The doc for com.sun.tools.attach.VirtualMachine (http://java.sun.com/javase/6/docs/jdk/api/attach/spec/index.html) says that methods VirtualMachine.loadAgent(String agent), VirtualMachine.loadAgentLibrary(String agent), VirtualMachine.loadAgentPath(String agent) throw NullPointerException if agent is null, but actually if agent is null these methods throw AgentLoadException.