Why don't I get a NullPointerException in Groovy in this case?

问题

I have this test code:

def test = null

test.each {  } 

Why don't I get any exception?

回答1:

The implementation of each tries to call the iterator method of it's target in a null-safe fashion. If each is called on a null object, or an object without an iterator method, nothing happens.

I haven't seen the source code, but it could look something like this§

Object each(Closure closure) {

  if (this?.respondsTo("iterator")) {

    def iterator = this.iterator()

    while (iterator.hasNext() {
      def item = iterator.next()
      closure(item)
    }
  }
  return this
}

§ In reality, this method is probably written in Java rather than Groovy

回答2:

A null value when using the each closure is the same as a collection with 0 elements. If you have the code

def test=null
test.each {println "In closure with value "+it}

The print statement won't execute. If you change test to

def test=[1,2,3]

you will get output.

来源：https://stackoverflow.com/questions/5472795/why-dont-i-get-a-nullpointerexception-in-groovy-in-this-case