问Groovy: this.metaClass与instance.metaClass
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Stack Overflow用户

提问于 2015-12-24 00:24:46

回答 1查看 1.1K关注 0票数 8

我在书中遇到了下面的groovy脚本代码。它给我带来了一些奇怪的结果。

class Person{
  def work(){
    println "work()"
  }
  def sports=['basketball','football','voleyball']
  def methodMissing(String name, args){
    if(name in sports){
        println "injected ${name} into Person class"
        Person instance=this
        println "this.metaClass:\t\t${this.metaClass}"
        println "instance.metaClass:\t${instance.metaClass}"
        assert this.metaClass==instance.metaClass
    }else{
        println "no such method:${name}() in Person class"
    }
  }
}
def jack=new Person()
jack.football()

它的输出如下：

injected football into Person class
this.metaClass:     groovy.lang.MetaClassImpl@245b4bdc[class Person]
instance.metaClass: org.codehaus.groovy.runtime.HandleMetaClass@245b4bdc[groovy.lang.MetaClassImpl@245b4bdc[class Person]]
Caught: Assertion failed: 
//I did not paste the detailed assertion here for simplicity

所以我很困惑：

为什么this.metaClass不等于instance.metaClass？
此外，我不能使用this.metaClass注入新方法；groovy告诉我，this.metaClass没有这样的属性，我打算注入它。
"org.codehaus.groovy.runtime.HandleMetaClass@245b4bdc[groovy.lang.MetaClassImpl@245b4bdcclass人是什么意思？我知道"245b4bdc“可能是对象指针。但是为什么HandleMetaClass和MetaClassImpl的指针值"245b4bdc“相同呢？

目前，我发现@245b4bdc不是“HandleMetaClass@245b4bdc对象引用”，所以MetaClassImpl@245b4bdc.不一定是同一个实例。我们可以使用Object.is()方法来判断它们是否相同。(我这样做了，结果是false)

groovy

this

metaclass

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回答 1

Stack Overflow用户

回答已采纳

发布于 2015-12-25 04:52:38

为什么是this.metaClass != instance.metaClass？它涉及到坡地的进入。

- When accessing an instance field from "**outside**", groovy actually calls the function getFieldName(). In my example, when I use "**instance**", i am at the **outside**; So **instance.metaClass** will call **instance.getMetaClass()**.
- When accessing an instance field from "**inside**", groovy simply directly access the field, getFieldName() is not called. In our example, when I use "**this**", i am at the "**inside**"; So "**this.metaClass**" will access "**metaClass**" directly.
- Finally, getMetaClass() returns a **HandleMetaClass** object while the internal metaClass is a **MetaClassImpl** object. So **this.metaClass!=instance.metaClass**.

为什么this.metaClass.say={->println "say"}将抛出MissingPropertyException？

- this.metaClass's type is MetaClassImpl
- MetaClassImpl is a low level class, which supports upper level classes(eg. HandleMetaClass) for injection. It's not meant for Developer to use directly, So it does not support the injection way: **xxxx.say={->println "say"}**.

代码示例(针对问题1)：

class Person{
  def work(){
    println "work()"
  }
  def sports=['basketball','football','voleyball']
  def methodMissing(String name, args){
    if(name in sports){
        Person instance=this

        println "this.metaClass:\n\t${this.metaClass}"
        println "instance.metaClass:\n\t${instance.metaClass}"
        //output: false
        println "this.metaClass.is(instance.metaClass):\n\t${this.metaClass.is(instance.metaClass)}"

        //output: true
        println "this.getMetaClass().is(instance.getMetaClass()):\n\t${this.getMetaClass().is(instance.getMetaClass())}"

    }else{
        println "no such method:${name}() in Person class"
    }
  }
}
def jack=new Person()
jack.football()
jack.football()

代码示例(针对问题2)：

class Cat{}
    def a=new groovy.lang.MetaClassImpl(Cat)
try{
    a.say={->println "say"}
}catch(MissingPropertyException e){
    println "[Fail]\n\tcan not inject method say() into MetaClassImpl class.\n"
}

def b=new org.codehaus.groovy.runtime.HandleMetaClass(a)
println b
b.say={->println "[say]"}
println "[OK]\n\tcan inject method say() into HandleMetaClass class\n"
def method=b.getMetaMethod("say")
method.invoke(this)