code: public void setMessageResolver(final IMessageResolver messageResolver) {
          Validate.notNull(messageResolver, "Message Resolver cannot be null");
          checkNotInitialized();
          this.messageResolvers.clear();
          this.messageResolvers.add(messageResolver);
      }
docs: <p>
  Sets a single message resolver for this template engine.
  </p>
  <p>
  Calling this method is equivalent to calling {@link #setMessageResolvers(Set)}
  passing a Set with only one message resolver.
  </p>
  <p>
  This operation can only be executed before processing templates for the first
  time. Once a template is processed, the template engine is considered to be
  <i>initialized</i>, and from then on any attempt to change its configuration
  will result in an exception.
  </p>
  
  @param messageResolver the message resolver to be set.