Java classpath and directories

The java classpath and directories has always been quite a sad story. Of course you were always able to pass a directory of classes to the jvm like this:

java -classpath classes Main

But what you usually deal with these days are jars. Often quite a bunch of them. So fair enough – let’s add a directory with jars as well:

java -classpath classes:lib Main

Up until java 5 all you got was a ClassNotFoundException because java did not search for jars but classes in there. It just ignored the jars. So what pretty much everyone ended up doing is providing yet another shell script to build up the classpath and pass it to the jvm via command line. Of course for bigger projects this let to…

java -classpath lib/commons-logging-1.1.1.jar:lib/commons-jci-core-1.0.jar:commons-io-1.2.jar:lib/junit-3.8.2.jar:lib/maven-project-2.0.8.jar:lib/rhino-js-1.6.5.jar: ...

I guess you see where I am going …an unmanageable mess of a command line. But thankfully times have changed. Starting with java 6 (mustang) you can now use wildcards in the classpath:

java -classpath classes:lib/'*' Main

Naturally this means that the order of the jars is implicit. Which in turn means you need to be extra careful when there are classpath clashes. But in general I think this is a nice feature that should have been there from day one …and I just found out about it.

http://vafer.org/blog/20081203024812/