springmvc核心之HandlerMethodReturnValueHandler

在springmvc当中@ResponseBody的做用估计不少人都知道，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页面或者其余视图。若是不加，那么它就返回了一个具体的视图，如jsp/html等。springmvc是如何作到这个效果的呢？其实springmvc为处理各类返回值提供了不少的处理类，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，而且将处理好的值返回给model，还能够处理该返回什么视

 

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，而且提供了自定义的HandlerMethodReturnValueHandler的入口（因此咱们能够开发本身的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

针对一种类型

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，若是以上的都不知足就会进入

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);//请求是否已经彻底在处理程序中处理过，这里设置为true代表已经处理过，无需其余处理器处理。若是为fasle，则继续流转到对应的视图，若是设置为true，则不会再流转到其余处理器，默认是false。

public void handleReturnValue(Object returnValue, MethodParameter returnType,

ModelAndViewContainer mavContainer, NativeWebRequest webRequest)

throws IOException, HttpMediaTypeNotAcceptableException {

//若是没有视图,则必须设置为true，不然会返回视图层

mavContainer.setRequestHandled(true);

if (returnValue != null) {

writeWithMessageConverters(returnValue, returnType, webRequest);

}

}

 

使用案例：

1、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;

}

 

2、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;

        }

    }

}

 

3、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>

 

4、在controller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);

    }

}

参考：https://my.oschina.net/kings0/blog/735449

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