SpringMVC重要接口（二）HandlerMethodReturnValueHandler

我们在上一节提到，SpringMVC方法中的参数都是靠HandlerMethodArgumentResolver解析的，那返回值呢。

我们都使用过@ResponseBody注解，也知道它的作用，那又是谁来解析这个注解，使之输出我们想要的东西，本节我们一起来剖析org.springframework.web.method.support.HandlerMethodReturnValueHandler。

package org.springframework.web.method.support;
 
import org.springframework.core.MethodParameter;
import org.springframework.web.context.request.NativeWebRequest;
 
/**
 * Strategy interface to handle the value returned from the invocation of a
 * handler method .
 *
 * @author Arjen Poutsma
 * @since 3.1
 * @see HandlerMethodArgumentResolver
 */
public interface HandlerMethodReturnValueHandler {
 
    /**
     * Whether the given {@linkplain MethodParameter method return type} is
     * supported by this handler.
     * @param returnType the method return type to check
     * @return {@code true} if this handler supports the supplied return type;
     * {@code false} otherwise
     */
    boolean supportsReturnType(MethodParameter returnType);
 
    /**
     * Handle the given return value by adding attributes to the model and
     * setting a view or setting the
     * {@link ModelAndViewContainer#setRequestHandled} flag to {@code true}
     * to indicate the response has been handled directly.
     * @param returnValue the value returned from the handler method
     * @param returnType the type of the return value. This type must have
     * previously been passed to {@link #supportsReturnType} which must
     * have returned {@code true}.
     * @param mavContainer the ModelAndViewContainer for the current request
     * @param webRequest the current request
     * @throws Exception if the return value handling results in an error
     */
    void handleReturnValue(Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;
 
}

HandlerMethodArgumentResolver接口同样包含两个接口方法

• supportsReturnType 
  通过该方法我们如果需要对某个参数进行处理 只要此处返回true即可， 
  通过MethodParameter可以获取该方法返回值上的一些信息， 如方法返回值中的注解信息等

@Override
public boolean supportsReturnType(MethodParameter returnType) {
    return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) || returnType.hasMethodAnnotation(ResponseBody.class));
}

@Override
public boolean supportsReturnType(MethodParameter returnType) {
    return ModelAndView.class.isAssignableFrom(returnType.getParameterType());
}

• resolveArgument 
  实现对返回值的处理操作。

SpringMVC自己的 
HandlerMethodReturnValueHandler有哪些，并且会以什么样的顺序执行呢？ 
其实定义在RequestMappingHandlerAdapter里：

/**
 * Return the list of return value handlers to use including built-in and
 * custom handlers provided via {@link #setReturnValueHandlers}.
 */
private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {
    List<HandlerMethodReturnValueHandler> handlers = new ArrayList<HandlerMethodReturnValueHandler>();
 
    // Single-purpose return value types
    handlers.add(new ModelAndViewMethodReturnValueHandler());
    handlers.add(new ModelMethodProcessor());
    handlers.add(new ViewMethodReturnValueHandler());
    handlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters()));
    handlers.add(new StreamingResponseBodyReturnValueHandler());
    handlers.add(new HttpEntityMethodProcessor(getMessageConverters(),
                this.contentNegotiationManager, this.requestResponseBodyAdvice));
    handlers.add(new HttpHeadersReturnValueHandler());
    handlers.add(new CallableMethodReturnValueHandler());
    handlers.add(new DeferredResultMethodReturnValueHandler());
    handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));
 
    // Annotation-based return value types
    handlers.add(new ModelAttributeMethodProcessor(false));
    handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),
            this.contentNegotiationManager, this.requestResponseBodyAdvice));
 
    // Multi-purpose return value types
    handlers.add(new ViewNameMethodReturnValueHandler());
    handlers.add(new MapMethodProcessor());
 
    // Custom return value types
    if (getCustomReturnValueHandlers() != null) {
        handlers.addAll(getCustomReturnValueHandlers());
    }
 
    // Catch-all
    if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {
            handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));
        }
    else {
        handlers.add(new ModelAttributeMethodProcessor(true));
    }
 
    return handlers;
}

HandlerMethodReturnValueHandler子类注解 or 类org.springframework.web.servlet.mvc.method.annotation.ModelAndViewMethodReturnValueHandlerModelAndVieworg.springframework.web.method.annotation.ModelMethodProcessorModelorg.springframework.web.servlet.mvc.method.annotation.ViewMethodReturnValueHandlerVieworg.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitterReturnValueHandlerResponseEntityorg.springframework.web.servlet.mvc.method.annotation.StreamingResponseBodyReturnValueHandlerStreamingResponseBodyorg.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessorHttpEntityorg.springframework.web.servlet.mvc.method.annotation.HttpHeadersReturnValueHandlerHttpHeadersorg.springframework.web.servlet.mvc.method.annotation.CallableMethodReturnValueHandlerCallableorg.springframework.web.servlet.mvc.method.annotation.DeferredResultMethodReturnValueHandler org.springframework.web.servlet.mvc.method.annotation.AsyncTaskMethodReturnValueHandlerWebAsyncTaskorg.spr[e-mail][e-mail]eHandlerCharSequence 、voidorg.springframework.web.method.annotation.MapMethodProcessorMap

---

<mvc:annotation-driven>
    <mvc:return-value-handlers>
        <bean class=""/>
   </mvc:return-value-handlers>
</mvc:annotation-driven>