Spring SpringMVC文件上传错误（二）

续上文，

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场景：

不上传文件提交表单。

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简述一下：表单请求的两种方式

1. 同步提交
2. 异步提交

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笔者之前写过的测试Demo：

HTML：

<form action="upload.do" method="post" enctype="multipart/form-data">  
    <input type="file" name="fileName" />
    <input type="submit" value="Submit" />
</form>  

控制器：

@RequestMapping(value = "/upload.do", method = RequestMethod.POST)
@ResponseBody
public Map<String, Object> add(@RequestParam(value="fileName") MultipartFile file){
    //...
    return null;
}

错误信息：

11:03:07.147 [http-nio-8080-exec-4] DEBUG o.s.w.s.m.a.AnnotationMethodHandlerExceptionResolver - Resolving exception from handler [com.xFee.serverAdmin.controller.ApkInfoController@25a5db]: org.springframework.beans.ConversionNotSupportedException: Failed to convert value of type 'java.lang.String' to required type 'org.springframework.web.multipart.MultipartFile'; nested exception is java.lang.IllegalStateException: Cannot convert value of type [java.lang.String] to required type [org.springframework.web.multipart.MultipartFile]: no matching editors or conversion strategy found
11:03:07.147 [http-nio-8080-exec-4] DEBUG o.s.w.s.m.a.ResponseStatusExceptionResolver - Resolving exception from handler [com.xFee.serverAdmin.controller.ApkInfoController@25a5db]: org.springframework.beans.ConversionNotSupportedException: Failed to convert value of type 'java.lang.String' to required type 'org.springframework.web.multipart.MultipartFile'; nested exception is java.lang.IllegalStateException: Cannot convert value of type [java.lang.String] to required type [org.springframework.web.multipart.MultipartFile]: no matching editors or conversion strategy found
11:03:07.147 [http-nio-8080-exec-4] DEBUG o.s.w.s.m.s.DefaultHandlerExceptionResolver - Resolving exception from handler [com.xFee.serverAdmin.controller.ApkInfoController@25a5db]: org.springframework.beans.ConversionNotSupportedException: Failed to convert value of type 'java.lang.String' to required type 'org.springframework.web.multipart.MultipartFile'; nested exception is java.lang.IllegalStateException: Cannot convert value of type [java.lang.String] to required type [org.springframework.web.multipart.MultipartFile]: no matching editors or conversion strategy found
11:03:07.147 [http-nio-8080-exec-4] DEBUG o.s.web.servlet.DispatcherServlet - Null ModelAndView returned to DispatcherServlet with name 'springServlet': assuming HandlerAdapter completed request handling
11:03:07.147 [http-nio-8080-exec-4] DEBUG o.s.web.servlet.DispatcherServlet - Successfully completed request

笔者忽略了，在同步提交的时候，Spring把空上传是做null来处理的，而在异步提交的时候，Spring把文件域的值当作空字符串看待的。在Spring做请求转换的时候（request–>MultipartHttpServletRequest）底层的TypeConverter接口实现对null和""是做了不同操作的。源码就不贴了，有兴趣的伙伴可以自行从：org.springframework.web.multipart.commons.CommonsMultipartResolver
这个类跟下去，就会发现差异。

解决方案

在配置文件中加入以下配置：

    <bean id="conversionService"
        class="org.springframework.context.support.ConversionServiceFactoryBean">
        <property name="converters">
            <list>
                <bean class="com.xFee.serverAdmin.utils.SpringMVCFileConverter"></bean>
            </list>
        </property>
    </bean>
    <mvc:annotation-driven conversion-service="conversionService" />

然后在list下的bean节点的class属性的路径下创建一个SpringMVCFileConverter：

public class SpringMVCFileConverter implements Converter<String, MultipartFile>  {

    public SpringMVCFileConverter() {
        super();
    }

    @Override
    public MultipartFile convert(String source) {
        return null;
    }
}

注：注意在引入上面配置文件的时候不要忘记引入其定义及约束文件。

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