SpringMVC自动封装List对象 —— 自定义参数解析器

2022-11-15 17:43:56

　　前台传递的参数为集合对象时，后台Controller希望用一个List集合接收数据。

　　原生SpringMVC是不支持，Controller参数定义为List类型时，接收参数会报如下错误：

org.springframework.beans.BeanInstantiationException: Failed to instantiate [java.util.List]: Specified class is an interface

at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:99) ~[spring-beans-4.3.16.RELEASE.jar:4.3.16.RELEASE]

at org.springframework.web.method.annotation.ModelAttributeMethodProcessor.createAttribute(ModelAttributeMethodProcessor.java:139) ~[spring-web-4.3.16.RELEASE.jar:4.3.16.RELEASE]

at org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor.createAttribute(ServletModelAttributeMethodProcessor.java:82) ~[spring-webmvc-4.3.16.RELEASE.jar:4.3.16.RELEASE]

at org.springframework.web.method.annotation.ModelAttributeMethodProcessor.resolveArgument(ModelAttributeMethodProcessor.java:106) ~[spring-web-4.3.16.RELEASE.jar:4.3.16.RELEASE]

at org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:121) ~[spring-web-4.3.16.RELEASE.jar:4.3.16.RELEASE]

at org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:158) ~[spring-web-4.3.16.RELEASE.jar:4.3.16.RELEASE]

at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:128) ~[spring-web-4.3.16.RELEASE.jar:4.3.16.RELEASE]

at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:97) ~[spring-webmvc-4.3.16.RELEASE.jar:4.3.16.RELEASE]

at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827) ~[spring-webmvc-4.3.16.RELEASE.jar:4.3.16.RELEASE]

at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738) ~[spring-webmvc-4.3.16.RELEASE.jar:4.3.16.RELEASE]

at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) ~[spring-webmvc-4.3.16.RELEASE.jar:4.3.16.RELEASE]

　　查看了一下源码，发现问题在于ModelAttributeMethodProcessor解析参数时，会先使用BeanUtils.instantiateClass方法创建一个对象实例来接收参数。然而List是一个接口，不能被实例化。于是我想到，既然自带的参数解析器不能解析，那就自定义一个参数解析器来实现这个功能。

　　List存在类型擦除，在运行期不能够通过反射来获取泛型，所以得有个办法获取泛型，我便定义了一个注解ListParam

/**

 * 强制申明List的泛型<br/>

 * 用于反射获取参数类型

 *

 * @author zengyuanjun

 *

 */

@Target(ElementType.PARAMETER)

@Retention(RetentionPolicy.RUNTIME)

@Documented

public @interface ListParam {

    public Class<?> value();

}

　　接下来写自定义的解析器ListArgumentResolver 。解析器需要实现HandlerMethodArgumentResolver接口，该接口有两个方法：

　　1. supportsParameter(MethodParameter parameter) 返回当前解析器是否支持该参数。

　　2. resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) 具体的参数解析实现。

　　先使用webRequest.getParameterValues(String paramName)获取request中的参数数组，遍历添加到List<MutablePropertyValues>中，以分配给各个对象。再循环使用ServletRequestDataBinder绑定PropertyValues进行类型转换，得到需要的对象集合。

import java.lang.reflect.Field;

import java.util.ArrayList;

import java.util.List;

import org.springframework.beans.BeanUtils;

import org.springframework.beans.MutablePropertyValues;

import org.springframework.core.MethodParameter;

import org.springframework.stereotype.Component;

import org.springframework.util.ClassUtils;

import org.springframework.web.bind.ServletRequestDataBinder;

import org.springframework.web.bind.WebDataBinder;

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.zyj.springboot.study.annotation.ListParam;

/**

 * List集合参数解析器 <br/>

 * @author zengyuanjun

 *

 */

@Component

public class ListArgumentResolver implements HandlerMethodArgumentResolver {

    @Override

    public boolean supportsParameter(MethodParameter parameter) {

        if (null != parameter.getParameterAnnotation(ListParam.class)

                && List.class.equals(parameter.getParameterType())) {

            return true;

        }

        return false;

    }

    @Override

    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,

            NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {

        String[] parameterValues = null;

        MutablePropertyValues mutablePropertyValues = null;

        Class<?> elementClass = getElementTypeFromAnnotation(parameter);

        List<MutablePropertyValues> mpvList = new ArrayList<MutablePropertyValues>();

        Field[] fields = elementClass.getDeclaredFields();

        for (Field field : fields) {

            parameterValues = webRequest.getParameterValues(field.getName());

            if (null == parameterValues) {

                continue;

            }

            for (int i = 0; i < parameterValues.length; i++) {

                if (mpvList.size() <= i) {

                    mutablePropertyValues = new MutablePropertyValues();

                    mutablePropertyValues.add(field.getName(), parameterValues[i]);

                    mpvList.add(mutablePropertyValues);

                } else {

                    mutablePropertyValues = mpvList.get(i);

                    mutablePropertyValues.add(field.getName(), parameterValues[i]);

                }

            }

        }

        String name = ClassUtils.getShortNameAsProperty(elementClass);

        Object attribute = null;

        WebDataBinder binder = null;

        ServletRequestDataBinder servletBinder = null;

        Object element = null;

        List<Object> actualParameter = new ArrayList<Object>(mpvList.size());

        for (int i = 0; i < mpvList.size(); i++) {

            attribute = BeanUtils.instantiateClass(elementClass);

            binder = binderFactory.createBinder(webRequest, attribute, name);

            servletBinder = (ServletRequestDataBinder) binder;

            servletBinder.bind(mpvList.get(i));

            element = binder.getTarget();

            actualParameter.add(binder.convertIfNecessary(element, elementClass, parameter));

        }

        return actualParameter;

    }

    private Class<?> getElementTypeFromAnnotation(MethodParameter parameter) {

        ListParam parameterAnnotation = parameter.getParameterAnnotation(ListParam.class);

        return parameterAnnotation.value();

    }

}

　　定义好解析器后，需要注入到RequestMappingHandlerAdapter中，这里要注意，自带的ServletModelAttributeMethodProcessor解析器是对List类型生效的！！！所以必须把自定义的解析器放到ServletModelAttributeMethodProcessor前面，我这里直接把自定义的解析器ListArgumentResolver放到了第一个。

import java.util.LinkedList;

import java.util.List;

import org.springframework.beans.BeansException;

import org.springframework.context.ApplicationContext;

import org.springframework.context.ApplicationContextAware;

import org.springframework.stereotype.Component;

import org.springframework.web.method.support.HandlerMethodArgumentResolver;

import org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter;

import com.zyj.springboot.study.resolver.ListArgumentResolver;

/**

 * 初始化将自定义的ListArgumentResolver注入到RequestMappingHandlerAdapter中

 * @author zengyuanjun

 *

 */

@Component

public class InitialListArgumentResolver implements ApplicationContextAware {

    @Override

    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {

        RequestMappingHandlerAdapter handlerAdapter = applicationContext.getBean(RequestMappingHandlerAdapter.class);

        List<HandlerMethodArgumentResolver> argumentResolvers = handlerAdapter.getArgumentResolvers();

        List<HandlerMethodArgumentResolver> resolvers = new LinkedList<HandlerMethodArgumentResolver>();

        resolvers.add(new ListArgumentResolver());

        resolvers.addAll(argumentResolvers);

        handlerAdapter.setArgumentResolvers(resolvers);

    }

}

　　然后？就没有然后了，使用时在List参数前加上ListParam注解，申明一下类型就好。最后还是给个使用的小例子吧！

　　页面定义了多个user对象的信息，它们的name是重复的。

<!DOCTYPE html>

<html>

<head>

<meta charset="UTF-8" />

<title>(-.-) ZYJ (*-*)</title>

</head>

<body>

    <form action="http://localhost:8080/addUser">

        <input name="id" value="1"></input>

        <input name="userName" value="user1"></input>

        <input name="telephone" value="13411111111"></input>

        <input name="birthDay" value="2018-11-01"></input>

        <input name="id" value="2"></input>

        <input name="userName" value="user2"></input>

        <input name="telephone" value="15822222222"></input>

        <input name="birthDay" value="2018-11-02"></input>

        <input name="id" value="3"></input>

        <input name="userName" value="user3"></input>

        <input name="telephone" value="18033333333"></input>

        <input name="birthDay" value="2018-11-03"></input>

    </form>

<script src="https://cdn.bootcss.com/jquery/3.3.1/jquery.min.js"></script>

<script>

    $(function(){

        $.ajax({

            url : $('form').attr('action'),

            type : 'post',

            data : $('form').serialize(),

            success : function(data){

                alert(data);

            }

        })

    })

</script>

</body>

</html>

　　后台controller使用List<User>接收，注意要加上@ListParam(User.class)申明类型。

@RestController

public class UserContoller {

    @Autowired

    private UserService userService;

    @RequestMapping("/addUsers")

    public Object addUsers(@ListParam(User.class) List<User> users){

        return userService.addUsers(users);

    }

}

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