java使用教程如何实现文件的上传和下载功能 java使用教程的文件传输操作指南​

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

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.IOException;
import java.util.List;

public class UploadServlet extends HttpServlet {
    private static final String UPLOAD_DIRECTORY = "upload";
    private static final int MAX_FILE_SIZE = 1024 * 1024 * 40; // 40MB
    private static final int MAX_REQUEST_SIZE = 1024 * 1024 * 50; // 50MB

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 检查是否是multipart/form-data请求
        if (!ServletFileUpload.isMultipartContent(request)) {
            response.getWriter().println("Error: Form must has enctype=multipart/form-data");
            return;
        }

        // 配置上传参数
        DiskFileItemFactory factory = new DiskFileItemFactory();
        // 设置内存临界值 - 超过后将产生临时文件并存储于临时目录中
        factory.setSizeThreshold(1024 * 3);
        // 设置临时存储目录
        factory.setRepository(new File(System.getProperty("java.io.tmpdir")));

        ServletFileUpload upload = new ServletFileUpload(factory);

        // 设置最大文件上传值
        upload.setFileSizeMax(MAX_FILE_SIZE);

        // 设置最大请求值 (包含文件和表单数据)
        upload.setSizeMax(MAX_REQUEST_SIZE);

        // 中文处理
        upload.setHeaderEncoding("UTF-8");

        // 构造临时路径来存储上传的文件
        // 这个路径相对当前应用的目录
        String uploadPath = getServletContext().getRealPath("./" + 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
            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);

                        // 保存文件到硬盘
                        item.write(storeFile);
                        request.setAttribute("message", "文件上传成功!");
                    }
                }
            }
        } catch (Exception ex) {
            request.setAttribute("message", "文件上传失败: " + ex.getMessage());
        }
        getServletContext().getRequestDispatcher("/message.jsp").forward(request, response);
    }
}

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;

public class DownloadServlet extends HttpServlet {
    private static final String DOWNLOAD_DIRECTORY = "upload"; //与上传目录一致

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String filename = request.getParameter("filename");
        if (filename == null || filename.isEmpty()) {
            response.getWriter().println("Error: Filename parameter is missing.");
            return;
        }

        String filePath = getServletContext().getRealPath("./" + DOWNLOAD_DIRECTORY) + File.separator + filename;
        File downloadFile = new File(filePath);

        if (!downloadFile.exists()) {
            response.getWriter().println("Error: File not found.");
            return;
        }

        String mimeType = getServletContext().getMimeType(filePath);
        if (mimeType == null) {
            // 设置为二进制数据类型
            mimeType = "application/octet-stream";
        }

        response.setContentType(mimeType);
        response.setContentLength((int) downloadFile.length());

        // 设置头部信息
        String headerKey = "Content-Disposition";
        String headerValue = String.format("attachment; filename=\"%s\"", downloadFile.getName());
        response.setHeader(headerKey, headerValue);

        try (FileInputStream inStream = new FileInputStream(downloadFile);
             OutputStream outStream = response.getOutputStream()) {

            byte[] buffer = new byte[4096];
            int bytesRead;
            while ((bytesRead = inStream.read(buffer)) != -1) {
                outStream.write(buffer, 0, bytesRead);
            }
        } catch (IOException e) {
            // 处理IO异常
            response.getWriter().println("Error during file download: " + e.getMessage());
        }
    }
}