Tomcat之启动报错分析

今天同事的tomcat启动时报错如下：

严重: ContainerBase.addChild: start: 
org.apache.catalina.LifecycleException: Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext[/docs]]
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:162)
	at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:899)
	at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:875)
	at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:652)
	at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:1260)
	at org.apache.catalina.startup.HostConfig$DeployDirectory.run(HostConfig.java:2002)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:439)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
	at java.util.concurrent.FutureTask.run(FutureTask.java:138)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
	at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.NoSuchMethodError: javax.servlet.ServletContext.getClassLoader()Ljava/lang/ClassLoader;
	at org.apache.catalina.startup.WebappServiceLoader.load(WebappServiceLoader.java:96)
	at org.apache.catalina.startup.ContextConfig.processServletContainerInitializers(ContextConfig.java:1579)
	at org.apache.catalina.startup.ContextConfig.webConfig(ContextConfig.java:1280)
	at org.apache.catalina.startup.ContextConfig.configureStart(ContextConfig.java:888)
	at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:388)
	at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:117)
	at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:90)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5522)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:145)

 主要原因就是因为tomcat类加载的时候加载servlet-api.jar这个jar包出了问题，但是反编译软件看servlet-api.jar是有ServletContext类中有getClassLoader这个方法。那么，肯定就是tomcat加载了其他地方的servlet-api.jar包中的ServletContext类，那么就需要了解Tomcat的类加载模型。

首先，tomcat的类模型是双亲委派模型，如下图

其中启动类加载器（通过JNI实现，无法获得其实力）、扩展类加载器是jvm自带的，其中启动类加载器加载的是%JAVA_RUNTIME_JRE%/lib该路径下的jar包，只会加载符合名称的一些jar包，例如rt.jar。扩展类加载器加载的是%JAVA_RUNTIME_JRE%/lib/ext/路径下的jar包。

其余CatalinaClassLoader、SharedClassLoader和WebAppClassLoader类加载路径在tomcat/conf/catalina.properties配置文件中(common.loader,server.loader,shared.loader).

该文参考于http://blog.****.net/czmacd/article/details/54017027