原来 SpringBoot 3.x 日志优化实战是这样做滴！

@Override
public void initialize(LoggingInitializationContext initializationContext,
		String configLocation, LogFile logFile) {
	// 如传递了日志配置文件，则使用指定文件
	if (StringUtils.hasLength(configLocation)) {
		initializeWithSpecificConfig(initializationContext, configLocation, logFile);
		return;
	}
	// 没传递日志配置文件，使用约定方式
	initializeWithConventions(initializationContext, logFile);
}

private void initializeWithSpecificConfig(
		LoggingInitializationContext initializationContext, String configLocation,
		LogFile logFile) {
	// 处理日志配置文件中的占位符
	configLocation = SystemPropertyUtils.resolvePlaceholders(configLocation);
	loadConfiguration(initializationContext, configLocation, logFile);
}

private void initializeWithConventions(
		LoggingInitializationContext initializationContext, LogFile logFile) {
	// 获取自初始化的日志配置文件，该方法会使用getStandardConfigLocations抽象方法得到的文件数组
	// 然后进行遍历，如果文件存在，返回对应的文件目录。注意这里的文件指的是classpath下的文件
	String config = getSelfInitializationConfig();
	// 如果找到对应的日志配置文件并且logFile为null(logFile为null表示只有console会输出)
	if (config != null && logFile == null) {
		// 调用reinitialize方法重新初始化
		// 默认的reinitialize方法不做任何处理，logback,log4j和log4j2覆盖了这个方法，会进行处理
		reinitialize(initializationContext);
		return;
	}
	// 如果没有找到对应的日志配置文件
	if (config == null) {
		// 获取日志配置文件
		// 该方法与getSelfInitializationConfig方法的区别在于getStandardConfigLocations方法得到的文件数组内部遍历的逻辑
		// getSelfInitializationConfig方法直接遍历并判断classpath下是否存在对应的文件
		// getSpringInitializationConfig方法遍历后判断的文件名会在后缀前加上 "-spring" 字符串
		// 比如查找logback.xml文件，getSelfInitializationConfig会直接查找classpath下是否存在logback.xml文件，而getSpringInitializationConfig方法会判断classpath下是否存在logback-spring.xml文件
		config = getSpringInitializationConfig();
	}
	// 如找到对应日志配置文件
	if (config != null) {
		// 调用抽象方法，子类实现
		loadConfiguration(initializationContext, config, logFile);
		return;
	}
	// 还没找到日志配置文件，调用抽象方法加载
	loadDefaults(initializationContext, logFile);
}

protected abstract String[] getStandardConfigLocations();

protected abstract void loadConfiguration(
		LoggingInitializationContext initializationContext, String location,
		LogFile logFile);

protected abstract void loadDefaults(
		LoggingInitializationContext initializationContext, LogFile logFile);

@Override
protected String[] getStandardConfigLocations() {
	return new String[] { "logback-test.groovy", "logback-test.xml", "logback.groovy",
			"logback.xml" };
}

@Override
protected void loadConfiguration(LoggingInitializationContext initializationContext,
		String location, LogFile logFile) {
	// 调用父类Slf4JLoggingSystem的方法
	super.loadConfiguration(initializationContext, location, logFile);
	// 获取slf4j内部的LoggerContext
	LoggerContext loggerContext = getLoggerContext();
	// logback环境的一些配置配置处理
	stopAndReset(loggerContext);
	try {
		configureByResourceUrl(initializationContext, loggerContext,
				ResourceUtils.getURL(location));
	}
	catch (Exception ex) {
		throw new IllegalStateException(
				"Could not initialize Logback logging from " + location, ex);
	}
	List<Status> statuses = loggerContext.getStatusManager().getCopyOfStatusList();
	StringBuilder errors = new StringBuilder();
	for (Status status : statuses) {
		if (status.getLevel() == Status.ERROR) {
			errors.append(errors.length() > 0 ? "\n" : "");
			errors.append(status.toString());
		}
	}
	if (errors.length() > 0) {
		throw new IllegalStateException(
				"Logback configuration error " + "detected: \n" + errors);
	}
}

// 没找到日志配置文件的话使用loadDefaults方法加载
@Override
protected void loadDefaults(LoggingInitializationContext initializationContext,
		LogFile logFile) {
	// 获取slf4j内部的LoggerContext
	LoggerContext context = getLoggerContext();
	stopAndReset(context);
	LogbackConfigurator configurator = new LogbackConfigurator(context);
	context.putProperty("LOG_LEVEL_PATTERN",
			initializationContext.getEnvironment().resolvePlaceholders(
					"${logging.pattern.level:${LOG_LEVEL_PATTERN:%5p}}"));
	// 构造默认的console Appender。如果logFile不为空，还会构造file Appender
	new DefaultLogbackConfiguration(initializationContext, logFile)
			.apply(configurator);
	context.setPackagingDataEnabled(true);
}

// logback的清除工作
@Override
public void cleanUp() {
	super.cleanUp();
	getLoggerContext().getStatusManager().clear();
}

// 动态设置logger的level
@Override
public void setLogLevel(String loggerName, LogLevel level) {
	getLogger(loggerName).setLevel(LEVELS.get(level));
}

// 清除后的一些工作
// ShutdownHandler会调用LoggerContext的stop方法
@Override
public Runnable getShutdownHandler() {
	return new ShutdownHandler();
}

@SuppressWarnings({ "unchecked", "rawtypes" })
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
  this.resourceLoader = resourceLoader;
  Assert.notNull(primarySources, "PrimarySources must not be null");
  this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
  this.webApplicationType = deduceWebApplicationType();
  setInitializers((Collection) getSpringFactoriesInstances(
      ApplicationContextInitializer.class));
  // 使用工厂加载机制找到这些Listener
  setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
  this.mainApplicationClass = deduceMainApplicationClass();
}

@Override
public void onApplicationEvent(ApplicationEvent event) {
	if (event instanceof ApplicationStartedEvent) {
		// 先得到LoggingSystem，再调用beforeInitialize
		onApplicationStartedEvent((ApplicationStartedEvent) event);
	}
  ...
}

private void onApplicationStartedEvent(ApplicationStartedEvent event) {
	// get会从下面那段static代码块得到Map中遍历
	// 如对应的key(key是某个类的全名)在classloader中存在，构造该key对应的value对应的LoggingSystem
	this.loggingSystem = LoggingSystem
			.get(event.getSpringApplication().getClassLoader());
	this.loggingSystem.beforeInitialize();
}


static {
  Map<String, String> systems = new LinkedHashMap<>();
  systems.put("ch.qos.logback.core.Appender",
      "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);
}

logging:
  file: /tmp/temp.log

logging:
  path: /tmp

@Override
public String toString() {
	// 如果配置了logging.file，直接使用该文件
	if (StringUtils.hasLength(this.file)) {
		return this.file;
	}
	// 否则使用logging.path目录，在该目录下创建spring.log日志文件
	String path = this.path;
	if (!path.endsWith("/")) {
		path = path + "/";
	}
	return StringUtils.applyRelativePath(path, "spring.log");
}

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <!--<pattern>%d{YYYY-MM-dd HH:mm:ss.SSS} [%thread] %-5level %logger[%line] &#45;&#45; %msg%n</pattern>-->
            <pattern>%d{YYYY-MM-dd} [%thread] %-5level %logger[%line] -- %msg%n</pattern>
        </encoder>
    </appender>

    <root level="INFO">
        <appender-ref ref="STDOUT"/>
    </root>
</configuration>

logging:
  config: classpath:my-logback.xml

public static LoggingSystem get(ClassLoader classLoader) {
	// SYSTEM_PROPERTY静态变量是LoggingSystem的类全名
	String loggingSystem = System.getProperty(SYSTEM_PROPERTY);
	if (StringUtils.hasLength(loggingSystem)) {
		if (NONE.equals(loggingSystem)) { // None静态变量是值是none
			return new NoOpLoggingSystem();
		}
		return get(classLoader, loggingSystem);
	}
	for (Map.Entry<String, String> entry : SYSTEMS.entrySet()) {
		if (ClassUtils.isPresent(entry.getKey(), classLoader)) {
			return get(classLoader, entry.getValue());
		}
	}
	throw new IllegalStateException("No suitable logging system located");
}

-Dorg.springframework.boot.logging.LoggingSystem=none