Add Admin SDK snippet -- getting service account credentials

Change-Id: I2e3bd92d5ca97fba1ad6af194dac26ac48b99967

Author: kevinthecheung

admin/build.gradle

@@ -0,0 +1,19 @@
+group 'com.google.firebase.example'
+version '1.0'
+
+apply plugin: 'java'
+apply plugin: 'application'
+
+mainClassName = 'com.google.firebase.example.Main'
+
+sourceCompatibility = 1.8
+
+repositories {
+    mavenCentral()
+    mavenLocal()
+}
+
+dependencies {
+    // Firebase Admin Java SDK
+    compile 'com.google.firebase:firebase-admin:5.1.0'
+}

admin/src/main/java/com/google/firebase/example/Main.java

@@ -0,0 +1,41 @@
+package com.google.firebase.example;
+
+import com.google.firebase.auth.FirebaseCredential;
+import com.google.firebase.auth.FirebaseCredentials;
+import com.google.firebase.auth.GoogleOAuthAccessToken;
+import com.google.firebase.internal.NonNull;
+import com.google.firebase.tasks.OnCompleteListener;
+import com.google.firebase.tasks.Task;
+
+import java.io.FileInputStream;
+import java.io.FileNotFoundException;
+import java.io.IOException;
+
+public class Main {
+
+    public void getServiceAccountAccessToken() throws FileNotFoundException, IOException{
+        // [START get_service_account_tokens]
+        FileInputStream serviceAccount = new FileInputStream("path/to/serviceAccountKey.json");
+        FirebaseCredential credential = FirebaseCredentials.fromCertificate(serviceAccount);
+        credential.getAccessToken().addOnCompleteListener(
+                new OnCompleteListener<GoogleOAuthAccessToken>() {
+                    @Override
+                    public void onComplete(@NonNull Task<GoogleOAuthAccessToken> task) {
+                        if (task.isSuccessful()) {
+                            String accessToken = task.getResult().getAccessToken();
+                            long expirationTime = task.getResult().getExpiryTime();
+                            // Attach accessToken to HTTPS request in the
+                            //   "Authorization: Bearer" header
+                            // After expirationTime. you must generate a new access
+                            //   token
+                        } else {
+                            // Error
+                        }
+                    }});
+        // [END get_service_account_tokens]
+    }
+
+    public static void main(String[] args) {
+        // write your code here
+    }
+}

settings.gradle

@@ -1 +1,2 @@
-include ':database'
+include ':admin'
+include ':database'