Groovy Mixins?

问题:

I'm trying to mix-in a class in my Groovy/Grails app, and I'm using the syntax defined in the docs, but I keep getting an error.

I have a domain class that looks like this:

class Person {
  mixin(ImagesMixin)

  // ...
}

It compiles fine, but for some reason it won't work. The file containing ImagesMixin is located in my /src/groovy/ directory.

I've tried it using Groovy versions 1.5.7 and 1.6-RC1 without any luck. Does anyone know what I'm doing wrong?

stacktrace:

2008-12-30 17:58:25.258::WARN:  Failed startup of context org.mortbay.jetty.webapp.WebAppContext@562791{/FinalTransmission,/home/kuccello/Development/workspaces/lifeforce/FinalTransmission/web-app}
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'pluginManager' defined in ServletContext resource [/WEB-INF/applicationContext.xml]: Invocation of init method failed; nested exception is java.lang.ExceptionInInitializerError
    at java.security.AccessController.doPrivileged(Native Method)
    at RunApp_groovy$_run_closure2_closure7.doCall(RunApp_groovy:67)
    at RunApp_groovy$_run_closure2_closure7.doCall(RunApp_groovy)
    at Init_groovy$_run_closure6.doCall(Init_groovy:131)
    at RunApp_groovy$_run_closure2.doCall(RunApp_groovy:66)
    at RunApp_groovy$_run_closure2.doCall(RunApp_groovy)
    at RunApp_groovy$_run_closure1.doCall(RunApp_groovy:57)
    at RunApp_groovy$_run_closure1.doCall(RunApp_groovy)
    at gant.Gant.dispatch(Gant.groovy:271)
    at gant.Gant.this$2$dispatch(Gant.groovy)
    at gant.Gant.invokeMethod(Gant.groovy)
    at gant.Gant.processTargets(Gant.groovy:436)
    at gant.Gant.processArgs(Gant.groovy:372)
Caused by: java.lang.ExceptionInInitializerError
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:169)
    at Episode.class$(Episode.groovy)
    at Episode.<clinit>(Episode.groovy)
    ... 13 more
Caused by: groovy.lang.MissingMethodException: No signature of method: static Person.mixin() is applicable for argument types: (java.lang.Class) values: {class ImagesMixin}
    at Broadcast.<clinit>(MyClass.groovy:17)
    ... 17 more
2008-12-30 17:58:25.259::WARN:  Nested in org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'pluginManager' defined in ServletContext resource [/WEB-INF/applicationContext.xml]: Invocation of init method failed; nested exception is java.lang.ExceptionInInitializerError:
groovy.lang.MissingMethodException: No signature of method: Person.mixin() is applicable for argument types: (java.lang.Class) values: {class ImagesMixin}
    at Broadcast.<clinit>(Person.groovy:17)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:169)
    at Episode.class$(BelongsToMyClass.groovy)
    at Episode.<clinit>(BelongsToMyClass.groovy)
    at java.security.AccessController.doPrivileged(Native Method)
    at RunApp_groovy$_run_closure2_closure7.doCall(RunApp_groovy:67)
    at RunApp_groovy$_run_closure2_closure7.doCall(RunApp_groovy)
    at Init_groovy$_run_closure6.doCall(Init_groovy:131)
    at RunApp_groovy$_run_closure2.doCall(RunApp_groovy:66)
    at RunApp_groovy$_run_closure2.doCall(RunApp_groovy)
    at RunApp_groovy$_run_closure1.doCall(RunApp_groovy:57)
    at RunApp_groovy$_run_closure1.doCall(RunApp_groovy)
    at gant.Gant.dispatch(Gant.groovy:271)
    at gant.Gant.this$2$dispatch(Gant.groovy)
    at gant.Gant.invokeMethod(Gant.groovy)
    at gant.Gant.processTargets(Gant.groovy:436)
    at gant.Gant.processArgs(Gant.groovy:372)
2008-12-30 17:58:25.271::INFO:  Started SelectChannelConnector@0.0.0.0:8080

回答1:

Since Groovy 1.6 you can either apply a mixin at compile-time to a class using an annotation

@Mixin(ImagesMixin)
class Person {
}

Or you can apply the mixin at runtime like this:

def myMixin = ImagesMixin
Person.mixin myMixin

The latter approach is more dynamic as the class to mixin can be determined at runtime. Further information about Groovy mixins is available here.

In my experience, a lot of meta-programming of domain classes simply doesn't work. I don't exactly know why, but suspect it's due to the fact these classes are already very heavily meta-programmed by the Grails runtime. In general my approach is

• Try the meta-programming on a POGO in the Groovy console
• If that works, try it on a non-domain class in the Grails console
• If that works, try it on a domain class in the Grails console. If it doesn't work, then it must be because it's a domain class (rather than a problem with the syntax). At this point it's advisable to try and find another way of accomplishing your goal. If that's not possible, then use a combination of the Grails mailing list and/or Stackoverflow and/or the Grails source code to try and get the meta-programming working.

回答2:

I don't think you are using the proper mixin syntax. Try this:

class MyMixin {
    static doStuff(Person) {
        'stuff was done'
    }
}

class Person {}

Person.mixin MyMixin

new Person().doStuff()

回答3:

I guess what you've seen there is rather a proposal than a feature ;) Groovy does not support mixins out of the box in this way yet (if ever). But there is a 3rd party lib that can be used to emulate such a behavour: Injecto. And mixins can be defined using AST-Macros in the 1.6 version of Groovy (which is not final yet).

You should always check if your're reading the docs from the real groovy project or from the GroovyJSR project (which is rather a place where proposals are collected).

Another way is to use plain-old MOP to inject behaviour into groovy classes by modifying metaClasses.

Cheers

回答4:

In case this helps anyone, Following on from @virtualeyes comment above, I've found that

Person.doStuff()

fails unless you call the following first:

new Person().doStuff()
Person.doStuff()

after which, the static method on the class does seem to work (for me, using Grails 2.2.4) I guess it's to do with initialising the class, or something, but I tried:

Person.metaClass.initialize()
Person.doStuff()

and that didn't work!

回答5:

FYI: There is such a thing as "embedded" domains in Grails now, but it has problems. This is where you can logically include one domain as part of another and have its fields all occur physically in the one DB table. For example, if you find that you have the same subset of fields appearing in multiple tables, like street address/city/state/zip, you could define a StreetAddress domain and embed it. One of the current problems is that Grails will still create a street_address table in addition to embedding its fields in the other tables (unless you play tricks). There are submitted patches pending for this, it seems.

回答6:

Grails domain objects are already heavily meta-programmed. Instead of the groovy mixin try:

@grails.util.Mixin(ImagesMixin)
    class Person {
}

回答7:

Use Traits!

Traits are the reason they removed support for mixins, because that are just better. They're basically abstract classes that are implemented. Allowing you to use multiple and operate them as partial classes.

trait A {
    void printSomething() {
        println "foobar"
    }
}

class B implements A {
    void printAnything() {
        printSomething()
    }
}

new B().printAnything() 

Try it out!