 Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
 
 
....

try {
    message = new EmptyBodyCheckingHttpInputMessage(inputMessage);

    for (HttpMessageConverter converter : this.messageConverters) {
        Class> converterType = (Class>) 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) 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。

三、@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。

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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  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>) 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();
}

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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。

四、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> messageConverters) {
 StringHttpMessageConverter stringConverter = new StringHttpMessageConverter();
 stringConverter.setWriteAcceptCharset(false);
 
 
messageConverters.add(new ByteArrayHttpMessageConverter());
messageConverters.add(stringConverter);
messageConverters.add(new ResourceHttpMessageConverter());
messageConverters.add(new SourceHttpMessageConverter());
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。

 com.fasterxml.jackson.core
 jackson-databind
 2.9.8
 compile
 true
 
 
 com.fasterxml.jackson.dataformat
 jackson-dataformat-cbor
 2.9.8
 compile
 true
 
 
 com.fasterxml.jackson.dataformat
 jackson-dataformat-smile
 2.9.8
 compile
 true
 
 
 com.fasterxml.jackson.dataformat
 jackson-dataformat-xml
 2.9.8
 compile
 true
 

六、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。

参考：
https://cloud.tencent.com/developer/article/1611093