在Android Studio中未使用适配器显示的微调器项目

Question

好日子的家伙！这是我第一次问一个问题，因为我几天都在解决这个问题...我正在开发一个使用微调器并连接到mysql服务器的Android应用程序..但它似乎下拉列表没有显示在微调器上......我找不到任何明确的解决方案。我希望有人能帮我...

package edu.sti.mobileinstructorevaluation;

import android.os.AsyncTask;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ArrayAdapter;
import android.widget.Spinner;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;


public class MainActivity extends ActionBarActivity {
    ArrayList<String> listItems=new ArrayList<>();
    ArrayAdapter<String> adapter;
    Spinner sp;
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        sp=(Spinner)findViewById(R.id.spinner);
        adapter=new ArrayAdapter<String>(this,R.layout.spinner_layout,R.id.txt,listItems);
        sp.setAdapter(adapter);

    }
    public void onStart(){
        super.onStart();
        BackTask bt=new BackTask();
        bt.execute();
    }
    private class BackTask extends AsyncTask<Void,Void,Void> {
        ArrayList<String> list;
        protected void onPreExecute(){
            super.onPreExecute();
            list=new ArrayList<>();
        }
        protected Void doInBackground(Void...params){
            InputStream is=null;
            String result="";
            try{
                HttpClient httpclient=new DefaultHttpClient();
                HttpPost httppost= new HttpPost("http://10.0.2.2:8080/mobieva/getcourse.php");
                HttpResponse response=httpclient.execute(httppost);
                HttpEntity entity = response.getEntity();
                // Get our response as a String.
                is = entity.getContent();
            }catch(IOException e){
                e.printStackTrace();
            }

            //convert response to string
            try{
                BufferedReader reader = new BufferedReader(new InputStreamReader(is,"utf-8"));
                String line = null;
                while ((line = reader.readLine()) != null) {
                    result+=line;
                }
                is.close();
                //result=sb.toString();
            }catch(Exception e){
                e.printStackTrace();
            }
            // parse json data
            try{
                JSONArray jArray =new JSONArray(result);
                for(int i=0;i<jArray.length();i++){
                    JSONObject jsonObject=jArray.getJSONObject(i);
                    // add interviewee name to arraylist
                    list.add(jsonObject.getString("cname"));
                }
            }
            catch(JSONException e){
                e.printStackTrace();
            }
            return null;
        }
        protected void onPostExecute(Void result){
            listItems.addAll(list);
            adapter.notifyDataSetChanged();
        }
    }
}