Custom reference resolver tutorial : creating a grammar

Introduction

In my previous blog entry, I presented an example on the new reference resolver add-on of ExtVal. It was able to read from a property from a MBean and use that value for validation.

As it was designed as a Reference Resolver, you had to specify a parameter at the cross field validation annotation of ExtVal where you needed the resolver. This is not friendly if you have designed some kind of resolver that you would use on a lot of locations.  Therefore the add-on has foreseen the option to create a new Reference Grammar. In this text I'll explain you how you can convert a reference resolver to a reference grammar.

Coding it

Basically, there are 2 small differences between a Reference Resolver and a Reference Grammar and you can use almost the same code in both situations. You only need to structure it a bit different.  Besides the registration of your grammar with the ExtVal add-on, see further, you should extend from the AbstractReferenceGrammar class. This class has a type parameter that needs to be specified.  In our case, it is just a simple object that holds the bean and property name. If you remember from my previous text, we invented a new grammar 'mbean@property' and the object just holds the 2 parts of the grammar (before and after the @-sign).

Your grammar has to implement 4 methods, so the empty grammar looks like this

@InvocationOrder(100)

public class MBeanGrammar extends AbstractReferenceGrammar

{

    @Override

    public void initGrammar()

    {

    }

    @Override

    protected boolean targetNotCompatibleWithGrammar(String targetValue)

    {

    }

    @Override

    protected MBeanGrammarData determineReference(String targetValue, MetaDataEntry metadataEntry)

    {

    }

    @Override

    public ReferencedValue resolveReference(MBeanGrammarData targetValue, MetaDataEntry metadataEntry)

    {

    }

}

where the (MBeanGrammarData object holds the bean and property name the user specified.

The initGrammar method is called when you register the grammar with the add-on.  You can use this interception point to do any initialization required to have a proper function of the grammar.  In our case we grab a reference to the MBean server we will be using for the resolving of the value.  Important thing to remember is that this method is just executed once.

When the ExtVal add-on encounters an annotation related to the cross field validation feature, it will ask to all the grammars if it can handle the specified reference.  Our implementation should return false for the targetNotCompatibleWithGrammar method when it contains the @-sign (our grammar can handle the reference) and true when we don't know it (like some EL-reference).  The first grammar that says that it can handle, will be used by the add-on.  How is the order determined? Grammars can be annotated with @InvocationOrder and the value you specify determines the order.  The default grammars (JavaBean and EL) have values of 950 and 1000.  So when we specify a value of 100, our new grammar is the first to be contacted to resolve the reference.

If we have a reference that we should handle, the add-on calls the determineReference method next. There we have the opportunity to parse the reference according to the grammar.  In our case it is just splitting the reference around the @-sign and put it into the MBeanGrammarData instance.

As a last step, the resolveReference method is called where we get the MBeanGrammarData instance we prepared in the previous step and we need to resolve the reference.  In our case we contact the MBean server, ask for the correct bean and extract the value of the property.  That value is wrapped nicely into the ReferencedValue and returned.

So the main difference is that the reference resolver contained all the logic in the resolveReference method and in the grammar version, we have the functionality split into 4 methods. You can find the code here.

Configuration

This is an additional step we need to do to integrate our grammar in the ExtVal add-on.  With the following 2 lines, placed in a startupListener, the Reference Grammar is registered and ready to use.

 ExtValContext extValContext = ExtValContext.getContext();

        extValContext.getModuleConfiguration(AdvancedCrossValidationAddonConfiguration.class).addReferenceGrammar(new MBeanGrammar());

Testing it

This version behaves exactly the same as our previous example, so I refer to the other text on how you can test it.

Conclusion

As easy it was to create a Reference Resolver, as easy it is to create a reference Grammar.  The grammar has the advantage that you don’t have to specify a parameter at the cross field validation annotation. But on the other side, if you only using it on a few locations, the add-on will ask your code for every reference, if you understand it.  So based on the concrete situation you have to decide what you need, a resolver or a reference.

Custom reference resolver tutorial

Introduction


In the previous blog text, I described the cross field validation feature of ExtVal. In the meantime, the reference resolver add-on is released and this text shows how you can write your custom reference resolver.
And since the usage of the add-on API is easy, we are aiming a little bit higher with our example as a simple hello world. I'll explain you how you can write a resolver that checks a value stored in an MBean object.

Coding it

The reference resolver api is described here in the project documentation. There is just one method that you need to implement, resolveReference. The first parameter contains the string defined in the value attribute of the annotation (or method is repeatedly called for each string defined in the case of multiple values)

ReferencedValue resolveReference(final String targetValue, final MetaDataEntry metadataEntry);

So, first we have to 'invent' our indication how we will describe the property of the MBean where the value is stored. A possibility is using mbean@property where mbean is just the name in a certain node of our MBean server.

Our implementation of the method needs to do 3 things

1. Get a reference to the MBean server of our environment
2. Split the parameter in 2 parts (around the @ sign)
3. Lookup the MBean and retrieve the value of the specified property.

If you aren't familiar with accessing JMX beans, you can find various documents on internet which explain it in detail. For this example, you can have a look at the source code placed in an apache extra repository (link). The implementation of our resolver is in the class org.os890.extval.addon.demo.resolver.MBeanReferenceResolver.

Before we can use this new resolver, we have to define a validationParameter so that we are able to use it in the cross field annotations of ExtVal. It is described in the add-on documentation on the referenceResolver page.

In our case, it results in:

public class MBeanReferenceResolver implements ReferenceResolver
{
    public interface MBeanReferenceResolverParameter extends ValidationParameter
    {
       @ParameterKey
       public Class KEY = ReferenceResolver.class;

       @ParameterValue
        public Class referenceResolverClass = MBeanReferenceResolver.class;
    }

     @Override
      public ReferencedValue resolveReference(String targetValue, MetaDataEntry metadataEntry)
      {
         // Implementation details omitted
       }
}

Now we can use this resolver in all cross field annotations like @Equals. In the example you can find a usage in the class org.os890.extval.addon.demo.view.BackingBean.

@Equals(value = "demo@TargetValue", parameters = MBeanReferenceResolver.MBeanReferenceResolverParameter.class)
private String source;

This property is linked to an inputField on a screen in the example.

Testing it

Now we are ready to run the example. If we press the 'Submit' button, a comparison will be made between the entered value and the value which is by default set in the MBean, which is 'ExtVal'. If they don't match, you see the message that the input is different.

This default value is set by the class org.os890.extval.addon.demo.startup.MBeanResolverStartupListener which is defined as JSF PhaseListener in the faces config file.

The following steps makes this example interesting. You can start up the default JMX console of your environment (look for the JConsole program) or any other JMX console that is able to connect to your (application) server. You will find between all the other MBeans, the one created by the demo application in the org.os890.extval.reference.addon node. It is called demo. The property TargetValue can be changed to another value, let's say 'Test'. If you now go back to your browser you will see that the comparison now only succeed with the new value 'Test' that you have entered in the JMX bean.

Conclusion
 This example shows how easy it is to create a new reference resolver and integrate it in ExtVal.  With the add-on you can create any reference you like that needs to be used by the cross field validation feature of ExtVal.

Cross field validation with JSF and ExtVal

Introduction
JSF has support for validation built in, but it is limited to a single field. Is it required, is it in the correct range, has it the correct format and so on.

With the MyFaces Extensions Validator framework (ExtVal in short), there is support for cross field validation. You can verify if two fields have the same or different content. Check the relative order of two dates and so on. For the feature we need 2 fields we can compare and this is done by specifying a reference in the annotation on one field to the other field.

And although the current possibilities to specify the other field are sufficient in 99% of your cases, I was already thinking about a mechanism for two years that allows greater flexibility and easier custom implementations.

ReferenceResolver
And since I now have almost completed the Reference Resolver add-on for ExtVal, the time has come to start blogging about the possibilities.

The add-on introduces an interface, the ReferenceResolver, when it comes to resolving the references to the other field in ExtVal. This allows the developer to implement any kind of reference he like. But since the default ExtVal functionality is enough in 99% of the cases, the requirement of the add-on was that it should recognize and correctly evaluates the current system. Backwards compatibility was in this case a requirement.

The Apache Commons JEXL project proved to be the ideal candidate. With some extensions, it correctly handles the default references of ExtVal and allowed some advanced references out of the box.

Cross field validation overview
But the details of the add-on will be explained in the next blog entry/entries I'll make. First I need to explain some concepts of the cross field validation of ExtVal before I can continue with the explanation of the add-on.

So the rest of this blog entry will only explain the default functionality and concepts of ExtVal. You don't need the add-on for this.

An example makes it more clear.

-- Managed Bean
@ManagedBean(name="bean")
@RequestScoped
public class ClassicGrammarBean {

     @Equals(value = "target")
     private String source;

     private String target;

     // setters and getters omitted
}

-- screen

<h:inputText id="source" value="#{bean.source}" label="source"/>
<h:inputText id="target" value="#{bean.target}" label="target"/>

If you use the above code, posting the screen will result in an error except when the contents in the 2 fields are the same.

How does it work? ExtVal installs a proxy for each renderer defined in the JSF system. That way, it can perform some action anytime there is a decode or encode request. One of the actions is that after the standard decode method is called, the value is 'verified' against the annotations that are present on the property in the managed bean to which the field is linked.

How is cross field validation then implemented? Another action which is performed after the decode, is that the value is stored by the ProcessedInformationRecorder for later usage. After the JSF validation lifecycle phase, there is a check for any cross field annotations on the properties used in the screen. If so, the required checks are performed based on the values stored by the ProcessedInformationRecorder. Later on I'll make some comments on this principle.

Reference methods
There are 2 options to refer to the other field that is needed in the cross field validation. One is based on EL expression, but first I'll explain the other one.

local reference (javaBean reference)
The example which is presented earlier in this text, is an example of the local reference type. In the value attribute of the annotation, we specify the property name of the other property we like to use in our comparison. Besides the referring to other simple properties (like target in the example) we can reference properties in another class as long as the object is a local property in the bean.

As always, the example makes it very clear what I mean.

public class SubBean {
     private String targetInSub;

     // setter and getter omitted
}

@ManagedBean(name="bean")
@RequestScoped
public class ClassicGrammarBean {

     @Equals(value = "subBean.targetInSub")
     private String source;

     private SubBean subBean;

     // setters and getters ommitted
}

But not all kind of java references are supported. Besides the above 2 example, the map notation is also supported. But only with a fixed key value, a literal and not a reference to another property. The contents in the value attribute of the annotation then looks like mapProperty.key where key is the string literal of the map key.

EL reference
The El references are more common because we all use them in the JSF pages. But the disadvantage is that the bean name is placed in a string attribute of the annotation. Changing the bean name will break your validation rules, which isn't the case if you can use the local reference method. But there you have the problems when you rename the properties of the bean.

Validation mode
Knowing the possible reference methods isn't enough to understand how the cross field validation works. You basically point to 2 properties of objects but the values aren't yet pushed into them. This is done in the update model phase of the JSF lifecycle which comes after the validation phase.

A very important piece of the cross field functionality is the matching of the EL references used in the JSF pages to link a component to a managed bean property and the reference specified in the attribute of the annotations.

In the case the reference of the annotation couldn't be matched to a value found in the current request, the reference is evaluated and the value stored in the model, the managed bean property in this case, is taken. This is called the model validation mode.

Conclusion
The new Reference Resolver add-on for ExtVal makes it possible to reference any kind of other value for the cross field validation functionality of ExtVal. If you have exotic requirement regarding the other party in the validation rule, you can implement them yourself using the ReferenceResolver interface.

And by default, by using the fine JEXL engine, the backward compatibility is guaranteed and opens the gate for very complex references.

In the next blog entry I'll explain how you can use the value of an MBean in the cross field validation with the new add-on.