Richmond JUG April 21st

I went to my second Java User Group meeting last night. Kevin Smith gave a talk on Service-oriented Architecture (SOA) security. He talked about using interceptors and about access control based on services, requests, or individual records (or even individual pieces of data), and about loose coupling. And that's really what it all comes down to: loose coupling. All the buzz, hype, and mania over SOA really just boils down to the simple practice of not tying all of your code up in knots. Write little pieces of code that do one thing and anywhere one piece depends on another piece, use a layer of abstraction (an interface in the Object-oriented world or a service in SOA-speak) so that component A isn't tied to the implementation of component B, but rather to a specification that could be satisfied by components C or D instead. When the components don't depend on each other directly, it's more likely you'll be able to reuse one or more of them later.

Taking Command of Your GWT Application

I love interfaces in Java. When used properly, they make for code that is highly cohesive and loosely coupled. Classes that implement interfaces or declare method parameters with interfaces are easier to reuse. One of my favorite interfaces in the Google Web Toolkit is Command. Command has one method:

public void execute();

That's it. A Command can be executed. Command, like the HasValue interface in the GWT Incubator is a very simple interface. So often in programming it is these simple building blocks that give us the greatest opportunities. Let's build on Command and another basic component, Button.

public class CommandButton extends Button {
  public CommandButton(final String html, final Command command) {
    super(html);
    addClickListener(new ClickListener() {
      public void onClick(Widget sender) {
        command.execute();
      }
    });
  }
}

There. Now we can create Buttons that execute a Command when clicked. If your application uses a lot of Buttons, that saves a lot of code for adding ClickListeners. And the code you might've put in those anonymous ClickListeners is now safely encapsulated inside a reusable Command class. I use some variation on CommandButton in almost every application I build. Here's another one I use:

public class CommandLabel extends Label {
  public CommandButton(final String text, final Command command) {
    super(html);
    addClickListener(new ClickListener() {
      public void onClick(Widget sender) {
        command.execute();
      }
    });
  }
}

Almost identical to CommandButton, isn't it? This one just uses a text Label instead of a Button. You can do the same with an Image if you want (and I have). I even wrote one with a ToggleButton.

At some point, I'm going to find the time to start a library project with all of these simple Widget extensions.

public int getValue()

This week I committed my first change as a member of the GWT Incubator project. That change was to introduce an interface, HasValue. This by itself is no monumental achievement. The interface is very simple:

public interface HasValue<T> {
 T getValue();
 void setValue(T value);
}

But this simple interface will facilitate the creation of more complex features such as libraries for data binding or validation.

HasValue provides a layer of abstraction around Widgets, allowing access to the underlying data model through a common API. HasValue works with simple Widgets and data types as well as complex data types and composite Widgets.

Consider two Widgets (both from the Incubator) for selecting a Date value:

DatePicker picker = new DatePicker();
DropDownListBox<Date> dropDown = new DropDownListBox<Date>();

Both implement HasValue<Date> so you could, for example, write a BirthdayValidator that ensures the selected Date is before today. This validator would have a HasValue<Date> parameter and could validate either of the two Widgets above, or any other Widget implementing the HasValue<Date> interface, without additional coding to support those Widgets.

Right now HasValue exists only in the Incubator, but there is support for including it in the next GWT release, version 1.6. Hopefully that release will see HasValue implemented by most of the standard GWT Widgets. In the mean time, I would love to see how GWT application developers use HasValue in their own projects. If you haven't worked with the Incubator before, you'll want to setup the project locally so you can build the latest features. Here's where to start:

Setting up the GWT Incubator project