同步知识复习

看了Oracle论坛上一篇文章

https://forums.oracle.com/forums/thread.jspa?threadID=1540893

搜到这些

http://whitesock.iteye.com/blog/162342

http://java.sun.com/developer/technicalArticles/javase/finalization/

http://hi.baidu.com/tengfeitengfei/blog/item/927258d89060893b33fa1c80.html