Hi Alexander,
Here you go:
function transformDocument(callback, document) {
var builder = callback.documentBuilder().copy(document);
var Files = Java.type("java.nio.file.Files");
var Paths = Java.type("java.nio.file.Paths")
var fullPath = document.getStringMetadataValue("fullPath");
var myPath = Paths.get(fullPath);
var BooleanDocumentFieldValue = Java.type("com.hds.ensemble.sdk.model.BooleanDocumentFieldValue");
builder.setMetadata("fileExists", BooleanDocumentFieldValue.builder().setBoolean(Files.exists(myPath)).build());
return builder.build();
}