SpringMVC常用接口之HandlerMethodArgumentResolver

在初学springmvc框架时,我就一直有一个疑问,为什么controller方法上竟然可以放这么多的参数,而且都能得到想要的对象,比如HttpServletRequestHttpServletResponse,各种注解@RequestParam@RequestHeader@RequestBody@PathVariable@ModelAttribute等。相信很多初学者都曾经感慨过。

这篇文章就是讲解处理这方面内容的
org.springframework.web.method.support.HandlerMethodArgumentResolver接口。

ServletRequestMethodArgumentResolver和ServletResponseMethodArgumentResolver
处理了自动绑定HttpServletRequest和HttpServletResponse

RequestParamMapMethodArgumentResolver处理了@RequestParam
RequestHeaderMapMethodArgumentResolver处理@RequestHeader
PathVariableMapMethodArgumentResolver处理了@PathVariable
ModelAttributeMethodProcessor处理了@ModelAttribute
RequestResponseBodyMethodProcessor处理了@RequestBody

我们可以模仿springmvc的源码,实现一些我们自己的实现类,而方便我们的代码开发。

接口说明

package org.springframework.web.method.support;

import org.springframework.core.MethodParameter;
import org.springframework.web.bind.WebDataBinder;
import org.springframework.web.bind.support.WebDataBinderFactory;
import org.springframework.web.context.request.NativeWebRequest;

public interface HandlerMethodArgumentResolver {
    //用于判定是否需要处理该参数分解,返回true为需要,并会去调用下面的方法resolveArgument。
    boolean supportsParameter(MethodParameter parameter);
    //真正用于处理参数分解的方法,返回的Object就是controller方法上的形参对象。
    Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
            NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception;

}

示例1
本示例显示如何 优雅地将传入的信息转化成自定义的实体传入controller方法。

post 数据:
first_name = Bill
last_name = Gates
初学者一般喜欢类似下面的代码:

package com.demo.controller;

import javax.servlet.http.HttpServletRequest;

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

import com.demo.domain.Person;
import com.demo.mvc.annotation.MultiPerson;

import lombok.extern.slf4j.Slf4j;

@Slf4j
@Controller
@RequestMapping("demo1")
public class HandlerMethodArgumentResolverDemoController {

    @ResponseBody
    @RequestMapping(method = RequestMethod.POST)
    public String addPerson(HttpServletRequest request) {
        String firstName = request.getParameter("first_name");
        String lastName = request.getParameter("last_name");
        Person person = new Person(firstName, lastName);
        log.info(person.toString());
        return person.toString();
    }
}

这样的代码强依赖了javax.servlet-api的HttpServletRequest对象,并且把初始化Person对象这“活儿”加塞给了controller。代码显得累赘不优雅。在controller里我只想使用person而不想组装person,想要类似下面的代码:

@RequestMapping(method = RequestMethod.POST)
public String addPerson(Person person) {
  log.info(person.toString());
  return person.toString();
}

直接在形参列表中获得person。那么这该如实现呢?

我们需要定义如下的一个参数分解器:

package com.demo.mvc.component;

import org.springframework.core.MethodParameter;
import org.springframework.web.bind.support.WebDataBinderFactory;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.method.support.HandlerMethodArgumentResolver;
import org.springframework.web.method.support.ModelAndViewContainer;

import com.demo.domain.Person;

public class PersonArgumentResolver implements HandlerMethodArgumentResolver {

    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return parameter.getParameterType().equals(Person.class);
    }

    @Override
    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {

        String firstName = webRequest.getParameter("first_name");
        String lastName = webRequest.getParameter("last_name");
        return new Person(firstName, lastName);
    }

}

在supportsParameter中判断是否需要启用分解功能,这里判断形参类型是否为Person类,也就是说当形参遇到Person类时始终会执行该分解流程resolveArgument。

在resolveArgument中处理person的初始化工作。

注册自定义分解器:

传统XML配置:


      
        
      


    
          
    

示例2
加强版Person分解器,支持多个person对象。

post 数据:
person1.first_name = Bill
person1.last_name = Gates
person2.first_name = Steve
person2.last_name = Jobs
用前缀区分属于哪个person对象。
定义一个注解用于设定前缀:

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.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
public @interface MultiPerson {

    public String value();
}

参数分解器:

package com.demo.mvc.component;

import org.springframework.core.MethodParameter;
import org.springframework.web.bind.support.WebDataBinderFactory;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.method.support.HandlerMethodArgumentResolver;
import org.springframework.web.method.support.ModelAndViewContainer;

import com.demo.domain.Person;
import com.demo.mvc.annotation.MultiPerson;

public class MultiPersonArgumentResolver implements HandlerMethodArgumentResolver {

    @Override
    public boolean supportsParameter(MethodParameter parameter) {
        return parameter.hasParameterAnnotation(MultiPerson.class) && parameter.getParameterType().equals(Person.class);
    }

    @Override
    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
        MultiPerson annotation = parameter.getParameterAnnotation(MultiPerson.class);
        String firstName = webRequest.getParameter(annotation.value() + ".first_name");
        String lastName = webRequest.getParameter(annotation.value() + ".last_name");
        return new Person(firstName, lastName);
    }

}

controller:

@ResponseBody
@RequestMapping(value = "multi", method = RequestMethod.POST)
public String addPerson(@MultiPerson("person1") Person person1, @MultiPerson("person2") Person person2) {
  log.info(person1.toString());
  log.info(person2.toString());
  return person1.toString() + "\n" + person2.toString();
}

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