SpringMvc返回JSON的工作原理。

一:导入jar包。然后在springmvc.xml中配置上这么一句话

1 
2     

会自动注册DefaultAnnotationHandlerMapping与AnnotationMethodHandlerAdapter 两个bean,是spring MVC为@Controllers分发请求所必须的。
并提供了:数据绑定支持,@NumberFormatannotation支持,@DateTimeFormat支持,@Valid支持,读写XML的支持(JAXB),读写JSON的支持(Jackson)。

二:在方法中前面加上这么一个注解:

@ResponseBody
return返回的就是一个JSON的对象。而不是去跳转到页面上。
原理:使用HtppMessageConverter这个类。
      这个类负责将请求信息转换成一个对象。将对象输出为响应信息。

SpringMvc返回JSON的工作原理。_第1张图片

简介:

@RequestBody

作用: 

      i) 该注解用于读取Request请求的body部分数据,使用系统默认配置的HttpMessageConverter进行解析,然后把相应的数据绑定到要返回的对象上;

      ii) 再把HttpMessageConverter返回的对象数据绑定到 controller中方法的参数上。

使用时机:

A) GET、POST方式提时, 根据request header Content-Type的值来判断:

  •     application/x-www-form-urlencoded, 可选(即非必须,因为这种情况的数据@RequestParam, @ModelAttribute也可以处理,当然@RequestBody也能处理);
  •     multipart/form-data, 不能处理(即使用@RequestBody不能处理这种格式的数据);
  •     其他格式, 必须(其他格式包括application/json, application/xml等。这些格式的数据,必须使用@RequestBody来处理);

 

B) PUT方式提交时, 根据request header Content-Type的值来判断:

 

  •     application/x-www-form-urlencoded, 必须;
  •     multipart/form-data, 不能处理;
  •     其他格式, 必须;

说明:request的body部分的数据编码格式由header部分的Content-Type指定;

 

 

@ResponseBody

 

作用: 

      该注解用于将Controller的方法返回的对象,通过适当的HttpMessageConverter转换为指定格式后,写入到Response对象的body数据区。

使用时机:

      返回的数据不是html标签的页面,而是其他某种格式的数据时(如json、xml等)使用;

 1 /**
 2  * Strategy interface that specifies a converter that can convert from and to HTTP requests and responses.
 3  *
 4  * @author Arjen Poutsma
 5  * @author Juergen Hoeller
 6  * @since 3.0
 7  */
 8 public interface HttpMessageConverter {
 9 
10     /**
11      * Indicates whether the given class can be read by this converter.
12      * @param clazz the class to test for readability
13      * @param mediaType the media type to read, can be {@code null} if not specified.
14      * Typically the value of a {@code Content-Type} header.
15      * @return {@code true} if readable; {@code false} otherwise
16      */
17     boolean canRead(Class clazz, MediaType mediaType);
18 
19     /**
20      * Indicates whether the given class can be written by this converter.
21      * @param clazz the class to test for writability
22      * @param mediaType the media type to write, can be {@code null} if not specified.
23      * Typically the value of an {@code Accept} header.
24      * @return {@code true} if writable; {@code false} otherwise
25      */
26     boolean canWrite(Class clazz, MediaType mediaType);
27 
28     /**
29      * Return the list of {@link MediaType} objects supported by this converter.
30      * @return the list of supported media types
31      */
32     List getSupportedMediaTypes();
33 
34     /**
35      * Read an object of the given type form the given input message, and returns it.
36      * @param clazz the type of object to return. This type must have previously been passed to the
37      * {@link #canRead canRead} method of this interface, which must have returned {@code true}.
38      * @param inputMessage the HTTP input message to read from
39      * @return the converted object
40      * @throws IOException in case of I/O errors
41      * @throws HttpMessageNotReadableException in case of conversion errors
42      */
43     T read(Classextends T> clazz, HttpInputMessage inputMessage)
44             throws IOException, HttpMessageNotReadableException;
45 
46     /**
47      * Write an given object to the given output message.
48      * @param t the object to write to the output message. The type of this object must have previously been
49      * passed to the {@link #canWrite canWrite} method of this interface, which must have returned {@code true}.
50      * @param contentType the content type to use when writing. May be {@code null} to indicate that the
51      * default content type of the converter must be used. If not {@code null}, this media type must have
52      * previously been passed to the {@link #canWrite canWrite} method of this interface, which must have
53      * returned {@code true}.
54      * @param outputMessage the message to write to
55      * @throws IOException in case of I/O errors
56      * @throws HttpMessageNotWritableException in case of conversion errors
57      */
58     void write(T t, MediaType contentType, HttpOutputMessage outputMessage)
59             throws IOException, HttpMessageNotWritableException;
60 
61 }
62 

该接口定义了四个方法,分别是读取数据时的 canRead(), read() 和 写入数据时的canWrite(), write()方法。

 

在使用 标签配置时,默认配置了RequestMappingHandlerAdapter(注意是RequestMappingHandlerAdapter不是AnnotationMethodHandlerAdapter,详情查看Spring 3.1 document “16.14 Configuring Spring MVC”章节),并为他配置了一下默认的HttpMessageConverter:

ByteArrayHttpMessageConverter converts byte arrays.

    StringHttpMessageConverter converts strings.

    ResourceHttpMessageConverter converts to/from org.springframework.core.io.Resource for all media types.

    SourceHttpMessageConverter converts to/from a javax.xml.transform.Source.

    FormHttpMessageConverter converts form data to/from a MultiValueMap.

    Jaxb2RootElementHttpMessageConverter converts Java objects to/from XML — added if JAXB2 is present on the classpath.

    MappingJacksonHttpMessageConverter converts to/from JSON — added if Jackson is present on the classpath.

    AtomFeedHttpMessageConverter converts Atom feeds — added if Rome is present on the classpath.

    RssChannelHttpMessageConverter converts RSS feeds — added if Rome is present on the classpath.

ByteArrayHttpMessageConverter: 负责读取二进制格式的数据和写出二进制格式的数据;

StringHttpMessageConverter:   负责读取字符串格式的数据和写出二进制格式的数据;

 

ResourceHttpMessageConverter:负责读取资源文件和写出资源文件数据; 

FormHttpMessageConverter:       负责读取form提交的数据(能读取的数据格式为 application/x-www-form-urlencoded,不能读取multipart/form-data格式数据);负责写入application/x-www-from-urlencoded和multipart/form-data格式的数据;

 

MappingJacksonHttpMessageConverter:  负责读取和写入json格式的数据;

 

SouceHttpMessageConverter:                   负责读取和写入 xml 中javax.xml.transform.Source定义的数据;

Jaxb2RootElementHttpMessageConverter:  负责读取和写入xml 标签格式的数据;

 

AtomFeedHttpMessageConverter:              负责读取和写入Atom格式的数据;

RssChannelHttpMessageConverter:           负责读取和写入RSS格式的数据;

 

当使用@RequestBody和@ResponseBody注解时,RequestMappingHandlerAdapter就使用它们来进行读取或者写入相应格式的数据。

HttpMessageConverter匹配过程:

@RequestBody注解时: 根据Request对象header部分的Content-Type类型,逐一匹配合适的HttpMessageConverter来读取数据;

spring 3.1源代码如下:

 1 private Object readWithMessageConverters(MethodParameter methodParam, HttpInputMessage inputMessage, Class paramType)
 2             throws Exception {
 3 
 4         MediaType contentType = inputMessage.getHeaders().getContentType();
 5         if (contentType == null) {
 6             StringBuilder builder = new StringBuilder(ClassUtils.getShortName(methodParam.getParameterType()));
 7             String paramName = methodParam.getParameterName();
 8             if (paramName != null) {
 9                 builder.append(' ');
10                 builder.append(paramName);
11             }
12             throw new HttpMediaTypeNotSupportedException(
13                     "Cannot extract parameter (" + builder.toString() + "): no Content-Type found");
14         }
15 
16         List allSupportedMediaTypes = new ArrayList();
17         if (this.messageConverters != null) {
18             for (HttpMessageConverter messageConverter : this.messageConverters) {
19                 allSupportedMediaTypes.addAll(messageConverter.getSupportedMediaTypes());
20                 if (messageConverter.canRead(paramType, contentType)) {
21                     if (logger.isDebugEnabled()) {
22                         logger.debug("Reading [" + paramType.getName() + "] as \"" + contentType
23                                 +"\" using [" + messageConverter + "]");
24                     }
25                     return messageConverter.read(paramType, inputMessage);
26                 }
27             }
28         }
29         throw new HttpMediaTypeNotSupportedException(contentType, allSupportedMediaTypes);
30     }

@ResponseBody注解时: 根据Request对象header部分的Accept属性(逗号分隔),逐一按accept中的类型,去遍历找到能处理的HttpMessageConverter;

源代码如下:

private void writeWithMessageConverters(Object returnValue,
                HttpInputMessage inputMessage, HttpOutputMessage outputMessage)
                throws IOException, HttpMediaTypeNotAcceptableException {
            List acceptedMediaTypes = inputMessage.getHeaders().getAccept();
            if (acceptedMediaTypes.isEmpty()) {
                acceptedMediaTypes = Collections.singletonList(MediaType.ALL);
            }
            MediaType.sortByQualityValue(acceptedMediaTypes);
            Class returnValueType = returnValue.getClass();
            List allSupportedMediaTypes = new ArrayList();
            if (getMessageConverters() != null) {
                for (MediaType acceptedMediaType : acceptedMediaTypes) {
                    for (HttpMessageConverter messageConverter : getMessageConverters()) {
                        if (messageConverter.canWrite(returnValueType, acceptedMediaType)) {
                            messageConverter.write(returnValue, acceptedMediaType, outputMessage);
                            if (logger.isDebugEnabled()) {
                                MediaType contentType = outputMessage.getHeaders().getContentType();
                                if (contentType == null) {
                                    contentType = acceptedMediaType;
                                }
                                logger.debug("Written [" + returnValue + "] as \"" + contentType +
                                        "\" using [" + messageConverter + "]");
                            }
                            this.responseArgumentUsed = true;
                            return;
                        }
                    }
                }
                for (HttpMessageConverter messageConverter : messageConverters) {
                    allSupportedMediaTypes.addAll(messageConverter.getSupportedMediaTypes());
                }
            }
            throw new HttpMediaTypeNotAcceptableException(allSupportedMediaTypes);
        }

 

 
  

转载于:https://www.cnblogs.com/bulrush/p/8681106.html

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