2.盤點springmvc的經常使用接口之HandlerMethodReturnValueHandler

2.盤點springmvc的經常使用接口之HandlerMethodReturnValueHandler###

前言

上一章中介紹了HandlerMethodArgumentResolver來處理controller方法的參數綁定問題。一個請求中有入參的綁定處理，那麼固然也會有響應參數的處理，這一章就來說講處理返回對象的接口——org.springframework.web.method.support.HandlerMethodReturnValueHandler。

接口說明

public interface HandlerMethodReturnValueHandler {
	//檢驗是否支持本處理器處理，返回true會執行handleReturnValue
	boolean supportsReturnType(MethodParameter returnType);
  
	//處理返回值的方法，returnValue便是controller方法中的返回值
	void handleReturnValue(Object returnValue, MethodParameter returnType,
			ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;

示例1

controller

package com.demo.controller;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;

import com.demo.domain.Person;

@Controller
@RequestMapping("demo2")
public class HandlerMethodReturnValueHandlerDemoController {

	@ResponseBody
	@RequestMapping(method = RequestMethod.GET)
	public Person getPerson() {
		Person person = new Person("Bill", "Gates");
		return person;
	}
}

這個controller方法中返回了Person對象，若是使用默認的配置，將會把Person對象以json格式輸出。

{
  "firstName": "Bill",
  "lastName": "Gates"
}

那是由於使用了@ResponseBody註解，springmvc調用了RequestResponseBodyMethodProcessor，而RequestResponseBodyMethodProcessor實現了HandlerMethodReturnValueHandler接口的handleReturnValue方法。

@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
                              ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
  throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

  mavContainer.setRequestHandled(true);

  // Try even with null return value. ResponseBodyAdvice could get involved.
  writeWithMessageConverters(returnValue, returnType, webRequest);
}

看源碼可知，RequestResponseBodyMethodProcessor轉交給了HttpMessageConverter去處理，具體的是轉交給了MappingJackson2HttpMessageConverter，使用Jackson框架把person對象序列化成了json。

實現本身的HandlerMethodReturnValueHandler：

package com.demo.mvc.component;

import javax.servlet.http.HttpServletResponse;

import org.springframework.core.MethodParameter;
import org.springframework.util.Assert;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.method.support.HandlerMethodReturnValueHandler;
import org.springframework.web.method.support.ModelAndViewContainer;

import com.demo.domain.Person;

public class PersonReturnValueHandler implements HandlerMethodReturnValueHandler {

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

	@Override
	public void handleReturnValue(Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
		Assert.isInstanceOf(Person.class, returnValue);
		mavContainer.setRequestHandled(true);
		HttpServletResponse response = webRequest.getNativeResponse(HttpServletResponse.class);
		response.setContentType("text/plain;charset=utf-8");
      	Person person = (Person) returnValue;
		response.getWriter().write("通過HandlerMethodReturnValueHandler輸出的person:" + person);
	}
}

其中mavContainer.setRequestHandled(true);標誌着這次請求是不是由handler本身控制的，true表示本方法會響應請求。

使用本身的實現後，須要去除controller方法上的@ResponseBody註解，由於咱們再也不須要使用默認的處理器。

@RequestMapping(method = RequestMethod.GET)
public Person getPerson() {
  Person person = new Person("Bill", "Gates");
  return person;
}

發送請求GET http://localhost:8080/demo2

響應獲得

通過HandlerMethodReturnValueHandler輸出的person:Bill Gates

示例2

發揮想象，給controller的方法加上自定義註解輔助返回值響應。

設置Person前綴註解：

package com.demo.mvc.annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface PersonPrefix {

	public String value() default "";
}

controller

@PersonPrefix("rich man")
@RequestMapping(method = RequestMethod.GET)
public Person getPerson() {
  Person person = new Person("Bill", "Gates");
  return person;
}

handleReturnValue方法：

@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
  Assert.isInstanceOf(Person.class, returnValue);
  mavContainer.setRequestHandled(true);
  // 獲取方法上的註解
  PersonPrefix annotation = returnType.getMethodAnnotation(PersonPrefix.class);
  HttpServletResponse response = webRequest.getNativeResponse(HttpServletResponse.class);
  response.setContentType("text/plain;charset=utf-8");
  Person person = (Person) returnValue;
  response.getWriter().write(annotation.value() + " : " + person);
}

最後響應：

rich man : Bill Gates

友情連接：

盤點springmvc的經常使用接口目錄