Using Spring IoC to set up enum values

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<Class<? extends Enum>> enumClasses = new ArrayList<>();

public EnumAutowiringBeanFactoryPostProcessor(Class<? extends Enum>... enumClasses) {
Collections.addAll(this.enumClasses, enumClasses);
}

@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
for (Class<? extends Enum> 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;
}
}

}

I have done it in the following way:

@Component
public class MessageSourceHelper {
@Inject
public MessageSource injectedMessageSource;

public static MessageSource messageSource;

public static String getMessage(String messageKey, Object[] arguments, Locale locale) {
return messageSource.getMessage(messageKey, arguments, locale);
}

@PostConstruct
public void postConstruct() {
messageSource = injectedMessageSource;
}
}

That way you can easily use it in the enum to get messages in the following way:

MessageSourceHelper.getMessage(key, arguments, locale);

I have faced the same issue when I was working to localize my enum label in different locales.

**Enum Code:**

public enum Type {
SINGLE("type.single_entry"),
MULTIPLE("type.multiple_entry"),
String label;

Type(String label) {
this.label = label;
}

public String getLabel() {
String translatedString = I18NTranslator.getI18NValue(getLocale(), label);
return StringUtils.isEmpty(translatedString) ? label : translatedString;
}
}

My I18NTranslator class which basically load the message source to get localized content. `I18Ntransalator` class depends on `springContext` if you don't write you might face a peculiar bug. Some time might face a dependency related which causes null pointer exception. I had put a lot of effort to resolve this issue.

@Component
@DependsOn({"springContext"})
public class I18NTranslator {

private static MessageSource i18nMessageSource;

public static String getI18NValue(Locale locale, String key) {
if (i18nMessageSource != null)
return i18nMessageSource.getMessage(key, null, locale);
return key;
}

@PostConstruct
public void initialize() {
i18nMessageSource = SpringContext.getBean("i18nMessageSource", MessageSource.class);
}
}

We have to set the spring context

@Component
@Slf4j
public class SpringContext implements ApplicationContextAware {

private static ApplicationContext context;

public static <T extends Object> T getBean(Class<T> beanClass) {
return context.getBean(beanClass);
}

public static <T extends Object> T getBean(String beanClassName, Class<T> beanClass) {
return context.getBean(beanClassName, beanClass);
}

@Override
public void setApplicationContext(ApplicationContext context) throws BeansException {
SpringContext.context = context;
}
}

Now it is time to define the bean for I18NMessageSource.

@Configuration
public class LocaleConfiguration implements WebMvcConfigurer {

@Bean(name = "i18nMessageSource")
public MessageSource getMessageResource() {
ReloadableResourceBundleMessageSource messageResource = new ReloadableResourceBundleMessageSource();
messageResource.setBasename("classpath:i18n/messages");
messageResource.setCacheSeconds(3600);
messageResource.setDefaultEncoding("UTF-8");
return messageResource;
}

@Bean(name = "localeResolver")
public LocaleResolver getLocaleResolver() {
return new UrlLocaleResolver();
}

@Override
public void addInterceptors(InterceptorRegistry registry) {
//UrlLocalInterceptor is custom locale resolver based on header paramter.
UrlLocaleInterceptor localeInterceptor = new UrlLocaleInterceptor();
registry.addInterceptor(localeInterceptor);
}
}

PS: if you need the custom interceptor code I can share in the comment.
Defines all local properties files inside resources/i18n folder with messages prefix like `messages_en.properties` for english and `messages_fr.properties` fro french.