blocks|key|277432|text|来自Wikipedia|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|277433|277434|在面向对象编程中，元类是实例是类的类。就像普通类定义某些对象的行为一样，元类定义某些类及其实例的行为。并非所有面向对象的编程语言都支持元类。在这些类中，元类可以覆盖类行为的任何给定方面的程度各不相同。每种语言都有自己的元对象协议，这是一组控制对象、类和元类如何交互的规则。|blockquote|277435|277436|277437|和|277438|语言和工具中的|277439|支持|277440|277441|下面是一些支持元类的最重要的编程语言。Common+Lisp，通过CLOS+Groovy+Objective-C+Python+Perl，通过元类杂注，以及Moose+Ruby+Smalltalk一些不太流行的支持元类的语言，包括OpenJava、OpenC%2B%2B、OpenAda、CorbaScript、ObjVLisp、Object-Z、MODEL-K、XOTcl和MELDC.|277442|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Metaclass^0|2|9|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@$A|11|B|12|1|13]]|C|$]]|$1|D|3|-4|5|6|7|14|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|15|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|16|8|@]|9|@]|C|$]]|$1|I|3|-4|5|6|7|17|8|@]|9|@]|C|$]]|$1|J|3|K|5|6|7|18|8|@]|9|@]|C|$]]|$1|L|3|M|5|6|7|19|8|@]|9|@]|C|$]]|$1|N|3|O|5|G|7|1A|8|@]|9|@]|C|$]]|$1|P|3|-4|5|6|7|1B|8|@]|9|@]|C|$]]|$1|Q|3|R|5|6|7|1C|8|@]|9|@]|C|$]]|$1|S|3|-4|5|6|7|1D|8|@]|9|@]|C|$]]]|T|$U|$5|V|W|X|C|$Y|Z]]]]

from <a href="http://en.wikipedia.org/wiki/Metaclass">Wikipedia</a>

<blockquote>
 In object-oriented programming, a
 metaclass is a class whose instances
 are classes. Just as an ordinary class
 defines the behavior of certain
 objects, a metaclass defines the
 behavior of certain classes and their
 instances. Not all object-oriented
 programming languages support
 metaclasses. Among those that do, the
 extent to which metaclasses can
 override any given aspect of class
 behavior varies. Each language has its
 own metaobject protocol, a set of
 rules that govern how objects,
 classes, and metaclasses interact.
</blockquote>

and 

<blockquote>
 Support in languages and tools
 
 The following are some of the most
 prominent programming languages that
 support metaclasses. Common Lisp, via
 CLOS Groovy Objective-C Python Perl,
 via the metaclass pragma, as well as
 Moose Ruby Smalltalk Some less
 widespread languages that support
 metaclasses include OpenJava, OpenC++,
 OpenAda, CorbaScript, ObjVLisp,
 Object-Z, MODEL-K, XOTcl, and MELDC.
</blockquote>

blocks|key|279378|text|您可能正在考虑Groovy's+MetaClass|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|279379|Groovy中的MetaClass定义了任何给定的|279380|或MetaClass类的行为。MetaClass接口定义了两个部分。客户端API，它是通过扩展MetaObjectProtocol接口和与Groovy运行时系统的契约定义的。通常，编译器和Groovy运行时引擎与此类上的方法交互，而MetaClass客户端与MetaObjectProtocol接口定义的方法交互|blockquote|279381|279382|279383|279384|Groovy允许您在运行时为类分配行为和状态，而无需编辑原始源代码，它是在原始MetaClass之上的一层。|279385|这是Groovy用于extend+the+Java+JDK+objects的机制。|279386|示例：|style|BOLD|279387|Object.class.metaClass.explode{->+println+"Boom!+${delegate}+Exploded!"}
"SomeString".explode();
12345.explode();|code-block|syntax|javascript|279388|输出：|279389|279390|+Boom！SomeString爆炸了！|279391|279392|砰！12345爆炸了！|279393|有关更高级的用法，请阅读以下内容：MetaClasses|279394|entityMap|0|LINK|mutability|MUTABLE|url|http://groovy.codehaus.org/api/groovy/lang/MetaClass.html|1|http://groovy.codehaus.org/groovy-jdk/|2|http://webcache.googleusercontent.com/search?q=cache:groovy.codehaus.org/JN3525-MetaClasses^0|7|I|0|0|0|0|0|0|0|0|A|R|1|0|0|3|0|0|0|3|0|0|0|0|0|H|B|2|0^^$0|@$1|2|3|4|5|6|7|1K|8|@]|9|@$A|1L|B|1M|1|1N]]|C|$]]|$1|D|3|E|5|6|7|1O|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|1P|8|@]|9|@]|C|$]]|$1|I|3|-4|5|6|7|1Q|8|@]|9|@]|C|$]]|$1|J|3|-4|5|6|7|1R|8|@]|9|@]|C|$]]|$1|K|3|-4|5|6|7|1S|8|@]|9|@]|C|$]]|$1|L|3|M|5|6|7|1T|8|@]|9|@]|C|$]]|$1|N|3|O|5|6|7|1U|8|@]|9|@$A|1V|B|1W|1|1X]]|C|$]]|$1|P|3|Q|5|6|7|1Y|8|@$A|1Z|B|20|R|S]]|9|@]|C|$]]|$1|T|3|U|5|V|7|21|8|@]|9|@]|C|$W|X]]|$1|Y|3|Z|5|6|7|22|8|@$A|23|B|24|R|S]]|9|@]|C|$]]|$1|10|3|-4|5|6|7|25|8|@]|9|@]|C|$]]|$1|11|3|12|5|H|7|26|8|@]|9|@]|C|$]]|$1|13|3|-4|5|6|7|27|8|@]|9|@]|C|$]]|$1|14|3|15|5|6|7|28|8|@]|9|@]|C|$]]|$1|16|3|17|5|6|7|29|8|@]|9|@$A|2A|B|2B|1|2C]]|C|$]]|$1|18|3|-4|5|6|7|2D|8|@]|9|@]|C|$]]]|19|$1A|$5|1B|1C|1D|C|$1E|1F]]|1G|$5|1B|1C|1D|C|$1E|1H]]|1I|$5|1B|1C|1D|C|$1E|1J]]]]

You're probably thinking of <a href="http://groovy.codehaus.org/api/groovy/lang/MetaClass.html" rel="noreferrer">Groovy's MetaClass</a>:

<blockquote>
 A MetaClass within Groovy defines the
 behaviour of any given Groovy or Java
 class. The MetaClass interface defines
 two parts. The client API, which is
 defined via the extend
 MetaObjectProtocol interface and the
 contract with the Groovy runtime
 system. In general the compiler and
 Groovy runtime engine interact with
 methods on this class whilst MetaClass
 clients interact with the method
 defined by the MetaObjectProtocol
 interface
</blockquote>

<hr>

The Groovy MetaClass lets you assign behavior and state to Classes at runtime without editing the original source code, it's a layer above the original Class.

It's the mechanism used by Groovy to <a href="http://groovy.codehaus.org/groovy-jdk/" rel="noreferrer">extend the Java JDK objects</a>.

Example:

<pre><code>Object.class.metaClass.explode{-&gt; println "Boom! ${delegate} Exploded!"}
"SomeString".explode();
12345.explode();
</code></pre>

Output:

<blockquote>
 Boom! SomeString Exploded! 
 Boom! 12345 Exploded! 
</blockquote>

For more advanced usage, read this: <a href="http://webcache.googleusercontent.com/search?q=cache:groovy.codehaus.org/JN3525-MetaClasses" rel="noreferrer">MetaClasses</a>

What is the use of Meta-Class in Groovy and other OO programming languages?

问Groovy的MetaClass是用来做什么的？
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