springboot默认日志框架源码解析

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 {
   LoggingSystem getLoggingSystem(ClassLoader classLoader);
 
   static LoggingSystemFactory fromSpringFactories() {
      return new DelegatingLoggingSystemFactory(
            (classLoader) -> SpringFactoriesLoader.loadFactories(LoggingSystemFactory.class, classLoader));
   }
 
}
class DelegatingLoggingSystemFactory implements LoggingSystemFactory {
 
   private final Function> delegates;

   DelegatingLoggingSystemFactory(Function> delegates) {
      this.delegates = delegates;
   }
 
   @Override
   public LoggingSystem getLoggingSystem(ClassLoader classLoader) {
      List 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