Load, modify, and write an XML document in Groovy

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>
*/

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.

For the one who find the output empty, here is the solution:

def xml = file("${projectDir}/src/main/AndroidManifest.xml")
def manifest = new XmlSlurper().parse(file(xml))
manifest.@package = "com.newapp.id"
xml.withWriter {out->
    XmlUtil.serialize(manifest, out)
}