springmvc核心之HandlerMethodReturnValueHandler

在springmvc当中@ResponseBody的作用估计很多人都知道，在controller中使用了@ResponseBody就会返回对应的数据结果(json格式)，而不是jsp页面或者其他视图。如果不加，那么它就返回了一个具体的视图，如jsp/html等。springmvc是如何做到这个效果的呢？其实springmvc为处理各种返回值提供了很多的处理类，这些处理类大多类都是已ReturnValueHandler或者Processor(包含了参数处理)结尾，这些处理类的核心接口就是：
org.springframework.messaging.handler.invocation.HandlerMethodReturnValueHandler

/**
* Strategy interface to handle the value returned from the invocation of a
* handler method .
*
* @author Arjen Poutsma
* @since 3.1
*/
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(org.springframework.core.MethodParameter)}
* and it 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;

}

HandlerMethodReturnValueHandler包含2个方法：
1.supportsReturnType()决定了哪类类型的返回值将将使用该返回值处理器
2.handleReturnValue()则是主要处理返回值的处理逻辑，并且将处理好的值返回给model，还可以处理该返回什么视

HandlerMethodReturnValueHandler是初始化过程：
在初始化RequestMappingHandlerAdapter时候，springmvc默认初始化了一系列返回值处理器，并且提供了自定义的HandlerMethodReturnValueHandler的入口（所以我们可以开发自己的HandlerMethodReturnValueHandler）。

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 HttpEntityMethodProcessor(getMessageConverters(), this.contentNegotiationManager));
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));
// 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的匹配是按照初始化的顺序，请看下面的处理器以及对应的处理类型。

处理器 处理类型
针对一中类型
ModelAndViewMethodReturnValueHandler ModelAndView
ModelMethodProcessor Model
ViewMethodReturnValueHandler View
ResponseBodyEmitterReturnValueHandler ResponseEntity<ResponseBodyEmitter>
StreamingResponseBodyReturnValueHandler ResponseEntity<StreamingResponseBody>
HttpHeadersReturnValueHandler HttpHeaders
CallableMethodReturnValueHandler Callable
DeferredResultMethodReturnValueHandler DeferredResult、ListenableFuture、CompletionStage
AsyncTaskMethodReturnValueHandler WebAsyncTask
针对注解
ModelAttributeMethodProcessor @ModelAttribute(require=false)
RequestResponseBodyMethodProcessor @ResponseBody
处理多种类型
ViewNameMethodReturnValueHandler void、CharSequence(V4.2)
MapMethodProcessor Map
自定义返回值处理器
ModelAndViewResolverMethodReturnValueHandler 默认处理，如果以上的都不满足就会进入
ModelAttributeMethodProcessor @ModelAttribute(require=true)

下面以@ResponseBody进行讲解一下[url=mailto:br/>@ResponseBody的处理类是RequestResponseBodyMethodProcessor
@ResponseBody的处理类是RequestResponseBodyMethodProcessor
//处理支持的类型，也就是带ResponseBody注解的controller
return (returnType.getMethodAnnotation(ResponseBody.class) != null);
}

mavContainer.setRequestHandled(true);//请求是否已经完全在处理程序中处理过，这里设置为true表明已经处理过，无需其他处理器处理。如果为fasle，则继续流转到对应的视图，如果设置为true，则不会再流转到其他处理器，默认是false。

public void handleReturnValue(Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException {
//如果没有视图,则必须设置为true，否则会返回视图层
mavContainer.setRequestHandled(true);
if (returnValue != null) {
writeWithMessageConverters(returnValue, returnType, webRequest);
}
}

使用案例：
一、定义一个注解（加了注解就进行加密处理）
@Target ( {ElementType.TYPE, ElementType.METHOD})
@Retention (RetentionPolicy.RUNTIME)[url=mailto:br/>@Documented
@Documented
String encrypt() default true;
}

二、自定义处理器
CustomerHandlerMethodReturnValueHandler

public class CustomerHandlerMethodReturnValueHandler implements HandlerMethodReturnValueHandler {
@Override
public boolean supportsReturnType(MethodParameter returnType) {
return (returnType.getMethodAnnotation(ResponseBodyModel.class) != null);
}
@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
KingsResponseBody anno = returnType.getMethodAnnotation(KingsResponseBody.class);
mavContainer.setRequestHandled(true);
boolean encrypt = anno.encrypt();
HttpServletResponse response = webRequest.getNativeResponse(HttpServletResponse.class);
response.setContentType("text/json;charset=UTF-8");
try (PrintWriter out = response.getWriter()) {
if(encrypt){
//TODO 加密返回

}else{
//不加密返回
Gson jb = new Gson();
out.write(jb.toJson(returnValue));
out.flush();
}
} catch (IOException e) {
throw e;
}
}
}

三、在spring的配置文件中注册CustomerHandlerMethodReturnValueHandler处理器
<bean id="handler" class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter">
<property name="customReturnValueHandlers">
<list>
<bean class="com.customer.mvc.CustomerHandlerMethodReturnValueHandler"></bean>
</list>
</property>
</bean>

四、在ctroller使用该注解

@Controller
public class HandlerMethodReturnValueHandlerDemoController {

@RequestMapping (value="encrypt",method=GET)
@ResponseBodyModel（encrypt=false）
public List<Person> demo() {
Person p = new Person();
p.setName("huangjinjin");
Person p = new Person();
p.setName("huangzl");
return Lists.newArrayList(p,p);
}
}