@FunctionalInterface
public interface EnvironmentPostProcessor {

    /**
     * Post-process the given {@code environment}.
     * @param environment the environment to post-process
     * @param application the application to which the environment belongs
     */
    void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application);

}

/**
 * {@link SmartApplicationListener} used to trigger {@link EnvironmentPostProcessor
 * EnvironmentPostProcessors} registered in the {@code spring.factories} file.
 *
 * @author Phillip Webb
 * @since 2.4.0
 */
public class EnvironmentPostProcessorApplicationListener implements SmartApplicationListener, Ordered {
    /**
     * The default order for the processor.
     */
    public static final int DEFAULT_ORDER = Ordered.HIGHEST_PRECEDENCE + 10;
    private final DeferredLogs deferredLogs;
    private int order = DEFAULT_ORDER;
    private final EnvironmentPostProcessorsFactory postProcessorsFactory;
    /**
     * Create a new {@link EnvironmentPostProcessorApplicationListener} with
     * {@link EnvironmentPostProcessor} classes loaded via {@code spring.factories}.
     */
    public EnvironmentPostProcessorApplicationListener() {
        this(EnvironmentPostProcessorsFactory
                .fromSpringFactories(EnvironmentPostProcessorApplicationListener.class.getClassLoader()));
    }
    /**
     * Create a new {@link EnvironmentPostProcessorApplicationListener} with post
     * processors created by the given factory.
     * @param postProcessorsFactory the post processors factory
     */
    public EnvironmentPostProcessorApplicationListener(EnvironmentPostProcessorsFactory postProcessorsFactory) {
        this(postProcessorsFactory, new DeferredLogs());
    }
    EnvironmentPostProcessorApplicationListener(EnvironmentPostProcessorsFactory postProcessorsFactory,
            DeferredLogs deferredLogs) {
        this.postProcessorsFactory = postProcessorsFactory;
        this.deferredLogs = deferredLogs;
    }
    @Override
    public boolean supportsEventType(Class<? extends ApplicationEvent> eventType) {
        return ApplicationEnvironmentPreparedEvent.class.isAssignableFrom(eventType)
                || ApplicationPreparedEvent.class.isAssignableFrom(eventType)
                || ApplicationFailedEvent.class.isAssignableFrom(eventType);
    }
    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof ApplicationEnvironmentPreparedEvent) {
            onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
        }
        if (event instanceof ApplicationPreparedEvent) {
            onApplicationPreparedEvent((ApplicationPreparedEvent) event);
        }
        if (event instanceof ApplicationFailedEvent) {
            onApplicationFailedEvent((ApplicationFailedEvent) event);
        }
    }
    private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {
        ConfigurableEnvironment environment = event.getEnvironment();
        SpringApplication application = event.getSpringApplication();
        for (EnvironmentPostProcessor postProcessor : getEnvironmentPostProcessors(event.getBootstrapContext())) {
            postProcessor.postProcessEnvironment(environment, application);
        }
    }
    private void onApplicationPreparedEvent(ApplicationPreparedEvent event) {
        finish();
    }
    private void onApplicationFailedEvent(ApplicationFailedEvent event) {
        finish();
    }
    private void finish() {
        this.deferredLogs.switchOverAll();
    }
    List<EnvironmentPostProcessor> getEnvironmentPostProcessors(ConfigurableBootstrapContext bootstrapContext) {
        return this.postProcessorsFactory.getEnvironmentPostProcessors(this.deferredLogs, bootstrapContext);
    }
    @Override
    public int getOrder() {
        return this.order;
    }
    public void setOrder(int order) {
        this.order = order;
    }
}

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.env.EnvironmentPostProcessorApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

public class EnvironmentPostProcessorExample implements EnvironmentPostProcessor {
    private final YamlPropertySourceLoader loader = new YamlPropertySourceLoader();
    @Override
    public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
        Resource path = new ClassPathResource("com/example/myapp/config.yml");
        PropertySource<?> propertySource = loadYaml(path);
        environment.getPropertySources().addLast(propertySource);
    }
    private PropertySource<?> loadYaml(Resource path) {
        if (!path.exists()) {
            throw new IllegalArgumentException("Resource " + path + " does not exist");
        }
        try {
            return this.loader.load("custom-resource", path).get(0);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Failed to load yaml configuration from " + path, ex);
        }
    }
}