Servlet如何处理文件上传下载

在Java Web应用中，Servlet可以用来处理文件的上传和下载。这里分别介绍如何使用Servlet实现文件上传和下载的功能。

1. 文件上传

要实现文件上传功能，可以使用Apache Commons FileUpload库。首先需要将这个库添加到项目的依赖中。如果使用Maven，可以在pom.xml文件中添加以下依赖：

<dependency>
    <groupId>commons-fileupload</groupId>
    <artifactId>commons-fileupload</artifactId>
    <version>1.4</version>
</dependency>

接下来，创建一个HTML表单，用于文件上传：

<!DOCTYPE html>
<html>
<head>
    <title>File Upload</title>
</head>
<body>
    <form action="upload" method="post" enctype="multipart/form-data">
        <input type="file" name="file" />
        <input type="submit" value="Upload" />
    </form>
</body>
</html>

然后，在Servlet中处理文件上传：

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.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 FileUploadServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        if (ServletFileUpload.isMultipartContent(req)) {
            try {
                List<FileItem> items = new ServletFileUpload(new DiskFileItemFactory()).parseRequest(req);
                for (FileItem item : items) {
                    if (!item.isFormField()) {
                        String fileName = new File(item.getName()).getName();
                        String filePath = getServletContext().getRealPath("/") + "uploads/" + fileName;
                        File file = new File(filePath);
                        item.write(file);
                    }
                }
                resp.getWriter().println("File uploaded successfully.");
            } catch (Exception e) {
                e.printStackTrace();
                resp.getWriter().println("Error occurred while uploading the file.");
            }
        } else {
            resp.getWriter().println("Invalid request format.");
        }
    }
}

2. 文件下载

在Servlet中处理文件下载：

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.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;

@WebServlet("/download")
public class FileDownloadServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String fileName = req.getParameter("fileName");
        if (fileName == null || fileName.isEmpty()) {
            resp.getWriter().println("Invalid file name.");
            return;
        }

        String filePath = getServletContext().getRealPath("/") + "uploads/" + fileName;
        File file = new File(filePath);

        if (!file.exists()) {
            resp.getWriter().println("File not found.");
            return;
        }

        resp.setContentType("application/octet-stream");
        resp.setHeader("Content-Disposition", "attachment;filename=" + fileName);
        resp.setContentLength((int) file.length());

        FileInputStream fis = null;
        OutputStream os = null;
        try {
            fis = new FileInputStream(file);
            os = resp.getOutputStream();
            byte[] buffer = new byte[1024];
            int bytesRead;
            while ((bytesRead = fis.read(buffer)) != -1) {
                os.write(buffer, 0, bytesRead);
            }
        } finally {
            if (fis != null) {
                fis.close();
            }
            if (os != null) {
                os.close();
            }
        }
    }
}

这样，当用户访问/download?fileName=example.txt时，Servlet会处理文件下载请求，并将文件发送给客户端。注意替换example.txt为实际文件名。