SpringMVC文件上传模块失败

BUG

在编写文件上传模块时，服务器报了个状态码为500的错误：Unable to process parts as no multi-part configuration has been provided，由于之前没有处理过SpringMVC的图片上传，所以也看不懂是什么玩意儿（后来我才知道，我的配置里少了一行代码，直接导致这个错误）。

背景知识

我在前端写了个表单，用来选择文件和上传：

<form method="post" action="<c:url value="/student/commit"/>" enctype="multipart/form-data">
    <input type="text" name="name"/>
    <input type="file" name="file"/>
    <input type="submit"/>
</form>

在选择了文件，并按submit按钮后，前端就会将文件POST到后台服务器（路径是localhost:8080/student/commit）。

后台服务器的controller里有对应的处理方法：

@RequestMapping(value = "/commit", method = RequestMethod.POST)
public String commit(@RequestParam("name") String name,
                     @RequestParam("file") MultipartFile file) {

    logger.info(name);
    try {
        logger.info(file.getInputStream().toString());
    } catch (IOException e) {
        e.printStackTrace();
    }

    return "/student/commit";
}

它会打印前端传来的文件信息。

同时我还在SpringMVC的配置文件(spring-web.xml)里配置了对文件的支持：

<!--配置文件上传-->
<bean id="multipartResolver"
      class="org.springframework.web.multipart.support.StandardServletMultipartResolver">
</bean>

它令SpringMVC具有了识别文件上传的功能。

问题之谜

这么配置后，我运行项目，选择文件，点击上传按钮，浏览器里显示状态码为500，内容为Unable to process parts as no multi-part configuration has been provided的错误：

HTTP Status 500 - Request processing failed; nested exception is org.springframework.web.multipart.MultipartException: Could not parse multipart servlet request; nested exception is java.lang.IllegalStateException: Unable to process parts as no multi-part configuration has been provided

type Exception report

message Request processing failed; nested exception is org.springframework.web.multipart.MultipartException: Could not parse multipart servlet request; nested exception is java.lang.IllegalStateException: Unable to process parts as no multi-part configuration has been provided

description The server encountered an internal error that prevented it from fulfilling this request.

exception

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.springframework.web.multipart.MultipartException: Could not parse multipart servlet request; nested exception is java.lang.IllegalStateException: Unable to process parts as no multi-part configuration has been provided
						     org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:979)
						     org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:869)
						     javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
						     org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:843)
						     javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
						     org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
root cause

org.springframework.web.multipart.MultipartException: Could not parse multipart servlet request; nested exception is java.lang.IllegalStateException: Unable to process parts as no multi-part configuration has been provided
						      org.springframework.web.multipart.support.StandardMultipartHttpServletRequest.parseRequest(StandardMultipartHttpServletRequest.java:100)
						      org.springframework.web.multipart.support.StandardMultipartHttpServletRequest.<init>(StandardMultipartHttpServletRequest.java:78)
						      org.springframework.web.multipart.support.StandardServletMultipartResolver.resolveMultipart(StandardServletMultipartResolver.java:76)
						      org.springframework.web.servlet.DispatcherServlet.checkMultipart(DispatcherServlet.java:1089)
						      org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:928)
						      org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893)
						      org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:967)
						      org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:869)
						      javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
						      org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:843)
						      javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
						      org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
root cause

java.lang.IllegalStateException: Unable to process parts as no multi-part configuration has been provided
				 org.apache.catalina.connector.Request.parseParts(Request.java:2742)
				 org.apache.catalina.connector.Request.getParts(Request.java:2708)
				 org.apache.catalina.connector.RequestFacade.getParts(RequestFacade.java:1096)
				 org.springframework.web.multipart.support.StandardMultipartHttpServletRequest.parseRequest(StandardMultipartHttpServletRequest.java:85)
				 org.springframework.web.multipart.support.StandardMultipartHttpServletRequest.<init>(StandardMultipartHttpServletRequest.java:78)
				 org.springframework.web.multipart.support.StandardServletMultipartResolver.resolveMultipart(StandardServletMultipartResolver.java:76)
				 org.springframework.web.servlet.DispatcherServlet.checkMultipart(DispatcherServlet.java:1089)
				 org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:928)
				 org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893)
				 org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:967)
				 org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:869)
				 javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
				 org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:843)
				 javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
				 org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
note The full stack trace of the root cause is available in the Apache Tomcat/9.0.0.M6 logs.

Apache Tomcat/9.0.0.M6

搜寻其他人的方案，我发现我的方案并没有什么特殊的，到底问题出在哪里？

错误里的no multi-part configuration has been provided是什么意思，未提供文件上传的配置？

可我分明在spring-web.xml里加入了相应的配置。

我再仔细看，Could not parse multipart servlet request，没有servlet的配置？

谜题之解

我这才意识到，SpringMVC是依赖Servlet的，虽然SpringMVC能识别Servlet，但它底层的Servlet也许还不能识别文件上传。

于是我赶紧翻开SpringMVC官网的refference去看文档，果然，我发现了一些猫腻：

Once Servlet 3.0 multipart parsing has been enabled in one of the above mentioned ways you can add the StandardServletMultipartResolver to your Spring configuration

在配置SpringMVC的StandardServletMultipartResolver之前必须先配置Servlet，官网提供的方案有：

1. mark the DispatcherServlet with a “multipart-config” section in web.xml(在web.xml文件中的DispatcherServlet元素里，加上一个multipart-config)
2. with a javax.servlet.MultipartConfigElement in programmatic Servlet registration
3. in case of a custom Servlet class possibly with a javax.servlet.annotation.MultipartConfig annotation on your Servlet class

我选择了第一种方案，打开webapp/WEB-INFO目录下的web.xml，往DispatcherServlet里加了一行

<multipart-config/>

再运行项目，就可以正常地上传文件了。

web.xml全部内容如下：

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                      http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1"
         metadata-complete="true">


    <!--配置DispatcherServlet-->
    <servlet>
        <servlet-name>my-dispatcher</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>

        <!--让Servlet支持文件上传-->
        <multipart-config/>

        <!--springMVC需要加载的文件-->
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:spring/spring-*.xml</param-value>
        </init-param>
    </servlet>

    <servlet-mapping>
        <servlet-name>my-dispatcher</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>


</web-app>

直到最后，我才发现，解决这个BUG，只需要一行代码。