java实现下载功能的代码

以下是 Java 实现文件下载的代码：

import java.io.*;
import java.net.*;

public class FileDownloader {
    
    public static void main(String[] args) {
        String fileUrl = "http://example.com/file.pdf"; // 文件的 URL 地址
        String saveDir = "/home/user/downloads/"; // 文件保存的目录
        
        try {
            URL url = new URL(fileUrl);
            HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();
            int responseCode = httpConn.getResponseCode();
            
            // 检查服务器响应是否为 OK
            if (responseCode == HttpURLConnection.HTTP_OK) {
                // 获取文件名和文件大小
                String fileName = "";
                String disposition = httpConn.getHeaderField("Content-Disposition");
                
                if (disposition != null) {
                    int startIndex = disposition.indexOf("filename=");
                    if (startIndex > 0) {
                        fileName = disposition.substring(startIndex + 10, disposition.length() - 1);
                    }
                } else {
                    fileName = fileUrl.substring(fileUrl.lastIndexOf("/") + 1, fileUrl.length());
                }
                
                int fileSize = httpConn.getContentLength();
                
                // 创建输入流和输出流
                InputStream inputStream = httpConn.getInputStream();
                FileOutputStream outputStream = new FileOutputStream(saveDir + fileName);
                
                // 读取文件，并将文件保存到本地磁盘
                byte[] buffer = new byte[4096];
                int bytesRead = -1;
                int totalBytesRead = 0;
                
                while ((bytesRead = inputStream.read(buffer)) != -1) {
                    outputStream.write(buffer, 0, bytesRead);
                    totalBytesRead += bytesRead;
                }
                
                outputStream.close();
                inputStream.close();
                
                System.out.println("File downloaded successfully!");
            } else {
                System.out.println("Error: Server returned response code " + responseCode);
            }
            
            httpConn.disconnect();
        } catch (IOException ex) {
            System.out.println("Error: " + ex.getMessage());
        }
    }
}

请注意，如果要下载的文件需要身份验证或需要代理服务器，则需要先设置合适的 HTTP 请求头部和代理服务器。在这个示例代码中没有包含这些情况的处理。