Day: April 30, 2009

This tripped up a colleague today. When he mentioned it, about three other people (myself included) piped up with oh yeah, that caught me out a while back.

  java -cp 'a.jar:b.jar' -jar foo.jar

This completely ignores the classpath. It’s not blindingly obvious that this is going on. If you manage to wind your way down to java(1), you do see:

-jar

Execute a program encapsulated in a JAR file. The first argument is the name of a JAR file instead of a startup class name. In order for this option to work, the manifest of the JAR file must contain a line of the form Main-Class: classname. Here, classname identifies the class having the public static void main(String[] args) method that serves as your application’s starting point. See the Jar tool reference page and the Jar trail of the Java Tutorial for information about working with Jar files and Jar-file manifests.

When you use this option, the JAR file is the source of all user classes, and other user class path settings are ignored.

Note that JAR files that can be run with the “java -jar” option can have their execute permissions set so they can be run without using “java -jar”. Refer to Java Archive (JAR) Files.

(underlining mine)

This is fairly clear, but unless you’re used to looking for man pages, this might well not be found. In particular, some indication on the java -help output would be a boon.