给出Java web servlet文件上传与下载示例

在Java Web中，使用Servlet进行文件上传和下载的基本步骤如下：

**文件上传示例（使用HttpServletRequest和HttpServletResponse）**:

import javax.servlet.*;
import java.io.*;

public class FileUploadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 获取上传文件的参数
        String filePart = "file"; // 这通常是form表单中的file字段名
        Part filePartObj = request.getPart(filePart);
        
        if (filePartObj != null) {
            // 创建临时目录存储文件
            File dir = new File("uploads");
            if (!dir.exists()) {
                dir.mkdir();
            }
            
            // 获取文件名称和保存路径
            String fileName = Paths.get(dir.getAbsolutePath(), filePartObj.getName()).toString();

            // 将文件保存到服务器上
            InputStream inputStream = filePartObj.getInputStream();
            OutputStream outputStream = new FileOutputStream(fileName);
            byte[] buffer = new byte[4096];
            int length;
            while ((length = inputStream.read(buffer)) > 0) {
                outputStream.write(buffer, 0, length);
            }
            outputStream.close();
            inputStream.close();
        }

        // 返回成功响应
        response.getWriter().println("File uploaded successfully.");
    }
}

**文件下载示例（使用ServletOutputStream）**:

import javax.servlet.*;
import java.io.*;

public class FileDownloadServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String filePath = "/path/to/downloaded/file.txt"; // 替换为你实际的文件路径

        // 设置响应头信息，告知浏览器这是一个文件下载
        response.setContentType("application/octet-stream");
        response.setHeader("Content-Disposition", "attachment; filename=file.txt");

        // 读取文件内容并发送给浏览器
        FileInputStream fis = new FileInputStream(filePath);
        ServletOutputStream sos = response.getOutputStream();
        byte[] bytes = new byte[1024];
        int length;
        while ((length = fis.read(bytes)) != -1) {
            sos.write(bytes, 0, length);
        }
        fis.close();
        sos.flush();
        sos.close();
    }
}