http://wenku.baidu.com/link?url=tQPQ-dgm7NOkEGj_vemwtsPd6TJ6W3x6_0UBLgw61N982SwPlz-QFxqncsmPGqHwJAEF_HHYpdYCuU4la0lWatkL1GfMBZV50RBPQzMZ0ve
http://www.cnblogs.com/preftest/archive/2011/09/30/2196444.html
http://blog.csdn.net/dreajay/article/details/8628929