01-Jul-2016 10:49:05.875 WARNING [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesJdbc The web application [ROOT] registered the JDBC driver [com.mysql.jdbc.Driver] but failed to unregister it when the web application was stopped. To prevent a memory leak, the JDBC Driver has been forcibly unregistered
出现条件：web应用，使用spring，连接池用的bonecp。当修改一点代码，save后，spring会重新加载，然后问题就出现了。
在网上查看资料后，找到了该报告：https://issues.apache.org/jira/browse/DBCP-332
原文引用：
BasicDataSource's method close() doesn't deregister JDBC driver. This causes permgen memory leaks in web server environments, during context reloads. For example, using Tomcat 6.0.26 with Spring, and BasicDataSource declared in Spring context, there is a message printed at web application reload:
SEVERE: A web application registered the JBDC driver [com.mysql.jdbc.Driver] but failed to unregister it when the web application was stopped. To prevent a memory leak, the JDBC Driver has been forcibly unregistered.
翻译：
BasicDataSource中的close方法没有注销（deregister）jdbc驱动。在web
server环境中，当reload content时，就引起了堆内存泄露（permgen memory leaks）。例如，当在使用spring和tomcat 6.0.26时，BasicDataSource在Spring
context被声明了，这样就会有一条信息在web application reload的时候： SEVERE: A web
application registered the JBDC driver [com.mysql.jdbc.Driver] but
failed to unregister it when the web application was stopped. To
prevent a memory leak, the JDBC Driver has been forcibly unregistered.
该地址中 https://issues.apache.org/jira/browse/DBCP-332 提供了一种解决方案。
我并没有使用该方案，因为使用的是bonecp，所以我看了下bonecp的BoneCPDataSource的close方法，发现，其中有个
BoneCPConfig类，其中有这个属性：deregisterDriverOnClose。 于是我在spring的xml配置中，加入了该配置。