This command line tool validates your XML files for proper formatting. If they are not formatted correctly, it prints the difference and exits with an error. You can use it two ways: 1) to fail your build if any XML-ish files (for example, XML, XSD, XSL, or XHTML) are not formatted correctly, and 2) to format them correctly using --fix option.

Read this blog post first: XCOP—XML Style Checker.

Make sure you have Ruby installed and then install the tool:

$ gem install xcop

Run it locally and read its output:

$ xcop --help

To validate formatting of your XML files just pass their names as arguments:

$ xcop file1.xml file2.xml

If your files are not formatted correctly and xcop complains, you can ask it to "beautify" them, using --fix option:

$ xcop --fix broken-file.xml

To fix all files in the directory you can do (won't work if your file names contain spaces):

$ xcop --fix $(find . -name '*.xml')

You can put command line options into .xcop file in the directory where you start xcop. Each option should take a single line in the file. They all will be added to the list of options you specify. For example:

--license LICENSE.txt
--nocolor
--quiet
--include **/*
--exclude **/*.xsl
--exclude **/*.html

You can also create ~/.xcop file (in your personal home directory), which will also be read and added to the command line options.

This is what you need there:

require 'xcop/rake_task'
desc 'Run XCop on all XML/XSL files in all directories'
Xcop::RakeTask.new(:xcop) do |task|
  task.license = 'LICENSE.txt' # no license by default
  task.quiet = true # FALSE by default
  task.includes = ['**/*.xml', '**/*.xsl'] # xml|xsd|xhtml|xsl|html by default
  task.excludes = ['target/**/*'] # empty by default
end

Create new workflow file in repository under .github/workflows/xcop.yml:

---
name: XCOP
"on":
  # run on push to master events
  push:
    branches:
      - master
  # run on pull requests to master
  pull_request:
    branches:
      - master
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: g4s8/xcop-action@master

To customize license location or files pattern use action inputs license and files:

- uses: g4s8/xcop-action@master
  with:
    license: MY_LICENSE.txt
    files: "src/*.xml"

You can integrate it with the help of maven-antrun-plugin:

<project>
  [...]
  <build>
    [...]
    <plugins>
      [...]
      <plugin>
        <artifactId>maven-antrun-plugin</artifactId>
        <version>1.8</version>
        <executions>
          <execution>
            <phase>verify</phase>
            <configuration>
              <target>
                <apply executable="xcop" failonerror="true">
                  <arg value="--license"/>
                  <arg value="LICENSE.txt"/>
                  <fileset dir=".">
                    <include name="**/*.xml"/>
                    <include name="**/*.xsd"/>
                    <exclude name="target/**/*"/>
                    <exclude name=".idea/**/*"/>
                  </fileset>
                </apply>
              </target>
            </configuration>
            <goals>
              <goal>run</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
</project>

Something like this should work:

<project>
  [...]
  <target name="xcop">
    <apply executable="xcop" failonerror="true">
      <arg value="--license"/>
      <arg value="LICENSE.txt"/>
      <fileset dir=".">
        <include name="**/*.xml"/>
        <include name="**/*.xsd"/>
        <exclude name="target/**/*"/>
        <exclude name=".idea/**/*"/>
      </fileset>
    </apply>
  </target>
</project>

Read these guidelines. Make sure you build is green before you contribute your pull request. You will need to have Ruby 2.3+ and Bundler installed. Then:

$ bundle update
$ bundle exec rake

If it's clean and you don't see any error messages, submit your pull request.