How to change data in-flight, without a Debugger

Do you ever wish you could change the behavior of some object in the middle of a bunch of method calls?  I sometimes do.

Maybe you want to set some properties on an object to replicate a fault.  Maybe you’d like to force a specific return value from a method for some reason.  These are both very possible, without altering the source code for your application.

I like to do this through Byteman.  Byteman is a JBoss project that deals with bytecode injection.  This means that you can inject code you want to run, specifying where you want to run it.  It’s sort of like AOP, except I think it allows more freedom for the user.  It’s also pretty easy to use.

Here’s how:

1)  Make a helper class to do the work you want done
2)  Put the helper class in a .jar
3)  Write the Byteman rule that tells the helper class to do it’s thing

That’s it!  So here’s the example.

Part 1, the helper class

public void addMarker(Message msg){
System.out.println(“Changing Message contents.”);
Body msgBody = msg.getBody();
msgBody.add(“Hey, I’ve got changed content”);
System.out.println(“Done changing Message contents”);

As you can see, there is nothing fancy at all about that class.  It’s just a plain old Java class.  It has a method that expects to be passed an object, and it has code that will alter properties on that object.  Absolutely nothing out of the ordinary.

Part 2, put that class in a .jar
You can use your favorite IDE for this if you want.  I like to just use Java’s ‘jar’ utility.  To use it, you just place your command line interface in the ‘root’ directory for the compiled class (in this case, one level above ‘com’ of com/byteman/helper) and do something like ‘jar cvf HelperJar.jar *

Part 3, write your rule
Put your Helper.jar in $BYTEMAN/bin.  I like to put my scripts here, too.  So add something like this in a file you might call ‘MyRule1.btm’:
RULE Alter Message
CLASS org.jboss.soa.esb.actions.SystemPrintln
METHOD process
HELPER com.byteman.helper.MsgHelper
BIND msg = $1;
DO addMarker(msg);

That rule is pretty simple– it’s just telling Byteman to run the method ‘addMarker’, passing it the first argument (‘msg’) that was passed to SystemPrintln’s ‘process’ method.  There is some good documentation on the Byteman site to help you learn more things you can do with the scripting.

Finally, you attach Byteman to your running SOA-P server and install the rule.  I like to do that by finding SOA-P’s Process Id (pid) through ‘ps -ef |grep jboss’.
Once I have the pid, from $BYTEMAN/bin I run ‘ $PID’ and then submit the rule with ‘ $MYRULEFILE’, in this case MyRule1.btm.

That’s all there is to it!  You are then using Byteman, which lets you set hooks and alter data on the fly, without altering anybody else’s sourcecode.  You put whatever you want done in your helper method.

Have fun!