Android FTP Server: A Comprehensive Guide with Code Examples

Introduction

FTP (File Transfer Protocol) is a standard network protocol used for transferring files between a client and a server on a computer network. In the context of Android development, an Android FTP server allows users to transfer files to and from an Android device over a network.

In this article, we will explore how to create an Android FTP server using Java and the Apache MINA FTP server library. We will cover the basics of setting up an FTP server, handling client connections, and managing file transfers.

Prerequisites

To follow along, make sure you have the following prerequisites installed:

• Android Studio
• Java Development Kit (JDK)

Setting up the Project

1. Open Android Studio and create a new Android project.

2. Add the Apache MINA FTP server library to your project by including the following dependency in your app/build.gradle file:

dependencies {
    implementation 'org.apache.ftpserver:ftpserver-core:1.1.1'
}

3. Create a new Java class FTPServer that extends org.apache.ftpserver.FtpServerFactory:

import org.apache.ftpserver.FtpServer;
import org.apache.ftpserver.FtpServerFactory;

public class FTPServer {
    private static final int PORT = 2221;

    public static void main(String[] args) {
        FtpServerFactory serverFactory = new FtpServerFactory();
        serverFactory.addListener("default", new ListenerFactory().createListener(PORT));
        
        FtpServer server = serverFactory.createServer();
        try {
            server.start();
            System.out.println("FTP server started on port " + PORT);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Start the FTP Server

To start the FTP server, add the following code to your MainActivity class:

import android.os.Bundle;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    private FTPServer ftpServer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ftpServer = new FTPServer();
        ftpServer.start();

        Toast.makeText(this, "FTP server started", Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        ftpServer.stop();
    }
}

Handling Client Connections

To handle client connections, we need to implement a custom UserManager class that defines the authentication logic. Create a new Java class FTPUserManager:

import org.apache.ftpserver.ftplet.Authentication;
import org.apache.ftpserver.ftplet.FtpException;
import org.apache.ftpserver.ftplet.FtpRequest;
import org.apache.ftpserver.usermanager.UsernamePasswordAuthentication;
import org.apache.ftpserver.usermanager.impl.BaseUser;
import org.apache.ftpserver.usermanager.impl.WritePermission;

public class FTPUserManager implements UserManager {
    private static final String USERNAME = "admin";
    private static final String PASSWORD = "admin";
    private static final String HOME_DIRECTORY = "/sdcard";

    @Override
    public User getUserByName(String username) throws FtpException {
        if (username.equals(USERNAME)) {
            BaseUser user = new BaseUser();
            user.setName(USERNAME);
            user.setPassword(PASSWORD);
            user.setHomeDirectory(HOME_DIRECTORY);
            user.setMaxIdleTime(0);
            user.setEnabled(true);
            user.setAuthorities(Collections.singletonList(new WritePermission()));
            return user;
        }
        return null;
    }

    @Override
    public String[] getAllUserNames() throws FtpException {
        return new String[]{USERNAME};
    }

    // Implement other methods of the UserManager interface
}

Configure the FTP Server

To configure the FTP server, modify the FTPServer class as follows:

import org.apache.ftpserver.usermanager.UserManager;
import org.apache.ftpserver.usermanager.impl.PropertiesUserManagerFactory;

public class FTPServer {
    private static final int PORT = 2221;

    public static void main(String[] args) {
        FtpServerFactory serverFactory = new FtpServerFactory();
        serverFactory.addListener("default", new ListenerFactory().createListener(PORT));

        PropertiesUserManagerFactory userManagerFactory = new PropertiesUserManagerFactory();
        userManagerFactory.setFile(new File("myusers.properties"));

        UserManager userManager = userManagerFactory.createUserManager();
        serverFactory.setUserManager(userManager);

        FtpServer server = serverFactory.createServer();

        // Start the server
    }
}

Conclusion

In this article, we have explored how to create an Android FTP server using the Apache MINA FTP server library. We covered the basics of setting up an FTP server, handling client connections, and managing file transfers. By following the code examples and instructions provided, you should now have a working Android FTP server.

Remember to always consider security measures when deploying an FTP server, such as enforcing secure connections, implementing proper authentication mechanisms, and restricting access to sensitive areas of the device.