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Property Expression Language is used in the Property Models to access property values of object wrapped by model. Its syntax is very similar to the Object Graph Navigation Language (OGNL). (Wicket actually used to utilize the OGNL implementation until version 1.1.x. However, later it has been replaced by custom, better-performing, implementation of expression parser).

Current implementation of the property expression parser is represented by the class wicket.util.lang.PropertyResolver. This class supports the following expressions:

  • "property": This expression can be used to access a bean property with get and set method for a property named property. Alternatively, if a object wrapped by model is an instance of Map and there is no get property for given name, the expression will be used as a key of the given map.
  • "property1.property2": Both properties are looked up in the same way as mentioned above. If the property1 evaluates to null and there exists a setter with name property1 on the model object and the class representing the class representing property1 has a default constructor then a new instance will be constructed and assigned on the model object via the property1 and the property2 will be set on this new object.
  • "property.index": If the property is a List or Array then the second property can be used as a index on that list/array like this: 'mylist.0'. This kind of expression will be mapped on the getProperty(index) or setProperty(index, value) methods. If the object represents a List and the index is greater than its size of this list, the list will be expanded.

Index or map properties can be alternatively written as: "property[index]" or "property[key]".

For example, if we have a following class:

Person.java

Then we can use the following expressions:

  • name can be used to access getters and setter for the name property
  • parent.name can be used to retrieve or set the name of a parent. If there is no parent set (i. e. it is null), a new instance of Person is created and set on the model object. Thereafter a name property is get/set on this new instance.
From the mailing list

We started out with OGNL in the past but:
1) OGNL at one point took about 30% processor time of the whole request. We simplified and optimized and wrote OGNL out.
2) We feel it's not the recommended way of programming to rely on property expressions beyond simple navigations
3) By overriding wicket.model.AbstractPropertyModel#onGetObject(wicket.Component) and AbstractPropertyModel#onSetObject(Component, Object) users can provide their own resolving if [necessary].

Eelco Hillenius

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