It seems that the easiest would be to use AWS SDK with a tJava. The first operation would be to list the key (AWSKMSClient.listKeyResult) then use directly the SDK again with the decrypt function.
Java doc is here.
For the initial connection, both AWS KMS Client and any of the txxxxConnection implement AWSClientBuilder so with a bit of manipulation you can probably reuse the connection. Or you can get one of the component and change the code directly / create a custom component.