Ruby Runtime
Asciidoctor itself is implemented in Ruby and AsciidoctorJ is a wrapper that encapsulates Asciidoctor in a JRuby runtime. Even though AsciidoctorJ tries to hide as much as possible there are some points that you have to know and consider when using AsciidoctorJ.
Every Asciidoctor instance uses and initializes its own Ruby runtime. As booting a Ruby runtime takes a considerable amount of time it is wise to either use a single instance or pool multiple instances in case your program wants to render multiple documents instead of creating one Asciidoctor instance per conversion. Asciidoctor itself is thread-safe, so from this point of view there is no issue in starting only one instance.
The JRuby runtime can be configured in numerous ways to change the behavior as well as the performance. As the performance requirements vary between a program that only render a single document and quit and server application that run for a long time you should consider modifying these options for your own use case. AsciidoctorJ itself does not make any configurations so that you can modify like you think. A full overview of the options is available at github.com/jruby/jruby/wiki/ConfiguringJRuby.
To change the configuration of the JRuby instance you have to set the corresponding options as system properties before creating the Asciidoctor instance.
So to create an Asciidoctor instance for single use that does not try to JIT compile the Ruby code the option compile.mode should be set to OFF.
That means that you have to set the system property jruby.compile.mode to OFF:
System.setProperty("jruby.compile.mode", "OFF");
Asciidoctor asciidoctor = Asciidoctor.Factory.create();The default for this value is JIT which is already a reasonable value for multiple uses of the Asciidoctor instance.
In case you want to have direct access to the Ruby runtime instance that is used by a certain Asciidoctor instance you can use the class JRubyRuntimeContext to obtain the org.jruby.Ruby instance:
Asciidoctor asciidoctor = Asciidoctor.Factory.create();
Ruby ruby = JRubyRuntimeContext.get(asciidoctor);