java文件上传下载功能

Java文件上传下载功能可以通过Java Servlet API实现。以下是一个简单的实现步骤：

1. 在HTML表单中添加文件上传控件。
2. 在Servlet中处理文件上传请求，使用Apache Commons FileUpload库解析上传的文件。
3. 将上传的文件保存到服务器上的指定位置。
4. 在HTML页面中添加文件下载链接。
5. 在Servlet中处理文件下载请求，将文件从服务器上读取并发送到客户端。

以下是一个简单的Java文件上传下载示例代码：

import java.io.*;
import javax.servlet.ServletException;
import javax.servlet.http.*;
import org.apache.commons.fileupload.*;
import org.apache.commons.fileupload.disk.*;
import org.apache.commons.fileupload.servlet.*;

public class FileUploadServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    private static final String UPLOAD_DIRECTORY = "uploads";
    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

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 检查是否为文件上传请求
        if (!ServletFileUpload.isMultipartContent(request)) {
            PrintWriter writer = response.getWriter();
            writer.println("Error: Form must has enctype=multipart/form-data.");
            writer.flush();
            return;
        }

        // 配置上传参数
        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);

        // 构造上传文件保存路径
        String uploadPath = getServletContext().getRealPath("") + File.separator + UPLOAD_DIRECTORY;

        // 如果目录不存在则创建
        File uploadDir = new File(uploadPath);
        if (!uploadDir.exists()) {
            uploadDir.mkdir();
        }

        try {
            // 解析请求的内容提取文件数据
            @SuppressWarnings("unchecked")
            List<FileItem> formItems = upload.parseRequest(request);

            if (formItems != null && formItems.size() > 0) {
                // 迭代表单数据
                for (FileItem item : formItems) {
                    // 处理不在表单中的字段
                    if (!item.isFormField()) {
                        String fileName = new File(item.getName()).getName();
                        String filePath = uploadPath + File.separator + fileName;
                        File storeFile = new File(filePath);

                        // 在控制台输出文件的上传路径
                        System.out.println(filePath);

                        // 保存文件到硬盘
                        item.write(storeFile);
                        request.setAttribute("message", "File upload success!");
                    }
                }
            }
        } catch (Exception ex) {
            request.setAttribute("message", "File upload failed due to " + ex);
        }

        // 跳转到消息页面
        getServletContext().getRequestDispatcher("/message.jsp").forward(request, response);
    }
}