org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesJdbc The web application [xx] ...

异常详情：

31-Jan-2018 16:30:40.843 WARNING [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesJdbc The web application [spicy_dev] 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.
31-Jan-2018 16:30:40.843 WARNING [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesJdbc The web application [spicy_dev] 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.
31-Jan-2018 16:30:40.846 WARNING [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesThreads The web application [spicy_dev] appears to have started a thread named [Abandoned connection cleanup thread] 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(ReferenceQueue.java:143)
 com.mysql.jdbc.AbandonedConnectionCleanupThread.run(AbandonedConnectionCleanupThread.java:43)

以上标红的地方是两个重要的地方：在tomcat的lib目录下面的catalina.jar里面

其中WebappClassLoaderBase这个类的作用就是关闭一些信息，具体代码如下：

  protected void clearReferences()
  {
    clearReferencesJdbc();

    clearReferencesThreads();

    checkThreadLocalsForLeaks();

    if (this.clearReferencesRmiTargets) {
      clearReferencesRmiTargets();
    }

    IntrospectionUtils.clear();

    if (this.clearReferencesLogFactoryRelease) {
      LogFactory.release(this);
    }

    Introspector.flushCaches();

    TomcatURLStreamHandlerFactory.release(this);
  }

  private final void clearReferencesJdbc()
  {
    byte[] classBytes = new byte[2048];
    int offset = 0;
    try { InputStream is = getResourceAsStream("org/apache/catalina/loader/JdbcLeakPrevention.class"); Throwable localThrowable2 = null;
      try {
        int read = is.read(classBytes, offset, classBytes.length - offset);
        while (read > -1) {
          offset += read;
          if (offset == classBytes.length)
          {
            byte[] tmp = new byte[classBytes.length * 2];
            System.arraycopy(classBytes, 0, tmp, 0, classBytes.length);
            classBytes = tmp;
          }
          read = is.read(classBytes, offset, classBytes.length - offset);
        }
        Class lpClass = defineClass("org.apache.catalina.loader.JdbcLeakPrevention", classBytes, 0, offset, getClass().getProtectionDomain());

        Object obj = lpClass.getConstructor(new Class[0]).newInstance(new Object[0]);

        List driverNames = (List)obj.getClass().getMethod("clearJdbcDriverRegistrations", new Class[0]).invoke(obj, new Object[0]);

        for (String name : driverNames)
          log.warn(sm.getString("webappClassLoader.clearJdbc", new Object[] { getContextName(), name }));
      }
      catch (Throwable localThrowable1)
      {
        localThrowable2 = localThrowable1; throw localThrowable1;
      }
      finally
      {
        if (is != null) if (localThrowable2 != null) try { is.close(); } catch (Throwable x2) { localThrowable2.addSuppressed(x2); } else is.close();  
      }
    } catch (Exception e)
    {
      Throwable t = ExceptionUtils.unwrapInvocationTargetException(e);
      ExceptionUtils.handleThrowable(t);
      log.warn(sm.getString("webappClassLoader.jdbcRemoveFailed", new Object[] { getContextName() }), t);
    }
  }

  private void clearReferencesThreads()
  {
    Thread[] threads = getThreads();
    List executorThreadsToStop = new ArrayList();

    for (Thread thread : threads) {
      if (thread != null) {
        ClassLoader ccl = thread.getContextClassLoader();
        if (ccl != this)
          continue;
        if (thread == Thread.currentThread())
        {
          continue;
        }
        String threadName = thread.getName();

具体就是用来关闭一些连接操作的，接下来我们看它用引用的下一个重要的类JdbcLeakPrevention

package org.apache.catalina.loader;

import java.sql.Driver;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.List;

public class JdbcLeakPrevention
{
  public List<String> clearJdbcDriverRegistrations()
    throws SQLException
  {
    List driverNames = new ArrayList();

    HashSet originalDrivers = new HashSet();
    Enumeration drivers = DriverManager.getDrivers();
    while (drivers.hasMoreElements()) {
      originalDrivers.add(drivers.nextElement());
    }
    drivers = DriverManager.getDrivers();
    while (drivers.hasMoreElements()) {
      Driver driver = (Driver)drivers.nextElement();

      if (driver.getClass().getClassLoader() != getClass().getClassLoader())
      {
        continue;
      }

      if (originalDrivers.contains(driver)) {
        driverNames.add(driver.getClass().getCanonicalName());
      }
      DriverManager.deregisterDriver(driver);
    }
    return driverNames;
  }
}

代码不长，具体就是为了去除数据库驱动的。问题就是出现在这里，我们的异常发送在 com.mysql.jdbc.AbandonedConnectionCleanupThread.run(AbandonedConnectionCleanupThread.java:43)，

但是这个类里面还有一个shutdown方法没有被使用，所以造成一个线程在不断的查询数据库连接，而另一个线程在关闭数据库连接，所以就造成了以上的问题（想知道是哪个线程一直通讯数据库的请查看java.lang.ref.ReferenceQueue类）

好了不啰嗦了  直接上解决方案，思路就是使用一个ServletContextListener监听器在销毁方法里面使用 com.mysql.jdbc.AbandonedConnectionCleanupThread.shutdown 方法

参考博客：https://www.cnblogs.com/interdrp/p/5633020.html

解决方案：（这里我使用的是servlet3.0的注解）

package com.spicy.common.framework.listener;

import java.sql.Driver;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.Enumeration;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

import com.mysql.jdbc.AbandonedConnectionCleanupThread;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@WebListener
public class ContextFinalizer implements ServletContextListener {

private Logger logger = LoggerFactory.getLogger(this.getClass());
public void contextInitialized(ServletContextEvent sce) {
}

public void contextDestroyed(ServletContextEvent sce) {
Enumeration<Driver> drivers = DriverManager.getDrivers();
Driver d = null;
try {
while (drivers.hasMoreElements()) {
d = drivers.nextElement();
DriverManager.deregisterDriver(d);
logger.info(" 消除数据库连接驱动 --> : Driver {} deregistered", d);
}
} catch (SQLException ex) {
logger.error("Error: deregistering driver {} exceptionName:{} detail:{}", d, ex.getClass().getName(), ex.getMessage());
}finally {
try {
AbandonedConnectionCleanupThread.shutdown();
} catch (InterruptedException e) {
logger.error("Error: SEVERE problem cleaning up: " + e.getMessage());
}
}
}
}

结果：

31-Jan-2018 17:26:33.211 信息 [main] org.apache.catalina.core.StandardServer.await A valid shutdown command was received via the shutdown port. Stopping the Server instance.
31-Jan-2018 17:26:33.211 信息 [main] org.apache.coyote.AbstractProtocol.pause Pausing ProtocolHandler ["http-nio-8080"]
31-Jan-2018 17:26:33.590 信息 [main] org.apache.coyote.AbstractProtocol.pause Pausing ProtocolHandler ["ajp-nio-8009"]
31-Jan-2018 17:26:33.939 信息 [main] org.apache.catalina.core.StandardService.stopInternal Stopping service [Catalina]
23907 INFO  [2018-01-31 17:26:33]  Closing WebApplicationContext for namespace 'spring4mvc-servlet': startup date [Wed Jan 31 17:26:18 CST 2018]; parent: Root WebApplicationContext
23947 INFO  [2018-01-31 17:26:33]   消除数据库连接驱动 --> : Driver com.alibaba.druid.proxy.DruidDriver@2e5aa1e6 deregistered
23947 INFO  [2018-01-31 17:26:33]   消除数据库连接驱动 --> : Driver com.mysql.jdbc.Driver@6eaed403 deregistered
log4j:WARN No appenders could be found for logger (org.springframework.web.context.support.XmlWebApplicationContext).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
31-Jan-2018 17:26:34.029 信息 [main] org.apache.coyote.AbstractProtocol.stop Stopping ProtocolHandler ["http-nio-8080"]
31-Jan-2018 17:26:34.033 信息 [main] org.apache.coyote.AbstractProtocol.stop Stopping ProtocolHandler ["ajp-nio-8009"]
31-Jan-2018 17:26:34.035 信息 [main] org.apache.coyote.AbstractProtocol.destroy Destroying ProtocolHandler ["http-nio-8080"]
31-Jan-2018 17:26:34.037 信息 [main] org.apache.coyote.AbstractProtocol.destroy Destroying ProtocolHandler ["ajp-nio-8009"]
Disconnected from server