Classpath is ignored when using java -jar

I have an application that I packaged in a JAR, and I needed to add a directory to the classpath so my logging library Logback could find the configuration file I created (using a wizard). So I figured I could just add it using -cp:

java -cp mydir -jar myjar.jar

But no, the classpath doesn't get set when you use the -jar option (you can check this by calling System.getProperty("java.class.path")). It turns out you have to add it to the manifest file when creating the JAR:

Class-Path: classpath/

Also notice the slash at the end. It's not optional if you want to specify a directory. Don't ask me why, but it will only work if you add the slash.