什么是閉包
一個groovy閉包就像一個代碼塊或者方法指針，他是定義然后執行的一段代碼，但是他有一些特性：隱含變量，支持自由變量，支持currying 。
我們先來看看一些例子：
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def clos = { println "hello!" } |
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println "Executing the Closure:" |
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clos() //prints "hello!" |
在上面的例子中”hello!”是因為調用clos（）函數才打印出來的，而不是在定義的時候打印出來的。
參數
閉包的參數在->之前列出，比如：
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def printSum = { a, b -> print a+b } |
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printSum( 5, 7 ) //prints "12" |
如果閉包的參數是少於2個的話，那么 ->是可以省略的。
Parameter notes
A Closure without -> , i.e. {} , is a Closure with one argument that is implicitly named as ‘it’. (see below for details) In some cases, you need to construct a Closure with zero arguments, e.g. using GString for templating, defining EMC Property etc. You have to explicity define your Closure as { -> } instead of just { }
You can also use varargs as parameters, refer to the Formal Guide for details. A JavaScript-style dynamic args could be simulated, refer to the Informal Guide.
自由變量
閉包能引用在參數列表中沒有列出的變量，這些變量成為自由變量，他們的作用范圍在他們定義的范圍內：
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def myConst = 5 |
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def incByConst = { num -> num + myConst } |
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println incByConst(10) // => 15 |
另外一個例子：
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def localMethod() { |
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def localVariable = new java.util.Date() |
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return { println localVariable } |
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} |
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def clos = localMethod() |
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println "Executing the Closure:" |
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clos() //prints the date when "localVariable" was defined |
隱式變量
it
如果你有一個閉包但是只有一個參數，那么你可以省略這個參數，比如：
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def clos = { print it } |
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clos( "hi there" ) //prints "hi there" |
this, owner, and delegate
this : 和java中是一樣的, this refers to the instance of the enclosing class where a Closure is defined
owner : the enclosing object (this or a surrounding Closure)
delegate : by default the same as owner, but changeable for example in a builder or ExpandoMetaClass
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class Class1 { |
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def closure = { |
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println this.class.name |
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println delegate.class.name |
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def nestedClos = { |
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println owner.class.name |
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} |
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nestedClos() |
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} |
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} |
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def clos = new Class1().closure |
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clos.delegate = this |
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clos() |
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/* prints: |
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Class1 |
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Script1 |
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Class1$_closure1 */ |
閉包作為方法參數
當一個方法使用閉包作為最后一個參數的話，那么就可以內嵌一個閉包，比如：
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def list = ['a','b','c','d'] |
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def newList = [] |
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list.collect( newList ) { |
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it.toUpperCase() |
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} |
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println newList // ["A", "B", "C", "D"] |
在上面的collect方法中，他接受一個一個List和閉包，上面的代碼等價於下面的代碼：
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def list = ['a','b','c','d'] |
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def newList = [] |
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def clos = { it.toUpperCase() } |
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list.collect( newList, clos ) |
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assert newList == ["A", "B", "C", "D"] |
更多的信息：
Groovy繼承了 java.lang.Object並且許多的 Collection 和Map 的許多方法都接受閉包作為參數的 See GDK Extensions to Object for practical uses of Groovy's Closures.
See Also: