Tuesday Feb 03, 2009

JSR 245 MR: Part I - Expression Language (EL)

This is a quick update on JSP and EL (Expression Language) specification.

Although the EL specification has been separated from the JSP specification, for various reasons, we have not taken the last step to create a new JSR for EL. The proposed work on the JSP MR and EL MR has been filed under JSR-245. The details can be obtained from here.

The close date for the MR is March 3, 2009. Please send comments to me.

This blog covers the EL MR.

One of the most requested feature enhancement to EL is the ability to invoke methods in EL expressions. The MethodExpression in the current EL can be used to specify a method in the EL expression. For instance, in the following JSF code fragment,

    <h:commandButton action="#{trader.buy}" value="buy"/>
the method buy in the bean trader is specified in EL. This method is is not invoked here, but instead, it is passed to the tag handler, and can only be invoked from Java codes. Also, the parameter types of the method cannot be obtained from the EL expression, but must be specified in the TLD (Tag Library Descriptor). At the method invocation, the actual arguments (also not specified in the EL expression) are coerced to the corresponding parameter types and passed to the method. As you can see, there are a lot things that need to be done in Java code to make a method call.

The proposed EL MR allows for specifying the method arguments, as well as the invocation of the method in the EL expression itself. The required changes in EL syntax to do this is surprisingly little. All we need to do to is add (what else) a list of arguments in parenthesis to the method calls. For instance, we can added the argument "JAVA" to above JSF fragment:

    <h:commandButton action="#{trader.buy('JAVA')}" value="buy"/>

One thing nice about this new syntax is that you can tell immediately that this is a method call, and that the method takes one parameter of the type String. Furthermore, we also specify the actual argument to pass to the method when invoked. There is no need to specify the parameter types in the TLD, and there is no hidden arguments in the method call.

Note in the above code, the EL expression, being a deferred method expression, does not cause the method to be invoked. To actually invoke the method and retrieve the method return value, we can write in a JSP page:

    The stock price bought: ${trader.buy('JAVA')}

Very nice!

Included in the EL MR is another feature that is related to method invocation, namely that of customization of method invocations with the use of custom ELResolvers. The default behavior for method calls, such as #{trader.buy('JAVA')} is to look for the method buy, using reflection, in the bean trader. This default behavior can be altered with custom ELResovler. A new method invoke is thus added to ELResolver and this method is used to resolve the evaluation of method calls in expressions, much like the method getValue is used to resolve the evaluation of properties in expressions. By providing custom ELResolvers, one can make "calls" to methods that are not in the bean class. In a sense, this feature enables the definition and invocation of macros, and can lead to some interesting applications.

The default behavior for method calls is handled by the class javax.el.BeanELResolver. Using reflection, it looks up the method in the bean class. Currently overloaded methods are not supported, and the number of arguments specified in the EL expression must matched that in the method signature. This is so because we don't want to specify complicated overload resolution rules in the EL specification.

However, the method ELResolver.invoke has a paramTypes argument that can be used to specify the formal parameter types for the method. If this argument is non-null, then the method with the same name and parameter types will be selected for invocation. Although the EL syntax does not allow the specification of parameter types in the method call, it can be done quite easily with an extended syntax. Borrowing from Javascript (I think) one can write, for instance

to indicate a call to the method buy with String argument. This is currently not included in the MR, but if there are enough interest, it can be considered for the next version of the specification.

The source for the changes to EL api and implementation can be found from Unified Expression Language project. The basic functionalities have been implemented. The area that needs further work is in the overloaded method resolution, and performance.




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