Placeholder Providers

It's responsible for getting a value using a specific 'variable' in messages For example, if we set {receiver_name} in a message, we can replace it with the receiver name using a PlaceholderProvider.

In that variable, the parts are identifier (receiver) and the parameters (name), they are delimited by _. And we resolve that implementing a PlaceholderProvider

Definition

PlaceholderProvider is

public interface PlaceholderProvider<E> {
    
  @Nullable
  Object replace(ContextRepository ctx, E entity, String parameters);
  
}

where 'E' is the used entity type. In the above example, we can use a named class that can receive a message, for example, an User.

Implementing

Implementing the placeholder provider for the above example:

public class UserPlaceholderProvider implements PlaceholderProvider<User> {

    @Override
    @Nullable
    public Object replace(ContextRepository ctx, User entity, String param) {
        switch (param.toLowerCase()) {
            case "name": {
                return entity.getName();
            }
            case "age": {
                return entity.getAge();
            }
            case "another_property": {
                return entity.getAnotherProperty();
            }
            default: return null; // null indicates an invalid parameter
        }
    }

}

The method return type is Object for convenience. You can return numbers, strings, booleans and they are converted to a string using Object#toString. Null indicates that an invalid parameter was provided and the placeholder isn't replaced.

Registration

MessageHandler handler = MessageHandler.of(
    source,
    config -> {
      config.specify(User.class)
            .addProvider("user", new UserPlaceholderProvider())
            .addProvider("idk", new SomeOtherProvider());
    }
);