docs(samples): add auth with metadata server

samples/snippets/src/main/java/AuthWithCredentialsFromMetadataServer.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright 2022 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import com.google.api.gax.paging.Page;
+import com.google.auth.oauth2.ComputeEngineCredentials;
+import com.google.auth.oauth2.GoogleCredentials;
+import com.google.cloud.storage.Bucket;
+import com.google.cloud.storage.Storage;
+import com.google.cloud.storage.StorageOptions;
+import java.io.IOException;
+import java.security.GeneralSecurityException;
+
+public class AuthWithCredentialsFromMetadataServer {
+
+  public static void main(String[] args) throws IOException, GeneralSecurityException {
+    // TODO(Developer):
+    //  1. Replace the below variable.
+    //  2. Make sure you have the necessary permission to list storage buckets "storage.buckets.list"
+    String projectId = "your-google-cloud-project-id";
+
+    authWithCredentialsFromMetadataServer(projectId);
+  }
+
+  // Authenticating using Client libraries can be done in one of the following ways:
+  // 1. Implicit authentication with ADC (Application Default Credentials)
+  // 2. Explicit authentication by specifying the service account
+  // 3. Authentication with service account credentials obtained from a metadata server, like,
+  // Compute Engine or App Engine etc.,
+  // 4. Bring your own (BYO) access token
+  // 5. Using API keys (for libraries that support)
+  //
+  // In this snippet, we demonstrate "Authentication with service account credentials
+  // obtained from a metadata server".
+  public static void authWithCredentialsFromMetadataServer(String project) {
+
+    // This snippet demonstrates how to initialize Cloud Storage and list buckets.
+    // Note that the credentials are requested from the ComputeEngine metadata server.
+    Storage storage = initService(project);
+
+    System.out.println("Buckets:");
+    Page<Bucket> buckets = storage.list();
+    for (Bucket bucket : buckets.iterateAll()) {
+      System.out.println(bucket.toString());
+    }
+  }
+
+  // Initialize the Storage client by getting the Service account credentials
+  // from a Metadata server.
+  public static Storage initService(String projectId) {
+    // Explicitly request the service account credentials from the ComputeEngine metadata server.
+    GoogleCredentials credentials = ComputeEngineCredentials.create();
+
+    // Alternately, if executing within AppEngine, you can get credentials as follows:
+    // GoogleCredentials credentials = AppEngineCredentials.getApplicationDefault();
+
+    // Construct the Storage client.
+    // Note that, here we explicitly specify the service account to use.
+    return StorageOptions.newBuilder()
+        .setCredentials(credentials)
+        .setProjectId(projectId)
+        .build()
+        .getService();
+  }
+}

samples/snippets/src/main/java/AuthenticateExplicit.java

@@ -47,8 +47,10 @@ public static void main(String[] args) throws IOException, GeneralSecurityExcept
   // Authenticating using Client libraries can be done in one of the following ways:
   // 1. Implicit authentication with ADC (Application Default Credentials)
   // 2. Explicit authentication by specifying the service account
-  // 3. Bring your own (BYO) access token
-  // 4. Using API keys (for libraries that support)
+  // 3. Authentication with service account credentials obtained from metadata server, like,
+  // Compute Engine or App Engine etc.,
+  // 4. Bring your own (BYO) access token
+  // 5. Using API keys (for libraries that support)
   //
   // In this snippet, we demonstrate "Explicit authentication by specifying the service account".
   public static void authenticateExplicit(String project, String jsonCredentialPath, String scope)

samples/snippets/src/main/java/AuthenticateImplicitWithAdc.java

@@ -33,8 +33,10 @@ public static void main(String[] args) throws IOException {
   // Authenticating using Client libraries can be done in one of the following ways:
   // 1. Implicit authentication with ADC (Application Default Credentials)
   // 2. Explicit authentication by specifying the service account
-  // 3. Bring your own (BYO) access token
-  // 4. Using API keys (for libraries that support)
+  // 3. Authentication with service account credentials obtained from metadata server, like,
+  // Compute Engine or App Engine etc.,
+  // 4. Bring your own (BYO) access token
+  // 5. Using API keys (for libraries that support)
   //
   // In this snippet, we demonstrate "Implicit authentication with ADC".
   // ADC - Application Default Credentials