如何利用Java Servlet实现文件上传下载

要使用Java Servlet实现文件上传和下载，你需要遵循以下步骤：

文件上传

添加依赖：确保你的项目中包含了Apache Commons FileUpload库。如果你使用Maven，可以在pom.xml中添加以下依赖：

<dependency>
    <groupId>commons-fileuploadgroupId>
    <artifactId>commons-fileuploadartifactId>
    <version>1.4version>
dependency>
<dependency>
    <groupId>commons-iogroupId>
    <artifactId>commons-ioartifactId>
    <version>2.8.0version>
dependency>

创建Servlet：创建一个Servlet来处理文件上传请求。

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 {
    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

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Check that we have a file upload request
        boolean isMultipart = ServletFileUpload.isMultipartContent(request);

        if (!isMultipart) {
            response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Not a multipart request");
            return;
        }

        // Create a factory for disk-based file items
        DiskFileItemFactory factory = new DiskFileItemFactory();

        // Create a new file upload handler
        ServletFileUpload upload = new ServletFileUpload(factory);

        // Parse the request
        try {
            List formItems = upload.parseRequest(request);

            if (formItems != null && formItems.size() > 0) {
                for (FileItem item : formItems) {
                    if (!item.isFormField()) {
                        String fileName = new File(item.getName()).getName();
                        String filePath = UPLOAD_DIRECTORY + File.separator + fileName;
                        File storeFile = new File(filePath);

                        // Save the file on server
                        item.write(storeFile);
                        response.getWriter().println("File " + fileName + " has uploaded successfully!");
                    }
                }
            }
        } catch (Exception ex) {
            response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR, "There was an error: " + ex.getMessage());
        }
    }
}

配置web.xml（如果你不使用注解）：

<servlet>
    <servlet-name>FileUploadServletservlet-name>
    <servlet-class>com.example.FileUploadServletservlet-class>
servlet>
<servlet-mapping>
    <servlet-name>FileUploadServletservlet-name>
    <url-pattern>/uploadurl-pattern>
servlet-mapping>

创建上传表单：创建一个HTML表单来允许用户上传文件。

html>
<html>
<head>
    <title>File Uploadtitle>
head>
<body>
    <h1>Upload Fileh1>
    <form action="upload" method="post" enctype="multipart/form-data">
        <input type="file" name="file" />
        <input type="submit" value="Upload" />
    form>
body>
html>

文件下载

创建Servlet：创建一个Servlet来处理文件下载请求。

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

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

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String fileName = request.getParameter("file");
        if (fileName == null || fileName.isEmpty()) {
            response.sendError(HttpServletResponse.SC_BAD_REQUEST, "File name is required");
            return;
        }

        String filePath = DOWNLOAD_DIRECTORY + File.separator + fileName;
        File file = new File(filePath);

        if (!file.exists()) {
            response.sendError(HttpServletResponse.SC_NOT_FOUND, "File not found");
            return;
        }

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

        FileInputStream inStream = new FileInputStream(file);
        OutputStream outStream = response.getOutputStream();

        byte[] buffer = new byte[4096];
        int length;
        while ((length = inStream.read(buffer)) > 0) {
            outStream.write(buffer, 0, length);
        }

        inStream.close();
        outStream.flush();
    }
}

配置web.xml（如果你不使用注解）：

<servlet>
    <servlet-name>FileDownloadServletservlet-name>
    <servlet-class>com.example.FileDownloadServletservlet-class>
servlet>
<servlet-mapping>
    <servlet-name>FileDownloadServletservlet-name>
    <url-pattern>/downloadurl-pattern>
servlet-mapping>

创建下载链接：创建一个HTML页面来允许用户下载文件。

html>
<html>
<head>
    <title>File Downloadtitle>
head>
<body>
    <h1>Download Fileh1>
    <a href="download?file=example.txt">Download example.txta>
body>
html>

通过以上步骤，你可以实现一个基本的文件上传和下载功能。请确保在实际应用中对文件大小、类型等进行适当的验证和处理，以确保安全性和性能。