下午将log4j的版本由2.3升级到了2.9.1，并且改用slf4j接口，而不直接调用log4j2的接口。slf4j的好处有多个：
强制输出String，避免不规范代码。 例如直接调用log4j的方法log.info(obj),obj可以是个对象，但是当这个对象没有重写toString()方法时，则输出的只是hashcode值。强制输出String则可以避免这种情况。 日志模板功能. 例如直接使用log4j的时候经常写这样的代码logger.debug("Entry number: " + i + " is " + String.valueOf(entry[i]));这引发了两个问题： (1)需要编写拼接字符串的代码，使开发效率降低 (2)即使不需要输出这条日志（比如当前日志级别是ERROR时），也会执行拼接字符串的操作，消耗额外性能，占用额外内存 而slf4j使用日志模板功能解决了这两个问题： logger.debug("Entry number: {} is {}", i, String.valueOf(entry[i])); 不仅开发变得简单了，而且slf4j只会在此条日志确实需要输出时才会去拼装字符串。 并且在输出异常信息时也可以使用模板，不会妨碍stacktrace的输出： String s = "Hello world"; try { Integer i = Integer.valueOf(s); } catch (NumberFormatException e) { logger.error("Failed to format {}", s, e); } 下面问题来了：使用的slf4j-api版本是1.7.25，jdk是1.8.0_101, log4j2版本是2.9.1
启动程序报异常：
ERROR StatusLogger Caught java.lang.AbstractMethodError setting feature http://xml.org/sax/features/external-general-entities to false on DocumentBuilderFactory org.apache.xerces.jaxp.DocumentBuilderFactoryImpl@53251a66: java.lang.AbstractMethodError: javax.xml.parsers.DocumentBuilderFactory.setFeature(Ljava/lang/String;Z)V java.lang.AbstractMethodError: javax.xml.parsers.DocumentBuilderFactory.setFeature(Ljava/lang/String;Z)V at org.apache.logging.log4j.core.config.xml.XmlConfiguration.setFeature(XmlConfiguration.java:213) at org.apache.logging.log4j.core.config.xml.XmlConfiguration.disableDtdProcessing(XmlConfiguration.java:205) at org.apache.logging.log4j.core.config.xml.XmlConfiguration.newDocumentBuilder(XmlConfiguration.java:194) at org.apache.logging.log4j.core.config.xml.XmlConfiguration.<init>(XmlConfiguration.java:92) at org.apache.logging.log4j.core.config.xml.XmlConfigurationFactory.getConfiguration(XmlConfigurationFactory.java:46) at org.apache.logging.log4j.core.config.ConfigurationFactory$Factory.getConfiguration(ConfigurationFactory.java:451) at org.apache.logging.log4j.core.config.ConfigurationFactory$Factory.getConfiguration(ConfigurationFactory.java:385) at org.apache.logging.log4j.core.config.ConfigurationFactory.getConfiguration(ConfigurationFactory.java:260) at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:613) at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:634) at org.apache.logging.log4j.core.LoggerContext.start(LoggerContext.java:229) at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:152) at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:45) at org.apache.logging.log4j.LogManager.getContext(LogManager.java:194) at org.apache.logging.log4j.spi.AbstractLoggerAdapter.getContext(AbstractLoggerAdapter.java:122) at org.apache.logging.slf4j.Log4jLoggerFactory.getContext(Log4jLoggerFactory.java:43) at org.apache.logging.log4j.spi.AbstractLoggerAdapter.getLogger(AbstractLoggerAdapter.java:46) at org.apache.logging.slf4j.Log4jLoggerFactory.getLogger(Log4jLoggerFactory.java:29) at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:358) at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:155) at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:132) at org.apache.commons.logging.LogFactory.getLog(LogFactory.java:655) at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:301) at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:107) at org.mortbay.jetty.handler.ContextHandler.startContext(ContextHandler.java:549) at org.mortbay.jetty.servlet.Context.startContext(Context.java:136) at org.mortbay.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1282) at org.mortbay.jetty.handler.ContextHandler.doStart(ContextHandler.java:518) at org.mortbay.jetty.webapp.WebAppContext.doStart(WebAppContext.java:499) at org.mortbay.component.AbstractLifeCycle.start(AbstractLifeCycle.java:50) at org.mortbay.jetty.handler.HandlerWrapper.doStart(HandlerWrapper.java:130) at org.mortbay.jetty.Server.doStart(Server.java:224) at org.mortbay.component.AbstractLifeCycle.start(AbstractLifeCycle.java:50) at runjettyrun.Bootstrap.main(Bootstrap.java:97) ERROR StatusLogger Caught java.lang.AbstractMethodError setting feature http://xml.org/sax/features/external-parameter-entities to false on DocumentBuilderFactory org.apache.xerces.jaxp.DocumentBuilderFactoryImpl@53251a66: java.lang.AbstractMethodError: javax.xml.parsers.DocumentBuilderFactory.setFeature(Ljava/lang/String;Z)V java.lang.AbstractMethodError: javax.xml.parsers.DocumentBuilderFactory.setFeature(Ljava/lang/String;Z)V at org.apache.logging.log4j.core.config.xml.XmlConfiguration.setFeature(XmlConfiguration.java:213) at org.apache.logging.log4j.core.config.xml.XmlConfiguration.disableDtdProcessing(XmlConfiguration.java:206) at org.apache.logging.log4j.core.config.xml.XmlConfiguration.newDocumentBuilder(XmlConfiguration.java:194) at org.apache.logging.log4j.core.config.xml.XmlConfiguration.<init>(XmlConfiguration.java:92) at org.apache.logging.log4j.core.config.xml.XmlConfigurationFactory.getConfiguration(XmlConfigurationFactory.java:46) at org.apache.logging.log4j.core.config.ConfigurationFactory$Factory.getConfiguration(ConfigurationFactory.java:451) at org.apache.logging.log4j.core.config.ConfigurationFactory$Factory.getConfiguration(ConfigurationFactory.java:385) at org.apache.logging.log4j.core.config.ConfigurationFactory.getConfiguration(ConfigurationFactory.java:260) at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:613) at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:634) at org.apache.logging.log4j.core.LoggerContext.start(LoggerContext.java:229) at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:152) at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:45) at org.apache.logging.log4j.LogManager.getContext(LogManager.java:194) at org.apache.logging.log4j.spi.AbstractLoggerAdapter.getContext(AbstractLoggerAdapter.java:122) at org.apache.logging.slf4j.Log4jLoggerFactory.getContext(Log4jLoggerFactory.java:43) at org.apache.logging.log4j.spi.AbstractLoggerAdapter.getLogger(AbstractLoggerAdapter.java:46) at org.apache.logging.slf4j.Log4jLoggerFactory.getLogger(Log4jLoggerFactory.java:29) at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:358) at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:155) at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:132) at org.apache.commons.logging.LogFactory.getLog(LogFactory.java:655) at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:301) at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:107) at org.mortbay.jetty.handler.ContextHandler.startContext(ContextHandler.java:549) at org.mortbay.jetty.servlet.Context.startContext(Context.java:136) at org.mortbay.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1282) at org.mortbay.jetty.handler.ContextHandler.doStart(ContextHandler.java:518) at org.mortbay.jetty.webapp.WebAppContext.doStart(WebAppContext.java:499) at org.mortbay.component.AbstractLifeCycle.start(AbstractLifeCycle.java:50) at org.mortbay.jetty.handler.HandlerWrapper.doStart(HandlerWrapper.java:130) at org.mortbay.jetty.Server.doStart(Server.java:224) at org.mortbay.component.AbstractLifeCycle.start(AbstractLifeCycle.java:50) at runjettyrun.Bootstrap.main(Bootstrap.java:97)出现异常的第一反应是跑去Google问问咋回事，发现Apache log4j2讨论区描述的是这个问题在2.9.0出现了，在2.9.1版本已经解决，可是我用的就是2.9.1版本啊。。。没办法Google搞不定，第二反应是去调试下代码发现DocumentBuilderFactoryImpl在两个库中有，其中有一个库中的类并没有setFeature这个方法，而代码报异常应该就是进到这个方法了。于是知道应该是jar包冲突导致的这个异常，把这个类的包exclude掉吧 留下jdk中的rt.jar中的com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl，重新编译运行搞定了。
PS：以后遇到问题还是先自己调试下代码吧，看下究竟是哪里出的问题
