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

@ResponseBody的处理类是RequestResponseBodyMethodProcessor

public boolean supportsReturnType(MethodParameter returnType) {

//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)

@Documented

public @interface ResponseBodyModel {

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);

}

}