Verse Of The Day

Tuesday, April 29, 2008

Some Notes On The Jboss BeanShell Deployer

I've been playing a little with the BeanShell Deployer in Jboss lately. Technically, it's pretty cool. Drop a .bsh script in the deploy directory and the hot deployer picks it up just as if you dropped a .war/.ear/.sar file in there. If it's a simple script, the bsh deployer runs it immediately. Or you can implement any or all of the methods of the ScriptService interface (below) and your script will be deployed as a service mbean.
public interface ScriptService
extends org.jboss.system.Service
public String objectName ();
public String[] dependsOn ();
public Class[] getInterfaces ();

public void setCtx (ServiceMBeanSupport wrapper);

It was extremely easy to write a few scripts and drop them in the deploy directory. Of course, you can start with the requisite "Hello World" script, which upon deployment, prints out to the console. A few more minutes of scripting (adding the above methods) yields a service mbean that can be managed via JMX.

But then the obvious questions: Why? Why would I write mbeans using bsh instead of Java? What do I gain? What do I lose? Is it really any quicker?

I don't really have good answers for the why question yet. I did write a few services using bsh. They were easy enough to implement, but nothing that couldn't have been done in a similar time frame using Java. You lose compile time error checking, but you avoid some of the plumbing regarding build scripts, deployment descriptors, and building .ear or .sar files.

I did write a few services. One connected to a Websphere MQ (a.k.a. MQ Series) queue manager to get queue depths and print them to the Jboss console. I ripped most of the code right from an existing MQ tool I wrote a few years ago. So not the most useful thing in the world, but it proves out importing existing Java classes and the Java syntax. Another used JDBC to connect to an audit database and purge old transactions. Again, nothing that couldn't be done in regular old Java code in the same amount of time.

Overall, I didn't feel like I was saving any time. For one, the syntax is Java. That's great for code re-use (either cutting/pasting or importing libraries), but Java syntax doesn't have that "lightweight scripting" feel to it. Also, the lack of compile-time error checking doesn't save you much time if you have to run the script to find errors. Make a simple mistake like a type mismatch or failing to catch a specific exception, and your IDE and/or compiler will tell you about it. Do the same thing in a bsh script, and you find out after you deploy it and run it.

I think this could prove useful in the future, as one more tool in my programming toolbox, but have to admit I don't have a lot of use for it right now. Maybe I'm just not seeing the big picture, or just haven't run across the scenario where this is the ideal way to solve a problem. If you are using the bsh deployer, please let me know what you are doing with, or what types of problems you are solving with it.

Once I have a reason to use it, at least I know it is easy to do. And once the why question starts to have answers, maybe I'll explore writing a JRuby or Groovy deployer. Or actually, what would be really useful would be a SQL script deployer, so you could run ad-hoc queries against your Jboss DataSources without writing JDBC code or having to launch a DB tool like Squirrel or TOAD. Since Jboss deployers are service mbeans, maybe I could write the SQL deployer in bsh...

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