Java Doc for ELResolver.java in  » Scripting » JUEL » javax » el » Java Source Code / Java DocumentationJava Source Code and Java Documentation

While evaluating an expression, the ELResolver associated with the ELContext is consulted to do the initial resolution of the first variable of an expression. It is also consulted when a . or [] operator is encountered, except for the last such operator in a method expression, in which case the resultion rules are hard coded.

For example, in the EL expression ${employee.lastName}, the ELResolver determines what object employee refers to, and what it means to get the lastName property on that object.

Most methods in this class accept a base and property parameter. In the case of variable resolution (e.g. determining what employee refers to in ${employee.lastName}), the base parameter will be null and the property parameter will always be of type String. In this case, if the property is not a String, the behavior of the ELResolver is undefined.

In the case of property resolution, the base parameter identifies the base object and the property object identifies the property on that base. For example, in the expression ${employee.lastName}, base is the result of the variable resolution for employee and property is the string "lastName". In the expression ${y[x]}, base is the result of the variable resolution for y and property is the result of the variable resolution for x.

Though only a single ELResolver is associated with an ELContext, there are usually multiple resolvers considered for any given variable or property resolution. ELResolvers are combined together using CompositeELResolver s, to define rich semantics for evaluating an expression.

For the ELResolver.getValue , ELResolver.getType , ELResolver.setValue and ELResolver.isReadOnly methods, an ELResolver is not responsible for resolving all possible (base, property) pairs. In fact, most resolvers will only handle a base of a single type. To indicate that a resolver has successfully resolved a particular (base, property) pair, it must set the propertyResolved property of the ELContext to true. If it could not handle the given pair, it must leave this property alone. The caller must ignore the return value of the method if propertyResolved is false.

The ELResolver.getFeatureDescriptors and ELResolver.getCommonPropertyType methods are primarily designed for design-time tool support, but must handle invocation at runtime as well. The java.beans.Beans.isDesignTime method can be used to determine if the resolver is being consulted at design-time or runtime.

This assists tools in auto-completion and also provides a way to express that the resolver accepts a primitive value, such as an integer index into an array.

If this resolver handles the given (base, property) pair, the propertyResolved property of the ELContext object must be set to true by the resolver, before returning.

If this resolver handles the given (base, property) pair, the propertyResolved property of the ELContext object must be set to true by the resolver, before returning.

If this resolver handles the given (base, property) pair, the propertyResolved property of the ELContext object must be set to true by the resolver, before returning.

If this resolver handles the given (base, property) pair, the propertyResolved property of the ELContext object must be set to true by the resolver, before returning.

The attribute name of the named attribute in the FeatureDescriptor that specifies whether the variable or property can be resolved at runtime.

The attribute name of the named attribute in the FeatureDescriptor that specifies the runtime type of the variable or property.

This assists tools in auto-completion and also provides a way to express that the resolver accepts a primitive value, such as an integer index into an array. For example, the ArrayELResolver will accept any int as a property, so the return value would be Integer.class.

If the base parameter is null, the resolver must enumerate the list of top-level variables it can resolve.

The Iterator returned must contain zero or more instances of java.beans.FeatureDescriptor , in no guaranteed order. In the case of primitive types such as int, the value null must be returned. This is to prevent the useless iteration through all possible primitive values. A return value of null indicates that this resolver does not handle the given base object or that the results are too complex to represent with this method and the ELResolver.getCommonPropertyType method should be used instead.

Each FeatureDescriptor will contain information about a single variable or property. In addition to the standard properties, the FeatureDescriptor must have two named attributes (as set by the setValue method):

• ELResolver.TYPE - The value of this named attribute must be an instance of java.lang.Class and specify the runtime type of the variable or property.
• ELResolver.RESOLVABLE_AT_DESIGN_TIME - The value of this named attribute must be an instance of java.lang.Boolean and indicates whether it is safe to attempt to resolve this property at design-time. For instance, it may be unsafe to attempt a resolution at design time if the ELResolver needs access to a resource that is only available at runtime and no acceptable simulated value can be provided.

The caller should be aware that the Iterator returned might iterate through a very large or even infinitely large set of properties. Care should be taken by the caller to not get stuck in an infinite loop.

This is a "best-effort" list. Not all ELResolvers will return completely accurate results, but all must be callable at both design-time and runtime (i.e. whether or not Beans.isDesignTime() returns true), without causing errors.

The propertyResolved property of the ELContext is not relevant to this method. The results of all ELResolvers are concatenated in the case of composite resolvers.

If this resolver handles the given (base, property) pair, the propertyResolved property of the ELContext object must be set to true by the resolver, before returning. If this property is not true after this method is called, the caller should ignore the return value.

This is not always the same as getValue().getClass(). For example, in the case of an ArrayELResolver , the getType method will return the element type of the array, which might be a superclass of the type of the actual element that is currently in the specified array element.

If this resolver handles the given (base, property) pair, the propertyResolved property of the ELContext object must be set to true by the resolver, before returning. If this property is not true after this method is called, the caller should ignore the return value.

If this resolver handles the given (base, property) pair, the propertyResolved property of the ELContext object must be set to true by the resolver, before returning. If this property is not true after this method is called, the caller should ignore the return value.

If this resolver handles the given (base, property) pair, the propertyResolved property of the ELContext object must be set to true by the resolver, before returning. If this property is not true after this method is called, the caller can safely assume no value has been set.