Package org.jdesktop.beansbinding

Class ELProperty<S,​V>

  • Type Parameters:
    S - the type of source object that this ELProperty operates on
    V - the type of value that this ELProperty represents
    public final class ELProperty<S,​V>
extends PropertyHelper<S,​V>
    An implementation of Property that allows Java Beans properties of source objects to be addressed using a simple dot-separated path syntax within an EL expression. For example, to create a simple property representing a Person bean's mother's firstName: 

    
    ELProperty.create("${mother.firstName}")

    Note that BeanProperty is more suitable for such a simple property.

    To create a property representing the concatenation of a Person bean's firstName and lastName properties: 

    
    ELProperty.create("${firstName} ${lastName}");

    To create a property that is true or false depending on whether or not the Person's mother is older than 65: 

    
    BeanProperty.create("${mother.age > 65}");

    Paths specified in the EL expressions are resolved against the source object with which the property is being used.

    An instance of ELProperty is immutable and can be used with different source objects. When a PropertyStateListener is added to an ELProperty for a given source object, the ELProperty starts listening to all objects along the paths in the expression (based on that source object) for change notification, and reflects any changes by notifying the listener associated with the property for that source object. So, for example, if a PropertyStateListener is added to the property from the second example above for an object Duke, the PropertyStateListener is notified when either Duke's first name changes, or his last name changes. If a listener is added to the property from the third example, the PropertyStateListener is notified when either a change in Duke's mother or Duke's mother's age results in a change to the result of the expression.

    It is very important that any bean properties addressed via a ELProperty follow the Java Beans specification, including firing property change notification; otherwise, ELProperty cannot respond to change. As some beans outside of your control may not follow the Java Beans specification, ELProperty always checks the BeanAdapterFactory to see if a delegate provider has been registered to provide a delegate bean to take the place of an object for a given property. See the ext package level documentation for more details.

    When there are no PropertyStateListeners installed on an ELProperty for a given source, all Property methods act by evaluating the full expression, thereby always providing "live" information. On the contrary, when there are PropertyStateListeners installed, the beans along the paths, and the final value, are cached, and only updated upon notification of change from a bean. Again, this makes it very important that any bean property that could change along the path fires property change notification. Note: The setValue method is currently excluded from the previous assertion; with the exception of checking the cache to determine if the property is writeable, it always evaluates the entire expression. The result of this is that when working with paths containing beans that don't fire property change notification, you can end up with all methods (including getValue) working on cached information, but setValue working on the live expression. There are plans to resolve this inconsistency in a future release.

    Readability of an ELProperty for a given source is defined as follows: An ELProperty is readable for a given source if and only if the following is true for all paths used in the expression: a) each bean the path, starting with the source, defines a Java Beans getter method for the the property to be read on it AND b) each bean in the path, starting with the source and ending with the bean on which we read the final property, is non-null. The final value being null does not affect the readability.

    So, in the third example given earlier, the ELProperty is readable for Duke when all of the following are true: Duke defines a Java Beans getter for mother, Duke's mother defines a Java Beans getter for age, Duke is non-null, Duke's mother is non-null. The ELProperty is therefore unreadable when any of the following is true: Duke does not define a Java Beans getter for mother, Duke's mother does not define a Java Beans getter for age, Duke is null, Duke's mother is null.

    Writeability of an ELProperty for a given source is defined as follows: An ELProperty is writeable for a given source if and only if a) the EL expression itself is not read-only (ie. it is a simple expression involving one path such as "${foo.bar.baz}" AND b) each bean in the path, starting with the source and ending with the bean on which we set the final property, defines a Java Beans getter method for the property to be read on it AND c) the bean on which we set the final property defines a Java Beans setter for the property to be set on it AND d) each bean in the path, starting with the source and ending with the bean on which we set the final property, is non-null. The final value being null does not affect the writeability.

    So in the first example given earlier (a simple path), the ELProperty is writeable for Duke when all of the following are true: Duke defines a Java Beans getter for mother, Duke's mother defines a Java Beans setter for firstName, Duke is non-null, Duke's mother is non-null. The ELProperty is therefore unreadable when any of the following is true: Duke does not define a Java Beans getter for mother, Duke's mother does not define a Java Beans setter for firstName, Duke is null, Duke's mother is null. The second and third examples above both represent read-only ELExpressions and are therefore unwritable.

    In addition to working on Java Beans properties, any object in the paths can be an instance of Map. In this case, the Map's get method is used with the property name as the getter, and the Map's put method is used with the property name as the setter. ELProperty can only respond to changes in Maps if they are instances of ObservableMap.

    Some methods in this class document that they can throw PropertyResolutionException if an exception occurs while trying to evaluate the expression. The throwing of this exception represents an abnormal condition and if listeners are installed for the given source object, leaves the ELProperty in an inconsistent state for that source object. An ELProperty should not be used again for that same source object after such an exception without first removing all listeners associated with the ELProperty for that source object.

    Author:
    Shannon Hickey, Scott Violet

    • Method Detail

      • create

        public static final <S,​V> ELProperty<S,​V> create​(String expression)
        Creates an instance of ELProperty for the given expression.
        Parameters:
        expression - the expression
        Returns:
        an instance of ELProperty for the given expression
        Throws:
        IllegalArgumentException - if the path is null or empty
        PropertyResolutionException - if there's a problem with the expression

      • create

        public static final <S,​V> ELProperty<S,​V> create​(Property<S,​?> baseProperty,
                                                             String expression)
        Creates an instance of ELProperty for the given base property and expression. The expression is relative to the value of the base property.
        Parameters:
        baseProperty - the base property
        expression - the expression
        Returns:
        an instance of ELProperty for the given base property and expression
        Throws:
        IllegalArgumentException - if the path is null or empty
        PropertyResolutionException - if there's a problem with the expression

      • getValue

        public V getValue​(S source)
        Returns the value of this Property for the given source.

        See the class level documentation for the definition of readability.

        Specified by:
        getValue in class PropertyHelper<S,​V>
        Parameters:
        source - the source object on which to operate
        Returns:
        the value of this Property for the given source
        Throws:
        UnsupportedOperationException - if the Property is not readable for the given source
        PropertyResolutionException - if an exception occurs while evaluating the expression
        See Also:
        isReadable(S)

      • listeningStarted

        protected final void listeningStarted​(S source)
        Description copied from class: PropertyHelper
        Called when this PropertyHelper changes from having no listeners installed for the given source object to having listeners installed for the given source object. This is the ideal time for subclasses to install any listeners needed to track change on the source object.
        Overrides:
        listeningStarted in class PropertyHelper<S,​V>
        See Also:
        PropertyHelper.listeningStopped(S)

      • listeningStopped

        protected final void listeningStopped​(S source)
        Description copied from class: PropertyHelper
        Called when this PropertyHelper changes from having listeners installed for the given source object to having no listeners installed for the given source object. This is the ideal time for subclasses to remove any listeners that they've installed to track changes on the source object.
        Overrides:
        listeningStopped in class PropertyHelper<S,​V>
        See Also:
        PropertyHelper.listeningStopped(S)

      • toString

        public String toString()
        Returns a string representation of the ELProperty. This method is intended to be used for debugging purposes only, and the content and format of the returned string may vary between implementations. The returned string may be empty but may not be null.
        Overrides:
        toString in class Object
        Returns:
        a string representation of this ELProperty