feat: add code sample and test for getting an access token from an im…

…personated SA (#1289)

Co-authored-by: Tianzi Cai <tianzi@google.com>

samples/snippets/src/main/java/AccessTokenFromImpersonatedCredentials.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright 2023 Google LLC
+ *
+ * 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.
+ */
+
+// [START auth_cloud_accesstoken_impersonated_credentials]
+
+import com.google.auth.oauth2.GoogleCredentials;
+import com.google.auth.oauth2.ImpersonatedCredentials;
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.List;
+
+public class AccessTokenFromImpersonatedCredentials {
+
+  public static void main(String[] args) throws IOException {
+    // TODO(Developer): Replace the below variables before running the code.
+
+    // Provide the scopes that you might need to request access to Google APIs,
+    // depending on the level of access you need.
+    // This example uses the cloud-wide scope and uses IAM to narrow the permissions.
+    // https://cloud.google.com/docs/authentication/external/authorization-gcp
+    // For more information, see: https://developers.google.com/identity/protocols/oauth2/scopes
+    String scope = "https://www.googleapis.com/auth/cloud-platform";
+
+    // The name of the privilege-bearing service account for whom the credential is created.
+    String impersonatedServiceAccount = "name@project.service.gserviceaccount.com";
+
+    getAccessToken(impersonatedServiceAccount, scope);
+  }
+
+  // Use a service account (SA1) to impersonate as another service account (SA2) and obtain id token
+  // for the impersonated account.
+  // To obtain token for SA2, SA1 should have the "roles/iam.serviceAccountTokenCreator" permission
+  // on SA2.
+  public static void getAccessToken(
+      String impersonatedServiceAccount, String scope) throws IOException {
+
+    // Construct the GoogleCredentials object which obtains the default configuration from your
+    // working environment.
+    GoogleCredentials googleCredentials = GoogleCredentials.getApplicationDefault();
+
+    // delegates: The chained list of delegates required to grant the final accessToken.
+    // For more information, see:
+    // https://cloud.google.com/iam/docs/create-short-lived-credentials-direct#sa-credentials-permissions
+    // Delegate is NOT USED here.
+    List<String> delegates = null;
+
+    // Create the impersonated credential.
+    ImpersonatedCredentials impersonatedCredentials =
+        ImpersonatedCredentials.newBuilder()
+            .setSourceCredentials(googleCredentials)
+            .setTargetPrincipal(impersonatedServiceAccount)
+            .setScopes(Arrays.asList(scope))
+            .setLifetime(300)
+            .setDelegates(delegates)
+            .build();
+
+    // Get the OAuth2 token.
+    // Once you've obtained the OAuth2 token, you can use it to make an authenticated call.
+    impersonatedCredentials.refresh();
+    String accessToken = impersonatedCredentials.getAccessToken().getTokenValue();
+    System.out.println("Generated access token.");
+  }
+}
+// [END auth_cloud_accesstoken_impersonated_credentials]

samples/snippets/src/test/java/SnippetsIT.java

@@ -40,6 +40,8 @@ public class SnippetsIT {
 
   private static final String PROJECT_ID = System.getenv("GOOGLE_CLOUD_PROJECT");
   private static final String CREDENTIALS = System.getenv("GOOGLE_APPLICATION_CREDENTIALS");
+  private static final String impersonatedServiceAccount = "TODO";
+  private static final String scope = "https://www.googleapis.com/auth/cloud-platform";
   private ByteArrayOutputStream stdOut;
 
   // Check if the required environment variables are set.
@@ -62,18 +64,7 @@ public static void setup() throws IOException {
   }
 
   @AfterClass
-  public static void cleanup() {}
-
-  @Before
-  public void beforeEach() {
-    stdOut = new ByteArrayOutputStream();
-    System.setOut(new PrintStream(stdOut));
-  }
-
-  @After
-  public void afterEach() {
-    stdOut = null;
-    System.setOut(null);
+  public static void cleanup() {
   }
 
   // Get an id token from a Google service account.
@@ -93,6 +84,18 @@ private static String getIdTokenFromServiceAccount(
     return idToken.getTokenValue();
   }
 
+  @Before
+  public void beforeEach() {
+    stdOut = new ByteArrayOutputStream();
+    System.setOut(new PrintStream(stdOut));
+  }
+
+  @After
+  public void afterEach() {
+    stdOut = null;
+    System.setOut(null);
+  }
+
   @Test
   public void testIdTokenFromServiceAccount() throws IOException {
     IdTokenFromServiceAccount.getIdTokenFromServiceAccount(CREDENTIALS, "https://example.com");
@@ -107,6 +110,21 @@ public void testVerifyGoogleIdToken() throws IOException {
         idToken, "https://example.com", "https://www.googleapis.com/oauth2/v3/certs");
   }
 
+  @Test
+  public void testAccessTokenFromImpersonatedCredentials()
+      throws GeneralSecurityException, IOException {
+    AccessTokenFromImpersonatedCredentials.getAccessToken(impersonatedServiceAccount, scope);
+    assertThat(stdOut.toString()).contains("Generated access token.");
+  }
+
+  @Test
+  public void testIdTokenFromImpersonatedCredentials()
+      throws GeneralSecurityException, IOException {
+    IdTokenFromImpersonatedCredentials.getIdTokenUsingOAuth2(impersonatedServiceAccount, scope,
+        "https://example.com");
+    assertThat(stdOut.toString()).contains("Generated ID token.");
+  }
+
   @Test
   public void testIdTokenFromMetadataServer() throws GeneralSecurityException, IOException {
     IdTokenFromMetadataServer.getIdTokenFromMetadataServer("https://www.google.com");
@@ -116,7 +134,7 @@ public void testIdTokenFromMetadataServer() throws GeneralSecurityException, IOE
   @Test
   public void testAuthenticateImplicitWithAdc() throws IOException {
     AuthenticateImplicitWithAdc.authenticateImplicitWithAdc(PROJECT_ID);
-    assertThat(stdOut.toString()).contains("Listing instances complete");
+    assertThat(stdOut.toString()).contains("Listed all storage buckets.");
   }
 
   @Test