All the veneer libraries support picking up the default project id if an explicit one is not provided.
See Spanner , Datastore, Storage :
https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java

The deprecreated PubSub api supported it as well, v0.9.2-alpha does not.