handle duplicate values for Spring @RequestParam

Question

I have a Spring 3.2 Controller with basic request mappings like

@RequestMapping("/action")
public String action(@RequestParam("param") String param) {
    //do stuff...
    return "view";
}

This controller handles links created by non-technical business users. Sometimes the users mess it up and create links with duplicate parameters, e.g.,

www.example.com/action?param=value&param=value

The parameter is an exact duplicate and probably a copy/paste error.

My problem is that Spring is concatenating these dupes together, so that the url above will give "value,value" for param, when I want only "value".

What is a good way to detect and handle these duplicates? I know I could change all my @RequestParams to List<String>s and go from there, but that's a whole lot of boilerplate over dozens of request mappings.

Ideally there would be a way to intercept and modify the url parameters before Spring attempts to bind them -- but only for this controller.

Answer 1

I found that I can register a custom String property editor to do this.

class DuplicateParameterReducingPropertyEditor extends PropertyEditorSupport {

    Object value;

    @Override
    public void setValue(Object value) {
        if (value instanceof String[]) {
            String[] strings = (String[])value;
            Set<String> unique = Sets.newHashSet(strings);
            this.value = unique.toArray();
        } else {
            this.value = value;
        }
    }

    @Override
    public void setAsText(String text) throws IllegalArgumentException {
        this.value = text;
    }

    @Override
    public String getAsText() {
        return value.toString();
    }

    @Override
    public Object getValue() {
        return value;
    }

};

I added this to my controller:

@InitBinder
public void initBinder(WebDataBinder binder) {

    PropertyEditor stringEditor = new DuplicateParameterReducingPropertyEditor();
    binder.registerCustomEditor(String.class, stringEditor);
}

So whenever Spring encounters a @RequestParam-annotated String method argument, the PropertyEditor is invoked to transform the incoming data if needed. In the case of duplicate parameters, Spring passes a String[] of the values to the property editor setValue, which I can then manipulate.

This does have the results I am looking for. I'm not sure of all the implications of this, though, so I can't endorse it as good solution yet. Not having to alter any handler method signatures is a big plus though.

Answer 2

A good idea would be to extend AbstractNamedValueMethodArgumentResolver with your own strategy. Then the strategy could be used wherever you deem necessary.

This strategy only works for Spring 3.1+ which is not a problem for you since you are using Spring 3.2

Answer 3

I faced the same issue in Spring boot. Eventually I came up with this solution using converter, in case it helps anyone.

This method should be added as part of your WebMvcConfigurer class.

@Override
public void addFormatters(FormatterRegistry registry) {
    // Duplicate query parameters converter
    registry.addConverter(new Converter<String[], String>() {
        public String convert(String[] arr) {
            return arr[arr.length - 1];             // Return the last value
        }
    });
}