[英]Why is the value of the instance variable set in a Groovy closure not visible outside of it?
Question 题
Why does methodA()
print set within run
instead of set within closure
? 为什么methodA()
打印set within run
而不是set within closure
中set within closure
?
Code example 代码示例
class GroovyClosureVariableScopingTest extends Script {
String s
Object run() {
s = "set within run"
println "value of s in run(): $s"
[1].each {
s = "set within closure"
println "value of s in each()-closure: $s"
methodA()
}
}
void methodA() {
println "value of s in methodA(): $s"
}
}
Actual output 实际输出
value of s in run(): set within run
value of s in each()-closure: set within closure
value of s in methodA(): set within run // <- Surprised to see the original value
Expected output 预期产出
value of s in run(): set in run
value of s in each()-closure: set within closure
value of s in methodA(): set within closure // <- Whould have expected this value
Elaboration 精
I haven't quite understood how variable scoping works in the above example. 我还不太清楚变量作用域如何在上面的例子中起作用。 I would have expected s
to be a property (a public instance variable) of the class and I thought that I was assigning it a value from within the each
-closure which would be retained. 我本来希望s
是类的一个属性(一个公共实例变量),我认为我从each
-closure中分配了一个值,这个值将被保留。 But that does not seem to be the case. 但事实似乎并非如此。
Why does s
not keep the value which it was assigned from within the closure? 为什么s
不保留从闭包中分配的值?
Solution / workaround 解决方案/解决方法
Passing s
as argument to methodA()
works. 将s
作为参数传递给methodA()
有效。 See lines commented with // Change
. 请参阅// Change
评论的行。
class GroovyClosureVariableScopingTest extends Script {
String s
Object run() {
s = "set within run"
println "value of s in run(): $s"
[1].each {
s = "set within closure"
println "value of s in each()-closure: $s"
methodA(s) // Change
}
}
void methodA(s) { // Change
println "value of s in methodA(): $s"
}
}
Reference 参考
Closures - Formal Definition 闭包 - 正式定义
At the same time, the variables are still available normally to the enclosing scope, so the closure may read/change any such values, and code from the outer scope may read/change the same variables. 同时,变量仍然可以正常地用于封闭范围,因此闭包可以读取/更改任何这样的值,并且来自外部范围的代码可以读取/更改相同的变量。
Versions 版本
After user cfrick confirmed that it works on his machine, I ran it from the console and it indeed works. 用户cfrick确认它在他的机器上工作后,我从控制台运行它确实有效。
From console 从控制台
C:\temp>java -jar C:\...\lib\groovy-all-1.8.6.jar C:\temp\GroovyClosureVariableScopingTest.groovy
value of s in run(): set within run
value of s in each()-closure: set within closure
value of s in methodA(): set within closure // <- Here
With clean Eclipse installation 使用干净的Eclipse安装
But I still don't get the original script to run with a clean installation of 但我仍然没有得到原始脚本与干净安装运行
Summary 摘要
I still don't know why it doesn't work -- even with a clean installation of Eclipse. 我仍然不知道为什么它不起作用 - 即使是干净的Eclipse安装。 However, it works from the console, it works on cfrick's machine and there is a workaround. 但是,它可以在控制台上运行,它可以在cfrick的机器上运行,并且有一个解决方法。
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