Problem when logging in to android with AsyncTask

I am using MySQL as a database hosted on an Amazon server (AWS).

I am using the AsyncTask class to perform the search in the bank, but when I run on the mobile phone it enters the Exceptions, it seems to cut the connection to the bank.

Maybe the ResultSet is conflicting, because I use the same code to insert values into the bank with executeUpdate and it inserts certinhoooo

public class Tela_Login extends AppCompatActivity {
    ConnectionClass connectionClass;
    EditText editUsuario, editSenha;
    Button btLogin;
    TextView textCadastro;
    ProgressBar pbbar;

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

        editUsuario = (EditText) findViewById(R.id.editUsuario);
        editSenha = (EditText) findViewById(R.id.editSenha);
        textCadastro = (TextView) findViewById(R.id.textCadastro);
        btLogin = (Button) findViewById(R.id.btLogin);
        pbbar = (ProgressBar) findViewById(R.id.pbbar);
        pbbar.setVisibility(View.GONE);


        btLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                realizarLogin realizarLogin = new realizarLogin();
                btLogin.setClickable(false);
                realizarLogin.execute("");

            }
        });
    }
    public class realizarLogin extends AsyncTask<String,String,String>{
        String z = "";
        Boolean isSuccess = false;


        String usuario = editUsuario.getText().toString();
        String senha = editSenha.getText().toString();

        @Override
        protected void onPreExecute() {
            pbbar.setVisibility(View.VISIBLE);
        }

        @Override
        protected void onPostExecute(String r) {
            pbbar.setVisibility(View.GONE);
            Toast.makeText(Tela_Login.this, r, Toast.LENGTH_SHORT).show();

            if (isSuccess) {
                Toast.makeText(Tela_Login.this, r, Toast.LENGTH_SHORT).show();
            }

        }

        @Override
        protected String doInBackground(String... params) {
            if (usuario.trim().equals("") || senha.trim().equals(""))
                z = "Por favor os valores!";
            else {
                try {
                    Connection con = connectionClass.CONN();
                    if (con == null) {
                        z = "Erro ao conectar no banco de dados";
                    } else {
                        String query = "select * from Salao where email='" +usuario+"' and senha='" +senha+"'";
                        Statement stmt = con.createStatement();
                        ResultSet rs;
                        rs = stmt.executeQuery(query);

                        if (rs.next()) {
                            z = "VOCE CONSEGUIU";
                            isSuccess = true;
                        } else {
                            z = "Invalid Credentials";
                            isSuccess = false;
                        }

                    }
                } catch (Exception ex) {
                    isSuccess = false;
                    z = "Exceptions";
                }
            }
            btLogin.setClickable(true);
            return z;

        }
    }