spring json 模型默认值 spring自带的json转换

  关于json,xml的自动转换原理的核心就在messageConvert，前一篇我们已经分析到通过messageConvert对请求参数进行解析读取，那就续点分析。

            本节就以json的转换为例（xml类同只是不同的messageConvert）来进行分析，在我们对请求参数解析时候回顾下readWithMessageConverters方法

body = ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage);

通过converter读取请求消息

         那么问题来了，这些messageConverts哪来的呢？

         我们知道RequestMappingHandlerAdapter是请求处理的适配器，也就是请求之后处理具体逻辑的执行，关系到哪个类的哪个方法以及转换器等工作，这个类是我们讲的重点，其中它的属性messageConverters就是我们关注的重点

        RequestMappingHandlerAdapter有个默认构造函数

public RequestMappingHandlerAdapter() {
   StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();
   stringHttpMessageConverter.setWriteAcceptCharset(false);  // see SPR-7316

   this.messageConverters = new ArrayList<HttpMessageConverter<?>>(4);
   this.messageConverters.add(new ByteArrayHttpMessageConverter());
   this.messageConverters.add(stringHttpMessageConverter);
   this.messageConverters.add(new SourceHttpMessageConverter<Source>());
   this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
}

很明显在创建对象实例的时候 就对messageConverters进行了赋值，最后添加的AllEncompassingFormHttpMessageConverter    所有包含的FormHttpMessageConverter，看看究竟有哪些

if条件的值 则是

private static final boolean jaxb2Present =
      ClassUtils.isPresent("javax.xml.bind.Binder", AllEncompassingFormHttpMessageConverter.class.getClassLoader());

private static final boolean jackson2Present =
      ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", AllEncompassingFormHttpMessageConverter.class.getClassLoader()) &&
            ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator", AllEncompassingFormHttpMessageConverter.class.getClassLoader());

private static final boolean jackson2XmlPresent =
      ClassUtils.isPresent("com.fasterxml.jackson.dataformat.xml.XmlMapper", AllEncompassingFormHttpMessageConverter.class.getClassLoader());

private static final boolean gsonPresent =
      ClassUtils.isPresent("com.google.gson.Gson", AllEncompassingFormHttpMessageConverter.class.getClassLoader());

也就是当com.fasterxml.jackson.databind.ObjectMapper和com.fasterxml.jackson.core.JsonGenerator存在在classpath中才会去加载MappingJackson2HttpMessageConverter。（其他一样的道理）

现在是不是知道了 为什么 springMVC默认就支持json转换器，不过这里我用fastjson作为分析例子

messageConverter的read方法

public final T read(Class<? extends T> clazz, HttpInputMessage inputMessage) throws IOException {
   return readInternal(clazz, inputMessage);
}

FastJsonHttpMessageConverter的readInternal方法为：

protected Object readInternal(Class<? extends Object> clazz, //
                              HttpInputMessage inputMessage //
) throws IOException, HttpMessageNotReadableException {

    InputStream in = inputMessage.getBody();
    return JSON.parseObject(in, fastJsonConfig.getCharset(), clazz, fastJsonConfig.getFeatures());
}

可以看到就是把请求消息体输入流转换为json对象，这就是整个消息转化为json的过程

下面我们解析分析 是如何将@ResponseBody注解标准的Controller方法返回的java对象转换为json的

还记得之前RequestMappingHandlerAdapter中invokeAndHandle方法么，其中也包含了对返回值的处理

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

handleReturnValule方法的具体实现：

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

   HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
   if (handler == null) {
      throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
   }
   handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}

首先通过返回值以及返回类型搜索对应的returnValueHandler，其次再通过得到的handler对返回消息进行处理

handleRreturnValue的实现为

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

   mavContainer.setRequestHandled(true);

   // Try even with null return value. ResponseBodyAdvice could get involved.
   writeWithMessageConverters(returnValue, returnType, webRequest);
}

没有似曾相识的感觉 之前我们分析解析请求消息时候 有个readWithMessageConverters 现在又有个writeWithMessageConverters方法，那我们看看具体实现：

方法中代码太多，中间一段略过。。。。。

write方法是代码如下：

public final void write(final T t, MediaType contentType, HttpOutputMessage outputMessage)
      throws IOException, HttpMessageNotWritableException {

   final HttpHeaders headers = outputMessage.getHeaders();
   addDefaultHeaders(headers, t, contentType);

   if (outputMessage instanceof StreamingHttpOutputMessage) {
      StreamingHttpOutputMessage streamingOutputMessage =
            (StreamingHttpOutputMessage) outputMessage;
      streamingOutputMessage.setBody(new StreamingHttpOutputMessage.Body() {
         @Override
         public void writeTo(final OutputStream outputStream) throws IOException {
            writeInternal(t, new HttpOutputMessage() {
               @Override
               public OutputStream getBody() throws IOException {
                  return outputStream;
               }
               @Override
               public HttpHeaders getHeaders() {
                  return headers;
               }
            });
         }
      });
   }
   else {
      writeInternal(t, outputMessage);
      outputMessage.getBody().flush();
   }
}

我们重点关注writeInternal方法的实现，在FastJsonHttpMessageConverter中为

protected void writeInternal(Object obj, HttpOutputMessage outputMessage)
        throws IOException, HttpMessageNotWritableException {
    HttpHeaders headers = outputMessage.getHeaders();
    ByteArrayOutputStream outnew = new ByteArrayOutputStream();

    boolean writeAsToString = false;
    if (obj != null) {
        String className = obj.getClass().getName();
        if ("com.fasterxml.jackson.databind.node.ObjectNode".equals(className)) {
            writeAsToString = true;
        }
    }

    if (writeAsToString) {
        String text = obj.toString();
        OutputStream out = outputMessage.getBody();
        out.write(text.getBytes());
        if (fastJsonConfig.isWriteContentLength()) {
            headers.setContentLength(text.length());
        }
    } else {
        int len = JSON.writeJSONString(outnew, //
                fastJsonConfig.getCharset(), //
                obj, //
                fastJsonConfig.getSerializeConfig(), //
                fastJsonConfig.getSerializeFilters(), //
                fastJsonConfig.getDateFormat(), //
                JSON.DEFAULT_GENERATE_FEATURE, //
                fastJsonConfig.getSerializerFeatures());
        if (fastJsonConfig.isWriteContentLength()) {
            headers.setContentLength(len);
        }

        OutputStream out = outputMessage.getBody();
        outnew.writeTo(out);
    }


    outnew.close();
}

此方法中将Java对象转换为json字符串

其他消息转换器类似，如果读者感兴趣可自己去查看代码研究，这里就不作分析拉