Review SpeL relational operators documentation [SPR-14243]

[spring-projects-issues]

Stéphane Nicoll opened SPR-14243 and commented

http://docs.spring.io/spring-framework/docs/current/spring-framework-reference/html/expressions.html#expressions-operators has the following expression

boolean falseValue = parser.parseExpression(
        "'xyz' instanceof T(int)").getValue(Boolean.class);

It does not seem to work, maybe Integer should have been used?

---

Issue Links:

• SpEL Compiler: problems using primitives with instanceof operator [SPR-14250] #18823 SpEL Compiler: problems using primitives with instanceof operator

[spring-projects-issues]

Stéphane Nicoll commented

Andy Clement are the primitive types supported?

1 instanceof T(int) == false
1 instanceof T(Integer) == true
false instanceof T(boolean) == false
false instanceof T(Boolean) == true

The doc looks like they are but they don't seem to be ..

[spring-projects-issues]

Andy Clement commented

The primitives are supported but due to the auto boxing that goes on it can be hard to utilize them. For example:

This expression will evaluate to true: T(int)==T(Integer).TYPE

In those sample expressions in the previous comment the numbers and booleans are immediately boxed.

When you say the original expression doesn't work - how does it fail for you? Does it not parse? It is intended to evaluate to false and either using T(int) or T(Integer) will evaluate to false. It is true though that T(Integer) might be less misleading because it would then be failing just because the left hand operand is a string, rather than for the fact that it is a string and even if it were a number it would have been boxed.

In playing around with this I do think I uncovered a SpEL compiler issue when using these variants of T(..) - I'll raise a separate issue when I get a minute.

[spring-projects-issues]

Andy Clement commented

Raised #18823 to deal with the associated compiler issue.

[spring-projects-issues]

Stéphane Nicoll commented

Andy Clement sorry for the late reply. What is problematic in this case is the difference between primitive and wrapper types. All the four expressions above parse just fine but the outcome is wrong with a primitive and legit with a wrapper.

We have 'xyz' instanceof T(int) in the documentation which evaluates to false (as it should be). However I don't understand why 1 instanceof T(int) evaluates to false as well.

Thoughts?

[spring-projects-issues]

Andy Clement commented

Hey.

However I don't understand why 1 instanceof T(int) evaluates to false as well.

Because 1 is immediately boxed up to the wrapper type, primitives don't flow around as expressions are evaluated, when instanceof sees it, it is already boxed (and currently instanceof handling is pretty basic). So obviously I am talking about what the SpEL implementation code does right now. Could we change the code and make it work and return true? Yes, we could attempt to inspect the left hand object in the instanceof handling code and see if it is the boxed form of the primitive - and return true in that case. We are a bit out on our own here I'd say because we usually do-what-Java-does in most situations but with java you cannot ask for instanceof primitive or ask a primitive if it is an instance of a wrapper type, it won't compile.

Given that in other places we auto-unbox when necessary (e.g. when calling a method that needs an int but you have an Integer) - I could see the argument for similar auto-unboxing here to return true for the instanceof primitive test.

The auto-unboxing when necessary is probably one factor why these instanceof primitive expressions rarely come up - this is the first issue report to discuss it (IIRC) and that triggered from an imaginary use case in the docs. I'm not sure anyone uses it in practice for this purpose. I wonder what a real use case that required use of this construct looks like. If you want to raise an issue to improve instanceof handling for this case, that would be fine. But I'd also change the docs to a more realistic example involving a reference type.

[spring-projects-issues]

Stéphane Nicoll commented

Thanks Andy! I've polished the doc in this commit. Those examples were "imaginary" (actually samples used for the SpEL support in IntelliJ IDEA) and I think the new note in the documentation is enough for now. We can revisit later if a user complains.