springboot默认日志框架选择源码解析(推荐)

背景：

今天新生成一个springboot项目，然而启动日志，还有mybatis的详细日志无法打印出来，自写程序中打印的日志可以输出；网上找了很多资料，都没法解决问题；于是决定跟一下源码，弄清springboot日志相关的逻辑。

环境配置：macbook； intellij idea community edition 2020.03 ； gradle 6.8.3 jdk1.8 ；

gradle引用包如下：

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'
}

springboot 默认日志使用的是logback(引入spring-boot-starter包后，就自动引入了logback-core,logback-classic两个包，当然还有slf4j的包)，当springboot启动时，org.springframework.boot.context.logging.LoggingApplicationListener，该类211行注册的监控事件会被ApplicationStartingEvent触发；如下代码所示，会调用onApplicationStartingEvent初始化loggingSystem，而使用哪个日志组件，就要看loggingSystem初始化的值了

@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();
  }

如下图是org.springframework.boot.logging.LoggingSystem类里面get函数的内容：首先会从system.getProperty中获取className，新生成的项目，取到的这个值都为空，SYSTEM_PROPERTY是一个固定值，就是该类的名字；那么loggingSystem的值就是从SYSTEM_FACTORY.getLoggingSystem(classLoader);获取到的；接下来我们得看LoggingSystemFactory.fromSpringFactories.getLoggingSystem取的值是什么了；

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);
    }
  }

LoggingSystemFactory是一个接口，它的实现类在spring-boot-start有4个，其中3个是在内部内类实现，DelegatingLoggingSystemFactory（JavaLoggingSystem，Log4J2LoggingSystem，LogbackLoggingSystem，内部类实现）。上面SYSTEM_FACTORY的实现就是DelegatingLoggingSystemFactory这个类，如下代码中delegates的值为JavaLoggingSystem，Log4J2LoggingSystem，LogbackLoggingSystem；三个类具体的加载逻辑在SpringFactoriesLoader.loadFactories函数中，最终返回的loggingSystem就是前面函数返回列表中的第一个；SpringFactoriesLoader.loadFactories 才是决定springboot默认会使用哪个日志组件关键：该类是spring的核心组件类，在spring-core包中，org.springframework.core.io.support.SpringFactoriesLoader；loggingSystem的值=LogbackLoggingSystem

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;
  }

}

总结：虽然springboot会加载JavaLoggingSystem，Log4J2LoggingSystem，LogbackLoggingSystem三个日志实现类，但在选择时，还是会使用LogbackLoggingSystem作为它的日志框架

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