How to keep android applications always be logged in state?

Use Shared Preference for auto login functionality. When users log in to your application, store the login status into sharedPreference and clear sharedPreference when users log out.

Check every time when the user enters into the application if user status from shared Preference is true then no need to log in again otherwise direct to the login page.

To achieve this first create a class, in this class you need to write all the function regarding the get and set value in the sharedpreference. Please look at this below Code.

public class SaveSharedPreference 
{
    static final String PREF_USER_NAME= "username";

    static SharedPreferences getSharedPreferences(Context ctx) {
        return PreferenceManager.getDefaultSharedPreferences(ctx);
    }

    public static void setUserName(Context ctx, String userName) 
    {
        Editor editor = getSharedPreferences(ctx).edit();
        editor.putString(PREF_USER_NAME, userName);
        editor.commit();
    }

    public static String getUserName(Context ctx)
    {
        return getSharedPreferences(ctx).getString(PREF_USER_NAME, "");
    }
}

Now in the main activity (The “Activity” where users will be redirected when logged in) first check

if(SaveSharedPreference.getUserName(MainActivity.this).length() == 0)
{
     // call Login Activity
}
else
{
     // Stay at the current activity.
}

In Login activity if user login successful then set UserName using setUserName() function.