How to replace Spring boot's default logback config from a library? (see https://stackoverflow.com/questions/48539217/how-to-replace-spring-boots-default-logback-config-from-a-library)

SlogApplicationListener.java

import org.springframework.boot.SpringApplication;
import org.springframework.boot.context.event.ApplicationEnvironmentPreparedEvent;
import org.springframework.boot.logging.LoggingApplicationListener;
import org.springframework.boot.logging.LoggingSystem;
import org.springframework.boot.logging.logback.LogbackLoggingSystem;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.event.GenericApplicationListener;
import org.springframework.core.ResolvableType;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.MapPropertySource;
import org.springframework.core.env.MutablePropertySources;
import org.springframework.util.ReflectionUtils;
import org.springframework.util.StringUtils;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.Collections;

import static org.springframework.boot.logging.LoggingApplicationListener.CONFIG_PROPERTY;

/**
 * Load {@literal xxx-slog-logback.xml} using {@literal logging.config} if {@literal logging.config} is not set and no
 * logback configuration file is found.
 * <p>
 * Unfortunately, Spring does not allow a "starter" to ship a logback configuration file that would be loaded by default
 * if none is provided by the application. By running this listener just before {@link LoggingApplicationListener}, we
 * are able to inspect the logback configuration and use {@literal logging.config} to pass our own if none is detected.
 * <p>
 * This is hacky, but does the job. Getting ride of this listener would mean that we cannot ship our default
 * configuration with the starter and require from our user to create a {@literal logback.xml} file (which would include
 * our configuration). Doable but not a great developer experience.
 */
public class SLogApplicationListener implements GenericApplicationListener {

    private static final Class<?>[] EVENT_TYPES = {ApplicationEnvironmentPreparedEvent.class};

    private static final Class<?>[] SOURCE_TYPES = {SpringApplication.class, ApplicationContext.class};

    @Override
    public int getOrder() {
        return LoggingApplicationListener.DEFAULT_ORDER - 1;
    }

    @Override
    public boolean supportsEventType(ResolvableType eventType) {
        return isAssignableFrom(eventType.getRawClass(), EVENT_TYPES);
    }

    @Override
    public boolean supportsSourceType(Class<?> sourceType) {
        return isAssignableFrom(sourceType, SOURCE_TYPES);
    }

    private boolean isAssignableFrom(Class<?> type, Class<?>... supportedTypes) {
        return Arrays.stream(supportedTypes).anyMatch(supportedType -> supportedType.isAssignableFrom(type));
    }

    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof ApplicationEnvironmentPreparedEvent) {
            ApplicationEnvironmentPreparedEvent aepEvent = (ApplicationEnvironmentPreparedEvent) event;
            overrideLogbackConfig(aepEvent);
        }
    }

    private void overrideLogbackConfig(ApplicationEnvironmentPreparedEvent event) {
        ConfigurableEnvironment env = event.getEnvironment();
        String logConfig = env.getProperty(CONFIG_PROPERTY);

        if (StringUtils.hasLength(logConfig)) {
            return; // Never override user supplied logging.config
        }

        // Must be super careful to not initialize the LoggingSystem (ctor does nothing)
        ClassLoader classLoader = event.getSpringApplication().getClassLoader();
        LogbackLoggingSystem logbackLoggingSystem = getLogbackLoggingSystem(classLoader);

        if (logbackLoggingSystem == null) {
            return; // Logback is not active, don't touch anything
        }

        if (isLogbackConfPresent(logbackLoggingSystem)) {
            return; // A config file at well known location has been found, don't touch anything
        }

        setConfig(env.getPropertySources());
    }

    private LogbackLoggingSystem getLogbackLoggingSystem(ClassLoader classLoader) {
        LoggingSystem loggingSystem = LoggingSystem.get(classLoader);

        if (loggingSystem instanceof LogbackLoggingSystem) {
            return (LogbackLoggingSystem) loggingSystem;
        }

        return null;
    }

    private boolean isLogbackConfPresent(LogbackLoggingSystem loggingSystem) {
        Method method = ReflectionUtils.findMethod(LogbackLoggingSystem.class, "getSpringInitializationConfig");
        method.setAccessible(true);
        try {
            String ret = (String) method.invoke(loggingSystem);
            return ret != null;
        } catch (IllegalAccessException | InvocationTargetException e) {
            return false;
        }
    }

    private void setConfig(MutablePropertySources sources) {
        MapPropertySource mapPropertySource = new MapPropertySource(
                "xxx-slog-logging-config",
                Collections.singletonMap(CONFIG_PROPERTY, "classpath:xxx-slog-logback.xml"));

        sources.addLast(mapPropertySource);
    }
}