Java agents are based on facility, coming from the Java platform and the entry point to that is a java.lang instrument package, which provides services that allow agents to instrument programs running on the JVM. One thing to note is that there are non-Java languages that compile into Java bytecode (such as Scala, Clojure, and Kotlin), which means that the structure and shape of the bytecode for programs can be very different. Hence, while we don’t instrument the Java program itself, we use a very close representation of it. The interesting fact is, for Java programs, the structure of the bytecode is really close to the original Java program source code.

You may think how come the java byte code will be restructured so that agent can add the relevant code at the correct places. When you run the compiled HelloWorld application (HelloWorld.class), the agent could be viewed as a way to intercept classloaders behavior at runtime. Java agents are extremely powerful and also dangerous.īefore diving in, I will explain how a Java Agent intercepts a Class using the simple HelloWorld Example.Īs shown in the below diagram, Classloaders are in charge of loading classes from binary to in-memory. Java agents are a special type of class which, by using the Java Instrumentation API, can intercept applications running on the JVM, modifying their bytecode. Another interesting fact about Java agent is Application profilers are using the same technique at the backend to collect information of memory usage, memory leakage, and execution time of methods. Have you ever wondered how our Java code is hot-swapped in our IDE? It’s because of agents.

Java gives us a facility to do this using Java Agent. What you could do is dynamically attach some set of code to your application and carefully rewrite it so that the code dumps additional information you can log, or else you can dump the application stage into a text file. Every once in a while, it gets into a broken state, the error is hard to reproduce, and you need some more information out of the application.

Let’s say that you have an application running in production.

Jav spy agent