抽象类LoggingSystem指定了日志相关实例的生命周期和默认支持的日志框架。

/**
 * 其支持默认支持Logback，Log4J，JavaLogging
 */
public abstract class LoggingSystem {
    /**
     * 返回用户指定使用的或者默认的LoggingSystem,
     * 默认返回LogbackLoggingSystem
     */
    public static LoggingSystem get(ClassLoader classLoader) {
        String loggingSystem = System.getProperty(SYSTEM_PROPERTY);
        if (StringUtils.hasLength(loggingSystem)) {
            if (NONE.equals(loggingSystem)) {
                return new NoOpLoggingSystem();
            }
            return get(classLoader, loggingSystem);
        }
        return SYSTEMS.entrySet().stream()
                .filter((entry) -> ClassUtils.isPresent(entry.getKey(), classLoader))
                .map((entry) -> get(classLoader, entry.getValue())).findFirst()
                .orElseThrow(() -> new IllegalStateException(
                        "No suitable logging system located"));
    }
}

LoggingApplicationListener来调用LoggingSystem相关生命周期方法

public class LoggingApplicationListener {

      //确定了具体使用哪个日志框架，默认使用Logback
      private void onApplicationStartingEvent(ApplicationStartingEvent event) {
          this.loggingSystem = LoggingSystem
                  .get(event.getSpringApplication().getClassLoader());
          this.loggingSystem.beforeInitialize();
      }

      //对LoggingSystem进行初始化
      private void onApplicationEnvironmentPreparedEvent(
              ApplicationEnvironmentPreparedEvent event) {
          if (this.loggingSystem == null) {
              this.loggingSystem = LoggingSystem
                      .get(event.getSpringApplication().getClassLoader());
          }
          initialize(event.getEnvironment(), event.getSpringApplication().getClassLoader());
      }
      //将LoggingSystem注册到容器中
      private void onApplicationPreparedEvent(ApplicationPreparedEvent event) {
          ConfigurableListableBeanFactory beanFactory = event.getApplicationContext()
                  .getBeanFactory();
          if (!beanFactory.containsBean(LOGGING_SYSTEM_BEAN_NAME)) {
              beanFactory.registerSingleton(LOGGING_SYSTEM_BEAN_NAME, this.loggingSystem);
          }
      }
      //做一些清理相关的工作
      private void onContextClosedEvent() {
          if (this.loggingSystem != null) {
              this.loggingSystem.cleanUp();
          }
      }   
  }

Springboot中使用Logback框架进行日志打印，其配置文件是如何支持springProperty标签的？
LogbackLoggingSystem在初始化的过程中会加载相关的xml配置文件，
通过logback相关的解析器进行解析，并对解析器进行来扩展（SpringBootJoranConfigurator），使其支持pringProperty标签。

SpringBootJoranConfigurator 中定义了扩展的标签和对该标签的解析方式

class SpringBootJoranConfigurator extends JoranConfigurator {

        private LoggingInitializationContext initializationContext;

        SpringBootJoranConfigurator(LoggingInitializationContext initializationContext) {
            this.initializationContext = initializationContext;
        }

        @Override
        public void addInstanceRules(RuleStore rs) {
            super.addInstanceRules(rs);
            Environment environment = this.initializationContext.getEnvironment();
            rs.addRule(new ElementSelector("configuration/springProperty"),
                    new SpringPropertyAction(environment));
            rs.addRule(new ElementSelector("*/springProfile"),
                    new SpringProfileAction(this.initializationContext.getEnvironment()));
            rs.addRule(new ElementSelector("*/springProfile/*"), new NOPAction());
        }

    }