Validate to see if form has changed

Question

I am writing a validator which needs to test if a spring form object has changed.

In the validator if no changes have been made to the form, an error should be displayed.

Is there a spring mechanism to do this?

This is because a very expensive webservice update call is made when I submit, and I need to prevent the webservice call from being made if no changes have been made.

Cheers.

Answer 1

I'm not aware of any built-in Spring mechanism to handle this. I'd save a copy of the original object, and the modified object. I'd override the Form.equals() method appropriately (possibly delegating to org.apache.commons.lang.builder.EqualsBuilder.reflectionEquals(), if all of the fields are primitives/strings) and use Form.equals() to check whether the form had changed.

I have seen two distinct responses here which imply that overriding the "hashCode" method is a good way to go. It is not in the contract of hashCode() to guarantee inequality, this could lead to problems. Be sure to override equals() as well. Of course it's a one in a trillion chance of a hash collision, but it's poor design to rely on hashCode() alone when you're trying to check if two objects are different.

Answer 2

Override your hashCode() function to ensure a different value is returned when form values change. Save the return value of the function and test to see whether it's changed.

public class Person {
  String first;
  String last;
  int prevHash;
  @Override
  public int hashCode() {
    // does not include the value of currHash
    String stringHash = String.format("%1$-29s%2$-29s", first,last);
    return stringHash.hashCode();
  }
}

public class PersonValidator  implements Validator {
  public void validate(Object obj, Errors e) {
    Person p = (Person) obj;
    if (p.hashCode() == p.getPrevHash()) {
      e.reject("person", "unchanged");
    } 
  }
  ...
}

I don't think there's a Spring-provided validation test to check whether a form backing object has changed

Note that you could perform additional tests on the client side before allowing the user to submit the form.