Android Groovy metaclass

Question

I am trying writing some code on Groovy for Android and I stumbled upon a fact that I cannot use metaclass to add a property dynamically to an object:

it.mapMarker.metaClass.project = it

It fails with

java.lang.NoClassDefFoundError: Failed resolution of: Ljava/lang/invoke/MethodHandles;
        at org.codehaus.groovy.vmplugin.v7.IndyInterface.<clinit>(IndyInterface.java:81)
        at org.codehaus.groovy.vmplugin.v7.Java7.invalidateCallSites(Java7.java:31)
        at org.codehaus.groovy.reflection.ClassInfo.incVersion(ClassInfo.java:87)
        at groovy.lang.MetaClassImpl.incVersion(MetaClassImpl.java:3445)
        at groovy.lang.ExpandoMetaClass.performRegistryCallbacks(ExpandoMetaClass.java:931)
        at groovy.lang.ExpandoMetaClass.access$400(ExpandoMetaClass.java:254)
        at groovy.lang.ExpandoMetaClass$2.call(ExpandoMetaClass.java:860)
        at groovy.lang.ExpandoMetaClass.performOperationOnMetaClass(ExpandoMetaClass.java:813)
        at groovy.lang.ExpandoMetaClass.registerBeanProperty(ExpandoMetaClass.java:842)
        at groovy.lang.ExpandoMetaClass.setProperty(ExpandoMetaClass.java:791)
        at org.codehaus.groovy.runtime.HandleMetaClass.setProperty(HandleMetaClass.java:91)
        at org.codehaus.groovy.runtime.InvokerHelper.setProperty(InvokerHelper.java:191)
        at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.setProperty(ScriptBytecodeAdapter.java:480)

I guess the reason is that the class MethodHandle was only added in JDK 1.7.

Could you possibly think of the way I can overcome this? Surely I can just extend a class and give it the wanted field, but hey, that's probably missing the point of Groovy.

Answer 1

First of all, this is probably not a good idea to use runtime metaprogramming under Android. Not that it is not fun, but it is much slower than using @CompileStatic (because on Android we have no choice but leveraging reflection) and it's a better idea to rely on the static features of the language only (this doesn't prevent from using dynamic features like JSonSlurper on a case by case basis).

The error you are seeing will probably be fixed in Groovy 2.4.0-beta-4, but anyway I would not recommand to do this. Instead, I would favor using extension modules, which have the advantage of being compatible with static compilation. They will allow you to enhance existing classes too.

See for example the code that I used in my SpeakerTime demo: there's a module named extensions that provides extension methods to the Context class. I think it is the way to go.

Answer 2

You could always use a language such as SnapScript which is fully compatible with Android. Has all the features of Groovy and better support for remote debugging.