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轻松掌握Java上传下载：详解实用代码与技巧，助你轻松实现文件传输

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在互联网时代，文件的上传和下载是日常工作中不可或缺的环节。Java作为一种广泛使用的编程语言，提供了多种方式来实现文件的上传和下载。本文将详细介绍Java中实现文件上传和下载的实用代码与技巧，帮助你轻松掌握文件传输。

一、Java文件上传

1. 使用FileUpload类

FileUpload类是Java提供的一个简单易用的文件上传组件。以下是一个使用FileUpload类实现文件上传的示例代码：

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 FileUploadServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 设置文件上传的最大大小为5MB
        request.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=UTF-8");

        DiskFileItemFactory factory = new DiskFileItemFactory();
        ServletFileUpload upload = new ServletFileUpload(factory);
        upload.setSizeMax(5 * 1024 * 1024);

        List<FileItem> items = upload.parseRequest(request);
        for (FileItem item : items) {
            if (!item.isFormField()) {
                String fileName = item.getName();
                File file = new File("upload/" + fileName);
                item.write(file);
                item.delete();
            }
        }
        response.getWriter().println("文件上传成功！");
    }
}

2. 使用MultipartFile接口

Spring框架提供了MultipartFile接口，可以方便地处理文件上传。以下是一个使用MultipartFile接口实现文件上传的示例代码：

import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.multipart.MultipartFile;

@RestController
public class FileUploadController {
    @PostMapping("/upload")
    public String uploadFile(@RequestParam("file") MultipartFile file) {
        try {
            String fileName = file.getOriginalFilename();
            File saveFile = new File("upload/" + fileName);
            file.transferTo(saveFile);
            return "文件上传成功：" + fileName;
        } catch (IOException e) {
            e.printStackTrace();
            return "文件上传失败！";
        }
    }
}

二、Java文件下载

1. 使用HttpServletResponse输出流

以下是一个使用HttpServletResponse输出流实现文件下载的示例代码：

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.IOException;

public class FileDownloadServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String fileName = "example.txt";
        File file = new File("download/" + fileName);

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

        try (BufferedInputStream bis = new BufferedInputStream(new FileInputStream(file));
             ServletOutputStream os = response.getOutputStream()) {
            byte[] buffer = new byte[1024];
            int len;
            while ((len = bis.read(buffer)) != -1) {
                os.write(buffer, 0, len);
            }
        }
    }
}

2. 使用Spring框架的Resource接口

以下是一个使用Spring框架的Resource接口实现文件下载的示例代码：

import org.springframework.core.io.Resource;
import org.springframework.core.io.UrlResource;
import org.springframework.http.HttpHeaders;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class FileDownloadController {
    @GetMapping("/download/{fileName:.+}")
    public ResponseEntity<Resource> downloadFile(@PathVariable String fileName) {
        Resource resource = new UrlResource("classpath:download/" + fileName);
        if (resource.exists()) {
            return ResponseEntity.ok()
                    .header(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=\"" + resource.getFilename() + "\"")
                    .body(resource);
        } else {
            return ResponseEntity.notFound().build();
        }
    }
}

三、总结

本文详细介绍了Java中实现文件上传和下载的实用代码与技巧。通过学习本文，你可以轻松掌握文件传输，为你的项目开发带来便利。在实际应用中，你可以根据具体需求选择合适的方法，并结合其他技术实现更加复杂的文件上传和下载功能。

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