alts: Make GoogleDefaultChannelCredentials take a CallCredentials

alts/src/main/java/io/grpc/alts/GoogleDefaultChannelCredentials.java

@@ -44,30 +44,54 @@ private GoogleDefaultChannelCredentials() {}
    * as fallback.
    */
   public static ChannelCredentials create() {
-    ChannelCredentials nettyCredentials =
-        InternalNettyChannelCredentials.create(createClientFactory());
-    CallCredentials callCredentials;
-    try {
-      callCredentials = MoreCallCredentials.from(GoogleCredentials.getApplicationDefault());
-    } catch (IOException e) {
-      callCredentials = new FailingCallCredentials(
-          Status.UNAUTHENTICATED
-              .withDescription("Failed to get Google default credentials")
-              .withCause(e));
-    }
-    return CompositeChannelCredentials.create(nettyCredentials, callCredentials);
+    return newBuilder().build();
+  }
+
+  public static Builder newBuilder() {
+    return new Builder();
   }
 
-  private static InternalProtocolNegotiator.ClientFactory createClientFactory() {
-    SslContext sslContext;
-    try {
-      sslContext = GrpcSslContexts.forClient().build();
-    } catch (SSLException e) {
-      throw new RuntimeException(e);
+  /** Builder for {@link GoogleDefaultChannelCredentials} instances. */
+  public static final class Builder {
+    private CallCredentials callCredentials;
+
+    /** Construct GoogleDefaultChannelCredentials with a given call credential. */
+    public Builder setCallCredentials(CallCredentials callCreds) {
+      callCredentials = callCreds;
+      return this;
+    }
+
+    /** Build a GoogleDefaultChannelCredentials instance. */
+    public ChannelCredentials build() {
+      ChannelCredentials nettyCredentials =
+          InternalNettyChannelCredentials.create(createClientFactory());
+      if (callCredentials == null) {
+        CallCredentials callCreds;
+        try {
+          callCreds = MoreCallCredentials.from(GoogleCredentials.getApplicationDefault());
+        } catch (IOException e) {
+          callCreds =
+              new FailingCallCredentials(
+                  Status.UNAUTHENTICATED
+                      .withDescription("Failed to get Google default credentials")
+                      .withCause(e));
+        }
+        return CompositeChannelCredentials.create(nettyCredentials, callCreds);
+      }
+      return CompositeChannelCredentials.create(nettyCredentials, callCredentials);
+    }
+
+    private static InternalProtocolNegotiator.ClientFactory createClientFactory() {
+      SslContext sslContext;
+      try {
+        sslContext = GrpcSslContexts.forClient().build();
+      } catch (SSLException e) {
+        throw new RuntimeException(e);
+      }
+      return new GoogleDefaultProtocolNegotiatorFactory(
+          /* targetServiceAccounts= */ ImmutableList.<String>of(),
+          SharedResourcePool.forResource(HandshakerServiceChannel.SHARED_HANDSHAKER_CHANNEL),
+          sslContext);
     }
-    return new GoogleDefaultProtocolNegotiatorFactory(
-        /* targetServiceAccounts= */ ImmutableList.<String>of(),
-        SharedResourcePool.forResource(HandshakerServiceChannel.SHARED_HANDSHAKER_CHANNEL),
-        sslContext);
   }
 }