问题
I saw this in my generated GSP pages. What does the ? mean?
<g:textField name="name" value="${phoneInstance?.name}" />
回答1:
It's the "Safe Navigation Operator", which is a Groovy feature that concisely avoids null pointer exceptions. See http://docs.groovy-lang.org/latest/html/documentation/index.html#_safe_navigation_operator
In this case, if phoneInstance
is null, then it doesn't try to get the name
property and cause a NPE - it just sets the value of the field tag to null.
回答2:
The ?
operator allows null values in Groovy (and thusly, GSP). For example, normally in gsp,
<g:field name="amount" value="${priceDetails.amount}" />
If priceDetails
is null, this will throw a NullPointerException
.
If we use the ?
operator instead ...
<g:field name="amount" value="${priceDetails?.amount}" />
now the value of ${priceDetails?.amount}
is null, instead of throwing a null pointer exception.
回答3:
This is very important feature in Groovy. If the object is null (ie, "phoneInstance" is null) then it's provide "null" value. This feature is called "Safe Navigation Operator". Simply when we use this feature , No need of checking the object("phoneInstance") is null or not.
回答4:
the safe navigation operator (?.) returns null if the object on the left is null, otherwise it returns the value of the right member of that object. so phoneInstance?.name
is just shorthandn for phoneInstance == null ? null : phoneInstance.name
for example:
a = x?.y
is just shorthand for:
a = (x == null ? null : x.y)
which is shorthand for:
if(x == null){
a = null
} else {
a = x.y
}
来源:https://stackoverflow.com/questions/4581532/what-does-the-question-mark-mean-in-gsp-grails