There are situations where you want to change Java code not at the source code level but at runtime. For example when you want to instrument code for logging purposes but do not want to change the source code (because you might not have access to it). This can be done by using a Java Agent. For example, Dynatrace uses a Java agent to collect data from inside the JVM. Another example is the GraalVM tracing agent (here) which helps you create configuration for the generation of native images. Logging is one use-case but you can also more dramatically alter runtime code to obtain a completely different runtime behavior.
This blog post is not a step by step introduction for creating Java Agents. For that please take a look at the following. In this blog post I have created a Java agent which rewrites synchronized methods to use a ReentrantLock instead (see here). The use-case for this is to allow applications to use Project Loom's Virtual Threads more efficiently.
You can find the code of the agent here.
