Change class on runtime using javassist + spring + tomcat

Question

I want to add 1 annotation to a class with javassist library. The class is loaded once before and I know we can't reload such a class in regular cases. javassist HotSwapper can do that but i don't know which port is listening. I get a connection refused exception when calling this code:

HotSwapper hs = new HotSwapper(8000);  // 8000 is a port number.

What is the port? Java debug port? Can I use it when the code is in running mode? I don't know how i can reload the class in runtime. Is it possible when we are using spring and tomcat? How can I run my code before loading any of other jar files in class path?

Thank You

Answer 1

What is the port? Java debug port?

Yes, you need something like this in your VM parameters:

-agentlib:jdwp=transport=dt_socket,address=localhost:8000,server=y,suspend=y

Can I use it when the code is in running mode?

Yes. Agent does not impact performance signicantly (as I know, I'm not sure), but other things (which connected to jdwp port) of course may impact.

I don't know how i can reload the class in runtime. Is it possible when we are using spring and tomcat?

You can try to call reload method of HotSwapper . Check this example from javadoc ( link to javadoc ):

 CtClass clazz = ...
 byte[] classFile = clazz.toBytecode();
 HotSwapper hs = new HostSwapper(8000);  // 8000 is a port number.
 hs.reload("Test", classFile);