Java如何实现跨服务器上传下载文件

Java可以使用以下两种方式实现跨服务器上传下载文件：

1. 使用FTP协议：FTP协议是一种文件传输协议，可以实现跨服务器上传下载文件。Java中可以使用Apache Commons Net库来实现FTP文件传输。示例代码如下：

import org.apache.commons.net.ftp.*;

public class FtpExample {
    public static void main(String[] args) {
        String server = "ftp.example.com";
        int port = 21;
        String user = "username";
        String pass = "password";
        
        FTPClient client = new FTPClient();
        try {
            client.connect(server, port);
            client.login(user, pass);
            client.enterLocalPassiveMode();
            
            String remoteFile = "/path/to/remote/file";
            File localFile = new File("/path/to/local/file");
            
            OutputStream outputStream = new BufferedOutputStream(new FileOutputStream(localFile));
            boolean success = client.retrieveFile(remoteFile, outputStream);
            outputStream.close();
            
            if (success) {
                System.out.println("File downloaded successfully.");
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (client.isConnected()) {
                    client.logout();
                    client.disconnect();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

2. 使用HTTP协议：HTTP协议也可以用于文件传输，可以通过HTTP的POST和GET请求来实现文件上传和下载。Java中可以使用Apache HttpClient库来实现HTTP文件传输。示例代码如下：

import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.mime.MultipartEntityBuilder;
import org.apache.http.entity.mime.content.FileBody;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

public class HttpExample {
    public static void main(String[] args) {
        HttpClient httpClient = HttpClients.createDefault();
        
        // 文件上传
        HttpPost httpPost = new HttpPost("http://example.com/upload");
        FileBody fileBody = new FileBody(new File("/path/to/local/file"));
        MultipartEntityBuilder builder = MultipartEntityBuilder.create();
        builder.addPart("file", fileBody);
        httpPost.setEntity(builder.build());
        try {
            HttpResponse response = httpClient.execute(httpPost);
            String result = EntityUtils.toString(response.getEntity());
            System.out.println(result);
        } catch (IOException e) {
            e.printStackTrace();
        }
        
        // 文件下载
        HttpGet httpGet = new HttpGet("http://example.com/download?file=/path/to/remote/file");
        try {
            HttpResponse response = httpClient.execute(httpGet);
            HttpEntity entity = response.getEntity();
            if (entity != null) {
                InputStream inputStream = entity.getContent();
                OutputStream outputStream = new BufferedOutputStream(new FileOutputStream("/path/to/local/file"));
                byte[] buffer = new byte[1024];
                int len = 0;
                while ((len = inputStream.read(buffer)) != -1) {
                    outputStream.write(buffer, 0, len);
                }
                outputStream.close();
                inputStream.close();
                EntityUtils.consume(entity);
                System.out.println("File downloaded successfully.");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}