Using Spring IoC to set up enum values

Question

Is there a way to set up such enum values via Spring IoC at construction time?

What I would like to do is to inject, at class load time, values that are hard-coded i

Answer 1

OK, it's quite fiddly, but it CAN be done.

It's true that Spring cannot instantiate enums. But that's not a problem - Spring can also use factory methods.

This is the key component:

public class EnumAutowiringBeanFactoryPostProcessor implements BeanFactoryPostProcessor {

    private final List> enumClasses = new ArrayList<>();

    public EnumAutowiringBeanFactoryPostProcessor(Class... enumClasses) {
        Collections.addAll(this.enumClasses, enumClasses);
    }

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        for (Class enumClass : enumClasses) {
            for (Enum enumVal : enumClass.getEnumConstants()) {
                BeanDefinition def = new AnnotatedGenericBeanDefinition(enumClass);
                def.setBeanClassName(enumClass.getName());
                def.setFactoryMethodName("valueOf");
                def.getConstructorArgumentValues().addGenericArgumentValue(enumVal.name());
                ((BeanDefinitionRegistry) beanFactory).registerBeanDefinition(enumClass.getName() + "." + enumVal.name(), def);
            }
        }
    }
}

Then the following test class shows that it works:

@Test
public class AutowiringEnumTest {

    public void shouldAutowireEnum() {
        new AnnotationConfigApplicationContext(MyConig.class);

        assertEquals(AutowiredEnum.ONE.myClass.field, "fooBar");
        assertEquals(AutowiredEnum.TWO.myClass.field, "fooBar");
        assertEquals(AutowiredEnum.THREE.myClass.field, "fooBar");
    }

    @Configuration
    public static class MyConig {

        @Bean
        public MyClass myObject() {
            return new MyClass("fooBar");
        }

        @Bean
        public BeanFactoryPostProcessor postProcessor() {
            return new EnumAutowiringBeanFactoryPostProcessor(AutowiredEnum.class);
        }
    }

    public enum AutowiredEnum {
        ONE,
        TWO,
        THREE;

        @Resource
        private MyClass myClass;

    }

    public static class MyClass {

        private final String field;

        public MyClass(String field) {
            this.field = field;
        }
   }

}