UPDATE: How to Hotswap classes in the running jvm

If you have read my previous blog post about How to Hotswap classes in the running jvm, I am sure you will find Java Rebels even more interesting. It supports full class reloading support at runtime with method refactoring and all the stuffs that hotswap.jar could not do. Trust me, Java development has never been more fun. And if you are into spring development, it even supports spring configuration reloading without restarting the whole spring application context! Could it be better!!! I think its a must have feature for every java developer. I don’t know why sun is not incorporating this in the JVM? Just because java is not a scripting based language like php/ruby , doesn’t mean we can’t have fun like those guys! Speaking of fun, you can have some from this Java Rebel cartoon also

Only downside is JavaRebel is not free. There is a small fee that you have to pay. But let me tell you its worth it. And if you don’t trust me, you can always use the 30 Day trial version (although it will only take 3 days for you to become addicted to it).

NOTE: I don’t work for java rebel.I wish they took me in 🙂 it would be fun to develop such cool features that eases the life of so many developers.

How to Hotswap classes in the running jvm?

One of the major pain of every server side java developer is that you have to redeploy your application every time you made a small change to the java classes. Now if you are working with a web application which consists 1500 classes, about 150 servlets(basically webservices), imagine how much time tomcat is gonna take to deploy or even worse redeploy that application.(it takes about 6 mins on my p4 pc). This was a small trick I learned from Masum Bhai at Therap.
JVM comes with a debugging api which actually lets you to change the defination of a class at runtime. These are the following steps you have to do.

1> Download hotswap.jar and put it in the “run-classpath” of your ant script.
2> JVM must start with debugging parameter. So add the following parameters at your java start command:

      -Xint -Xdebug -Xrunjdwp:transport=dt_socket,address=${hotswap.port},server=y,suspend=n

For example, if you are using tomcat, you can add this line at the beginning of the catalina.sh

export JAVA_OPTS="-Xint -Xms256m -Xmx256m -Xdebug -Xrunjdwp:transport=dt_socket,address=8787,server=y,suspend=n"

alternatively , you can uset the “tomcat-start” ant task given below to start your tomcat. Just make sure you have the tomcat-home ant property defined.

 <!--enable timestamping upto second level (required for hotswapping)-->


 <!-- Use this when you want to see which files are being compiled -->







The rest is simple! When you change anything in a class/group of classes, just use “ant reload” and you will see the hotswap output like this:

ant reload
Buildfile: build.xml


   [echo] Compiling the application source code....
  [javac] Compiling 1 source file to /home/sajid/projects/cmsstandaloneportal/,tmp/cmsstandaloneportal/WEB-INF/classes

[hotswap] hotswapping 1 files from /home/sajid/projects/cmsstandaloneportal/,tmp/cmsstandaloneportal/WEB-INF/classes
[hotswap] hotswapping com.x.y.z.p.web.struts.QDispatchAction

Total time: 10 seconds</pre>
10 seconds! Thats all you need!

I have used this trick with Tomcat, JBoss, and standalone java applications and every time it worked like a charm! I can’t tell how much deployment time it saved me. So No doubt its my most favorite ant task to date!

Note Of Caution:
Hotswapping doesn’t work when you change the structure of a class (add new method/delete method/rename method). It only works when you change the logic inside a method. Thats what we do most of the time anyway, isn’t it?