java日志门面
1.slf4j是什么
slf4j全称为Simple Logging Facade for Java ,即java简单日志门面，渐渐地替换调了apache common logging。
The Simple Logging Facade for Java (SLF4J) serves as a simple facade or abstraction for various logging frameworks (e.g. java.util.logging, logback, log4j) allowing the end user to plug in the desired logging framework at deployment time.
理解为是java应用程序里的日志门面或者是各种日志框架的抽象，其中常用的实现有java.util.logging,logback,log4j，目前主流的是logback。
2.示例slf4j+logback
2.1.代码应用
级别依次是trace,debug,info,warn,error,越往下级别越高。
2.2.logback.xml配置
