类设计
LogFactory根据当前环境加载具体的Log实现：
1.从缓存中加载LogFactory
2.从系统属性org.apache.commons.logging.LogFactory 中加载LogFactory
3.从属性文件commons-logging.properties中的org.apache.commons.logging.LogFactory加载LogFactory
4.加载classpath下的实现
4.1加载用户自己的Log实现
4.2查找org.apache.commons.logging.impl.Log4JLogger
4.3查找org.apache.commons.logging.impl.Jdk14Logger
4.4查找org.apache.commons.logging.impl.Jdk13LumberjackLogger
4.5查找org.apache.commons.logging.impl.SimpleLog
