SpringBoot文件上传的使用以及原理

一、使用

1、构建文件上传表单

    

        邮箱
        
    
    

        名字
        
    
    

        头像
        
        
Example block-level help text here.
    
    

        生活照
        
    
    

        
             Check me out
        
    
    提交

2、文件上传代码

//表单提交必须用post请求    
@PostMapping("/upload")
public String upload(@RequestParam("email") String email,
                     @RequestParam("username") String username,
                     @RequestPart("headerImg") MultipartFile headerImg, // MultipartFile：用于上传文件功能   @RequestPart：获取表单里的文件项
                     @RequestPart("photos") MultipartFile[] photos) throws IOException {

    log.info("上传的信息：email={},username={},headerImg={},photos={}",
            email,username,headerImg.getSize(),photos.length);

    if(!headerImg.isEmpty()){ //isEmpty，getOriginalFilename，transferTo：都是MultipartFile接口里的方法
        String originalFilename = headerImg.getOriginalFilename();
        headerImg.transferTo(new File("D:\\cache\\"+originalFilename));//transferTo：文件保存（传输）到哪
    }
    if(photos.length>0){
        for (MultipartFile photo : photos) {
            if(!photo.isEmpty()){
                String originalFilename = photo.getOriginalFilename();
                photo.transferTo(new File("D:\\cache\\"+originalFilename));
            }
        }
    }
    return "main";
}
}

如果上传出现了超出限制的大小异常

org.apache.tomcat.util.http.fileupload.impl.FileSizeLimitExceededException: The field profile-photo exceeds its maximum permitted size of 1048576 bytes.

可以修改底层默认限制文件的大小:

spring.servlet.multipart.max-file-size=10 #单个文件最大的大小
#多文件上传时，总的一次提交的最大大小，默认是10MB，改成100
spring.servlet.multipart.max-request-size=100

二、原理

文件上传自动配置类是MultipartAutoConfiguration，MultipartProperties所有有关文件上传的配置都封装在MultipartProperties里

• springboot已经自动配置好了StandardServletMultipartResolver【文件上传解析器】它只能解析标准的以servlet的方式，相当于以servlet协议上传过来的文件。如果是自定义方式，直接往上传流的方式应该写自定义的文件上传解析器

  @Configuration(
    proxyBeanMethods = false
  )
  @ConditionalOnClass({Servlet.class, StandardServletMultipartResolver.class, MultipartConfigElement.class})
  @ConditionalOnProperty(
    prefix = "spring.servlet.multipart",
    name = {"enabled"},
    matchIfMissing = true
  )
  @ConditionalOnWebApplication(
    type = Type.SERVLET
  )
  @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 = {"multipartResolver"}
    )
    @ConditionalOnMissingBean({MultipartResolver.class})
    public StandardServletMultipartResolver multipartResolver() {
        StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();
        multipartResolver.setResolveLazily(this.multipartProperties.isResolveLazily());
        return multipartResolver;
    }
  }