How to access spring boot properties from freemarker template

Gonna answer myself :

Easiest way in spring-boot 1.3 is to overrides the FreeMarkerConfiguration class :

/**
 * Overrides the default spring-boot configuration to allow adding shared variables to the freemarker context
 */
@Configuration
public class FreemarkerConfiguration extends FreeMarkerAutoConfiguration.FreeMarkerWebConfiguration {

    @Value("${myProp}")
    private String myProp;

    @Override
    public FreeMarkerConfigurer freeMarkerConfigurer() {
        FreeMarkerConfigurer configurer = super.freeMarkerConfigurer();

        Map<String, Object> sharedVariables = new HashMap<>();
        sharedVariables.put("myProp", myProp);
        configurer.setFreemarkerVariables(sharedVariables);

        return configurer;
    }
}

One option in spring boot 2:

@Configuration
public class CustomFreeMarkerConfig implements BeanPostProcessor {

  @Value("${myProp}")
  private String myProp;

  @Override
  public Object postProcessBeforeInitialization(Object bean, String beanName)
  throws BeansException {
      if (bean instanceof FreeMarkerConfigurer) {
          FreeMarkerConfigurer configurer = (FreeMarkerConfigurer) bean;
          Map<String, Object> sharedVariables = new HashMap<>();
          sharedVariables.put("myProp", myProp);
          configurer.setFreemarkerVariables(sharedVariables);
      }
      return bean;
  }
}

Spring Boot 2.x changed class structure so it's no longer possible to subclass and keep the auto configuration like was possible with Spring Boot 1.x.