Why we have to use -jar option when running a self-contained jar

We can run a self-contained jar file with a command like this:

java -jar foo.jar

,where foo.jar contains a main class that is specified in META-INF/MANIFEST.MF with a Main-Class entry. What will happen if running if without -jar option? java will read foo.jar as a main class named jar in package foo:

C:\tmp>java foo.jar
Exception in thread "main" java.lang.NoClassDefFoundError: foo/jar

I guess that's the reason why -jar option is necessary to tell JVM to look for a jar file rather than a class file foo/jar.class. But realistically, how often people name their main class jar? Why can't we disallow this naming and then omit -jar option when running a jar file? JVM should be able to figure it out by .jar extension.

Tags: java, jar, java command