Google Sheets use API key instead of client_secret.json

An API key could only work when accessing the resources owned by the project that created the key.

For resources like spreadsheets, you're typically accessing resources owned by a user. It would be pretty awful if you got access to my private sheets simply by having an API key.

So no, I wouldn't expect there to be any way to avoid getting authorization to work with a user's documents. However, you should be able to use the Java OAuth library to retain the auth token so you can avoid needing to ask for it more than once. (Unless the user revokes access, of course.)

As DalmTo says, you can use service account credentials if you're trying to access resources owned by the project (or which the project can be granted access to). Note that if you're running on AppEngine, Google Kubernetes Engine or Google Compute Engine, the service account credentials for that environment should be available automatically.