How to use custom SpEL functions in @value with Spring Boot

App.java

package demo;

import static java.lang.reflect.Modifier.*;

import java.util.Arrays;
import java.util.Set;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.beans.factory.config.BeanExpressionResolver;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.context.expression.StandardBeanExpressionResolver;
import org.springframework.expression.spel.support.StandardEvaluationContext;
import org.springframework.util.ReflectionUtils;
import org.springframework.util.StringUtils;

@SpringBootApplication
public class App {

	public static void main(String[] args) {

		System.setProperty("someProperty", "a,b,c,a");

		BusinessComponent bc = SpringApplication.run(App.class, args).getBean(BusinessComponent.class);

		System.out.println(bc);
	}

	static class BusinessComponent {

		// Here we use our custom spel function
		@Value("#{#csvToSet(systemProperties['someProperty'])}")//
		Set<String> values;

		@Override
		public String toString() {
			return "BusinessComponent [values=" + values + "]";
		}
	}

	@Bean
	CustomSpelFunctionProvider customSpelFunctionProvider() {
		return new CustomSpelFunctionProvider(CustomSpelFunctions.class);
	}

	@Bean
	BusinessComponent businessComponent() {
		return new BusinessComponent();
	}

	public interface CustomSpelFunctions {

		// our custom SpEL function
		static Set<String> csvToSet(String line) {
			return StringUtils.commaDelimitedListToSet(line);
		}
	}

	/**
	 * A custom {@link BeanFactoryPostProcessor} that allows to register custom SpEL functions into the evaluation
	 * context.
	 * <p>
	 * We customize the {@link BeanExpressionResolver} used by the {@link ConfigurableListableBeanFactory} for the custom
	 * SpEL function registration.
	 */
	static class CustomSpelFunctionProvider implements BeanFactoryPostProcessor {

		private final Class<?>[] functionHolders;

		public CustomSpelFunctionProvider(Class<?>... functionHolders) {
			this.functionHolders = functionHolders;
		}

		@Override
		public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {

			beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver() {

				@Override
				protected void customizeEvaluationContext(StandardEvaluationContext evalContext) {

					Arrays.stream(functionHolders).forEach(cls -> ReflectionUtils.doWithMethods(cls, m -> {
						ReflectionUtils.makeAccessible(m);
						evalContext.registerFunction(m.getName(), m);
					}, m -> isPublic(m.getModifiers()) && isStatic(m.getModifiers())));
				}

			});
		}
	}
}