springmvc 视图解析器 原理 返回根据请求返回xml或者

之前公司的项目能什么都不配置就实现自动返回xml或者json(浏览器直接打开连接返回xml,ajax提交返回json),今天自己搭的框架也想实现这个效果,总结如下:
如果想实现什么都不配置需要导入以下两个jar

<dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>${jackson.version}</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.dataformat</groupId>
            <artifactId>jackson-dataformat-xml</artifactId>
            <version>${jackson.version}</version>
        </dependency>

为什么呢,因为在springmvc加载的时候 AnnotationDrivenBeanDefinitionParserl类里有一段代码

private static final boolean jackson2XmlPresent =
            ClassUtils.isPresent("com.fasterxml.jackson.dataformat.xml.XmlMapper", AnnotationDrivenBeanDefinitionParser.class.getClassLoader());
if (jackson2XmlPresent) {
                RootBeanDefinition jacksonConverterDef = createConverterDefinition(MappingJackson2XmlHttpMessageConverter.class, source);
                GenericBeanDefinition jacksonFactoryDef = createObjectMapperFactoryDefinition(source);
                jacksonFactoryDef.getPropertyValues().add("createXmlMapper", true);
                jacksonConverterDef.getConstructorArgumentValues().addIndexedArgumentValue(0, jacksonFactoryDef);
                messageConverters.add(jacksonConverterDef);
            }

如果spring扫描jar的时候扫到了jackson处理xml的这个包时,spingmvc会默认加载这个实现(视图解析器)
为了更熟悉springmvc原理,知其然更知其所以然,我查看了MappingJackson2XmlHttpMessageConverter这个类的代码,
发现它继承了AbstractHttpMessageConverter<T>这个类,因为之前我用的是fastjson提供的视图解析器FastJsonHttpMessageConverter,它继承的也是这个类,我看了它的代码,决定自己实现一个xml视图解析器(fastjson是解析json,我按照它的逻辑实现xml解析)
代码如下:

package cn.ddsxy.ddlx.common;
 
import com.thoughtworks.xstream.XStream;
import com.thoughtworks.xstream.io.xml.XppDriver;
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 java.io.IOException;
import java.nio.charset.Charset;
 
/**
 * 自定义xml返回数据
 */
public class CustomXmlHttpMessageConverter extends AbstractHttpMessageConverter<Object> {
 
    public final static Charset UTF8 = Charset.forName("UTF-8");
 
    private static XStream xstream = new XStream(new XppDriver());
 
    public CustomXmlHttpMessageConverter() {
        super(new MediaType("application", "xml"),
                new MediaType("text", "xml"),
                new MediaType("application", "*+xml"));
    }
 
    protected boolean supports(Class<?> clazz) {
        return true;
    }
 
    protected Object readInternal(Class<?> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
        xstream.alias(clazz.getSimpleName(), clazz);
        return xstream.fromXML(inputMessage.getBody());
    }
 
    protected void writeInternal(Object o, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        outputMessage.getBody().write(xstream.toXML(o).getBytes("utf-8"));
    }
}

使用的是Xstream这个jar处理xml同java对象的解析,这里有一个重点,springmvc是怎么自动返回xml或者json的呢
涉及到一个属性,大家看我实现类的构造器,传了一个数组MediaType,是的,springmvc就是根据这个来匹配请求头设置选择使用哪个视图解析器的!!!! 当然你也可以不直接在构造器里传参,AbstractHttpMessageConverter类有个属性supportedMediaTypes

<mvc:annotation-driven>
        <mvc:message-converters>
            <bean class="cn.ddsxy.ddlx.common.CustomXmlHttpMessageConverter">                                        
                <property name="supportedMediaTypes">
                    <list>
                        <!--<value>text/html;charset=UTF-8</value>-->
                        <value>application/json;charset=UTF-8</value>
                    </list>
                </property>
            </bean>
        </mvc:message-converters>
</mvc:annotation-driven>