“OGNL stands for Object-Graph Navigation Language; it is an expression language for getting and setting properties of Java objects” – this is a quotation from the OGNL project website (http://www.ognl.org/).

Different software projects, including Tapestry, use OGNL to simplify the basic operations of accessing properties of Java classes.

When in a page specification we write something like value=”currentDate”, Tapestry automatically treats whatever it finds inside of double quotes as an OGNL expression and asks OGNL to handle it. According to the rules of OGNL, ‘currentDate’ means that the getCurrentDate() method should exist somewhere in the page class – so Tapestry goes and invokes this method. Or it complains, if you forgot to write it. This is how it works in page specifications.

In page templates however, if you write value=”currentDate”, the contents between the double quotes are treated verbatim, as a literal value. Try to make this change and see what happens. Instead of going to the page class and invoking an appropriate method, Tapestry will simply use the ‘currentValue’ string as a value for the Insert component, so instead of the current date and time you will see:

Now is currentDate.

That's not very illuminating. So we have to tell Tapestry that ‘currentDate’ should be treated as an OGNL expression, and we use the ‘ognl:’ prefix to do this.

By the way, you are free to use the ‘ognl:’ prefix in page specification too:

<binding name="value" value="ognl:currentDate"/>

This will work perfectly well, however it is not required as OGNL is the default option here. If, however, you wanted to provide a literal value in a page specification, you would have to use either the ‘literal:’ prefix:

<binding name="value" value="literal:my birthday"/>

or single quotes inside of double quotes:

<binding name="value" value="’my birthday’"/>

This was a very simple example of an OGNL expression. However, imagine that a method called on the page class returned some Person object. Say, this object had an Address object as its property, and what we actually wanted to display is the town property of the Address object. In Java code, to obtain the desired value, we would write something like:

person.getAddress().getTown()

In OGNL this looks simpler:

person.address.town

This is again a basic example, but as we advance to the more involved parts of this tutorial, you will see some more complicated and powerful examples of OGNL expressions. For now, to complete our first acquaintance with OGNL, let’s do a simple experiment.

Add another method to the Home.page class:

public String getSomeMessage() {

   return ". Welcome to Tapestry!";

}

Then change the OGNL expression we use to obtain a value for our Insert component to be:

“ognl:currentDate + someMessage”

Run the application and you should see something like this:

To resolve the expression we have passed to it, OGNL invoked two methods on the page class, getCurrentDate() and getSomeMessage(), and put the results returned from them together. The resulting string was used as a value for the insert component. Here you can already see that OGNL can be quite helpful.

But let’s return to the two ways of defining a component: explicit component and declared component. Which approach is better? Well, it depends, first of all, on the component itself, but also on your personal preferences.