Myfaces 2.0.5 : Disable validation if no value entered in field

With these it helps This is already the default behaviour. Your problem is caused elsewhere. Perhaps you used an int instead of Integer or String, so that it defaults to 0 instead of null. Or perhaps you used an Integer while running the webapp on Tomcat/JBoss wherein the default Apache EL implementation would implicitly coerce it to 0 instead of null. You can turn it off by adding the following VM argument:

-Dorg.apache.el.COERCE_TO_ZERO=false

I think the issue was by ths following , Give your order button an id, and when changing the input value to enable it:

$("#order-button").removeAttr('disabled');

$("#order-button").attr('disabled', 'disabled');

it fixes the issue You need to initialize count value every time ,because you want to check that value is the statement of having 0 value or not.Put it that setting value in .change() function

$(document).ready(function() {
   $('input').change(function()  {
      var count = 0;
      $("input").each(function() {           
         if($(this).val() == "0")
         {
          count = 1;
         }
       });          

       if(count == 0)
            {

          $("#submitBtn").removeAttr("disabled","disabled");

            }
        else
            {
          $("#submitBtn").attr("disabled","disabled");                          
            }
      });
   });

To fix this issue You can use Hibernate Validator to validate the @RequestParam of your controller.
Add this dependency to your pom.xml

<dependency>
    <groupId>org.hibernate.validator</groupId>
    <artifactId>hibernate-validator</artifactId>
    <version>6.0.10.Final</version>
</dependency>

@RestController
@RequestMapping("/")
@Validated
public class Controller {
    // ...
}

@RequestMapping(value = "/convert", method = RequestMethod.GET, produces = "application/json")
    public ExchangeRateDTO converting(@RequestParam("fromCurrency") @NotNull @NotBlank @Size(max = 10) String fromCurrency,
                                      @RequestParam("toCurrency") String toCurrency,
                                      @RequestParam("amount") String amount) throws IOException {
        if (!Currency.getAvailableCurrencies().contains(Currency.getInstance(fromCurrency))) {

        }
        BigDecimal convertedAmount = currencyExchangeService.convert(fromCurrency, toCurrency, new BigDecimal(amount));