What is Java bytecode injection?
What exactly is Java开发者_JS百科 bytecode injection and why would one use it?
Java code compiles into bytecode (Foo.java ->> Foo.class
). Bytecode injection is modifying Foo.class
at 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.
First google search gives an interesting read Java Programming: Bytecode Injection
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