How to add validation for file type in grails - validation

Question

I have a domain object with following:
class Color {
String name
String fileLocation
static constraints = {
name (nullable: false, blank: false)
}
}
In my controller I'm doing the following:
def save() {
def colorInstance = new Color(params)
if (colorInstance.save(flush: true)) {
def file = request.getFile("myfile")
if (!file.empty && uploadService.isFileAllowed(file)) {
uploadService.uploadFile(file, file.originalName, "folderName")
}
}
else {
render (view: "create", model: [coorInstance: colorInstance])
}
}
This all works fine however, I'm not sure how to throw an error when the uploaded file isn't what is allowed. i.e. uploadService.isFileAllowed(file) returns false ??
How can I throw an error back to the user saying
Uploaded file isn't allowed
when uploadService.isFileAllowed(file) returns false ?
Note:
The isFileAllowed method is reading first few bytes of a file to determine what type of file it is.

Answer 1

What if you save an error message to flash memory and then render it on the page if it exists? See this post for help.
if (!file.empty && uploadService.isFileAllowed(file)) {
uploadService.uploadFile(file, file.originalName, "folderName")
} else {
flash.error = "Uploaded file isn't allowed"
}

Answer 2

apply this login in your controller
String fileName = "something.ext";
int a = fileName.lastIndexOf(".");
String extName = fileName.substring(a);
System.out.println(fileName.substring(a));
ArrayList<String> extList = new ArrayList<String>();
extList.add("jpg");
extList.add("jpeg");
extList.add("png");
if(extList.contains(extName))
{
System.out.println("proceed");
}
else{
System.out.println("throw exception");
}

Answer 3

So if isFileAllowed returns false or the file is empty, it will add an error to the colorInstance to the fileLocation property. It will only upload the file if the colorInstance validates successfully (to prevent files uploaded for unsaved objects).
As a side note, I prefer saving files in tables partly for this reason. It makes validation much less clunky and its impossible to have a disconnect between your objects and the files. - Just my 2c.
def save() {
def colorInstance = new Color(params)
def file = request.getFile("myfile")
if (!file.empty && uploadService.isFileAllowed(file)) {
if (colorInstance.validate()) {
uploadService.uploadFile(file, file.originalName, "folderName")
}
}
else {
colorInstance.errors.rejectValue('fileLocation','error.message.code.here')
}
if (colorInstance.save(flush: true)) {
//do whatever here
}
else {
render (view: "create", model: [coorInstance: colorInstance])
}
}