Spring Boot 默认使用的是 Logback 的日志框架、Logback 的组件主要通过 Spring Boot ApplicationListener 启动的

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// LoggingApplicationListener
@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();
   }
}

onApplicationStartingEvent

创建 LoggingSystem

private void onApplicationStartingEvent(ApplicationStartingEvent event) {this.loggingSystem = LoggingSystem.get(event.getSpringApplication().getClassLoader());
   this.loggingSystem.beforeInitialize();
}

LoggingSystem 是 Spring Boot 抽象的日志系统接口

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"));
}

private static LoggingSystem get(ClassLoader classLoader, String loggingSystemClass) {try {ClasssystemClass = ClassUtils.forName(loggingSystemClass, classLoader);
    Constructorconstructor = systemClass.getDeclaredConstructor(ClassLoader.class);
    constructor.setAccessible(true);
    return (LoggingSystem) constructor.newInstance(classLoader);
  }
  catch (Exception ex) {throw new IllegalStateException(ex);
  }
}

SYSTEMS的值如下、默认情况下创建的就是 LogbackLoggingSystem

private static final MapSYSTEMS;

static {Mapsystems = new LinkedHashMap<>();
   systems.put("ch.qos.logback.classic.LoggerContext",
         "org.springframework.boot.logging.logback.LogbackLoggingSystem");
   systems.put("org.apache.logging.log4j.core.impl.Log4jContextFactory",
         "org.springframework.boot.logging.log4j2.Log4J2LoggingSystem");
   systems.put("java.util.logging.LogManager", "org.springframework.boot.logging.java.JavaLoggingSystem");
   SYSTEMS = Collections.unmodifiableMap(systems);
}

接下来进入到 LogbackLoggingSystem#beforeInitialize 方法

@Override
public void beforeInitialize() {LoggerContext loggerContext = getLoggerContext();
   if (isAlreadyInitialized(loggerContext)) {  return;
   }
   super.beforeInitialize();
   loggerContext.getTurboFilterList().add(FILTER);
}

通过 StaticLoggerBinder.getSingleton() 创建 LoggerContext

private LoggerContext getLoggerContext() {ILoggerFactory factory = StaticLoggerBinder.getSingleton().getLoggerFactory();
   Assert.isInstanceOf(LoggerContext.class, factory,
         String.format(
               "LoggerFactory is not a Logback LoggerContext but Logback is on "
                     + "the classpath. Either remove Logback or the competing "
                     + "implementation (%s loaded from %s). If you are using "
                     + "WebLogic you will need to add 'org.slf4j' to "
                     + "prefer-application-packages in WEB-INF/weblogic.xml",
               factory.getClass(), getLocation(factory)));
   return (LoggerContext) factory;
}

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进入到静态代码块的 init 方法

void init() {try {try {new ContextInitializer(defaultLoggerContext).autoConfig();
        } catch (JoranException je) {Util.report("Failed to auto configure default logger context", je);
        }
        // logback-292
        if (!StatusUtil.contextHasStatusListener(defaultLoggerContext)) {StatusPrinter.printInCaseOfErrorsOrWarnings(defaultLoggerContext);
        }
        contextSelectorBinder.init(defaultLoggerContext, KEY);
        initialized = true;
    } catch (Exception t) {// see LOGBACK-1159
        Util.report("Failed to instantiate [" + LoggerContext.class.getName() + "]", t);
    }
}

最终会执行下面的代码、尝试获取默认配置文件、如果不存在则通过 SPI 获取 Configurator 实现类、如果还是没有、则使用默认的配置 BasicConfigurator

public void autoConfig() throws JoranException {
    StatusListenerConfigHelper.installIfAsked(loggerContext);
    URL url = findURLOfDefaultConfigurationFile(true);
    if (url != null) {
        configureByResource(url);
    } else {
        Configurator c = EnvUtil.loadFromServiceLoader(Configurator.class);
        if (c != null) {
            try {
                c.setContext(loggerContext);
                c.configure(loggerContext);
            } catch (Exception e) {
                throw new LogbackException(String.format("Failed to initialize Configurator: %s using ServiceLoader", c != null ? c.getClass()
                                .getCanonicalName() : "null"), e);
            }
        } else {
            BasicConfigurator basicConfigurator = new BasicConfigurator();
            basicConfigurator.setContext(loggerContext);
            basicConfigurator.configure(loggerContext);
        }
    }
}

onApplicationEnvironmentPreparedEvent

初始化阶段

private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {if (this.loggingSystem == null) {  this.loggingSystem = LoggingSystem.get(event.getSpringApplication().getClassLoader());
   }
   initialize(event.getEnvironment(), event.getSpringApplication().getClassLoader());
}

protected void initialize(ConfigurableEnvironment environment, ClassLoader classLoader) {new LoggingSystemProperties(environment).apply();
   this.logFile = LogFile.get(environment);
   if (this.logFile != null) {  this.logFile.applyToSystemProperties();
   }
   this.loggerGroups = new LoggerGroups(DEFAULT_GROUP_LOGGERS);
   initializeEarlyLoggingLevel(environment);
   initializeSystem(environment, this.loggingSystem, this.logFile);
   initializeFinalLoggingLevels(environment, this.loggingSystem);
   registerShutdownHookIfNecessary(environment, this.loggingSystem);
}

主要就是从 environment 对象中设置相关属性值到 System 中、然后判断是否设置了 log file

最主要的 initializeSystem

private void initializeSystem(ConfigurableEnvironment environment, LoggingSystem system, LogFile logFile) {LoggingInitializationContext initializationContext = new LoggingInitializationContext(environment);
   String logConfig = environment.getProperty(CONFIG_PROPERTY);
   if (ignoreLogConfig(logConfig)) {  system.initialize(initializationContext, null, logFile);
   }
   else {  try { system.initialize(initializationContext, logConfig, logFile);
      }
      catch (Exception ex) { Throwable exceptionToReport = ex;
         while (exceptionToReport != null && !(exceptionToReport instanceof FileNotFoundException)) {exceptionToReport = exceptionToReport.getCause();
         }
         exceptionToReport = (exceptionToReport != null) ? exceptionToReport : ex;
         // NOTE: We can't use the logger here to report the problem
         System.err.println("Logging system failed to initialize using configuration from '" + logConfig + "'");
         exceptionToReport.printStackTrace(System.err);
         throw new IllegalStateException(ex);
      }
   }
}

正常情况下、我们都没有配置 logConfig 进入到下面的方法

@Override
	public void initialize(LoggingInitializationContext initializationContext, String configLocation, LogFile logFile) {LoggerContext loggerContext = getLoggerContext();
		if (isAlreadyInitialized(loggerContext)) {	return;
		}
		super.initialize(initializationContext, configLocation, logFile);
		loggerContext.getTurboFilterList().remove(FILTER);
		markAsInitialized(loggerContext);
		if (StringUtils.hasText(System.getProperty(CONFIGURATION_FILE_PROPERTY))) {	getLogger(LogbackLoggingSystem.class.getName()).warn("Ignoring '" + CONFIGURATION_FILE_PROPERTY
					+ "' system property. Please use 'logging.config' instead.");
		}
	}

进入到父类的 initialize 方法中

@Override
public void initialize(LoggingInitializationContext initializationContext, String configLocation, LogFile logFile) {if (StringUtils.hasLength(configLocation)) {  initializeWithSpecificConfig(initializationContext, configLocation, logFile);
      return;
   }
   initializeWithConventions(initializationContext, logFile);
}

private void initializeWithConventions(LoggingInitializationContext initializationContext, LogFile logFile) {String config = getSelfInitializationConfig();
   if (config != null && logFile == null) {  // self initialization has occurred, reinitialize in case of property changes
      reinitialize(initializationContext);
      return;
   }
   if (config == null) {  config = getSpringInitializationConfig();
   }
   if (config != null) {  loadConfiguration(initializationContext, config, logFile);
      return;
   }
   loadDefaults(initializationContext, logFile);
}

正常建议使用的是 xxx-spring.xml 的配置文件、那么我们进入到 loadConfiguration 中

@Override
protected void loadConfiguration(LoggingInitializationContext initializationContext, String location,
      LogFile logFile) {super.loadConfiguration(initializationContext, location, logFile);
   LoggerContext loggerContext = getLoggerContext();
   stopAndReset(loggerContext);
   try {  configureByResourceUrl(initializationContext, loggerContext, ResourceUtils.getURL(location));
   }
   catch (Exception ex) {  throw new IllegalStateException("Could not initialize Logback logging from " + location, ex);
   }
   Liststatuses = loggerContext.getStatusManager().getCopyOfStatusList();
   StringBuilder errors = new StringBuilder();
   for (Status status : statuses) {  if (status.getLevel() == Status.ERROR) { errors.append((errors.length() >0) ? String.format("%n") : "");
         errors.append(status.toString());
      }
   }
   if (errors.length() >0) {  throw new IllegalStateException(String.format("Logback configuration error detected: %n%s", errors));
   }
}

private void configureByResourceUrl(LoggingInitializationContext initializationContext, LoggerContext loggerContext,
      URL url) throws JoranException {if (url.toString().endsWith("xml")) {  JoranConfigurator configurator = new SpringBootJoranConfigurator(initializationContext);
      configurator.setContext(loggerContext);
      configurator.doConfigure(url);
   }
   else {  new ContextInitializer(loggerContext).configureByResource(url);
   }

我们发现了一个 SpringBootJoranConfigurator 这个配置一看就是 Spring Boot 相关的

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增加了一些 Spring Boot 的规则、非常明显、用于从 environment 中获取属性填充到 logback 配置文件中

一起看下 logback-spring.xml 文件就能感受到了

logs/${logName}.logtruelogs/${logName}-%d{yyyy-MM-dd}.%i.log.zip100MB73GB[%date{yyyy-MM-dd HH:mm:ss}] [%-5level] [%logger:%line] --%mdc{client} %msg%nDEBUG[ %-5level] [%date{yyyy-MM-dd HH:mm:ss}] %logger{96} [%line] - %msg%n
 UTF-8DEBUG

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