springBoot中上传文件MultipartFile报空的问题

springBoot中上传文件MultipartFile报空的问题

1.问题描述
在通过表单上传文件到后台时，后台通过MultipartFile获取到的文件为null。
2.问题原因
通过查找资料发现springboot自带的MultipartFile和Multipart冲突。springBoot启动时，当不存在id为multipartResolver的bean时，会自动导入一个MultipartResolver类型的名为multipartResolver的bean。
DispatcherServletAutoConfiguration.java源码

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
@ConditionalOnWebApplication
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(EmbeddedServletContainerAutoConfiguration.class)
public class DispatcherServletAutoConfiguration {
	/*
	 * The bean name for a DispatcherServlet that will be mapped to the root URL "/"
	 */
	public static final String DEFAULT_DISPATCHER_SERVLET_BEAN_NAME = "dispatcherServlet";
 
	/*
	 * The bean name for a ServletRegistrationBean for the DispatcherServlet "/"
	 */
	public static final String DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME = "dispatcherServletRegistration";
 
	@Configuration
	@Conditional(DefaultDispatcherServletCondition.class)
	@ConditionalOnClass(ServletRegistration.class)
	@EnableConfigurationProperties(WebMvcProperties.class)
	protected static class DispatcherServletConfiguration {
 
		private final WebMvcProperties webMvcProperties;
 
		public DispatcherServletConfiguration(WebMvcProperties webMvcProperties) {
			this.webMvcProperties = webMvcProperties;
		}
 
		@Bean  //就是这个bean啦
		@ConditionalOnBean(MultipartResolver.class)
		@ConditionalOnMissingBean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
		public MultipartResolver multipartResolver(MultipartResolver resolver) {
			// Detect if the user has created a MultipartResolver but named it incorrectly
			return resolver;
		}
 
	}
}

在另一个配置类MultipartAutoConfiguration.java中满足条件时会注入两个bean。

@Configuration
@ConditionalOnClass({ Servlet.class, StandardServletMultipartResolver.class,
		MultipartConfigElement.class })
@ConditionalOnProperty(prefix = "spring.http.multipart", name = "enabled",
		matchIfMissing = true)
@EnableConfigurationProperties(MultipartProperties.class)
public class MultipartAutoConfiguration {
 
	private final MultipartProperties multipartProperties;
 
	public MultipartAutoConfiguration(MultipartProperties multipartProperties) {
		this.multipartProperties = multipartProperties;
	}
 
	@Bean
	@ConditionalOnMissingBean({ MultipartConfigElement.class,
			CommonsMultipartResolver.class })
	public MultipartConfigElement multipartConfigElement() {
		return this.multipartProperties.createMultipartConfig();
	}
 
	@Bean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
	@ConditionalOnMissingBean(MultipartResolver.class)
	public StandardServletMultipartResolver multipartResolver() {
		StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();
		multipartResolver.setResolveLazily(this.multipartProperties.isResolveLazily());
		return multipartResolver;
	}
 
}

3.解决办法
排除springboot自动导入配置的冲突问题。

@EnableAutoConfiguration(exclude = {MultipartAutoConfiguration.class})

注入一个id为multipartResolver的bean

@Bean(name = "multipartResolver")
    public MultipartResolver multipartResolver() {
        CommonsMultipartResolver resolver = new CommonsMultipartResolver();
        resolver.setDefaultEncoding("UTF-8");
        //resolveLazily属性启用是为了推迟文件解析，以在在UploadAction中捕获文件大小异常
        resolver.setResolveLazily(true);
        resolver.setMaxInMemorySize(40960);
        //上传文件大小 5M 5*1024*1024
        resolver.setMaxUploadSize(5 * 1024 * 1024);
        return resolver;
    }

后台参数前添加注解

@RequestParam("image") MultipartFile image