In case of debugging externally started application, the debuger very often selects wrong platform to debug against, so
stepping through JDK is usually completely unusable. If the debugger can't detect correct platform (which it should,
IMHO, it can perform virtually any code in the debugged VM and evaluating System.getProperty("java.class.path") should
tell it offhand it is offering wrong sources), the user needs to be able to override the debugger's decision.