Spring form validation with conversion - spring

Question

In a form I take a comma separated list of integers and convert them to an ArrayList<Integer>. This works fine. My form-backing object is:
public class FormDto {
#NotNull
private ArrayList<Integer> nList;
// getters and setters
}
In the form I have:
<input type="text" class="form-control" id="nList" th:field="*{nList}" required="required"/>
Apparently spring automatically converts the string into an ArrayList. Just in case I also created a Converter:
public class CSVStringToListConverter implements Converter<String, ArrayList<Integer>>{
#Override
public ArrayList<Integer> convert(String arg0) {
ArrayList<Integer> list = new ArrayList<Integer>();
for (String s : arg0.split(",")) {
list.add(Integer.parseInt(s));
}
return list;
}
}
which I add in
#Configuration
#EnableWebMvc
public class MvcConfig extends WebMvcConfigurerAdapter implements ApplicationContextAware {
#Override
public void addFormatters(FormatterRegistry registry) {
registry.addConverter(new CSVStringToListConverter());
}
}
Is this all I have to do in order to make converters work? As I wrote, the conversion also works without this converter, so I am unsure whether my converter is actually used or whether it still uses the built-in converter.
My main question however is how to validate the input String (not the ArrayList)? For form fields without conversion I just do that in the controller with:
#PostMapping("/submitForm")
public String formSubmit(#ModelAttribute #Valid SortingExplicitDto sortingExplicitDto, BindingResult result, Model model) {
if(result.hasErrors()) {
model.addAttribute("formDto", new formDto());
return "submitForm";
}
}
Is it possible to validate the input String in the same way, or do I have to do that with Javascript or in the converter?
addition
After a few more tests, I noticed that it does use my custom converter, which does answer my first question. Still I am wondering how to deal with the validation. If I enter some random string which are not integers separated by comma, I receive an exception:
Failed to convert property value of type [java.lang.String] to required type [java.util.ArrayList] for property nList; nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [#javax.validation.constraints.NotNull java.util.ArrayList<java.lang.Integer>] for value ,2,3; nested exception is java.lang.NumberFormatException: For input string:
Also I can do all my validation inside the CSVStringToListConverter and throw an exception if validation fails. I believe I should then catch the exception in the controller and somehow display a nice human readable message in the form. In the html, I display errors (including the one above) with:
<span id="errornList" class="alert alert-danger" th:if="${#fields.hasErrors('nList')}" th:errors="*{nList}">NList error</span>
How do I catch the exception in the controller and put a nice error message there?

Answer 1

Your converter can be this way:
#Component
public class CSVStringToListConverter implements Converter<String, List<Integer>> {
#Autowired
private Errors errors;
#Override
public List<Integer> convert(String arg0) {
List<Integer> list = new ArrayList<Integer>();
for (String s : arg0.split(",")) {
try {
list.add(Integer.parseInt(s));
} catch (NumberFormatException e) {
list = Collections.emptyList();
errors.rejectValue("fieldName", "fieldName.invalid", "Invalid integer in list");
}
}
return list;
}}
Register the Converter in a Configuration class
#Configuration
public class WebConfig implements WebMvcConfigurer {
#Override
public void addFormatters(FormatterRegistry registry) {
registry.addConverter(new CSVStringToListConverter());
}
}
I only Injected the "Errors" class in order to display "nice human readable message in the form" as you wish to have.
Use "org.springframework.validation.BindException" implementation of Errors class, since Errors is an interface, preferably in a configuration class and then the injection should work.