Question

What exactly is Java bytecode injection and why would one use it?

Answer 1

Java code compiles into bytecode (Foo.java ->> Foo.class). Bytecode injection is modifying Foo.classat runtime to inject code into it right before its loaded and run.

Imagine a scenario where I want to find out how many times method

public void bar();

is invoked in Foo.class. I could write an agent using java.lang.instrument that intercepts Foo.class during class load, modifies it using ASM so that bar() calls com.amir.agent.incrementCount() on method entry.

Now I can run my program:

$java -javagent:MyAgent Foo  //assuming Foo has a main() defined

and at runtime, Foo.class will not only do its normal work, it will also do work I defined in MyAgent each time bar() is invoked.

For a pointer on writing an agent from scratch, start here.