📚 Learning and exploring the metaprogramming features of Java.

Metaprogramming is "programming the program". One of Java's key features is its dynamic-ness. There are things like the Reflection API, Java agents, invokedynamic, Proxy, and annotation processing (often used for codegen). Together, these tools enable metaprogramming for Java and other JVM languages. I would like to learn more about these tools so that I can get even more leverage out of Java. In fact, the richness of these tools is a unique advantage of Java/JVM. Most languages have fewer tools.

NOTE: This project was developed on macOS. It is for my own personal use.

Follow these instructions to compile and run the program.

1. Pre-requisite: Java 21
2. Compile the program:

   • ./build.sh

3. Run the program:

   • ./run.sh

Alternatively, configure your favorite IDE (like Intellij) to look at the src/ for the source code and use the IDE to compile and run the program.

General clean-ups, TODOs and things I wish to implement for this project:

• Do something with classpath scanning. Can I find all classes that extend Record, for example?
• Do something with MethodHandles. No idea how this works.
• Do something with dynamic Proxies
• Do something with JSR-223 or JShell to execute dynamic code at runtime and somehow side effect the program state. This is possible, right?
• HOLD (I started this a long time ago, but I completely forgot about this codebase) Do something with Java agents (premain or dynamic)
  • Can I replace a method definition using the agent?
  • Reconcile this project with my other project java-agents-playground. Somehow when I did java-agents-playground I completely had forgotten about this project. I don't need the overlap on agents, but I still want to explore the other topics like reflection, invokedynamic, and Proxy. Those should best exist as standalone projects. This is a reminder.

• The Java Tutorials: "The Reflection API"
• The Java Tutorials: "Using JAR Files: The Basics"
• Package java.lang.instrument
  • This describes Java agents.
• Java Magazine: "Understanding Java method invocation with invokedynamic"
• JavaDoc for Proxy
• JavaDoc for "java.lang.instrument"
  • This is the only official Java document I can find on agents. I'm looking for more information and for examples. The OpenTelemetry project has a Java agent and the code is available at open-telemetry/opentelemetry-java-instrumentation on GitHub. It is ostensibly a good example of a sophisticated Java agent but there is little in the way of documentation that explains the implementation. Also we can count on Baeldung to have a Guide To Instrumentation which shows a simple agent example!