引言：如何在SpringMVC中统一对返回的Json进行加密？”。
postHandler方法处理。实际这是行不通的，因为当程序运行到该方法，是在返回数据之后，渲染页面之前，所以这时候HttpServletResponse中的输出流已经关闭了，自然无法在对返回数据进行处理。MethodArgumentResolver和HandlerMethodReturnValueHandler，还是接下来要提到的HttpMessageConverter。@RequestBody和@ResponseBody，通过这两个注解，可以在Controller中直接使用Java对象作为请求参数和返回内容，而完成这之间转换作用的便是HttpMessageConverter。
package org.springframework.http.converter;
import java.io.IOException;
import java.util.List;
import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
public interface HttpMessageConverter<T> {
/**
* Indicates whether the given class can be read by this converter.
* @param clazz the class to test for readability
* @param mediaType the media type to read, can be {@code null} if not specified.
* Typically the value of a {@code Content-Type} header.
* @return {@code true} if readable; {@code false} otherwise
*/
boolean canRead(Class<?> clazz, MediaType mediaType);
/**
* Indicates whether the given class can be written by this converter.
* @param clazz the class to test for writability
* @param mediaType the media type to write, can be {@code null} if not specified.
* Typically the value of an {@code Accept} header.
* @return {@code true} if writable; {@code false} otherwise
*/
boolean canWrite(Class<?> clazz, MediaType mediaType);
/**
* Return the list of {@link MediaType} objects supported by this converter.
* @return the list of supported media types
*/
List<MediaType> getSupportedMediaTypes();
/**
* Read an object of the given type form the given input message, and returns it.
* @param clazz the type of object to return. This type must have previously been passed to the
* {@link #canRead canRead} method of this interface, which must have returned {@code true}.
* @param inputMessage the HTTP input message to read from
* @return the converted object
* @throws IOException in case of I/O errors
* @throws HttpMessageNotReadableException in case of conversion errors
*/
T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
throws IOException, HttpMessageNotReadableException;
/**
* Write an given object to the given output message.
* @param t the object to write to the output message. The type of this object must have previously been
* passed to the {@link #canWrite canWrite} method of this interface, which must have returned {@code true}.
* @param contentType the content type to use when writing. May be {@code null} to indicate that the
* default content type of the converter must be used. If not {@code null}, this media type must have
* previously been passed to the {@link #canWrite canWrite} method of this interface, which must have
* returned {@code true}.
* @param outputMessage the message to write to
* @throws IOException in case of I/O errors
* @throws HttpMessageNotWritableException in case of conversion errors
*/
void write(T t, MediaType contentType, HttpOutputMessage outputMessage)
throws IOException, HttpMessageNotWritableException;
}HttpMessageConverter接口提供了5个方法：canRead
- ：判断该转换器是否能将请求内容转换成Java对象
canWrite
- ：判断该转换器是否可以将Java对象转换成返回内容
getSupportedMediaTypes
- ：获得该转换器支持的MediaType类型
read
- ：读取请求内容并转换成Java对象
write
- ：将Java对象转换后写入返回内容
read和write方法的参数分别有有HttpInputMessage和HttpOutputMessage对象，这两个对象分别代表着一次Http通讯中的请求和响应部分，可以通过getBody方法获得对应的输入流和输出流。
当前SpringMVC中已经默认提供了相当多的转换器，如上图，其中常用的有：
名称 | 作用 | 读支持MediaType | 写支持MediaType |
ByteArrayHttpMessageConverter | 数据与字节数组的相互转换 |
| application/octet-stream |
StringHttpMessageConverter | 数据与String类型的相互转换 | text/* | text/plain |
FormHttpMessageConverter | 表单与MultiValueMap的相互转换 | application/x-www-form-urlencoded | application/x-www-form-urlencoded |
SourceHttpMessageConverter | 数据与javax.xml.transform.Source的相互转换 | text/xml和application/xml | text/xml和application/xml |
MarshallingHttpMessageConverter | 使用Spring的Marshaller/Unmarshaller转换XML数据 | text/xml和application/xml | text/xml和application/xml |
MappingJackson2HttpMessageConverter | 使用Jackson的ObjectMapper转换Json数据 | application/json | application/json |
MappingJackson2XmlHttpMessageConverter | 使用Jackson的XmlMapper转换XML数据 | application/xml | application/xml |
BufferedImageHttpMessageConverter | 数据与java.awt.image.BufferedImage的相互转换 | Java I/O API支持的所有类型 | Java I/O API支持的所有类型 |
SpringMVC自己的
HttpMessageConverter还是定义在RequestMappingHandlerAdapter里，并且被MethodArgumentResolver和HandlerMethodReturnValueHandler解析数据的时候所用到。
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());
}
这里面四个
HttpMessageConverter
，重点介绍下
AllEncompassingFormHttpMessageConverter
，看源码。
public class AllEncompassingFormHttpMessageConverter extends FormHttpMessageConverter {
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());
public AllEncompassingFormHttpMessageConverter() {
addPartConverter(new SourceHttpMessageConverter<Source>());
if (jaxb2Present && !jackson2XmlPresent) {
addPartConverter(new Jaxb2RootElementHttpMessageConverter());
}
if (jackson2Present) {
addPartConverter(new MappingJackson2HttpMessageConverter());
} else if (gsonPresent) {
addPartConverter(new GsonHttpMessageConverter());
}
if (jackson2XmlPresent) {
addPartConverter(new MappingJackson2XmlHttpMessageConverter());
}
}
}
它的主要作用是，从类加载器去查找相关的类，只有这些转换器需要的类（jar包被你引入了）存在，那么转换器才会被加载进去，比如我们在项目中引入了
1. <dependency>
2. <groupId>com.fasterxml.jackson.core</groupId>
3. <artifactId>jackson-databind</artifactId>
4. <version>${jackson.version}</version>
5. </dependency>
MappingJackson2HttpMessageConverter被加入到了SpringMVC中，后面在使用@RequestBody和@ResponseBody，RequestResponseBodyMethodProcessor起作用的时候，它就会寻找能够匹配的Converter，进而找到MappingJackson2HttpMessageConverter用来转换对象。
那如果我们引入了
1. <dependency>
2. <groupId>com.fasterxml.jackson.dataformat</groupId>
3. <artifactId>jackson-dataformat-xml</artifactId>
4. <version>${jackson.version}</version>
5. </dependency>那也就预示着MappingJackson2XmlHttpMessageConverter被加入转换器队列中了，那么你使用@ResponseBody，响应类型为Content-Type: application/xml;charset=UTF-8，估计是默认请求头Accept直接为application/xml,具体源码我就不去翻了。
当用户发送请求后，@RequestBody注解会读取请求body中的数据，通过获取请求头Header中的Content-Type来确认请求头的数据格式，从而来为请求数据适配合适的转换器。例如contentType:applicatin/json，那么转换器会适配MappingJackson2HttpMessageConverter。响应时候的时候同理，@ResponseBody注解会通过检测请求头Header中Accept属性来适配对应响应的转换器。
我这里因为没找到合适的例子去实现自己的HttpMessageConverter，但想到了另外一个可以拿来用的例子。
@RequestMapping(value="simple")
public String simple(){
return "abc中国";
}abc中国.jsp的页面，由于找不到，会报404。
但当我们稍微修改如下：
@RequestMapping(value="simple")
@ResponseBody
public String simple(){
return "abc中国";
}@ResponseBody，当我们再次发起请求的时候，请求头Header中Accept为
text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8StringHttpMessageConverter，并完成转换，输出的直接是返回值，然而我们却发现结果中文乱码了，这是因为该类默认支持的编码格式是ISO-8859-1。
package org.springframework.http.converter;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.List;
import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.util.StreamUtils;
public class StringHttpMessageConverter extends AbstractHttpMessageConverter<String> {
public static final Charset DEFAULT_CHARSET = Charset.forName("ISO-8859-1");那我们应当如何使之支持UTF-8呢，这也是接下来我们所需要介绍的，应当如何覆盖SpringMVC自己的转换器，或者新增自定义的转换器。
1. <mvc:annotation-driven>
2. <mvc:message-converters>
3. <bean class="org.springframework.http.converter.StringHttpMessageConverter">
4. <property name="supportedMediaTypes">
5. <list>
6. <value>text/plain;charset=UTF-8</value>
7. <value>text/html;charset=UTF-8</value>
8. </list>
9. </property>
10. </bean>
11. </mvc:message-converters>
12. </mvc:annotation-driven>
如果我们不想使用系统默认的转换器，还可以去掉系统给你定义的，然后重新定义你所需要的转换器，如下：
1. <mvc:annotation-driven>
2. <mvc:message-converters register-defaults="false">
3. <bean class="org.springframework.http.converter.StringHttpMessageConverter">
4. <property name="supportedMediaTypes">
5. <list>
6. <value>text/plain;charset=UTF-8</value>
7. <value>text/html;charset=UTF-8</value>
8. </list>
9. </property>
10. </bean>
11. </mvc:message-converters>
12. </mvc:annotation-driven>
我个人的意见是，如果没有特别的需要，不要重新定义自己的转换器，我们大可覆盖或者新增就行了。
