Spring Boot日志模块的设计
首先我们需要关注LoggingApplicationListener这个应用监听器，它的官方说明如下：
An ApplicationListener that configures the LoggingSystem. If the environment contains a logging.config property it will be used to bootstrap the logging system, otherwise a default configuration is used. Regardless, logging levels will be customized if the environment contains logging.level.* entries and logging groups can be defined with logging.group.
