tomcat的一个常见错误

alonli

　　现象：
　　上次同事在热部署应用到liferay服务器上时，遇到了以下错误：

• Sep 21, 2012 3:19:06 AM org.apache.catalina.startup.HostConfig deployDirectory
  
• 
  
• INFO: Deploying web application directory /home/portal/liferay-container-1.0.0-SNAPSHOT/standalone/liferay-portal/community-edition-liferay-portal-tomcat-6.1.0/liferay-portal-6.1.0-ce-ga1/tomcat-7.0.23/webapps/paas_integration_portlet
  
• 
  
• Sep 21, 2012 3:19:06 AM org.apache.catalina.core.StandardContext startInternal
  
• 
  
• SEVERE: Error listenerStart
  
• 
  
• Sep 21, 2012 3:19:06 AM org.apache.catalina.core.StandardContext startInternal
  
• 
  
• SEVERE: Context [/paas_integration_portlet] startup failed due to previous errors
  
• 
  
• Sep 21, 2012 3:19:06 AM org.apache.catalina.loader.WebappClassLoader checkThreadLocalMapForLeaks
  
• 
  
• SEVERE: The web application [/paas_integration_portlet] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@3e043340]) and a value of type [java.lang.Class] (value [class oracle.sql.AnyDataFactory]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.
  
• 
  
• Sep 21, 2012 3:19:06 AM org.apache.catalina.loader.WebappClassLoader checkThreadLocalMapForLeaks
  
• 
  
• SEVERE: The web application [/paas_integration_portlet] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@5c0ce8f]) and a value of type [java.lang.Class] (value [class oracle.sql.TypeDescriptorFactory]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.
  
• 
  
• Sep 21, 2012 3:19:06 AM org.apache.coyote.AbstractProtocol start
  
• 
  
• INFO: Starting ProtocolHandler ["http-bio-8080"]
  
• 
  
• Sep 21, 2012 3:19:06 AM org.apache.coyote.AbstractProtocol start
  
• 
  
• INFO: Starting ProtocolHandler ["ajp-bio-8009"]
  
• 
  
• Sep 21, 2012 3:19:06 AM org.apache.catalina.startup.Catalina start
  
• 
  
• INFO: Server startup in 37724 ms
  

　　基于我的以前经验，这是"类加载器泄露“的问题，具体细节参见我的以下邮件回复：
Reason:




This is very common in Tomcat .







We all know (1) ThreadLocal is used for holding variables per thread.



                         (2) Tomcat supports multi-threads.
                        (3) The default GC mechanism for tomcat is “object reference”











So  the process is ,when we redeploy a portlet ,then the WebClassloader  will load all the necessary classes and jars which required by this  portlet ,and the threadlocal is used for identifying the thread . But  when we destroy the portlet(redeploy or remove) ,though the portlet is  removed ,but the threadlocal  value didn’t remove ,(which means the  object reference number still >0) ,so the classes loaded by  WebappClassLoader can’t not be gc , that’s a very famous topic   “Classloader leak”







My highlighted part is my understanding to this problem and it can explains what Judith has highlighted .














Solution:







“Classloader  leak” issue happened since the first version of tomcat ,and it always a  challenging work , as I know ,since tomcat 7 (because our Liferay 6.1  based on tomcat 7) has added some detection mechanism to detect the  classloader leak issue ,that’s why it report “SEVERE”   ,if you familiar  with tomcat 5 or 6 ,it just shutdown without any useful information.







Up  to now just as I know ,we have no solution about it ,maybe we need to  new version of tomcat to fix it .  If often meet with it ,I suggest we  re-start the liferay.














Referenced Articles:







http://wiki.apache.org/tomcat/MemoryLeakProtection#customThreadLocal



https://issues.apache.org/bugzilla/show_bug.cgi?id=49159
http://www.javacodegeeks.com/2012/05/threading-stories-threadlocal-in-web.html










Charles