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POChange a method at runtime via a hot swap mechanism
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copied!<p>Assume we have a trivial Java program that consists of just one class:</p> <pre><code>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); } } </code></pre> <p>After it's compiled and run, output will be this:</p> <blockquote> <p>Today is a nice day with a number 0</p> <p>Today is a nice day with a number 1</p> <p>Today is a nice day with a number 2</p> <p>Today is a nice day with a number 3</p> <p>...</p> </blockquote> <p>My question: does there exist (or is there on the horizon) some way to swap <code>replacable</code> method at runtime? Something like writing another version of <code>HelloWorld</code> with a new version of <code>replacable</code>, compiling it and then the old version in an already running JVM?</p> <p>So, if I write the new version like this:</p> <pre><code>private static void replacable(int i) { System.out.println("Today is an even nicer day with a number " + i); } </code></pre> <p>is there something similar to <a href="http://spawnlink.com/articles/rules-of-hot-code-swapping/">Erlang's hot code swapping</a> where I can do this:</p> <ol> <li>run original program</li> <li>write modified version</li> <li>using a command line program, connect to running JVM and replace existing method</li> </ol> <p>so that, during runtime, this will happen:</p> <blockquote> <p>Today is a nice day with a number 15000</p> <p>Today is a nice day with a number 15001</p> <p>Today is an even nicer day with a number 15002</p> <p>Today is an even nicer day with a number 15003</p> <p>...</p> </blockquote> <p>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.</p> <p>Note: I know that technologies like bytecode manipulation (<a href="http://cglib.sourceforge.net/">cglib</a>), <a href="http://www.eclipse.org/aspectj/">aspectJ</a>, <a href="http://www.zeroturnaround.com/jrebel/">jRebel</a>, <a href="http://java.sun.com/javase/technologies/core/mntr-mgmt/javamanagement/">JMX</a>, hotswapping of methods in Java EE etc. exist, but they aren't what I'm thinking of. Think of Erlang.</p>

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