Java won't look for classes in a jar unless it's told to. Class files are
searched on the classpath, and if this includes the current directory, then
class files in the current directory should be found. However, jar's in the
current directory are not searched. You need to explicitly add a jar to the
classpath.  Jars in ext are found because it's always searched.

For example, if I create a simple class and put it in a jar, let's call the
class ClassA and the jar ajar.jar. To create the jar I do:

$ javac ClassA.java
$ jar cf ajar.jar ClassA.class

To be sure that the ClassA is used from the jar I remove both ClassA.java and
ClassA.class.

Then I create a main class which uses ClassA, lets call it MainClass and put
this in MainClass.java. I compile MainClass.java as:

$ javac -cp ajar.jar MainClass.java

and run it as:

$ java -cp ajar.jar:. MainClass

Note that I've had to specify both the jar and "." in the classpath so that java
can locate classes in the jar and in the current directory.

What you want to do is possible.