前言

檔案上傳在web開發中很多地方都會用到，如使用者頭像上傳，商品圖片上傳。檔案上傳的請求的 content-type 必須為 multipart/form-data

請求內容

SpringMVC處理

SpringMVC中提供了兩種檔案解析器，CommonsMultipartResolver 和 StandardServletMultipartResolver。

示例

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

CommonsMultipartResolver

這種方式依賴於Apache的fileupload元件

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

簡單原始碼如下

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;

}
 

核心為FileUpload類，將請求內容解析為一個包含檔名稱和檔案內容的物件。

StandardServletMultipartResolver

Servlet3.0對檔案上傳提供了支援

public interface HttpServletRequest extends ServletRequest {
/**
     * 獲取檔案
     */
    public Part getPart(String name) throws IOException,
            ServletException;

/**
     * 獲取一個請求上傳的所有檔案
     */
    public Collection<Part> getParts() throws IOException,
            ServletException;
}

Tomcat中對檔案上傳的實現

/**
 * 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;
    }

    // 解析出檔案內容
    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中對檔案上傳的實現就是參考的Apache的commons-fileupload元件

SpringBoot預設使用的StandardServletMultipartResolver來處理檔案上傳