springboot默认日志框架选择源码解析(推荐)

dependencies {
  compile "com.alibaba:fastjson:1.2.75"
  compile "mysql:mysql-connector-java"
 
  //spring
  compile("org.springframework.boot:spring-boot-starter")
  compile("org.mybatis.spring.boot:mybatis-spring-boot-starter:2.1.4")
  compile("org.springframework.boot:spring-boot-starter-web")
  compile("org.springframework.boot:spring-boot-starter-actuator")
 
  //lombok
  compileOnly 'org.projectlombok:lombok'
  annotationProcessor 'org.projectlombok:lombok'
 
  //test
  testCompile('org.springframework.boot:spring-boot-starter-test')
  testImplementation 'io.projectreactor:reactor-test'
}

@Override
  public void onApplicationEvent(ApplicationEvent event) {
    if (event instanceof ApplicationStartingEvent) {
      onApplicationStartingEvent((ApplicationStartingEvent) event);
    }
    else if (event instanceof ApplicationEnvironmentPreparedEvent) {
      onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
    }
    else if (event instanceof ApplicationPreparedEvent) {
      onApplicationPreparedEvent((ApplicationPreparedEvent) event);
    }
    else if (event instanceof ContextClosedEvent
        && ((ContextClosedEvent) event).getApplicationContext().getParent() == null) {
      onContextClosedEvent();
    }
    else if (event instanceof ApplicationFailedEvent) {
      onApplicationFailedEvent();
    }
  }
 
  private void onApplicationStartingEvent(ApplicationStartingEvent event) {
    this.loggingSystem = LoggingSystem.get(event.getSpringApplication().getClassLoader());
    this.loggingSystem.beforeInitialize();
  }

public static final String SYSTEM_PROPERTY = LoggingSystem.class.getName();
private static final LoggingSystemFactory SYSTEM_FACTORY = LoggingSystemFactory.fromSpringFactories();
public static LoggingSystem get(ClassLoader classLoader) {
    String loggingSystemClassName = System.getProperty(SYSTEM_PROPERTY);
    if (StringUtils.hasLength(loggingSystemClassName)) {
      if (NONE.equals(loggingSystemClassName)) {
        return new NoOpLoggingSystem();
      }
      return get(classLoader, loggingSystemClassName);
    }
    LoggingSystem loggingSystem = SYSTEM_FACTORY.getLoggingSystem(classLoader);
    Assert.state(loggingSystem != null, "No suitable logging system located");
    return loggingSystem;
  }
 
  private static LoggingSystem get(ClassLoader classLoader, String loggingSystemClassName) {
    try {
      Class<?> systemClass = ClassUtils.forName(loggingSystemClassName, classLoader);
      Constructor<?> constructor = systemClass.getDeclaredConstructor(ClassLoader.class);
      constructor.setAccessible(true);
      return (LoggingSystem) constructor.newInstance(classLoader);
    }
    catch (Exception ex) {
      throw new IllegalStateException(ex);
    }
  }

public interface LoggingSystemFactory {
 
  /**
  * Return a logging system implementation or {@code null} if no logging system is
  * available.
  * @param classLoader the class loader to use
  * @return a logging system
  */
  LoggingSystem getLoggingSystem(ClassLoader classLoader);
 
  /**
  * Return a {@link LoggingSystemFactory} backed by {@code spring.factories}.
  * @return a {@link LoggingSystemFactory} instance
  */
  static LoggingSystemFactory fromSpringFactories() {
   return new DelegatingLoggingSystemFactory(
      (classLoader) -> SpringFactoriesLoader.loadFactories(LoggingSystemFactory.class, classLoader));
  }
 
}
class DelegatingLoggingSystemFactory implements LoggingSystemFactory {
 
  private final Function<ClassLoader, List<LoggingSystemFactory>> delegates;
 
  /**
  * Create a new {@link DelegatingLoggingSystemFactory} instance.
  * @param delegates a function that provides the delegates
  */
  DelegatingLoggingSystemFactory(Function<ClassLoader, List<LoggingSystemFactory>> delegates) {
   this.delegates = delegates;
  }
 
  @Override
  public LoggingSystem getLoggingSystem(ClassLoader classLoader) {
   List<LoggingSystemFactory> delegates = (this.delegates != null) ? this.delegates.apply(classLoader) : null;
   if (delegates != null) {
     for (LoggingSystemFactory delegate : delegates) {
      LoggingSystem loggingSystem = delegate.getLoggingSystem(classLoader);
      if (loggingSystem != null) {
        return loggingSystem;
      }
     }
   }
   return null;
  }
 
}