Firebase Cloud Messagingの中身をログに出力する

AndroidManifest.xml

...
<receiver
    android:name=".FirebaseDataReceiver"
    android:exported="true"
    android:permission="com.google.android.c2dm.permission.SEND">
    <intent-filter>
        <action android:name="com.google.android.c2dm.intent.RECEIVE"/>
    </intent-filter>
</receiver>
...

FirebaseDataReceiver.java

package com.example.firebase;

import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.content.WakefulBroadcastReceiver;
import android.util.Log;

public class FirebaseDataReceiver extends WakefulBroadcastReceiver {

    private final String TAG = FirebaseDataReceiver.class.getName();

    public void onReceive(Context context, Intent intent) {

        Bundle bundle = intent.getExtras();

        for (String key : bundle.keySet()) {
            Object value = bundle.get(key);
            String valueString = (value != null) ? value.toString() : "<NULL>";
            Log.d(TAG, "KEY: " + key + ", VALUE: " + valueString);
        }
    }
}