错误为:
<strong><span style="color:#ff0000;">java.lang.IllegalArgumentException: <session-config> element is limited to 1 occurrence</span></strong>
	at org.apache.catalina.startup.SetSessionConfig.begin(WebRuleSet.java:718)
	at org.apache.tomcat.util.digester.Digester.startElement(Digester.java:1276)
	at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.startElement(AbstractSAXParser.java:506)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanStartElement(XMLDocumentFragmentScannerImpl.java:1323)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:2717)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:607)
	at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:489)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:835)
	at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:764)
	at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(XMLParser.java:123)
	at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1210)
	at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(SAXParserImpl.java:568)
	at org.apache.tomcat.util.digester.Digester.parse(Digester.java:1537)
	at org.apache.catalina.startup.ContextConfig.parseWebXml(ContextConfig.java:1742)
	at org.apache.catalina.startup.ContextConfig.webConfig(ContextConfig.java:1174)
	at org.apache.catalina.startup.ContextConfig.configureStart(ContextConfig.java:828)
	at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:302)
	at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)
	at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:90)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5148)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1525)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1515)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
	at java.util.concurrent.FutureTask.run(FutureTask.java:166)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:724)

查找了半天,发现Tomcat 官网上面也有这个bug,但是一看人家提交bug时间是2011-06-21 01:36,几年过去了,肯定修复了. 

ctrl-f : <session-config> 发现,web.xml里面存在多个. 删除只留一个就解决了. !   web.xml 从别的项目copy过来,忘记已经设置过了,导致的. 记录下