Question

Assume we have a trivial Java program that consists of just one class:

public class HelloWorld {

   private static void replacable(int i) {
      System.out.println("Today is a nice day with a number " + i);
   }        

   public static void main(String[] args) throws Exception {
      for(int i = 0; i < 100000; ++i) {
      replacable(i);
      Thread.sleep(500);
   }
}

After it’s compiled and run, output will be this:

Today is a nice day with a number 0

Today is a nice day with a number 1

Today is a nice day with a number 2

Today is a nice day with a number 3

…

My question: does there exist (or is there on the horizon) some way to swap replacable method at runtime? Something like writing another version of HelloWorld with a new version of replacable, compiling it and then the old version in an already running JVM?

So, if I write the new version like this:

private static void replacable(int i) {
   System.out.println("Today is an even nicer day with a number " + i);
}  

is there something similar to Erlang’s hot code swapping where I can do this:

1. run original program
2. write modified version
3. using a command line program, connect to running JVM and replace existing method

so that, during runtime, this will happen:

Today is a nice day with a number 15000

Today is a nice day with a number 15001

Today is an even nicer day with a number 15002

Today is an even nicer day with a number 15003

…

Assume that above program is standalone, runs in a standard Java SE environment, there is nothing else on classpath, so it’s almost a Hello world style program.

Note: I know that technologies like bytecode manipulation (cglib), aspectJ, jRebel, JMX, hotswapping of methods in Java EE etc. exist, but they aren’t what I’m thinking of. Think of Erlang.

Answer 1

You can either use the open-source HotSpot VM or the commercial JRebel IDE plugin to easily achieve your goal (view comparison table here).