xxx 警告 [http-nio-8080-exec-56] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesJdbc The web application [/ssm] registered the JDBC driver [com.alibaba.druid.proxy.DruidDriver] but failed to unregister it when the web application was stopped. To prevent a memory leak, the JDBC Driver has been forcibly unregistered.
xxx 警告 [http-nio-8080-exec-56] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesJdbc The web application [/ssm] registered the JDBC driver [com.mysql.cj.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.
xxx 警告 [http-nio-8080-exec-56] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesThreads The web application [/ssm] appears to have started a thread named [mysql-cj-abandoned-connection-cleanup] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
java.lang.Object.wait(Native Method)
java.lang.ref.ReferenceQueue.remove(Unknown Source)
com.mysql.cj.jdbc.AbandonedConnectionCleanupThread.run(AbandonedConnectionCleanupThread.java:85)
java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
java.lang.Thread.run(Unknown Source)
xxx 信息 [mysql-cj-abandoned-connection-cleanup] org.apache.catalina.loader.WebappClassLoaderBase.checkStateForResourceLoading Illegal access: this web application instance has been stopped already. Could not load []. The following stack trace is thrown for debugging purposes as well as to attempt to terminate the thread which caused the illegal access.
java.lang.IllegalStateException: Illegal access: this web application instance has been stopped already. Could not load []. The following stack trace is thrown for debugging purposes as well as to attempt to terminate the thread which caused the illegal access.
at org.apache.catalina.loader.WebappClassLoaderBase.checkStateForResourceLoading(WebappClassLoaderBase.java:1348)
at org.apache.catalina.loader.WebappClassLoaderBase.getResource(WebappClassLoaderBase.java:1007)
at com.mysql.cj.jdbc.AbandonedConnectionCleanupThread.checkThreadContextClassLoader(AbandonedConnectionCleanupThread.java:117)
at com.mysql.cj.jdbc.AbandonedConnectionCleanupThread.run(AbandonedConnectionCleanupThread.java:84)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
说明：
这个严重警告不会影响项目和程序的正常运行，但是日志里放着有些扎眼。
原因：
tomcat 6.0.25以后引入了内存泄露侦测，对于垃圾回收不能处理的对像，它就会写日志输出。
解决方法1：
查看电脑的内存使用情况，若电脑内存已超过75%或更高，关闭部分软件或重启电脑，在执行项目。一般会好。
解决方法2：
把下面这个Listener给注释了。这样就不会再去检查是否内存不够不用这些情况了。
<!-- Prevent memory leaks due to use of particular java/javax APIs-->
<Listener className="org.apache.catalina.core.JreMemoryLeakPreventionListener"/>;
