Java.lang.instrument related question.

Question

Hi, all
Small question regardign java.lang.instrument.
java.lang.instrument package docs describes how the customer transformer ClassFileTransformer may be introduces using *-javaagent:{jar.file | class.name}* JVM command line arg. Looks rather obvious.
My question is as follows. This is about the JVM startup definitions. IDEs (for instace, IDEA I use) are able to reload recompiled classes during remote dubugging (the debuggee process is started with standard remote debug flags). How do they do it? - I mean how do they introduce their ClassFileTransformer to use it's redefine() during jmv's runtime?
Thanx in advance for any inputs
Sincerely,
Jabb

Answer 1

Security: you might not want all classes loaded off e. g. the internet to have full access to your computer.
Runtime code generation: you can make a class loader that rather generates java byte code than reads it from a source; think about optimizing arithmetic expression interpreters that you give an expression as a string like "x^8 + log(x)*sqrt(8+y)".. instead of generating the expression tree and traversing it every time values for x and y change, you could transparently generate a class that has a method that takes the variables as parameters and computes the value of the expression much much faster...