How to perform validation on @RequestParam in Spring

Question

I'm a newbie in java spring. I have checked for solutions online and I can't seem to get my hands on anything helpful.

I have a form that has been bound to an Entity, however, I have a field in that form that was not bound, which I receive as a requestParam. So when the field is submitted I need to validate that parameter to ensure it's not empty.

    @PostMapping("/store")
    public String store(@Valid Quote quote,BindingResult result, Model model,@RequestParam(required=false) String [] tagsFrom) {
        
        if(result.hasErrors()) {
            model.addAttribute("jsontags", returnTagsAsJson(tagRepo.findAll()));
            return "admin/quotes/create";
            
        }
        List<Tag> listtags = new ArrayList<Tag> ();

        for(String tag : tagsFrom) {
            
            Tag theTag = new Tag();
                    
            theTag.setTag(tag);
            theTag.setCreatedAt(new Date());
            theTag.setUpdatedAt(new Date());
                    
                    
            if(tagRepo.findByTag(tag) == null) {
                 tagRepo.save(theTag);
            }
                    
            listtags.add(tagRepo.findByTag(tag));
        
        }
        
        
            quote.setTags(listtags);

            quoteRepo.save(quote);
        
        return "redirect:/dashboard/quotes";
    }

What I have tried; I created a custom validation and added the annotation to the parameter but that gave an error "The annotation @EmptyArrayString is disallowed for this location"

public String store(@Valid Quote quote,BindingResult result, Model model,
      @RequestParam(required=false) @EmptyArrayString String [] tagsFrom)

I have tried using @NotEmpty on the parameter which throws NullPointerException

I need a solution that allows me to display the error on the HTML form like this

 <span th:if="${#fields.hasErrors('tags')}" 
                    th:errors="${quote.tags}" class="errors">
 </span>

Answer 1

So when the field is submitted I need to validate that parameter to ensure it's not empty.

,@RequestParam(required=false) String [] tagsFrom

By default, required is set to true. So, if the URL must have the param, you shouldn't do required=false .

String [] tagsFrom implies you expect a bunch of tag params. But, is it of the form
http://localhost:xxx?param=1,2,3 or
http://localhost:xxx?param1=1&param2="stringvalue" ?

For the first one, you can have the mapping method as:
public String store(...@RequestParam List<String> param)

For the second one, you can do:
public String store(...@RequestParam Map<String,String> allQueryParameters)

You can then do your necessary validations.

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