{"id":6740,"date":"2012-09-24T20:12:48","date_gmt":"2012-09-24T14:42:48","guid":{"rendered":"https:\/\/2thenew.xyz\/blog\/?p=6740"},"modified":"2015-08-26T15:28:57","modified_gmt":"2015-08-26T09:58:57","slug":"metaprogramming-with-metaclass","status":"publish","type":"post","link":"https:\/\/2thenew.xyz\/blog\/metaprogramming-with-metaclass\/","title":{"rendered":"MetaProgramming with MetaClass"},"content":{"rendered":"

As we all know that groovy<\/strong> is a dynamic programming language, so we can add any method to any class at runtime. First, let\u2019s see a small and simple example of metaProgamming<\/strong> given below, in which we are adding a method (isEvent) into an Integer class.<\/p>\n

[groovy]
\nInteger.metaClass.isEven = { -> \/\/ only (->) sign indicates that isEven() method is no argument method
\n delegate%2 == 0
\n}
\n[\/groovy]<\/p>\n

When we see the above program few questions arise in our mind, what is metaClass<\/strong>? What is delegate<\/strong>?? How metaprogramming<\/strong> is actually working?? How to disable MetaProgramming<\/strong>?? isEven() method a static or instance method?? etc… Let\u2019s move on to every question one by one.<\/p>\n

1. What is metaClass<\/strong>?? In Groovy<\/strong> Language, every object has an object of MetaClass<\/strong> class with name metaClass<\/strong>. This metaClass<\/strong> object is responsible for holding all the information related to that object. Whenever you perform any operation on that object, Groovy’s<\/strong> dispatch mechanism routes the call through that Metaclass<\/strong> object(metaClass<\/strong>). So if you want to change the behaviour of any object\/class, you will have to alter the MetaClass<\/strong> object attached to that class\/object, and it will alter the behaviour of that class\/object at run time.<\/p>\n

2. What is delegate<\/strong>?? We can say delegate<\/strong> is reference of that object who is invoking that isEven() method.<\/p>\n

3. How metaprogramming<\/strong> is actually working?? Whenever we call a method of class\/object, it first gets information of that object from metaClass<\/strong> attached to it, and then calls the appropriate method. In above program we are asking Integer metaClass<\/strong>, that there is an isEven() method in integer class, whose definition is followed by it. Now when we will call \u201canyInteger.isEven()\u201d, it will excute isEven() method and will return true\/false accordingly, e.g.<\/p>\n

[groovy]
\n9.isEven() \/\/ false
\n8.isEven() \/\/ true
\n[\/groovy]<\/p>\n

4. How to disable MetaProgramming<\/strong>?? Once a method is added into any class\/object, that method will be available in complete application. If you want remove that method from a particular situation you will have to assign null<\/strong> into Integer.metaClass<\/strong>. e.g.<\/p>\n

[groovy]
\nInteger.metaClass = null
\n7.isEven() \/\/ will throw MissingMethodException
\n[\/groovy]<\/p>\n

5. isEven() method is static or instance method?? isEven() is an instance method. To add a static method at run time, you will have to follow signature given below:<\/p>\n

[groovy]
\nInteger.metaClass.static.isEven = { number ->
\n number%2 == 0
\n}
\nInteger.isEven(1) \/\/ false
\nInteger.isEven(2) \/\/ true
\n[\/groovy]<\/p>\n

6. isEven() method is added to that object only?? No, isEven() method is added to all the Integer objects, if we want to add isEven() in a particular object only then we have to use that object reference .metaClass instead of Integer.metaClass, e.g.<\/p>\n

[groovy]
\nInteger aNumber = 9
\naNumber.metaClass.isEven = { ->
\n delegate%2 == 0
\n}
\nprintln aNumber.isEven() \/\/ false
\nprintln 2.isEven() \/\/ will throw MissingMethodException.
\n[\/groovy]<\/p>\n

7. How to add multiple methods?? To add the multiple method into single class see below example:<\/p>\n

[groovy]
\nInteger.metaClass {
\n isEven { -> delegate%2 == 0 }
\n isOdd { -> delegate%2 != 0 }
\n \/\/ other methods
\n}<\/p>\n

println 6.isEven() \/\/ true
\nprintln 6.isOdd() \/\/ false
\n[\/groovy]<\/p>\n

More Blogs by Me<\/a><\/strong><\/p>\n","protected":false},"excerpt":{"rendered":"

As we all know that groovy is a dynamic programming language, so we can add any method to any class at runtime. First, let\u2019s see a small and simple example of metaProgamming given below, in which we are adding a method (isEvent) into an Integer class. [groovy] Integer.metaClass.isEven = { -> \/\/ only (->) sign […]<\/p>\n","protected":false},"author":52,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"iawp_total_views":17,"footnotes":""},"categories":[7],"tags":[885,9,632,145],"class_list":["post-6740","post","type-post","status-publish","format-standard","hentry","category-grails","tag-delegate","tag-groovy","tag-metaclass","tag-metaprogramming"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t