Log the work of the software without having access to its source code?

Question

Java

Brief description of the task: there is some software (in java, without source code) from some foreign developer who either does not want to make changes to the source code, or will do it for a very long time. There are security requirements for the end system based on this software: user access to certain objects must be logged. The current version of the program cannot do this.
What is the easiest way to implement this functionality?
At the moment, the following solution is being considered: decompile followed by code injection through the use of JBoss Javassist + java agent.
It's possible that there is an even shorter way/easier implementation/existing solutions.
Ask those in the know to advise.

Answer 1

aspects, load time weaving

Answer 2

as already mentioned above -
look at aspectj aspects, run an application with an aspect that will write all called methods to the log file, then decide which methods you are interested in, make aspects that write their arguments, etc.

Answer 3

Check out jboss byteman

Answer 4

Are the objects we are talking about entities in the program or does something else also know about them (well, records in the database, files)? If the latter, then you can set up an audit using the OS and database server tools.