Springboot在tomcat Undeploying，或者容器中关闭时老是提示如下警告，虽然不会影响程序的运行，但是还是想去掉这些不必要的异常：

xxx 警告 [http-nio-8080-exec-56] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesJdbc The web application [gasbj] 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 [gasbj] 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 [gasbj] 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)

发生这种异常的原因有两个：

一是Tomcat在 6.0.24 后加入了一个 memory leak detection 检查机制，当我们程序中引入了JDBC 4.0 驱动时会自动注册到Tomcat容器中，但是却没有提供自动销毁机制。

二是我们Mysql驱动中自带的AbandonedConnectionCleanupThread类，会在我们加载jdbc驱动时启动一个newSingleThreadExecutor的线程池，这个也不会自动释放。

解决方式如下（注册一个ServletContextListener，注意使用@ServletComponentScan来扫描）：

package com.iwhere.gasbj.config.listener;

import lombok.extern.slf4j.Slf4j;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.sql.Driver;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.Enumeration;

@WebListener
@Slf4j
public class JdbcUnregisterListener implements ServletContextListener {
    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        try {
            log.info("Calling MySQL AbandonedConnectionCleanupThread checkedShutdown");
            // Or com.mysql.jdbc.AbandonedConnectionCleanupThread
            Class cls = Class.forName("com.mysql.cj.jdbc.AbandonedConnectionCleanupThread");
            Method method = cls.getMethod("checkedShutdown");
            method.invoke(null);
        } catch (ClassNotFoundException | NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
            log.error("Cannot call MySQL AbandonedConnectionCleanupThread.checkedShutdown!", e);
        }

        // Now deregister JDBC drivers in this context's ClassLoader:
        // Get the webapp's ClassLoader
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        // Loop through all drivers
        Enumeration<Driver> drivers = DriverManager.getDrivers();
        while (drivers.hasMoreElements()) {
            Driver driver = drivers.nextElement();
            if (driver.getClass().getClassLoader() == cl) {
                // This driver was registered by the webapp's ClassLoader, so deregister it:
                try {
                    log.info("Deregistering JDBC driver {}", driver);
                    DriverManager.deregisterDriver(driver);
                } catch (SQLException ex) {
                    log.error("Error deregistering JDBC driver {}", driver, ex);
                }
            } else {
                // driver was not registered by the webapp's ClassLoader and may be in use elsewhere
                log.trace("Not deregistering JDBC driver {} as it does not belong to this webapp's ClassLoader", driver);
            }
        }
    }
}