The Oracle JDeveloper has support for debugging java stored procedures. You will need to compile the java classes using the -g option so that the debugging information is generated in teh class files, i.e.
javac $JAVA_OPTS -g file_names
After that, you will need to write a dummy pl/sql package-procedure that invokes this java stored procedure. Then right click on the pl/sql procedure name and select 'Debug'.
After that you can step through the pl/sql and java code just as if you were debugging a regular java class. Since you compiled the java class using the -g option, you can watch variables, evaluate expressions etc. If you didn't use the -g option, you will still be able to step through the code in the debugger, but you will not be able to watch the variables/expressions.