Ant buildfiles are just text files, so the most straightforward way to create an Ant buildfile in Eclipse is:
By default the Ant editor only has an association with
build.xml named files.
As the user you can designate the Ant editor to be considered for all xml
*.xmland the Ant editor.
Now, as long as the file has a .xml extension, Eclipse will consider it to be a possible Ant buildfile, and will enable Ant-related actions when it is selected. Until a file has Ant buildfile content, you will need to open it using Open With > Ant Editor.
An example is the use of project builders, where an Ant buildfile is created for an Eclipse plug-in that contains predefined targets that are useful for deploying the plug-in.