Validate XML with VS Code

I just wanted to know if my xml syntax was valid, and I didn't want to have to install and setup Java.

XML Tools from Qub worked great for me, I highly recommend it:
https://marketplace.visualstudio.com/items?itemName=qub.qub-xml-vscode

If you need to validate xml against a schema, some of the other more popular extensions might be better

For validation against a schema there is XML extension by Red Hat https://marketplace.visualstudio.com/items?itemName=redhat.vscode-xml this no longer requires Java since v0.15.0.

It supports a few ways of associating an *.xml with an *.xsd or *.dtd see the docs at https://github.com/redhat-developer/vscode-xml/blob/master/docs/Validation.md#validation-with-xsd-grammar

Most methods involve modifying the files or creating a catalog file but you can also associate files using settings.json.

XSD

"xml.fileAssociations": [
   {
       "pattern": "foo.xml",
       "systemId": "foo.xsd"
   }
]

DTD

"xml.fileAssociations": [
  {
      "pattern": "foo.xml",
      "systemId": "foo.dtd"
  }
]

File paths support wildcards and VS Code variables ${workspaceFolder}, ${fileDirname}, ${fileBasenameNoExtension}

For validating XML Schemas in VS Code there is a package called XML Language Support.