在用SpringMVC时，加入文件上传后出现了.NullPointerException异常

在练习今天的项目时，出现了 HTTP Status 500 - Request processing failed; nested exception is java.lang.NullPointerException异常，在web里，没有加上传文件之前一切运行正常，但是加了文件上传以后，就出现了这种异常，我把上传文件的那部分反复看，就是觉得没毛病~。
代码异常如下：
HTTP Status 500 - Request processing failed; nested exception is java.lang.NullPointerException
type Exception report

message Request processing failed; nested exception is java.lang.NullPointerException

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 java.lang.NullPointerException
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:982)
org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
root cause

java.lang.NullPointerException
com.yueqian.ssm.common.ImageUtils.upload(ImageUtils.java:29)
com.yueqian.ssm.controller.UserController.addUser(UserController.java:51)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
java.lang.reflect.Method.invoke(Unknown Source)
org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:221)
org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:136)
org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:114)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)
org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)

然后，又去看了下其他的视频，终于发现了加入文件上传出现错误的原因；
之前提交表单的代码是这样的：

在加入文件上传以后的提交表单中应该加入：enctype="multipart/form-data"
及修改后的代码是这样的

加入这段代码后，文件可以正常上传了，问题解决了。