Raible Designs

Posted by Jason Carreira on February 25, 2004 at 07:04 PM MST #

There are a couple of way to execute an OGNL expression, depending on how much information you want to cache for efficiency. Also OGNL supports "get" and "set" operations; as such OGNL is a binding language rather than just an expression language. Of course it also serves admirably as an expression language for the purpose to which you are putting it.

The simplest way is to use the ognl.Ognl class to evaluate the expression against a root object:

    ognl.Ognl.getValue("user.name", rootObject)
    ognl.Ognl.getValue("bean.findCustomers().{? location == \"US\" && orders.amount > 1000 }", rootObject);

This will parse then evaluate the expression against the given root object.

For greater efficiency you can pre-compile the expression using the parseExpression() method:

    Object expression = ognl.Ognl.parseExpression("user.name");
    ognl.Ognl.getValue(expression, rootObject);

For repeated evaluations (such as you might have in a JSP tag that evaluates many times), you definitely want to cache the parsed expression. For your immediate purposes this will be a good test of how well it works within your framework.

This is just the simplest way of using OGNL. There is also a "context" object that allows you to put variable values in scope, and also track the current state of the expression evaluation.

There are many extension points that allow you to control how values are extracted from objects that you will definitely want to investigate:

• Class resolvers allow custom class resolution. Sometimes (especially in webapps) the normal Class.forName() does not suffice.
• Property Accessors allow you to extend the notion of properties to other domains than JavaBeans. A good example of this is a property accessor for the servlet classes that support the setAttribute()/getAttribute() pattern. Session, context, etc. attributes can be accessed simply by referring to property (i.e. "session.foo" will call HttpSession.getAttribute("foo") instead of looking for a getFoo() method).
• Elements accessors control how objects are iterated. The default elements accessors provide iteration over standard collections classes, native arrays, etc. There is also an Elements accessor for numbers that will produce sequences. Custom elements accessors allow you to produce iterations from things that normally are not iteratable (like the numeric accessor).
• Type converters allow you to customize how objects are converted to other types in method calls and return values. Back to the servlet example you often face conversion from String to other objects during form post binding. The default type converter will convert between numeric types and to/from String and numeric types.
• Null handlers allow you to customize how null return values and null properties are handled, giving you an opportunity to intercept these null values and replace them with other objects at runtime.
• Method accessors are not used very often, but they allow you customize the way methods are called.
• Member access allows you to inject accessibility code in controlled circumstances so that you can lower normal Java protection for certain cases. This is useful, for example, when writing code that does serialization and needs to reach into classes to retrieve private members. The default OGNL class for this does nothing, so normal Java rules apply.

Please feel free to ask any further questions you have to either me or the OGNL mailing list (see http://www.ognl.org for details)

Posted by Drew Davidson on February 27, 2004 at 10:33 AM MST #