HttpMessageConverter

基本介绍

我们进行Web开发，但是前端传入的 JSON 数据到底是如何被解析成 Java 对象作为 API入参的，后端返回的结果又是如何把 Java 对象解析成返回给前端的，在整个数据流转的过程中，这些工作都是由谁来完成的呢？

从请求报文到对象和从对象到响应报文的转换。而在源码内部，其实这种灵活的消息转换机制就是利用HttpMessageConverter来实现的。

针对不同的参数类型，Spring提供了一些基础的参数解析器，其中有基于注解的解析器，也有基于特定类型的解析器，当然也有兜底默认的解析器，如果已有的解析器不能满足解析要求，Spring也提供了支持用户自定义解析器的扩展点，源码如下：

org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#getDefaultArgumentResolvers

private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
   List<HandlerMethodArgumentResolver> resolvers = new ArrayList<HandlerMethodArgumentResolver>();

   // Annotation-based argument resolution 基于注解
   resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
   resolvers.add(new RequestParamMapMethodArgumentResolver());
   resolvers.add(new PathVariableMethodArgumentResolver());
   resolvers.add(new PathVariableMapMethodArgumentResolver());
   resolvers.add(new MatrixVariableMethodArgumentResolver());
   resolvers.add(new MatrixVariableMapMethodArgumentResolver());
   resolvers.add(new ServletModelAttributeMethodProcessor(false));
   resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
   resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice));
   resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
   resolvers.add(new RequestHeaderMapMethodArgumentResolver());
   resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
   resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));
   resolvers.add(new SessionAttributeMethodArgumentResolver());
   resolvers.add(new RequestAttributeMethodArgumentResolver());

   // Type-based argument resolution 基于类型
   resolvers.add(new ServletRequestMethodArgumentResolver());
   resolvers.add(new ServletResponseMethodArgumentResolver());
   resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
   resolvers.add(new RedirectAttributesMethodArgumentResolver());
   resolvers.add(new ModelMethodProcessor());
   resolvers.add(new MapMethodProcessor());
   resolvers.add(new ErrorsMethodArgumentResolver());
   resolvers.add(new SessionStatusMethodArgumentResolver());
   resolvers.add(new UriComponentsBuilderMethodArgumentResolver());

   // Custom arguments 用户自定义
   if (getCustomArgumentResolvers() != null) {
      resolvers.addAll(getCustomArgumentResolvers());
   }

   // Catch-all 兜底默认
   resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
   resolvers.add(new ServletModelAttributeMethodProcessor(true));

   return resolvers;
}

这些参数解析器大致分为4类

1.基于Name---从URI（路径变量）、HttpServletRequest、HttpSession、Header、Cookie…等中根据名称key来获取值

2.数据类型是Map的----从URI（路径变量）、HttpServletRequest、HttpSession、Header、Cookie…等中获取数据，只是参数类型是Map的

3.固定参数类型---参数比如是SessionStatus, ServletResponse, OutputStream, Writer, WebRequest, MultipartRequest, HttpSession, Principal, InputStream等

4.基于ContentType的消息转换器---利用HttpMessageConverter将输入流转换成对应的参数

注解命名	对应的解析器
@PathVariable	PathVariableMethodArgumentResolver	基于Name的
@RequestParam	RequestParamMethodArgumentResolver	基于Name的
@RequestBody	RequestResponseBodyMethodProcessor	基于ContentType的消息转换器
@RequestPart	RequestPartMethodArgumentResolver	基于ContentType的消息转换器

这是HttpMessageConverter消息转换器的策略接口也是扩展点，可以自定义扩展实现，也可以通过继承AbstractHttpMessageConverterpackage org.springframework.http.converter;去实现

import java.io.IOException;
import java.util.List;
import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.lang.Nullable;

public interface HttpMessageConverter<T> {
    boolean canRead(Class<?> var1, @Nullable MediaType var2);

    boolean canWrite(Class<?> var1, @Nullable MediaType var2);

    List<MediaType> getSupportedMediaTypes();

    T read(Class<? extends T> var1, HttpInputMessage var2) throws IOException, HttpMessageNotReadableException;

    void write(T var1, @Nullable MediaType var2, HttpOutputMessage var3) throws IOException, HttpMessageNotWritableException;
}

HttpMessageConverter<T> 的实现类

DispatcherServlet 默认装配的是 RequestMappingHandleAdapter，而 RequestMappingHandlerAdapter 默认装配如下 HttpMessageConverter：

public RequestMappingHandlerAdapter() {
    StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
    stringHttpMessageConverter.setWriteAcceptCharset(false);
    this.messageConverters = new ArrayList(4);
    this.messageConverters.add(new ByteArrayHttpMessageConverter());
    this.messageConverters.add(stringHttpMessageConverter);

    try {
        this.messageConverters.add(new SourceHttpMessageConverter());
    } catch (Error var3) {
    }

    this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
}