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The ChainBuilder Service is a built-in service used to implement of one of the most useful of the Gang Of Four design patterns, the chain of responsibility.

With the chain of responsibility design pattern, a complex process is broken down into many individual steps. Each step is a command (see command pattern). A key part of this is that the commands are expected to implement some common interface. The commands are also carefully arranged into a specific order.

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Because this pattern is used so often inside Tapestry, a built-in service exists to create implementations of the pattern as needed. The ChainBuilder service takes care of all the work:

public interface ChainBuilder
{
  <T> T build(Class<T> commandInterface, List<T> commands);
}

All that generics parameterization just ensures that the command interface matches the items in the list, and confirms that a single instance of the command interface will be returned.

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This can be used inside a service builder method. Nothing says a service builder method just has to instantiate a class; it is only required to return an appropriate object. We can just let the ChainBuilder service create that object.

  public static MyChainService build(List<MyChainService> commands,
    @InjectService("ChainBuilder")
    ChainBuilder chainBuilder)
  {
     return chainBuilder.build(MyChainService.class, commands);
  }

Here, the behavior of the MyChainService is defined by its configuration: an ordered list of MyChainService commands that are contributed by one or more modules.

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ChainBuilder will reuse the fabricated class for any number of chains of the same command interface.