java写一段文件上传的代码

好的，以下是一个Java文件上传的示例代码，使用了Apache Commons FileUpload库：

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

import javax.servlet.http.HttpServletRequest;
import java.io.File;
import java.util.List;

public class FileUploadHandler {

    private static final int MEMORY_THRESHOLD   = 1024 * 1024 * 3;  // 3MB
    private static final int MAX_FILE_SIZE      = 1024 * 1024 * 40; // 40MB
    private static final int MAX_REQUEST_SIZE   = 1024 * 1024 * 50; // 50MB

    public static void handleFileUpload(HttpServletRequest request, String uploadDir) throws Exception {
        if (!ServletFileUpload.isMultipartContent(request)) {
            throw new Exception("不是multipart/form-data类型的表单");
        }

        DiskFileItemFactory factory = new DiskFileItemFactory();
        factory.setSizeThreshold(MEMORY_THRESHOLD);
        factory.setRepository(new File(System.getProperty("java.io.tmpdir")));

        ServletFileUpload upload = new ServletFileUpload(factory);
        upload.setFileSizeMax(MAX_FILE_SIZE);
        upload.setSizeMax(MAX_REQUEST_SIZE);

        String uploadPath = request.getServletContext().getRealPath("") + File.separator + uploadDir;
        File uploadDirFile = new File(uploadPath);
        if (!uploadDirFile.exists()) {
            uploadDirFile.mkdir();
        }

        List<FileItem> formItems = upload.parseRequest(request);
        if (formItems != null && formItems.size() > 0) {
            for (FileItem item : formItems) {
                if (!item.isFormField()) {
                    String fileName = new File(item.getName()).getName();
                    String filePath = uploadPath + File.separator + fileName;
                    File storeFile = new File(filePath);
                    item.write(storeFile);
                }
            }
        }
    }
}

使用时，需要调用`handleFileUpload`方法，并传入HttpServletRequest对象和上传文件的保存目录：

String uploadDir = "uploads";
FileUploadHandler.handleFileUpload(request, uploadDir);

注意：需要在`web.xml`文件中添加文件上传的配置，如下所示：

<servlet>
    <servlet-name>fileUpload</servlet-name>
    <servlet-class>org.apache.catalina.servlets.WebdavServlet</servlet-class>
    <init-param>
        <param-name>debug</param-name>
        <param-value>0</param-value>
    </init-param>
    <init-param>
        <param-name>listings</param-name>
        <param-value>false</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>fileUpload</servlet-name>
    <url-pattern>/upload/*</url-pattern>
</servlet-mapping>

其中，`<servlet-class>`节点中的`WebdavServlet`可以替换为其他文件上传的Servlet类。