SpringMVC+ajax+FormData文件上传报java.lang.NullPointerException

た入场券 2022-03-29 09:54 454阅读 0赞

### SpringMVC+ajax+FormData文件上传报java.lang.NullPointerException错误解决思路 ###

github项目地址: [https://github.com/weiyangtang/StudyPlan\_JavaWeb][https_github.com_weiyangtang_StudyPlan_JavaWeb]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXlhbmdfdGFuZw_size_16_color_FFFFFF_t_70]

#### 1.springmvc-servlet.xml配置文件要加上下面的标记 ####

<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
            
            <property name="maxUploadSize">
                <value>5242880</value>
            </property>
        </bean>

**小心坑:bean id一定要是multipartResolver,不能改一个字,否则Spring容器无法调用文件上传的组件**

#### 2.form表单没有设置`enctype="multipart/form-data"` ####

<form id="updateForm" enctype="multipart/form-data">
    		<p>
    			用户信息：<input type="text" id='userName' name="userName">
    			上传文件：<input type="file" id='files' name="uploadFile" />
    		</p>
    		<button type="button" onclick="submitForm()">ajax提交</button>
    		<button type="button" onclick="submitFormAppend()">ajaxformdata提交</button>
    		 <input type="submit" value="提交" />
    	</form>

#### 3.最低级的错误:控制层的文件名和form表单的name不一致 ####

举个例子

<form id="updateForm" enctype="multipart/form-data">
    		<p>
    			用户信息：<input type="text" id='userName' name="userName">
    			上传文件：<input type="file" id='files' name="files" />
    		</p>
    		<button type="button" onclick="submitForm()">ajax提交</button>
    	</form>

@RequestMapping(value = {  "/upload" })
    	public  String fileUpload(@RequestParam MultipartFile uploadFile) { 
    		String uploadPath =System.getProperty("studyPlan.root")+ File.separator + uploadFile.getOriginalFilename();
    		try { 
    			FileOutputStream fs = new FileOutputStream(uploadPath);
    			fs.write(uploadFile.getBytes());
    			fs.close();
    			System.out.println(uploadPath);
    		} catch (Exception e) { 
    			e.printStackTrace();
    		}
    		return null;
    	}

> **此时form表单input file 的name是name="files"**  
> **controller 是@RequestParam MultipartFile uploadFile**  
> **回忆一下,SpringMVC @RequestParam的默认值**

@RequestParam(value="username")String user
    Spring 会将表单上传的name=username 的参数绑定到 user对象
    如果value的值 和要绑定的对象名称一致时就可以省略,缺省
    @RequestParam String username
    问题就这里了,当你form表单的name和你要绑定的数据对象名字不一致,就无法绑定,这个对象就为空了,要使用这个对象就会报nullPoint错误了

> 概括成 一句话:html的input file name="" 要和controller的绑定的参数一致

> 如果同学有兴趣看看我写的项目,包含了很多文件上传的功能  
> 项目下载地址  
> [https://github.com/weiyangtang/StudyPlan\_JavaWeb][https_github.com_weiyangtang_StudyPlan_JavaWeb]  
> ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXlhbmdfdGFuZw_size_16_color_FFFFFF_t_70 1]  
> ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXlhbmdfdGFuZw_size_16_color_FFFFFF_t_70 2]

[https_github.com_weiyangtang_StudyPlan_JavaWeb]: https://github.com/weiyangtang/StudyPlan_JavaWeb
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXlhbmdfdGFuZw_size_16_color_FFFFFF_t_70]: /images/20220329/f6fafd4fac9a449cb5f4dfdebe557aef.png
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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXlhbmdfdGFuZw_size_16_color_FFFFFF_t_70 2]: /images/20220329/4f2f843bfa0e4af3b77352bf69e7824a.png