Fix nullability issue in queuechannel

[augur]

As per@Nullable annotation description,

Methods override should repeat parent {@code @nullable} annotations unless they behave differently.

QueueChannel inherits from AbstractPollableChannel, which has annotation @Nullable on doReceive, but QueueChannel lacked it. It presents a problem in some cases, see discussion here:
https://discuss.kotlinlang.org/t/override-java-method-return-type-nullability/13633

[garyrussell]

I am not sure I understand - seems to be a bug in the compiler.

Are you saying the @Nullable is not inherited by the subclass?

I thought you only had to add @Nullable on the subclass if the super class has @NonNull and the subclass overrides the nullability.

[augur]

I thought you only had to add @Nullable on the subclass if the super class has @NonNull and the subclass overrides the nullability.

Me too, but then I read description of this annotation: https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/lang/Nullable.html

Methods override should repeat parent @nullable annotations unless they behave differently.

Thus, if I understand correct, even if super class has this annotation, sub class also must have it, otherwise it would have no effect.

[garyrussell]

Hmmm - this works fine for me...

package com.example.demo;

import org.springframework.lang.Nullable;

public class Foo {

    @Nullable
    public Object getOne() {
        return "one";
    }

}

package com.example.demo;

public class Bar extends Foo {

    @Override
    public Object getOne() {
        return null;
    }

}

package kot

import com.example.demo.Bar

class Baz: Bar() {

    override fun getOne(): Any? {
        return "two"
    }

}

package kot

fun main() {
    val bar: Baz = Baz()
    val one: Any? = bar.one
    println(one)
}

What's the difference between this and your case?

[augur]

Have you tested with -Xjsr305=strict compiler flag? This is default flag in Spring Initializr generated projects (spring-io/initializr#591)
More info about flag here: https://kotlinlang.org/docs/reference/java-interop.html

[garyrussell]

Interesting, with that I get the error when subclassing QueueChannel but there are no complaints about my Foo/Bar/Baz friends.

I'll go ahead and merge this, but I fear there are many other places in the framework that will need to be addressed 😦

[garyrussell]

Cherry-picked as e5fadc3 to 5.1.x

[artembilan]

See @Inherited JavaDocs:

* <p>Note that this meta-annotation type has no effect if the annotated
 * type is used to annotate anything other than a class.  Note also
 * that this meta-annotation only causes annotations to be inherited
 * from superclasses; annotations on implemented interfaces have no
 * effect.

So, this @Nullable on super method has no effect on the overridden method.

[garyrussell]

No; I didn't mean @Inherited I meant that I expected that the Kotlin compiler would walk up the super classes to find the first annotation (like we do with Spring).

It does seem to silence Sonar, though.

[garyrussell]

Surprisingly, we only have 12 classes with abstract methods marked with @Nullable.

It won't catch them all but fixing those shouldn't be too bad; I'll start tomorrow.

Of course, some of them have many, many subclasses.