1:GC roots
http://www.yourkit.com/docs/11/help/gc_roots.jsp
2:Garbage collection roots
http://publib.boulder.ibm.com/infocenter/hctool/v1r0/index.jsp?topic=%2Fcom.ibm.java.diagnostics.memory.analyzer.doc%2Fgcroots.html
特别注意点包括java local