Is Java really "Write once, run anywhere" ?

As many of us generally know, Java is popular for it's cross-platform portability "Write once, run anywhere", but i wanted to give it a test and see if it's truly one. I tried to run WebSphere 7 Application Server itself using Sun JRE 1.6.0 instead of IBM J9 VM which is bundled with AppServer and see if it works with cross vendor JVM on the same platform. I had to make couple of changes, the startServer.sh script adds IBM JVM specific arugments ( -Xshareclasses:name=webspherev70_%g,groupAccess,nonFatal -Xscmx50M )which i had to remove , changed the WAS_HOME/java to point to Sun's JAVA_HOME and set the environment variables as set in setupCmdline.sh and startServer.sh and ran the process from the cmdline with the huge list of arguments with the one i got from the process string when it's started from the startServer.sh script.

The AppServer failed to start with the following exception,

java.lang.NoClassDefFoundError: com/ibm/wsspi/buffermgmt/WsByteBufferPoolManager
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:621)

and

[20:48:03:436 PDT] 0000000a ServerCollabo E WSVR0121E: An exception occurred getting a socket for port 38930 on hostname waslinux with an IP address of 192.168.1.10.

I added the the AppServer/plugins/* directory to the classpath and even to -Djava.ext.dirs, but this time the JVM just exits and getting terminated without writing anything into the logs. I tried different things to find the actual root of the failure by enabling verbose:jni traces, strace and using jdb, but couldn't find anything useful and ultimately gave up. Note the same method of starting websphere using IBM J9 VM from the cmdline seems to work. Both Sun JDK and Open JDK just terminates for no reason.

I also ran apache tomcat server on both IBM's and Sun JVM and it seems to run fine without problems.

It seems like WebSphere AppServer java code is not compatible to run in other vendor JVM's, hence the promise of Java hasn't come true at least in this case where it's just "write once , run anywhere as long as you stick to the same JVM vendor that you used to develop and test " :) .


*If anyone had tried and been successful please comment on my post, I would really like to run and see as i can use some of the tools like jvisualvm, jmap, jps, jstack , etc which gets bundled with Sun JDK and not with IBM.