MainActivity.java

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.KeyEvent;
import android.content.Context;

import com.google.android.gms.gcm.GoogleCloudMessaging;
import com.google.android.gms.iid.InstanceID;

import java.io.IOException;

public class MainActivity extends Activity {

    /** Google Cloud Messagingオブジェクト */
    private GoogleCloudMessaging gcm;
    /** コンテキスト */
    private Context context;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        context = getApplicationContext();

        gcm = GoogleCloudMessaging.getInstance(this);
        registerInBackground();
    }


    private void registerInBackground() {
        new AsyncTask<Void, Void, String>() {
            @Override
            protected String doInBackground(Void... params) {
                String regId = "";
                try {
                    if (gcm == null) {
                        gcm = GoogleCloudMessaging.getInstance(MainActivity.this);
                    }
                    InstanceID instanceID = InstanceID.getInstance(context);
                    // senderIdはプロジェクト番号
                    regId = instanceID.getToken("senderId", GoogleCloudMessaging.INSTANCE_ID_SCOPE, null);

                } catch (IOException ex) {
                    ex.printStackTrace();
                }
                return regId;
            }

            @Override
            protected void onPostExecute(String regId) {
                Log.d("hoge", regId);
            }
        }.execute(null, null, null);
    }
}