feat: Universe Domain Environment Variable Support

Author: lqiu96

gax-java/gax/src/main/java/com/google/api/gax/rpc/EndpointContext.java

@@ -47,6 +47,7 @@
 @InternalApi
 @AutoValue
 public abstract class EndpointContext {
+  private static final String GOOGLE_CLOUD_UNIVERSE_DOMAIN = "GOOGLE_CLOUD_UNIVERSE_DOMAIN";
   private static final String INVALID_UNIVERSE_DOMAIN_ERROR_TEMPLATE =
       "The configured universe domain (%s) does not match the universe domain found in the credentials (%s). If you haven't configured the universe domain explicitly, `googleapis.com` is the default.";
   public static final String UNABLE_TO_RETRIEVE_CREDENTIALS_ERROR_MESSAGE =
@@ -213,7 +214,9 @@ private String determineUniverseDomain() {
       if (universeDomain() != null && universeDomain().isEmpty()) {
         throw new IllegalArgumentException("The universe domain value cannot be empty.");
       }
-      // Override with user set universe domain if provided
+      // Override with configured universe domain if provided. If the universe domain is configured
+      // by the user, the universe domain will either be the settings or from the env var. The
+      // value from ClientSettings has priority.
       return universeDomain() != null ? universeDomain() : Credentials.GOOGLE_DEFAULT_UNIVERSE;
     }
 
@@ -283,6 +286,11 @@ String mtlsEndpointResolver(
     }
 
     public EndpointContext build() throws IOException {
+      // If the universe domain wasn't configured explicitly in the settings, check the
+      // environment variable for the value
+      if (universeDomain() == null) {
+        setUniverseDomain(System.getenv(GOOGLE_CLOUD_UNIVERSE_DOMAIN));
+      }
       // The Universe Domain is used to resolve the Endpoint. It should be resolved first
       setResolvedUniverseDomain(determineUniverseDomain());
       setResolvedEndpoint(determineEndpoint());

gax-java/gax/src/test/java/com/google/api/gax/rpc/EndpointContextTest.java

@@ -340,6 +340,21 @@ public void endpointContextBuild_gdchFlow_noUniverseDomain_customEndpoint() thro
         .isEqualTo(Credentials.GOOGLE_DEFAULT_UNIVERSE);
   }
 
+  // JUnit has limitations with mocking Environment Variable. For this test, we simply replicate the
+  // behavior in `build()` by setting the universe domain to an env var value. Keeping this test
+  // case scenario if a future solution for testing env vars exists.
+  @Test
+  public void endpointContextBuild_universeDomainEnvVarSet() throws IOException {
+    String envVarUniverseDomain = "random.com";
+    EndpointContext endpointContext =
+        defaultEndpointContextBuilder
+            .setUniverseDomain(envVarUniverseDomain)
+            .setClientSettingsEndpoint(null)
+            .build();
+    Truth.assertThat(endpointContext.resolvedEndpoint()).isEqualTo("test.random.com:443");
+    Truth.assertThat(endpointContext.resolvedUniverseDomain()).isEqualTo(envVarUniverseDomain);
+  }
+
   @Test
   public void hasValidUniverseDomain_gdchFlow_anyCredentials() throws IOException {
     Credentials noCredentials = NoCredentialsProvider.create().getCredentials();