Monday, September 17, 2007

Trinidad versus Tomahawk component architecture

My blog has moved, please update your links. You can find this article here

I have been using Tomahawk quite a while and building several custom components with it. Relatively recently, I have incorporated Trinidad in a new project I am working on due to its AJAX support and large number of components.

Yesterday, I attempted to incorporate maven-faces-plugin into my project from the Trinidad code base. After trying it out, I have decided to back it out of my project, partially due to the amount of work, and mostly due to the architecture of Trinidad components.

Why is Tomahawk a better component development platform?

Yes, a bold statement, but I wanted to get your attention. Trinidad has a great offering, a lot of components, AJAX support and a good set of APIs, but there may be a large architectural flaw. This flaw is the architecture of Components and renderers around the FacesBean.


What I found is that Tomahawk follow an OO paradigm, but Trinidad breaks OOD standards. One of the major aspects of effective OOD is encapsulation. This basically means that the functionality an Object provides comes from the component itself. Tomahawk has a special map behind the getAttributes() method of each component. This map delegates the attributes to the property getter and setter methods of the component. Take for example:

// MyComponent comp; String value = comp.getMyProperty(); value = comp.getAttributes().get("myProperty");

These two lines of code are identical in terms of functionality, they both call "getMyProperty()" on MyComponent. This means that component developers know that the getter and setter methods of their component will be called, and they have the control to provide any functionality that is needed besides just storing and retrieving a value (they can manipulate values, check for illegal arguments, etc.). This illustrates a good comprehension of encapsulation, that the functions of MyComponent provide the functionality of MyComponent.

Trinidad works in a completely different way. Each UIXComponent has a FacesBean instance that is tied to a static Type that dictates what values are stored during state processing of the component. The getter and setter methods of the components retrieve and store values into this FacesBean. The break in encapsulation is that this FacesBean is made public. That means that people can access the FacesBean and get values without a reference to the component. Encapsulation is lost, the component no longer has any control over the functionality for its own properties. To make matters worse, all of the Trinidad JSP Tag classes and the Trinidad renders use this FacesBean directly, taking the component completely out of the picture.

Code generation

There were other issues that I had with the maven-faces-plugin. The biggest, is that this plugin drastically affects your project layout and architecture. Unlike the Tomahawk code generator which injects code into a component's source, Trinidad actually generates the entire component. This means that the Trinidad components have to be their own maven project. Furthermore, there is a two step process which requires a third project (i.e. trinidad-build, trinidad-api and trinidad-impl). After refactoring most of my code into this layout, the plugin still didn't work. It wasn't generating the taglib.xml files. Unfortunately the plugin is not documented


My conclusion from my research is to not use maven-faces-plugin for my project. For my custom components that extend Trinidad components, I am forced to use the FacesBean architecture or else risk breaking the Trinidad renderers, or at least not being compliant with the design.

To the Trinidad team:

Adam and the Trinidad team, what do you feel about inversing the FacesBean? What I mean by this, is that currently the get/set component methods store the values into the FacesBean. What about taking the design of the Tomahawk components and having the FacesBean call the get/set methods on the component to get their values and have the get/set methods of the component actually do the work? Since the code is generated, the cascade of the change shouldn't be too bad. Only people that have extended these components without using maven-faces-plugin would be affected.

Since this would break backwards-compatibility, Trinidad's major version could be bumped.

Updates and responses to comments



Yes, I know that the performance will be better and the generated code smaller, but is that a good reason for breaking standard OOD? With this model, the component's properties are reduced to the security and functionality of public fields.

I am a very strong believer in usage of OOD for Java objects and the current design forgoes the UIComponent design by having the renderers bypass the component and the component attributes by delegating component properties to the FacesBean.

Is the efficiency of FacesBean worth the cost of bypassing encapsulation and reducing a component's methods to not much more than a Map?

JDK 1.5 has made reflection much faster, and if the Method references were cached, it would be a much smaller performance hit. Take EJB3 for example, it uses reflection plus annotations to get and set values for database access. Even though there is a performance hit, the API remains viable for production use.

Instead of using FacesBean, renderers could use the attribute map instead, this would ensure that renderers remain component agnostic. For example:

public void encodeBegin(FacesContext facesContext, UIComponent comp) { Map attrs = comp.getAttributes(); Renderer delegate = getMyRendererDelegate(); delegate.renderStuff(facesContext, attrs); }


public void encodeBegin(FacesContext facesContext, UIComponent comp) { Map attrs = comp.getAttributes(); String styleClass = toString(attrs.get("styleClass")); } protected String toString(Object obj) { return obj == null ? null : obj.toString(); }

In these crude examples, the renderer doesn't care what the component is, only that the component may (or may not) provide a specific attributes. In the first example, the delegation renderer can access the attributes to get a styleClass for example. The second example shows a class accessing a property via the attributes

With this paradigm, a renderer developer may choose to access the component directly (cast the component to one it supports) or use the attributes. If it uses the attributes, it choose to be able to support behaviors rather than types. In this way, renderers that are used as delegates could simply rely on the attribute map.

In terms of performance, the attribute may could cache, in a transient variable, the Method to call for the get and/or set property accessors. The component could choose to use FacesBean, or just store the value in a member variable. I know there is some performance overhead, but this is a side effect of a good encapsulation with the flexibility that you desire from the loose-binding a.k.a. late binding you desire for renderers.



The reason FacesBean violates OOD is that it delegates the behavior of the UIComponent to the FacesBean. The UIComponent no longer has any control over its own properties and is no longer aware of external accesses to its own data. Also, the way that an object chooses to serialize its data (saveState, restoreState behaviors in JSF), is an internal, not external function.

With the FacesBean being a public object, a piece of code can easily change the value of a property to something illegal, and the component has no opportunity to validate the new value.

Lastly, the component has no control over removing attributes. With proper use of encapsulation, a component could remove an attribute, maintaining the get/set properties as deprecated but store the value elsewhere. With the FacesBean, the component now is permanently bound to the data that it serializes, as people now code against the PropertyKey names instead of the exposed properties. The way that the FacesBean architecture is made, the private fields of UIXComponent have been made public.



Good point on the sub-classing FacesBean, perhaps I should give that option more thought. If I have the incentive and time, I may decide to create a new blog entry on customizing the FacesBean to create validation methods, the ability to create access methods (handle set/get) as a proof of concept.