Load, modify, and write an XML document in Groovy

There's a pretty exhaustive set of examples for reading/writing XML using Groovy here. Regarding the loading/saving the data to/from a file, the various methods/properties that Groovy adds to java.io.File, should provide the functionality you need. Examples include:

File.write(text)
File.text
File.withWriter(Closure closure) 

See here for a complete list of these methods/properties.

If you want to use the XmlSlurper:

//Open file
def xml = new XmlSlurper().parse('/tmp/file.xml')

//Edit File e.g. append an element called foo with attribute bar
xml.appendNode {
   foo(bar: "bar value")
}

//Save File
def writer = new FileWriter('/tmp/file.xml')

//Option 1: Write XML all on one line
def builder = new StreamingMarkupBuilder()
writer << builder.bind {
  mkp.yield xml
}

//Option 2: Pretty print XML
XmlUtil.serialize(xml, writer)

Note: XmlUtil can also be used with the XmlParser as used in @John Wagenleitner's example.

References:

• Option 1
• Option 2

You can just modify the node's value property to modify the values of elements.

/* input:
<root>
  <foo>
    <bar id="test">
      test
    </bar>
    <baz id="test">
      test
    </baz>
  </foo>
</root>
*/

def xmlFile = "/tmp/test.xml"
def xml = new XmlParser().parse(xmlFile)
xml.foo[0].each { 
    it.@id = "test2"
    it.value = "test2"
}
new XmlNodePrinter(new PrintWriter(new FileWriter(xmlFile))).print(xml)

/* output:
<root>
  <foo>
    <bar id="test2">
      test2
    </bar>
    <baz id="test2">
      test2
    </baz>
  </foo>
</root>
*/