网络资源获取不到

package com.example.administrator.recyclerview;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.RecyclerView;

import com.example.administrator.recyclerview.bead.ImageData;
import com.example.administrator.recyclerview.utils.JsonUtil;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import javax.security.auth.callback.Callback;

import okhttp3.Call;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class MainActivity extends AppCompatActivity {
    private List<String> resList = new ArrayList<>();
    RecyclerView rv;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        rv=findViewById(R.id.recyclerView);
        GridLayoutManager gridLayoutManager=new GridLayoutManager(this,3);
        rv.setLayoutManager(gridLayoutManager);
        
        requestData();
        Adapter adapter=new Adapter(resList,this);
        rv.setAdapter(adapter);
    }

    private void requestData() {
        OkHttpClient mOkHttpClient=new OkHttpClient();
        Request.Builder requestBuilder=new Request.Builder().url("http://www.imooc.com/api/teacher?type=2&page=1");
        Request request =requestBuilder.build();
        Call mcall=mOkHttpClient.newCall(request);
        mcall.enqueue(new okhttp3.Callback() {
            @Override
            public void onFailure(Call call, IOException e) {

            }

            @Override
            public void onResponse(Call call, Response response) throws IOException {
                String res=response.body().string();
                ImageData imageData=(ImageData) JsonUtil.fromJson(res,ImageData.class);
                List<ImageData.DataBean> data=imageData.getData();
                for(int i=0;i<data.size();i++){
                    ImageData.DataBean dataBean=data.get(i);
                    String picBig=dataBean.getPicSmall();
                    resList.add(picBig);
                }
            }
        });
    }
}

有什么问题吗 ，一直在onFail（）