Should getters be validated when they are called?

The expert group is agonizing on a specific issue. We need your feedback. Should getters be considered regular methods and thus be validated when called?

The problem

Existing applications put Bean Validation constraints on properties (ie getters). If we enable validations when getters are called, some applications might fail and Bean Validation would not be backward compatible. Besides, it is unlikely that you want to validate genuine getters when they are called. These are state, not operations for the most part.

First off what does it mean to be a getter. A method is a getter if:

  • its name starts with is, has no parameter and its return type is Boolean
  • or its name starts with get and has not parameter

If in your service (say a CDI bean), you have an action method with no parameter and starting with get, and if you have added constraints to validate the return value upon method call, we cannot differentiate this action method from a genuine getter.

We have several solutions to work around the problem and we would like to know which one you prefer.

Solutions

We can use a few levers to work around the issue:

  • ask you to enable method validation explicitly
  • offer a coarse or fine grained solution to change the default behavior

Solution 1: enable method validation out of the box

If method validation is enabled out of the box then the sensible default is to exclude getters from method validation.

This approach is friendly out of the box and will work as expected most of the time (except for action methods with no parameter, starting with get and with constraints on the return value).

The downside of this approach is that in this very specific case where an action method is also a getter, method validation would be disabled out of the box and a manual intervention would be necessary.

You can change the default approach in two ways:

Solution 1.a: global flag

Use a global flag to disable method validation entirely or ask for getters to be validated upon call. You would use validation.xml for that:

<method-validation mode="INCLUDE_GETTERS"/>

There is no way to change the behavior for a specific (set of) class.

Solution 1.b: fine grained flag

An alternative solution is to change method validation behavior in a much more fine-grained approach:

  • set the default approach globally in validation.xml
  • set or override the setting for a given package (including sub-packages?) via @ValidateOnCall as a package annotation (or validation.xml)
  • set or override the setting for a given class via @ValidateOnCall as a type annotation (or validation.xml)
  • set or override the setting for a given method via @ValidateOnCall as a method annotation (or validation.xml)

A @ValidateOnCall annotation can be overridden in validation.xml like we do for constraints declarations.

public class AwesomeService {
    // not a getter - validated by default
    @NotNull Currency provideMainCurrency(@ISO @NotNull String country) { ... }

    // not a getter - validated by default
    @NotNull Currency getAlternativeCurrencies(@ISO @NotNull String country) { ... }

    // getter - must use @ValidateOnCall to activate
    @ValidateOnCall(mode=INCLUDE_GETTERS)
    @NotNull getAllCurrencies() { ... }
}

Note that, we could put @ValidateOnCall(mode=INCLUDE_GETTERS) on the package of service classes

@ValidateOnCall(mode=INCLUDE_GETTERS)
package com.acme.gladiator.action;

In this case, getAllCurrencies() does not need to be annotated with @ValidateOnCall.

Solution 2: disable method validation out of the box

In this situation, a user wanting to enable method validation needs to both:

  • add the constraints on methods
  • add the flag to enable method validation

The method validation flag would both allow it to be enabled and decide if getters should be considered.

This approach is the least surprise approach as nothing is happening that you have not explicitly asked for. The drawback is that it requires a manual intervention to enable method validation in a given archive which is not groovy.

Solution 2.a: global flag

For all archives using method validation, a META-INF/validation.xml file must be added. The file would contain the explicit setting:

<method-validation mode="INCLUDE_GETTER"/>

There is no way to change the behavior for a specific (set of) classes.

Solution 2.b: fine grained flag

As described in the previous section, we could enable method validation at the package, class and method level using either a @ValidateOnCall annotation or via the validation.xml. In this approach, validation.xml is not mandatory to enable method validation provided that you use @ValidateOnCall in your code.

So what's your favorite?

My personal favorite is to enable non-getter method validation out of the box and offer fine-grained options to override the behavior. That's solution 1.b. My reasoning is the following:

  • I want ease of use and method validation enabled by default
  • actions methods named like a getter, with no parameter and constraints on its return value will be rare - return value constraint are less common than parameter methods

Some in the expert group do prefer solution 2.a or 2.b.

What's your take? And why do you prefer this approach?