Will JVM only pick up .jar files?

Question

When I give the -classpath argument to java to start will it pick up only files which have .jar extensions or will it attempt to look into other files in -classpath path as well?

Meaning if I specify -classpath to be /mypath which contains

/mypath/IAmANormalJar.jar
/mypath/IAmAJarWithoutExtension

where IAmAJarWithoutExtension is an actual jar file but without the .jar extension. Will only /mypath/IAmANormalJar.jar be loaded by JVM or will /mypath/IAmAJarWithoutExtension be loaded as well?

Answer 1

http://docs.oracle.com/javase/7/docs/technotes/tools/solaris/classpath.html

Classpath entries that are neither directories nor archives (.zip or .jar files) nor * are ignored.

The wildcard symbol explained:

Class path entries can contain the basename wildcard character , which is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR. For example, the class path entry foo/ specifies all JAR files in the directory named foo. A classpath entry consisting simply of * expands to a list of all the jar files in the current directory. Files will be considered regardless of whether or not they are hidden (that is, have names beginning with '.').

Answer 2

Java classpath requires jar to be completed specified with extension. Without the extension Java treats that as a directory to which there are class files present to be added to the Java classpath. The Wikipedia article on the Java classpath provides some good information in this regard:

https://en.wikipedia.org/wiki/Classpath_(Java)

Answer 3

It will only pick up files with the extension .class, .jar or .zip. One trick I used to use when I wanted to flip between two jars for testing would be to add another extension to the file I wanted to be "out". If the version I wanted to test was mylib.jar, I would modify the old one as mylib.jar.old and they could co-exist in the directory.