spring mvc：03-@RequestBody和@ResponseBody原理解析

1. 概述

在SpringMVC的使用时，往往会用到@RequestBody和@ResponseBody两个注解，尤其是处理ajax请求必然要使用@ResponseBody注解。这两个注解对应着Controller方法的参数解析和返回值处理，开始时都是只知其用，不知原理。我们来看个例子。

@RequestMapping("/requestBody")
public void requestBody(@RequestBody String body, Writer writer) throws IOException{
    writer.write(body);
}

@RequestMapping(value="/responseBody", produces="application/json")
@ResponseBody
public Map<String, Object> responseBody(){
    Map<String, Object> retMap = new HashMap<>();
    retMap.put("param1", "abc");
    return retMap;
}

第一个requestBody请求，使用@RequestBody将HTTP请求体转换成String类型，第二个responseBody请求，将Map对象转换成json格式输出到HTTP响应中。这两个请求方法没有什么特殊，就是一个在参数前加了@RequestBody注解，一个在方法上加了@ResponseBody注解。而这两个注解是怎么完成HTTP报文信息同Controller方法中对象的转换的呢？

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。

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：

SpringMVC为@RequestBody和@ResponseBody两个注解实现了统一处理类RequestResponseBodyMethodProcessor，实现了HandlerMethodArgumentResolver和HandlerMethodReturnValueHandler两个接口。

由上一篇文章我们可以知道，Controller方法被封装成ServletInvocableHandlerMethod类，并且由invokeAndHandle方法完成请求处理。

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
        Object... providedArgs) throws Exception {

    // 执行请求
    Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
    
    // 返回值处理
    try {
        this.returnValueHandlers.handleReturnValue(
                returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
    }
    catch (Exception ex) {
        if (logger.isTraceEnabled()) {
            logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
        }
        throw ex;
    }
}

public Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
        Object... providedArgs) throws Exception {
    // 参数解析
    Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
    // invoke Controller方法
    Object returnValue = doInvoke(args);
    return returnValue;
}

在invoke Controller方法的前后分别执行了方法参数的解析和返回值的处理，我们分别来看。

2. 参数解析

private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer,
        Object... providedArgs) throws Exception {

    MethodParameter[] parameters = getMethodParameters();
    Object[] args = new Object[parameters.length];

    // 遍历所有参数，逐个解析
    for (int i = 0; i < parameters.length; i++) {
        MethodParameter parameter = parameters[i];
        parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
        args[i] = resolveProvidedArgument(parameter, providedArgs);
        if (args[i] != null) {
            continue;
        }

        // 参数解析器解析HTTP报文到参数
        if (this.argumentResolvers.supportsParameter(parameter)) {
            args[i] = this.argumentResolvers.resolveArgument(
                    parameter, mavContainer, request, this.dataBinderFactory);
            continue;
        }
    }
    return args;
}

getMethodArgumentValues方法中的argumentResolvers就是多个HandlerMethodArgumentResolver的集合体，supportsParameter方法寻找参数合适的解析器，resolveArgument调用具体解析器的resolveArgument方法执行。

我们从RequestResponseBodyMethodProcessor看看@RequestBody的解析过程。RequestResponseBodyMethodProcessor的supportsParameter定义了它支持的参数类型，即必须有@RequestBody注解。

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public boolean supportsParameter(MethodParameter parameter) {
    return parameter.hasParameterAnnotation(RequestBody.class);
}

再来看resolveArgument方法

public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
        NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {

    parameter = parameter.nestedIfOptional();
    // 通过HttpMessageConverter读取HTTP报文
    Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
    String name = Conventions.getVariableNameForParameter(parameter);

    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());
        }
    }
    mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());

    return adaptArgumentIfNecessary(arg, parameter);
}

具体实现由HttpMessageConverter来完成

protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
        Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {

    ....

    try {
        inputMessage = new EmptyBodyCheckingHttpInputMessage(inputMessage);

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

    ....

    return body;
}

代码部分省略了，关键部分即是遍历所有的HttpMessageConverter，通过canRead方法判断转换器是否支持对参数的转换，然后执行read方法完成转换。

3. 返回值处理

完成Controller方法的调用后，在ServletInvocableHandlerMethod的invokeAndHandle中，使用返回值处理器对返回值进行转换。

1 2	this.returnValueHandlers.handleReturnValue( returnValue, getReturnValueType(returnValue), mavContainer, webRequest);

这里的returnValueHandlers也是HandlerMethodReturnValueHandler的集合体HandlerMethodReturnValueHandlerComposite

public void handleReturnValue(Object returnValue, MethodParameter returnType,
        ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {

    // 选择合适的HandlerMethodReturnValueHandler，如果没有用@ResposeBody注解和用了注解其返回值处理器肯定不同
    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方法遍历所有HandlerMethodReturnValueHandler，调用其supportsReturnType方法选择合适的HandlerMethodReturnValueHandler，然后调用其handleReturnValue方法完成处理。

这里还是以RequestResponseBodyMethodProcessor来分析下@ResponseBody的处理，它的具体实现在AbstractMessageConverterMethodProcessor抽象基类中。

public boolean supportsReturnType(MethodParameter returnType) {
    return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||
            returnType.hasMethodAnnotation(ResponseBody.class));
}

RequestResponseBodyMethodProcessor要求方法上有@ResponseBody注解或者方法所在的Controller类上有@ResponseBody的注解。这就是常常用@RestController注解代替@Controller注解的原因，因为@RestController注解自带@ResponseBody。

handleReturnValue方法实际也是调用HttpMessageConverter来完成转换处理

public void handleReturnValue(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(T value, MethodParameter returnType,
        ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
        throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

    ....

    if (selectedMediaType != null) {
        selectedMediaType = selectedMediaType.removeQualityValue();
        for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
            // 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 (messageConverter.canWrite(valueType, selectedMediaType)) {
                outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
                        (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
                        inputMessage, outputMessage);
                if (outputValue != null) {
                    addContentDispositionHeader(inputMessage, outputMessage);
                    // 执行返回值转换
                    ((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);
                    ...
                }
                return;
            }
        }
    }
    ....
}

使用canWrite方法选择合适的HttpMessageConverter，然后调用write方法完成转换。

我们看看传入的参数 ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);

protected ServletServerHttpResponse createOutputMessage(NativeWebRequest webRequest) {
    //获取HttpServletResponse
    HttpServletResponse response = (HttpServletResponse)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;

    public ServletServerHttpResponse(HttpServletResponse servletResponse) {
        Assert.notNull(servletResponse, "HttpServletResponse must not be null");
        //将获取的HttpServletResponse作为ServletServerHttpResponse的属性值
        this.servletResponse = servletResponse;
        this.headers = new ServletServerHttpResponse.ServletResponseHttpHeaders();
    }
}

public interface ServletResponse {
    String getCharacterEncoding();

    String getContentType();
    
    //ServletResponse有一个输出流对象，保存需要相应客户端的字节流
    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();
}

我们具体看看 ((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);

protected void writeInternal(Object obj, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
    HttpHeaders headers = outputMessage.getHeaders();
    //创建一个数组字节流缓冲对象
    ByteArrayOutputStream outnew = new ByteArrayOutputStream();
    //将obj对象转换成JSON并写入ByteArrayOutputStream中
    int len = JSON.writeJSONString(outnew, this.fastJsonConfig.getCharset(), obj, this.fastJsonConfig.getSerializeConfig(), this.fastJsonConfig.getSerializeFilters(), this.fastJsonConfig.getDateFormat(), JSON.DEFAULT_GENERATE_FEATURE, this.fastJsonConfig.getSerializerFeatures());
    headers.setContentLength((long)len);
    //获取ServletResponse的输出流对象    
    OutputStream out = outputMessage.getBody();
    //将转换后的outnew写入ServletResponse的输出流对象，这样就可以给客户端响应数据了
    outnew.writeTo(out);
    outnew.close();
}

public OutputStream getBody() throws IOException {
    this.bodyUsed = true;
    this.writeHeaders();
    //获取ServletResponse的输出流对象    
    //ServletOutputStream getOutputStream() throws IOException;
    return this.servletResponse.getOutputStream();
}

最后我们看看JSON是怎么将obj对象转换成JSON对象的流

就是做一些循环拼接。

至此我们基本走完了一个HTTP请求报文经过处理后到HTTP响应报文的转换过程。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());
    }
}

对于json或xml的转换方式，只要引入了jackson的依赖，即可自动发现，并注册相关的转换器。

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.9.0</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.dataformat</groupId>
    <artifactId>jackson-dataformat-xml</artifactId>
    <version>2.9.0</version>
</dependency>