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Spring源码分析(10)——SpringMVC参数绑定和返回值解析

HTTP请求到达Controller层时，需要将请求中的参数转换成java可识别的参数，等Controller处理完成后，又需要将java的返回值转换为HTTP内容返回给用户。

先回顾SpringMVC调用Controller的入口位置，这里会调用HandlerAdapter，让HandlerAdapter调用实际的Controller方法。但这个方法可能包含不同的参数和返回值，就需要对参数和返回值进行绑定。

DispatcherServlet.java
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protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
  // 获取HandlerAdapter
  HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
  // 调用拦截器的preHandle
  if (!mappedHandler.applyPreHandle(processedRequest, response)) {
    return;
  }
  // 实际调用Controller
  mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
  // 调用拦截器的postHandle
  mappedHandler.applyPostHandle(processedRequest, response, mv);
}

一、初始化

常用的一个URL对应一个方法使用的是RequestMappingHandlerAdapter，这也是要研究的重点。这个类在afterPropertiesSet()方法会初始化参数绑定工具HandlerMethodArgumentResolver和返回值绑定工具HandlerMethodReturnValueHandler

RequestMappingHandlerAdapter.java
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@Override
public void afterPropertiesSet() {
  initControllerAdviceCache();

  if (this.argumentResolvers == null) {
    List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
    this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
  }
  if (this.initBinderArgumentResolvers == null) {
    List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
    this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
  }
  if (this.returnValueHandlers == null) {
    List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
    this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
  }
}

二、普通方法的参数绑定过程

一路跟着doDispatch方法的handle下去，在使用反射调用实际Handle方法之前，会对方法的每次参数进行绑定，待Handle处理完成之后，又会对返回值进行处理。

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@Override
@Nullable
public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
    throws Exception {
  return handleInternal(request, response, (HandlerMethod) handler);
}
@Override
  protected ModelAndView handleInternal(HttpServletRequest request,
      HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
......
      mav = invokeHandlerMethod(request, response, handlerMethod);
......
  return mav;
}
@Nullable
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
      HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
......
    invocableMethod.invokeAndHandle(webRequest, mavContainer);
......
}
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {
    // 调用方法并获得返回值
    Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
......
    // 对返回值进行处理
    this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
......
  }
protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
      Object... providedArgs) throws Exception {
    MethodParameter[] parameters = getMethodParameters();
    Object[] args = new Object[parameters.length];
    for (int i = 0; i < parameters.length; i++) {
      // 找到能处理该参数的resolver
      if (!this.resolvers.supportsParameter(parameter)) {
        throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
      }
      // 参数绑定
      args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
    }
    return args;
  }

要使用哪个resolver来绑定参数呢？在RequestMappingHandlerAdapter初始化时已经默认定义了一系列resolver，如果任意一个能支持解析参数，就会使用它来解析参数。

常用的HandlerMethodArgumentResolver有这些：

  • RequestResponseBodyMethodProcessor：解析注释了@RequestBody类型的参数

  • PathVariableMethodArgumentResolver：解析注释了@PathVariable而且不是Map类型的参数

  • RequestHeaderMethodArgumentResolver：解析注释了@RequestHeader的Map类型的参数

  • RequestParamMethodArgumentResolver：解析注释了@RequestParam的参数

  • ModelMethodProcessor：解析Model类型的参数

RequestMappingHandlerAdapter
HandlerMethodArgumentResolver
Controller Method
根据不同的注解或参数类型来选择参数处理器
所有处理器都定义在RequestMappingHandlerAdapter中
HandlerMethodReturnValueHandler
根据不同的注解或返回值来选择返回值处理器
所有处理器都定义在RequestMappingHandlerAdapter
实际调用Controller方法
拦截器
preHandle
拦截器
postHandle

三、RESTful方法的参数绑定过程

如果一个方法使用了@ResponseBody注解，将会使用RequestResponseBodyMethodProcessor来对参数和返回值进行处理。

以RequestResponseBodyMethodProcessor的参数绑定举例：从messageConverters中取出一个合适的来进行参数绑定，这和选择ArgumentResolver的套路如出一辙。

RequestResponseBodyMethodProcessor.java
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@Override
public boolean supportsParameter(MethodParameter parameter) {
  return parameter.hasParameterAnnotation(RequestBody.class);
}
@Override
  public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
      NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
......
    Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
......
    return adaptArgumentIfNecessary(arg, parameter);
  }
@Override
protected <T> Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter parameter,
    Type paramType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
......
  Object arg = readWithMessageConverters(inputMessage, parameter, paramType);
......
  return arg;
}
@Nullable
  protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
      Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {

      for (HttpMessageConverter<?> converter : this.messageConverters) {
        Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
        GenericHttpMessageConverter<?> genericConverter =
            (converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
        if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
            (targetClass != null && converter.canRead(targetClass, contentType))) {
        cif (message.hasBody()) {
            HttpInputMessage msgToUse = getAdvice().beforeBodyRead(message, parameter, targetType, converterType);
            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;
        }
      }
    return body;
  }

哦对了，这个messageConverter一般需要自己进行配置，一般这样配置：

servlet.xml
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<bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter">
    <property name="messageConverters">
        <list>
            <ref bean="mappingJacksonHttpMessageConverter"/>
        </list>
    </property>
</bean>
<bean id="mappingJacksonHttpMessageConverter" class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
    <property name="supportedMediaTypes">
        <list>
            <value>text/html;charset=UTF-8</value>
        </list>
    </property>
</bean>
RequestMappingHandlerAdapter
HandlerMethodArgumentResolver
HttpMessageConverter
Controller Method
根据不同的注解或参数来选择参数处理器
默认是RequestResponseBodyMethodProcessor
HandlerMethodReturnValueHandler
HttpMessageConverter
根据accept-type类型来选择Converter
一般是MappingJackson2HttpMessageConverter
根据accept-type类型来选择Converter
一般是MappingJackson2HttpMessageConverter
根据不同的注解或返回值来选择返回值处理器
默认是RequestResponseBodyMethodProcessor
实际调用Controller方法
# Spring源码
Spring源码分析(11)——SpringBoot源码
Spring源码分析(9)——SpringMVC九大组件
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