使用 @ResponseBody 自定义 HttpMessageConverter 来做 Json 事情

嗯...很难找到答案，我不得不追踪这么多线索寻找不完整的信息，所以我想把完整的答案贴在这里会很好.这样下一个人寻找这个就容易多了.

首先，我必须实现定制的HttpMessageConverter:

package net.iogui.web.spring.converter;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.io.StringWriter;
import java.io.Writer;
import java.nio.charset.Charset;

import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.http.converter.AbstractHttpMessageConverter;
import org.springframework.http.converter.HttpMessageNotReadableException;
import org.springframework.http.converter.HttpMessageNotWritableException;

import com.google.gson.Gson;
import com.google.gson.JsonSyntaxException;

public class GsonHttpMessageConverter extends AbstractHttpMessageConverter<Object> {

    private Gson gson = new Gson();

    public static final Charset DEFAULT_CHARSET = Charset.forName("UTF-8");

    public GsonHttpMessageConverter(){
        super(new MediaType("application", "json", DEFAULT_CHARSET));
    }

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

        try{
            return gson.fromJson(convertStreamToString(inputMessage.getBody()), clazz);
        }catch(JsonSyntaxException e){
            throw new HttpMessageNotReadableException("Could not read JSON: " + e.getMessage(), e);
        }

    }

    @Override
    protected boolean supports(Class<?> clazz) {
        return true;
    }

    @Override
    protected void writeInternal(Object t, 
                                 HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {

        //TODO: adapt this to be able to receive a list of json objects too

        String json = gson.toJson(t);

        outputMessage.getBody().write(json.getBytes());
    }

    //TODO: move this to a more appropriated utils class
    public String convertStreamToString(InputStream is) throws IOException {
        /*
         * To convert the InputStream to String we use the Reader.read(char[]
         * buffer) method. We iterate until the Reader return -1 which means
         * there's no more data to read. We use the StringWriter class to
         * produce the string.
         */
        if (is != null) {
            Writer writer = new StringWriter();

            char[] buffer = new char[1024];
            try {
                Reader reader = new BufferedReader(new InputStreamReader(is, "UTF-8"));
                int n;
                while ((n = reader.read(buffer)) != -1) {
                    writer.write(buffer, 0, n);
                }
            } finally {
                is.close();
            }
            return writer.toString();
        } else {
            return "";
        }
    }

}

Then I had to strip off the annnotaion-driven tag and configure all by my own hands on the spring-mvc configuration file:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xsi:schemaLocation="http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <!-- Configures the @Controller programming model -->

    <!-- To use just with a JSR-303 provider in the classpath 
    <bean id="validator" class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean" />
    -->

    <bean id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean" />

    <bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
        <property name="webBindingInitializer">
            <bean class="net.iogui.web.spring.util.CommonWebBindingInitializer" />
        </property>
        <property name="messageConverters">
            <list>
                <bean class="org.springframework.http.converter.ByteArrayHttpMessageConverter" />
                <bean class="org.springframework.http.converter.StringHttpMessageConverter" />
                <bean class="org.springframework.http.converter.ResourceHttpMessageConverter" />
                <bean class="net.iogui.web.spring.converter.GsonHttpMessageConverter" />
                <bean class="org.springframework.http.converter.xml.SourceHttpMessageConverter" />
                <bean class="org.springframework.http.converter.xml.XmlAwareFormHttpMessageConverter" />
                <!-- bean class="org.springframework.http.converter.xml.Jaxb2RootElementHttpMessageConverter" /-->
            </list>
        </property>
    </bean>
    <bean id="handlerMapping" class="org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping" />


    <context:component-scan base-package="net.iogui.teste.web.controller"/>

    <!-- Forwards requests to the "/" resource to the "login" view -->
    <mvc:view-controller path="/" view-name="home"/>

    <!-- Handles HTTP GET requests for /resources/** by efficiently serving up static resources in the ${webappRoot}/resources/ directory -->
    <mvc:resources mapping="/resources/**" location="/resources/" />

    <bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/>
        <property name="prefix" value="/WEB-INF/view/"/>
        <property name="suffix" value=".jsp"/>
    </bean>

</beans>

请注意，要使Formater和Validator正常工作，我们还必须构建自定义webBindingInitializer:

package net.iogui.web.spring.util;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.core.convert.ConversionService;
import org.springframework.validation.Validator;
import org.springframework.web.bind.WebDataBinder;
import org.springframework.web.bind.support.WebBindingInitializer;
import org.springframework.web.context.request.WebRequest;

public class CommonWebBindingInitializer implements WebBindingInitializer {

    @Autowired(required=false)
    private Validator validator;

    @Autowired
    private ConversionService conversionService;

    @Override
    public void initBinder(WebDataBinder binder, WebRequest request) {
        binder.setValidator(validator);
        binder.setConversionService(conversionService);
    }

}

An Interesting thing to see is that In order to make the configuration work without the annotaion-driven tag, we have to manually configure a AnnotationMethodHandlerAdapter and a DefaultAnnotationHandlerMapping. And in order to make the AnnotationMethodHandlerAdapter capable of handling formatting and validation, we had to configure a validator, a conversionService and to build a custom webBindingInitializer.

I hope all this helps someone else besides me.

On my desperate search, this @Bozho post was extremely util. I am also grateful to @GaryF couse his answer took me to the @Bozho post. To you that are trying to do this in Spring 3.1, see @Robby Pond answer.. A lot easier, isn't it?