引言： 诡异而少见的java.lang.ClassNotFoundException: ch.qos.lorgback.classic.PatternLayout异常错误信息，在启动的时候，出现了异常，哪里出了问题呢？
1. 背景介绍
项目是Web项目，使用了slf4j-api, 使用logback做为默认的日志包；之前一直工作正常，在将日志路径调整到了pom.xml之后，忽然就蹦出来一个异常错误信息。
pom.xml中引用的maven片段:
<dependency><artifactId>slf4j-api</artifactId><groupId>org.slf4j</groupId><version>1.7.10</version></dependency><dependency><groupId>ch.qos.logback</groupId><artifactId>logback-classic</artifactId><version>1.1.3</version><exclusions><exclusion><artifactId>slf4j-api</artifactId><groupId>org.slf4j</groupId></exclusion></exclusions></dependency>
2. 问题的出现
在某一天，忽然出现了以下问题，
17:03:02,893 |-INFO in c.q.l.core.rolling.DefaultTimeBasedFileNamingAndTriggeringPolicy – Setting initial period to Fri Apr 24 17:03:02 CST 201517:03:02,893 |-ERROR in ch.qos.logback.core.joran.action.NestedComplexPropertyIA – Could not create component [layout] of type [ch.qos.lorgback.classic.PatternLayout] java.lang.ClassNotFoundException: ch.qos.lorgback.classic.PatternLayoutat java.lang.ClassNotFoundException: ch.qos.lorgback.classic.PatternLayoutat at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1702)at at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1547)at at ch.qos.logback.core.util.Loader.loadClass(Loader.java:125)at at ch.qos.logback.core.joran.action.NestedComplexPropertyIA.begin(NestedComplexPropertyIA.java:100)at at ch.qos.logback.core.joran.spi.Interpreter.callBeginAction(Interpreter.java:275)at at ch.qos.logback.core.joran.spi.Interpreter.startElement(Interpreter.java:147)at at ch.qos.logback.core.joran.spi.Interpreter.startElement(Interpreter.java:129)at at ch.qos.logback.core.joran.spi.EventPlayer.play(EventPlayer.java:50)at at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:149)at at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:135)at at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:99)at at ch.qos.logback.core.joran.GenericConfigurator.doConfigure(GenericConfigurator.java:49)at at ch.qos.logback.classic.util.ContextInitializer.configureByResource(ContextInitializer.java:77)at at ch.qos.logback.classic.util.ContextInitializer.autoConfig(ContextInitializer.java:152)at at org.slf4j.impl.StaticLoggerBinder.init(StaticLoggerBinder.java:85)at at org.slf4j.impl.StaticLoggerBinder.<clinit>(StaticLoggerBinder.java:55)at at org.slf4j.LoggerFactory.bind(LoggerFactory.java:142)at at org.slf4j.LoggerFactory.performInitialization(LoggerFactory.java:121)at at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:332)at at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:284)at at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:156)at at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:132)at at org.apache.commons.logging.LogFactory.getLog(LogFactory.java:274)at at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:282)at at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:112)at at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4939)at at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5434)at at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)at at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1559)at at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1549)at at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)at at java.util.concurrent.FutureTask.run(FutureTask.java:138)at at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)at at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)at at java.lang.Thread.run(Thread.java:662)17:03:02,893 |-ERROR in ch.qos.logback.core.joran.spi.Interpreter@49:22 – no applicable action for [pattern], current ElementPath is [[configuration][appender][layout][pattern]]17:03:02,903 |-INFO in ch.qos.logback.core.rolling.RollingFileAppender[ThirdPartyAppender] – Active log file name: /opt/app/outpay/logs/es-thirdparty.log.2015-04-24.log17:03:02,903 |-INFO in ch.qos.logback.core.rolling.RollingFileAppender[ThirdPartyAppender] – File property is set to [null]17:03:02,903 |-WARN in ch.qos.logback.core.rolling.RollingFileAppender[ThirdPartyAppender] – Encoder has not been set. Cannot invoke its init method.
从日志上看，在Spring web context启动的过程中，需要查找默认的日志框架，结果在最终的结果中，没有找到期望的类ch.qos.lorgback.classic.PatternLayout。经过确认，这个类确实是在我们系统的类库之中，但是为什么却报没有找到的异常呢？
3. 问题的分析与解决
为什么在Spring web context的加载中报错，但是类确实是存在于系统中呢？最终的解决是加载的顺序，就是在启动web context的类库之时，类库确实无法被访问到。
回想到pom.xml中加载的类库，确实是可以被限制在不同的访问scope中的，即scope限定了在不同的时候，应用所可以访问的类库包。
君子无故，玉不去身。
