Android OkHttpを使った同期通信でエラー

OkHttpを使って同期通信を実行すると、エラーになってしまいます。
下記がそのコードなのですが、間違っている個所があったら教えていただきたいです。

java
1public class MainActivity extends AppCompatActivity {
2
3    @Override
4    protected void onCreate(Bundle savedInstanceState) {
5        super.onCreate(savedInstanceState);
6
7        String url = "http://xxxx/xxxx.php";
8
9        Request request = new Request.Builder()
10                .url(url)
11                .build();
12
13        OkHttpClient okHttpClient = new OkHttpClient().newBuilder().build();
14
15        try  {
16
17            Response response = okHttpClient.newCall(request).execute();
18            //ここで落ちる
19
20            int responseCode = response.code();
21            System.out.println("responseCode: " + responseCode);
22
23            if (!response.isSuccessful()) {
24                System.out.println("error!!");
25            }
26            if (response.body() != null) {
27                String body = response.body().string();
28                System.out.println("body: " + body);
29            }
30        }catch (IOException e){
31            e.printStackTrace();
32        }
33    }
34}
35

エラーメッセージ

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.android_test.babesnaky, PID: 1837
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android_test.babesnaky/com.android_test.babesnaky.MainActivity}: android.os.NetworkOnMainThreadException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
Caused by: android.os.NetworkOnMainThreadException
at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1513)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:389)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:230)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:212)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:436)
at java.net.Socket.connect(Socket.java:621)
at okhttp3.internal.platform.AndroidPlatform.connectSocket(AndroidPlatform.java:71)
at okhttp3.internal.connection.RealConnection.connectSocket(RealConnection.java:247)
at okhttp3.internal.connection.RealConnection.connect(RealConnection.java:167)
at okhttp3.internal.connection.StreamAllocation.findConnection(StreamAllocation.java:257)
at okhttp3.internal.connection.StreamAllocation.findHealthyConnection(StreamAllocation.java:135)
at okhttp3.internal.connection.StreamAllocation.newStream(StreamAllocation.java:114)
at okhttp3.internal.connection.ConnectInterceptor.intercept(ConnectInterceptor.java:42)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
at okhttp3.internal.cache.CacheInterceptor.intercept(CacheInterceptor.java:94)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
at okhttp3.internal.http.BridgeInterceptor.intercept(BridgeInterceptor.java:93)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
at okhttp3.internal.http.RetryAndFollowUpInterceptor.intercept(RetryAndFollowUpInterceptor.java:125)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
at okhttp3.RealCall.getResponseWithInterceptorChain(RealCall.java:264)
at okhttp3.RealCall.execute(RealCall.java:93)
at com.android_test.babesnaky.MainActivity.onCreate(MainActivity.java:53)
at android.app.Activity.performCreate(Activity.java:7136)
at android.app.Activity.performCreate(Activity.java:7127)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)