// org.springframework.boot.logging.LoggingApplicationListener
private void onApplicationStartedEvent(ApplicationStartedEvent event) {
    this.loggingSystem = LoggingSystem
            .get(event.getSpringApplication().getClassLoader());
    this.loggingSystem.beforeInitialize();
}

// org.springframework.boot.logging.LoggingSystem
public static LoggingSystem get(ClassLoader classLoader) {
    String loggingSystem = System.getProperty(SYSTEM_PROPERTY);// null
    if (StringUtils.hasLength(loggingSystem)) {
        if (NONE.equals(loggingSystem)) {
            return new NoOpLoggingSystem();
        }
        return get(classLoader, loggingSystem);
    }
    for (Map.Entry entry : SYSTEMS.entrySet()) {
        if (ClassUtils.isPresent(entry.getKey(), classLoader)) {
            return get(classLoader, entry.getValue());
        }
    }
    throw new IllegalStateException("No suitable logging system located");
}


/**
 * A System property that can be used to indicate the {@link LoggingSystem} to use.
 */
public static final String SYSTEM_PROPERTY = LoggingSystem.class.getName();

private static final Map SYSTEMS;

static {
    Map 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);
}

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

    /**
 * Initialize the logging system according to preferences expressed through the
 * {@link Environment} and the classpath.
 * @param environment the environment
 * @param classLoader the classloader
 */
protected void initialize(ConfigurableEnvironment environment,
        ClassLoader classLoader) {
    new LoggingSystemProperties(environment).apply();
    LogFile logFile = LogFile.get(environment);
    if (logFile != null) {
        logFile.applyToSystemProperties();
    }
    initializeEarlyLoggingLevel(environment);
    initializeSystem(environment, this.loggingSystem, logFile);
    initializeFinalLoggingLevels(environment, this.loggingSystem);
    registerShutdownHookIfNecessary(environment, this.loggingSystem);
}

private void initializeWithConventions(
        LoggingInitializationContext initializationContext, LogFile logFile) {
        // 获取logback自己支持的配置文件
    String config = getSelfInitializationConfig();
    if (config != null && logFile == null) {
        // self initialization has occurred, reinitialize in case of property changes
        reinitialize(initializationContext);
        return;
    }
    // 如果没有则检测spring相关的logback配置
    if (config == null) {
        config = getSpringInitializationConfig();
    }
    if (config != null) {
        loadConfiguration(initializationContext, config, logFile);
        return;
    }
    loadDefaults(initializationContext, logFile);
}

// org.springframework.boot.logging.logback.LogbackLoggingSystem
@Override
protected String[] getStandardConfigLocations() {
    return new String[] { "logback-test.groovy", "logback-test.xml", "logback.groovy",
            "logback.xml" };
}

/**
 * Return the spring config locations for this system. By default this method returns
 * a set of locations based on {@link #getStandardConfigLocations()}.
 * @return the spring config locations
 * @see #getSpringInitializationConfig()
 */
protected String[] getSpringConfigLocations() {
    String[] locations = getStandardConfigLocations();
    for (int i = 0; i ) {
        String extension = StringUtils.getFilenameExtension(locations[i]);
        locations[i] = locations[i].substring(0,
                locations[i].length() - extension.length() - 1) + "-spring."
                + extension;
    }
    return locations;
}