The extension mechanism - overview and caveats

The extension mechanism is a great addition to Java for Java2. It allows you to install jar files in a way which means the end to classpath fiddling in almost all cases. The idea is very simple - put all the extra jar files you need to use into one directory, and let Java pick them up automatically. You can set this directory (or many of them) yourself at the command line if you wish, by setting the system property of java.ext.dirs. For example:
java -Djava.ext.dirs=c:\mylibs mypackage.MyClass
will make Java load all the jar files within c:\mylibs. Note that to achieve this effect for compilation, you need
javac -extdirs c:\mylibs (rest of normal javac command line)

By default, Java will use the jre/lib/ext directory of the currently running JRE. (Note that some VM installers don't create this directory, so you may need to add it by hand.) The one problem with this is that on Windows, you may not be using the JRE you think you are... When the JDK is installed, 2 JREs are installed - by default, they are in c:\jdk1.3 and c:\Program Files\JavaSoft\JRE\1.3 (that's for 1.3, of course). A link to the latter JRE is also placed in the c:\WINNT\system32 directory (or the equivalent, depending on which version of Windows you're using). This means that unless you're careful, you'll be using a different JRE for compiling code than for running it. This means that you'll either have to put your jar files in two places (which is a maintenance nightmare) or make sure you use the JDK version for both compiling and running. I do this by putting c:\jdk1.3\bin right at the start of my path environment variable, before any system directories. Alternatively you could rename the java.exe in the system32 directory, for example. Note that the place to put jar files for the JDK (for both compiling and running) is c:\jdk1.3\jre\lib\ext, not c:\jdk1.3\lib\ext.

The other slight wrinkle with the extensions mechanism is obvious but can still catch everyone out: if you have multiple versions of the same classes in your extensions directory, you're asking for trouble. For instance, having both jsdk.jar from the servlet 2.0 development kit and servlet.jar from the servlet 2.2 development kit in the extensions directory is just asking for trouble. Similarly some XML parsers/factories don't like to mix and match - it's best to make sure you know which you need and only have that available.

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