Question

When exactly Java looks for dependencies of a Jar file at Run Time?

At the very beginning of running it?
When it tries to Initialize a Class which has some dependency?
Or any other time?

Answer 1

The Java Language Specification, §12.3 says (my emphasis):

This specification allows an implementation flexibility as to when linking activities (and, because of recursion, loading) take place, provided that the semantics of the language are respected, that a class or interface is completely verified and prepared before it is initialized, and that errors detected during linkage are thrown at a point in the program where some action is taken by the program that might require linkage to the class or interface involved in the error.

For example, an implementation may choose to resolve each symbolic reference in a class or interface individually, only when it is used (lazy or late resolution), or to resolve them all at once while the class is being verified (static resolution). This means that the resolution process may continue, in some implementations, after a class or interface has been initialized.

Now, the specification says that a JVM can do a range of things, but clearly, any given JVM does do one particular thing. Marko’s answer says that "all class files in the classpath may be loaded even before the main method starts executing", and he’s right, but the fact is that no JVM actually does that.

I believe that what actually happens in the Sun JVM is that things are loaded as late as possible. Whenever a class is initialized, then any classes it refers to need to be loaded and verified, but they don’t need to be initialized until they themselves are actually used. I appreciate that this is not a very detailed or authoritative answer.