Using the dot at operator to fetch a XML attribute

[bgoetzmann]

Hello,

I ask me a simple question about the use of the .@ operation for accessing a XML attribute.
Here a simple example of parsing a XML document using XmlSlurper class:

def xml = '''
        <test>
                <elm att="attr">elm</elm>
        </test>
'''
def doc = new groovy.util.XmlSlurper().parseText(xml)
println doc.elm.@att // displays 'attr'

In this example, we can use the .@ operator, normally used to access directly to a class attribute, to get the attribute value.

So, my question is: why does it work, and how?


Thank you.

Bertrand.

[bgoetzmann]

Finally I found the reason by loooking at the Groovy's groovy.util.slurpersupport.GPathResult class source code. The groovy.util.XmlSlurper().parseText() method returns a descendant of GPathResult and even doc.elm is a GPathResult.
This class uses a special meta class that permits to change the behavior of the .@ operator.

Suppose you have a Test class:

class Test {
}

this script

def t = new Test()
println t.@attr

fails by a "groovy.lang.MissingFieldException: No such field: attr for class: Test", unless you have a field "attr" defined in the Test class.

Following the example of GPathResult, you can change this behavior by defining a custom meta class:

MetaClass newMetaClass = new DelegatingMetaClass(metaClass) {
    @Override
    public Object getAttribute(Object object, String attribute) {
        "my$attribute"
    }            
}

def t = new Test()
t.metaClass = newMetaClass
println t.@hello

Now, there is no error, and "myhello" is displayed!


Bertrand.

bgoetzmann wrote

Hello,

I ask me a simple question about the use of the .@ operation for accessing a XML attribute.
Here a simple example of parsing a XML document using XmlSlurper class:

def xml = '''
        <test>
                <elm att="attr">elm</elm>
        </test>
'''
def doc = new groovy.util.XmlSlurper().parseText(xml)
println doc.elm.@att // displays 'attr'

In this example, we can use the .@ operator, normally used to access directly to a class attribute, to get the attribute value.

So, my question is: why does it work, and how?


Thank you.

Bertrand.