從原始碼分析 SpringBoot 的 LoggingSystem → 它是如何繫結日誌元件的

青石路發表於2024-08-22

開心一刻

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成功給我逗笑了

前情回顧

SpringBoot2.7 霸王硬上弓 Logback1.3 → 不甜但解渴 實現了 spring-boot 2.x.xlogback 1.3.x 的整合,分兩步

  1. 關閉 Spring Boot 的 LoggingSystem
  2. 配置檔案用 logback.xml

從示例看,整合是成功的;但有些問題是沒有分析的,比如

  1. System.setProperty("org.springframework.boot.logging.LoggingSystem", "none") 是如何生效的
  2. Spring Boot 的 LoggingSystem 是如何與日誌元件繫結的
  3. Spring Boot 預設依賴 3 個日誌元件:logback、log4j、jul,為什麼預設啟用的是 logback,而非其它兩個?

基於如上 3 個問題,我們一起去翻一翻 Spring Boot 的原始碼;在看原始碼之前,我先帶大家回顧一些內容,方便下文的原始碼分析

  1. 設計模式之觀察者模式 → 事件機制的底層原理

    講了觀察者模式的實現,以及在 JDK 中的應用(JDK 事件模型)、Spring 中的應用(事件機制);大家可以重點看下 Spring 的那個案例,使用非常簡單,總結一句就是

    SpringBoot 啟動過程中傳送的事件,所有 ApplicationListener 都會收到(即 onApplicationEvent 方法會被呼叫)

  2. spring-boot-2.0.3啟動原始碼篇一 - SpringApplication構造方法

    大家不要通篇去讀,重點看 getSpringFactoriesInstances,與本文息息相關的歸納成一句

    查詢類路徑下全部的 META-INF/spring.factories 的檔案路徑,並載入所有 spring.factories 中的內容到 SpringFactoriesLoader 的 cache 中,然後從快取中獲取 ApplicationListener 型別的類並進行例項化

下文是基於 Spring Boot 預設情況下的原始碼分析,而非整合 logback 1.3.x 的原始碼分析,大家注意下

整合 logback 1.3.x 需要關閉 Spring Boot 的 LoggingSystem,那還分析個毛

原始碼分析

問題來了,從哪開始跟?我就不繞圈子了,從 LoggingApplicationListener 開始跟,首先它在 META-INF/spring.factories

spring.factories_LoggingApplicationListener

其次它實現了 ApplicationListener

LoggingApplicationListener類圖

那麼 Spring Boot 在啟動過程中會例項化 LoggingApplicationListener,Spring Boot 啟動過程中傳送的事件都會來到 LoggingApplicationListeneronApplicationEvent 方法

@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) {
		onContextClosedEvent((ContextClosedEvent) event);
	}
	else if (event instanceof ApplicationFailedEvent) {
		onApplicationFailedEvent();
	}
}

Spring Boot 啟動過程分不同的階段,在每個階段都會傳送對應階段的事件,LoggingApplicationListener 針對這些事件會有不同的處理,我們暫且只需要關注以下事件

ApplicationStartingEvent,對應的處理方法:onApplicationStartingEvent

ApplicationEnvironmentPreparedEvent,對應的處理方法:onApplicationEnvironmentPreparedEvent

ApplicationPreparedEvent,對應的處理方法:onApplicationPreparedEvent

onApplicationStartingEvent

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

方法很簡單,獲取日誌系統,然後呼叫其 beforeInitialize 方法,我們跟進 LoggingSystem.get

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;
}

打個斷點除錯下,你們就會發現 SYSTEM_PROPERTY 的值是 org.springframework.boot.logging.LoggingSystem

system_property

從系統屬性中獲取 org.springframework.boot.logging.LoggingSystem ,是不是和

System.setProperty("org.springframework.boot.logging.LoggingSystem", "none") 是如何生效的

對應上了?如果獲取的值是 none,直接返回 NoOpLoggingSystem 例項

/**
 * {@link LoggingSystem} that does nothing.
 */
static class NoOpLoggingSystem extends LoggingSystem {

	@Override
	public void beforeInitialize() {

	}

	@Override
	public void setLogLevel(String loggerName, LogLevel level) {

	}

	@Override
	public List<LoggerConfiguration> getLoggerConfigurations() {
		return Collections.emptyList();
	}

	@Override
	public LoggerConfiguration getLoggerConfiguration(String loggerName) {
		return null;
	}

}

全是空實現,相當於關閉了 Spring Boot 的 LoggingSystem;org.springframework.boot.logging.LoggingSystem 還可以設定成其他值,但需要有對應的實現。預設情況下 loggingSystemClassName 的值是 null ,會跳過 if 來到 SYSTEM_FACTORY.getLoggingSystem(classLoader);

@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;
}

這裡推薦用斷點除錯去跟原始碼,按 F7 之後會來到 LoggingSystemFactory#fromSpringFactories

/**
 * 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));
}

SpringFactoriesLoader.loadFactories 是不是很眼熟?(不眼熟的去看:spring-boot-2.0.3啟動原始碼篇一 - SpringApplication構造方法)此時它會做三件事

  1. 從 SpringFactoriesLoader#cache 中獲取 LoggingSystemFactory 型別的工廠類的類名列表

    spring.factories_LoggingSystemFactory

    之前已經載入到 SpringFactoriesLoader#cache 中,所以此時從快取中獲取;注意看三個實現類的順序,LogbackLoggingSystem.Factory 在最前面

  2. 例項化這些工廠類

  3. 對這些工廠類例項按 @Order 升序排序

    這三個工廠類的 @Order 值是一樣的,都是 @Order(Ordered.LOWEST_PRECEDENCE),所以順序不變,LogbackLoggingSystem.Factory 仍在最前面

    LoggingSystemFactory列表

回到 DelegatingLoggingSystemFactory#getLoggingSystem,對這些工廠類例項逐個遍歷,得到 LoggingSystem 立即返回,不再遍歷後面的工廠例項;第一個遍歷的的是 LogbackLoggingSystem.Factory,呼叫其 getLoggingSystem 方法

private static final boolean PRESENT = ClassUtils.isPresent("ch.qos.logback.classic.LoggerContext",
		Factory.class.getClassLoader());

@Override
public LoggingSystem getLoggingSystem(ClassLoader classLoader) {
	if (PRESENT) {
		return new LogbackLoggingSystem(classLoader);
	}
	return null;
}

ch.qos.logback.classic.LoggerContext 存在(即存在logback依賴),直接建立 LogbackLoggingSystem 例項並返回;至此 Spring Boot 的 LoggingSystem 確定將基於 logback,而非 log4j,也非 jul,問題

Spring Boot 的 LoggingSystem 是如何與日誌元件繫結的

Spring Boot 預設依賴 3 個日誌元件:logback、log4j、jul,為什麼預設啟用的是 logback,而非其它兩個?

是不是清楚了?LoggingSystem 確定為 LogbackLoggingSystem 後回到 LoggingApplicationListener#onApplicationStartingEvent 方法的第二行,即呼叫 LogbackLoggingSystem#beforeInitialize 方法

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

主要初始化 LoggerContext,跟進 getLoggerContext()

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;
}

StaticLoggerBinder 有沒有很熟悉?看下它的全類名:org.slf4j.impl.StaticLoggerBinder,在 logback-classic-1.2.12.jar 下 ,而 logback 1.3.x 沒有這個類

所以 spring-boot 2.x.x 預設不支援 logback 1.3.x

總結下,onApplicationStartingEvent 方法確定了日誌系統是 LogbackLoggingSystem

onApplicationEnvironmentPreparedEvent

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

很顯然 loggingSystem 不為 null,我們直接跟 initialize 方法

protected void initialize(ConfigurableEnvironment environment, ClassLoader classLoader) {
	getLoggingSystemProperties(environment).apply();
	this.logFile = LogFile.get(environment);
	if (this.logFile != null) {
		this.logFile.applyToSystemProperties();
	}
	// 日誌分組,暫不關注
	this.loggerGroups = new LoggerGroups(DEFAULT_GROUP_LOGGERS);
	// 設定早期日誌級別,主要debug和trace之間的抉擇
	initializeEarlyLoggingLevel(environment);
	// 初始化日誌系統
	initializeSystem(environment, this.loggingSystem, this.logFile);
	// 設定最終日誌級別
	initializeFinalLoggingLevels(environment, this.loggingSystem);
	registerShutdownHookIfNecessary(environment, this.loggingSystem);
}

我們暫時只關注 initializeSystem 方法

initializeSystempng

繼續往下跟,來到 LogbackLoggingSystem#initialize

LogbackLoggingSystem

繼續往下跟,來到 AbstractLoggingSystem#initialize

AbstractLoggingSystem

繼續往下跟,來到 AbstractLoggingSystem#initializeWithConventions

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);
}

其中 getSelfInitializationConfig() 就是從 classpath 下逐個尋找

logback-test.groovy, logback-test.xml, logback.groovy, logback.xml

這四個檔案,一旦找到則直接返回;因為找到了 logback.xml,所以來到第一個 if

initializeWithConventions-reinitialize

繼續跟進,來到 LogbackLoggingSystem#reinitialize

LogbackLoggingSystem

logback.xml 中的配置進行載入;至此,Spring Boot 的 LoggingSystem 與 Logback 的繫結就完成了,你們清楚了嗎?

我們重新回到 AbstractLoggingSystem#initializeWithConventions ,如果 classpath

logback-test.groovy, logback-test.xml, logback.groovy, logback.xml

這四個檔案都沒有,會來到 config = getSpringInitializationConfig();,逐步跟下去會來到 AbstractLoggingSystem#getSpringConfigLocations

protected String[] getSpringConfigLocations() {
	String[] locations = getStandardConfigLocations();
	for (int i = 0; i < locations.length; i++) {
		String extension = StringUtils.getFilenameExtension(locations[i]);
		locations[i] = locations[i].substring(0, locations[i].length() - extension.length() - 1) + "-spring."
				+ extension;
	}
	return locations;
}

這個方法大家都能看懂吧,locations 的值

logback-test.groovy, logback-test.xml, logback.groovy, logback.xml

逐個遍歷,然後進行拼接,最終得到

logback-test-spring.groovy, logback-test-spring.xml, logback-spring.groovy, logback-spring.xml

getSpringConfigLocations

同樣從 classpath 下逐個尋找,一旦找到直接返回;這也是為什麼我們的日誌配置檔案是 logback-spring.xml 也能生效的原因。我們可以給 Spring Boot 的日誌配置檔案排個優先順序

logback-test.groovy > logback-test.xml > logback.groovy > logback.xml > logback-test-spring.groovy > logback-test-spring.xml > logback-spring.groovy > logback-spring.xml

總結下,onApplicationEnvironmentPreparedEvent 完成了日誌系統的初始化(日誌配置檔案的載入)

onApplicationPreparedEvent

private void onApplicationPreparedEvent(ApplicationPreparedEvent event) {
	ConfigurableApplicationContext applicationContext = event.getApplicationContext();
	ConfigurableListableBeanFactory beanFactory = applicationContext.getBeanFactory();
	if (!beanFactory.containsBean(LOGGING_SYSTEM_BEAN_NAME)) {
		beanFactory.registerSingleton(LOGGING_SYSTEM_BEAN_NAME, this.loggingSystem);
	}
	if (this.logFile != null && !beanFactory.containsBean(LOG_FILE_BEAN_NAME)) {
		beanFactory.registerSingleton(LOG_FILE_BEAN_NAME, this.logFile);
	}
	if (this.loggerGroups != null && !beanFactory.containsBean(LOGGER_GROUPS_BEAN_NAME)) {
		beanFactory.registerSingleton(LOGGER_GROUPS_BEAN_NAME, this.loggerGroups);
	}
	if (!beanFactory.containsBean(LOGGING_LIFECYCLE_BEAN_NAME) && applicationContext.getParent() == null) {
		beanFactory.registerSingleton(LOGGING_LIFECYCLE_BEAN_NAME, new Lifecycle());
	}
}

程式碼不復雜,就是註冊了幾個 Bean 到 Spring 容器,其中的 loggingSystem 是我們暫時比較關注的,預設情況下其型別是:LogbackLoggingSystem

日誌列印

Spring Boot 的 LoggingSystem 完成與 Logback 的繫結後,它是如何使用然後列印日誌的呢?是不是也像

業務日誌列印樣例

這樣來使用的?那絕對不可能的!

絕對不可能

這麼使用的話,跟 Spring Boot 的 LoggingSystem 有雞毛的關係?我們來看下 Spring Boot 中日誌的使用,SpringApplication 179 行就用到了

SpringApplicaton179行

我們會發現 LogLogFactoryspring-jcl-5.3.31.jar 包下

spring-jcl_LogFactory

spring-jcl 類似 slf4j,也是一個日誌門面,本文不展開

跟進 LogFactory.getLog ,一路跟下去會來到 LogAdapter#createLog

public static Log createLog(String name) {
	switch (logApi) {
		case LOG4J:
			return Log4jAdapter.createLog(name);
		case SLF4J_LAL:
			return Slf4jAdapter.createLocationAwareLog(name);
		case SLF4J:
			return Slf4jAdapter.createLog(name);
		default:
			// Defensively use lazy-initializing adapter class here as well since the
			// java.logging module is not present by default on JDK 9. We are requiring
			// its presence if neither Log4j nor SLF4J is available; however, in the
			// case of Log4j or SLF4J, we are trying to prevent early initialization
			// of the JavaUtilLog adapter - e.g. by a JVM in debug mode - when eagerly
			// trying to parse the bytecode for all the cases of this switch clause.
			return JavaUtilAdapter.createLog(name);
	}
}

logApi 的值獲取如下

private static final String LOG4J_SPI = "org.apache.logging.log4j.spi.ExtendedLogger";

private static final String LOG4J_SLF4J_PROVIDER = "org.apache.logging.slf4j.SLF4JProvider";

private static final String SLF4J_SPI = "org.slf4j.spi.LocationAwareLogger";

private static final String SLF4J_API = "org.slf4j.Logger";

private static final LogApi logApi;

static {
	if (isPresent(LOG4J_SPI)) {
		if (isPresent(LOG4J_SLF4J_PROVIDER) && isPresent(SLF4J_SPI)) {
			// log4j-to-slf4j bridge -> we'll rather go with the SLF4J SPI;
			// however, we still prefer Log4j over the plain SLF4J API since
			// the latter does not have location awareness support.
			logApi = LogApi.SLF4J_LAL;
		}
		else {
			// Use Log4j 2.x directly, including location awareness support
			logApi = LogApi.LOG4J;
		}
	}
	else if (isPresent(SLF4J_SPI)) {
		// Full SLF4J SPI including location awareness support
		logApi = LogApi.SLF4J_LAL;
	}
	else if (isPresent(SLF4J_API)) {
		// Minimal SLF4J API without location awareness support
		logApi = LogApi.SLF4J;
	}
	else {
		// java.util.logging as default
		logApi = LogApi.JUL;
	}
}

根據優先順序逐個去類路徑下尋找類,找到了直接返回;Spring Boot 預設情況下用的是 SLF4J + Logback,所以 logApi 的值是 SLF4J_SPI,那麼 LogAdapter#createLog 的返回值的型別是 LogAdapter$Slf4jLocationAwareLog

LogAdapter$Slf4jLocationAwareLog

相當於完成了 spring-jclslf4j 的適配;這麼說來,Spring Boot 日誌還是走的 SLF4J + Logback ?跟 Spring Boot 的 LoggingSystem 有什麼關係呢?敬請期待下篇

未完待續

總結

  1. onApplicationStartingEvent

    確定日誌系統型別並建立對應的 LoggingSystem,預設情況下是 LogbackLoggingSystem

  2. onApplicationEnvironmentPreparedEvent

    完成日誌配置檔案的載入以及 LoggingSystem 的初始化

  3. Spring Boot 的日誌列印貌似與 LoggingSystem 沒有關係?下篇分析

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