第三周05

今天学习了在 Java Web 应用中实现文件上传和下载功能。对于文件上传，使用 Apache Commons FileUpload 库。首先下载commons-fileupload.jar和commons-io.jar并添加到项目的WEB-INF/lib目录。
创建一个用于处理文件上传的 Servlet：
import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.FileUploadException;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
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;

@WebServlet("/upload")
public class UploadServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
private static final String UPLOAD_DIR = "uploads";

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    boolean isMultipart = ServletFileUpload.isMultipartContent(request);
    if (isMultipart) {
        DiskFileItemFactory factory = new DiskFileItemFactory();
        String uploadPath = getServletContext().getRealPath("") + File.separator + UPLOAD_DIR;
        File uploadDir = new File(uploadPath);
        if (!uploadDir.exists()) {
            uploadDir.mkdir();
        }
        factory.setRepository(uploadDir);
        ServletFileUpload upload = new ServletFileUpload(factory);
        try {
            List<FileItem> items = upload.parseRequest(request);
            for (FileItem item : items) {
                if (!item.isFormField()) {
                    String fileName = new File(item.getName()).getName();
                    String filePath = uploadPath + File.separator + fileName;
                    File storeFile = new File(filePath);
                    item.write(storeFile);
                }
            }
            response.getWriter().println("File uploaded successfully.");
        } catch (FileUploadException | Exception e) {
            e.printStackTrace();
        }
    }
}

}

对于文件下载，编写一个 Servlet 来处理下载请求：
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
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;

@WebServlet("/download")
public class DownloadServlet extends HttpServlet {
private static final long serialVersionUID = 1L;

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String fileName = request.getParameter("fileName");
    String filePath = getServletContext().getRealPath("") + File.separator + "uploads" + File.separator + fileName;
    File file = new File(filePath);
    if (file.exists()) {
        response.setContentType("application/octet-stream");
        response.setHeader("Content-Disposition", "attachment; filename=\"" + fileName + "\"");
        ServletOutputStream outStream = response.getOutputStream();
        FileInputStream inStream = new FileInputStream(file);
        byte[] buffer = new byte[4096];
        int length;
        while ((length = inStream.read(buffer)) > 0) {
            outStream.write(buffer, 0, length);
        }
        inStream.close();
        outStream.close();
    } else {
        response.getWriter().println("File not found.");
    }
}

}

通过实现文件上传和下载功能，使 Web 应用具备了更丰富的交互能力，满足了用户在数据传输方面的常见需求。