Getting a file from a remote server in android

Question

I recently started working with Android and I have to write a code snippet in my android application that transfers a file via WiFi from a Raspberry pi connected to the same network as my android application. I am using the JSCH library and a code example given here.

I have tried to include this code in my android application. However, in the try/Except block, my application crashes at the session.connect() command. I can't seem to figure out the problem here. The code compiles and builds fine. I am running it directly on my android phone rather than an emulator. The application crashes on start-up, even though this activity is started with the push of a button.

Can someone help me out?

public class ft_mainpage extends AppCompatActivity{

Spinner spinner;
String[] paths = {"Red Tripple Band Pass","Water Level Index","Blue Tripple Band Pass", "RGB"};
TextView tv;

////////////////

////////////////
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_ft_mainpage);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

    tv = (TextView)findViewById(R.id.ft_mainpage_tv);
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(ft_mainpage.this, android.R.layout.simple_spinner_item, paths);
    spinner = (Spinner)findViewById(R.id.ft_main_spinner);
    spinner.setAdapter(adapter);
    spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> adapterView, View view, int i, long l) {
            int position = spinner.getSelectedItemPosition();
            switch(position){
                case 1:
                    tv.setText("RTBP - Fetching Information");

                    Session session = null;
                    Channel channel = null;
                    ChannelSftp channelSftp = null;

                    try{
                        JSch ssh = new JSch();
                        session = ssh.getSession("pi","10.0.1.170",22);
                        session.setPassword("raspberry");
                        java.util.Properties config = new java.util.Properties();
                        config.put("StrictHostKeyChecking", "no");
                        session.setConfig(config);
                        session.connect();
                        channel = session.openChannel("sftp");
                        channel.connect();
                        channelSftp = (ChannelSftp) channel;
                        channelSftp.cd("/home/pi/Desktop/");
                        byte[] buffer = new byte[1024];
                        BufferedInputStream bis = new BufferedInputStream(channelSftp.get("Test.java"));
                        File newFile = new File(Environment.DIRECTORY_DOWNLOADS);
                        OutputStream os = new FileOutputStream(newFile);
                        BufferedOutputStream bos = new BufferedOutputStream(os);
                        int readCount;
                        while ((readCount = bis.read(buffer)) > 0) {
                            System.out.println("Writing: ");
                            bos.write(buffer, 0, readCount);
                        }
                        bis.close();
                        bos.close();

                    }catch (Exception ex){
                        ex.printStackTrace();
                    }


                    break;
                case 0:
                    tv.setText("WLI");
                    break;
                case 2:
                    tv.setText("RTBP");
                    break;
                case 3:
                    tv.setText("RGB");
                    break;
            }
        }

        @Override
        public void onNothingSelected(AdapterView<?> adapterView) {
            int a =0;
        }
    });

}

Even if there is some error in the way I am fetching files, the code causing the error is in a try/Except block, so I believe the application should not crash and go to the except block. However, as mentioned above, the entire application crashes.

I have the following permissions in my Android Manifest

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="com.google.android.providers.gsf.permission.READ_GSERVICES" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />
<uses-permission android:name="android.permission.CHANGE_NETWORK_STATE" />
<uses-permission android:name="android.permission.VIBRATE" />

Answer 1

The comment of "a person" is right.

You have to do the network operation in the background, using AsyncTask or something like this.