The installer should always install and use the bundled JRE, even if the server already has an acceptable version of Java installed. This ensures that we run with known-good versions of Java.
Customers that want to use their own JRE can use the zip distribution.
Bitbucket Server only provides a bundled JRE if Java is not installed or available on the server.
If Java is already available, the default system Java is used and configured to be used in the service ("Java Virtual Machine:" field in the "Java" tab).