I hear a great big, “Huh?”

First, a little background on converters. (You may want to get the Grails source code and have directories open to the org.codehaus.groovy.grails.web.converters and grails.converter packages before diving into this.) When a JSON converter is created, it obtains a ConverterConfiguration from the global ConvertersConfigurationHolder. ConverterConfiguration has several properties which influence the conversion process, but the most interesting for this discussion is its prioritized set of ObjectMarshallers. ObjectMarshallers are what do the actual work of turning an object into a JSON representation. Each ObjectMarshaller handles a certain set of classes, like arrays, maps, beans, grails domain objects, etc. When the JSON object is ready to convert a data object, it calls config.getMarshaller(data). The config iterates through its list of marshallers, calling marshaller.supports(data) on each. Since multiple marshallers may be able to handle a specific class (like GroovyBeanMarshaller and DomainClassMarshaller can both handle domain classes), whichever marshaller has highest priority (i.e. is called first) will be invoked.

When the standard ConverterConfiguration is initialized, the DomainClassMarshaller is given higher priority than the GroovyBeanMarshaller. Thus, it handles the conversion of domain objects during normal Grails application execution even though domain objects are also groovy beans. However, the DomainClassMarshaller.supports() method has a slight twist to it. It depends on there being a GrailsApplication object registered in the ConverterUtil class in order to tell it that a particular object is a domain object. Without the GrailsApplication object, ConverterUtil.isDomainClass() always returns false, causing DomainClassMarshaller.supports() to also return false. The ConverterConfiguration next checks the GroovyBeanMarshaller. Its supports() method returns true, so it handles the conversion in this case.

So why is this a big deal? For most domain object, it probably isn’t. Both marshallers render the primary properties of the domain object. However, the DomainClassMarshaller only renders properties return by domainClass.getPersistentProperties() while GroovyBeanMarshaller renders all properties, including transient ones. Because I needed some transient properties in the JSON representation, my unit tests worked great (since there is no grailsApplication setup, so the GroovyBeanMarshaller did the full rendering), but then my code failed when I ran the integration tests and the real app (since the DomainClassMarshaller handled the rendering).

How do you fix this? It’s pretty easy once you know where to look. First, I wanted to make my unit tests behave like the real app, so I had to get them to fail. Unfortunately, the conversion classes are written in java, not groovy, so you can’t just do a quick override of the ConverterUtil metaclass to get it to return what you want. You actually need to setup a GrailsApplication object. Rather than trying to create and initialize a whole one though, I determined that I could create a small stub class that overrode the two methods I needed:


import org.codehaus.groovy.grails.commons.*
class GrailsApplicationStub extends DefaultGrailsApplication {
def artefacts = [(DomainClassArtefactHandler.TYPE):[:]]

GrailsClass getArtefact(String artefactType, String name) {
def retVal = artefacts[artefactType] ? artefacts[artefactType][name] : null
return retVal
}
}

When initializing the unit test, I created the stub and set in the particular domain class that I wanted to be recognized. Then register the application stub with the ConverterUtil class:


def grailsApp = new GrailsApplicationStub();
grailsApp.artefacts[DomainClassArtefactHandler.TYPE][YourClass.name] = new DefaultGrailsDomainClass(YourClass);
ConverterUtil.setGrailsApplication(grailsApp);

Instances of YourClass will now be handled by the DomainClassMarshaller in the unit test. When the unit test checks for the existence of a transient property in a JSON representation, it should fail.

Next, I needed to change the marshaller prioritization so that the desired domain class was handled by a regular GroovyBeanMarshaller. Since I always wanted the GroovyBeanMarshaller to handle the class, I inserted this code into my unit test setup (and later into BootStrap so the behavior would apply to the full app):


JSON.registerObjectMarshaller(YourClass, {o, c ->
new GroovyBeanMarshaller().marshalObject(o, c)
})


This creates a ClosureObjectMarshaller that handles YourClass objects and gives the new marshaller the default priority of 1 (which puts it at the top of the priority list). When processing an object, this closure marshaller simply passes it through to a GroovyBeanMarshaller. Since the new marshaller only handles YourClass objects, the rendering behavior for all other domain objects is not affected.

This resolved my original issue. The JSON rendering of my object is now correct when I run my app. I did go down a side path while coming to this final solution. If you only want to override how a YourClass object is rendered some of the time, you could instead register a named configuration for it like this:


JSON.createNamedConfig("FullYourClassRender") {cfg ->
cfg.registerObjectMarshaller(new ClosureOjectMarshaller(YourClass, {o, c ->
new GroovyBeanMarshaller().marshalObject(o, c)
}))
}

Then when you want to use that particular configuration, you wrap the JSON object calls in a use statement:


JSON.use("FullYourClassRender") {
render(contentType: "application/json", text: myObject as JSON)
}


Other configuration options are also available. Take a look at the JSON class to see them.

I hope this helps document the conversion process a bit.