前言

文件上传在web开发中很多地方都会用到，如用户头像上传，商品图片上传。文件上传的请求的 content-type 必须为 multipart/form-data

请求内容

SpringMVC处理

SpringMVC中提供了两种文件解析器，CommonsMultipartResolver 和 StandardServletMultipartResolver。

示例

@GetMapping("testFileUpload1")
  public void testFileUload1(MultipartFile file) {
    System.out.println(file);
  }

CommonsMultipartResolver

这种方式依赖于Apache的fileupload组件

<dependency>
   <groupId>commons-fileupload</groupId>
   <artifactId>commons-fileupload</artifactId>
   <version>1.4</version>
</dependency>

简单源码如下

public class CommonsMultipartResolver extends CommonsFileUploadSupport
		implements MultipartResolver, ServletContextAware {

	private boolean resolveLazily = false;

}

public abstract class CommonsFileUploadSupport {

	protected final Log logger = LogFactory.getLog(getClass());

	private final DiskFileItemFactory fileItemFactory;

	private final FileUpload fileUpload;

	private boolean uploadTempDirSpecified = false;

	private boolean preserveFilename = false;

}

核心为FileUpload类，将请求内容解析为一个包含文件名称和文件内容的对象。

StandardServletMultipartResolver

Servlet3.0对文件上传提供了支持

public interface HttpServletRequest extends ServletRequest {
/**
     * 获取文件
     */
    public Part getPart(String name) throws IOException,
            ServletException;

/**
     * 获取一个请求上传的所有文件
     */
    public Collection<Part> getParts() throws IOException,
            ServletException;
}

Tomcat中对文件上传的实现

/**
 * Wrapper object for the Coyote request.
 *
 * @author Remy Maucherat
 * @author Craig R. McClanahan
 */
public class Request implements HttpServletRequest {

/**
     * {@inheritDoc}
     */
    @Override
    public Collection<Part> getParts() throws IOException, IllegalStateException,
            ServletException {

        parseParts(true);

        if (partsParseException != null) {
            if (partsParseException instanceof IOException) {
                throw (IOException) partsParseException;
            } else if (partsParseException instanceof IllegalStateException) {
                throw (IllegalStateException) partsParseException;
            } else if (partsParseException instanceof ServletException) {
                throw (ServletException) partsParseException;
            }
        }

        return parts;
    }

    // 解析出文件内容
    private void parseParts(boolean explicit) {
       
      // Create a new file upload handler
            DiskFileItemFactory factory = new DiskFileItemFactory();
            try {
                factory.setRepository(location.getCanonicalFile());
            } catch (IOException ioe) {
                parameters.setParseFailedReason(FailReason.IO_ERROR);
                partsParseException = ioe;
                return;
            }
            factory.setSizeThreshold(mce.getFileSizeThreshold());

            ServletFileUpload upload = new ServletFileUpload();
            upload.setFileItemFactory(factory);
            upload.setFileSizeMax(mce.getMaxFileSize());
            upload.setSizeMax(mce.getMaxRequestSize());

            parts = new ArrayList<>();
            try {
                List<FileItem> items =
                        upload.parseRequest(new ServletRequestContext(this));
                int maxPostSize = getConnector().getMaxPostSize();
                int postSize = 0;
                Charset charset = getCharset();
                for (FileItem item : items) {
                    ApplicationPart part = new ApplicationPart(item, location);
                    parts.add(part);
               }

            } 
    }
}

Tomcat中对文件上传的实现就是参考的Apache的commons-fileupload组件

SpringBoot默认使用的StandardServletMultipartResolver来处理文件上传