Path googleapis#3401.

Author: rmehta19

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

@@ -272,9 +272,6 @@ private String determineUniverseDomain() {
 
     /** Determines the fully resolved endpoint and universe domain values */
     private String determineEndpoint() throws IOException {
-      if (shouldUseS2A()) {
-        return mtlsEndpoint();
-      }
       MtlsProvider mtlsProvider = mtlsProvider() == null ? new MtlsProvider() : mtlsProvider();
       // TransportChannelProvider's endpoint will override the ClientSettings' endpoint
       String customEndpoint =
@@ -312,6 +309,11 @@ private String determineEndpoint() throws IOException {
     /** Determine if S2A can be used */
     @VisibleForTesting
     boolean shouldUseS2A() {
+      // If mTLS endpoint is not available, skip S2A
+      if (Strings.isNullOrEmpty(mtlsEndpoint())) {
+        return false;
+      }
+
       // If EXPERIMENTAL_GOOGLE_API_USE_S2A is not set to true, skip S2A.
       String s2AEnv;
       s2AEnv = envProvider().getenv(S2A_ENV_ENABLE_USE_S2A);

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

@@ -508,6 +508,20 @@ void shouldUseS2A_customEndpointSetViaTransportChannelProvider_returnsFalse() th
     Truth.assertThat(defaultEndpointContextBuilder.shouldUseS2A()).isFalse();
   }
 
+  @Test
+  void shouldUseS2A_mtlsEndpointNull_returnsFalse() throws IOException {
+    EnvironmentProvider envProvider = Mockito.mock(EnvironmentProvider.class);
+    Mockito.when(envProvider.getenv(EndpointContext.S2A_ENV_ENABLE_USE_S2A)).thenReturn("true");
+    defaultEndpointContextBuilder =
+        defaultEndpointContextBuilder
+            .setEnvProvider(envProvider)
+            .setClientSettingsEndpoint("")
+            .setTransportChannelProviderEndpoint("")
+            .setUsingGDCH(false)
+            .setMtlsEndpoint(null);
+    Truth.assertThat(defaultEndpointContextBuilder.shouldUseS2A()).isFalse();
+  }
+
   @Test
   void shouldUseS2A_mtlsEndpointEmpty_returnsFalse() throws IOException {
     EnvironmentProvider envProvider = Mockito.mock(EnvironmentProvider.class);