How to declare array value in Kotlin annotations?

Question

I have some problem with creating my own annotations in Kotlin. I have to create some annotations and in some of them i need to declare values with array type. In java we can do this:

public @interface JoinTable {
...
    JoinColumn[] inverseJoinColumns() default {};
...
}

Where JoinColumn is also an annotation type.

I want to do something like that in Kotlin:

annotation class JoinTable(
    val name: String,
    val joinColumns: Array<JoinColumn>
)

I also tried to do this:

annotation class JoinTable(
    val name: String,
    val joinColumns: List<JoinColumn>
)

But my IDe says:

Invalid type of annotation member

What should i do?

Thank you!

Very good explanation for your first post btw!

guenhter
– guenhter

2017-07-06 13:47:41 +00:00
Commented Jul 6, 2017 at 13:47 — guenhter
– guenhter, Commented Jul 6, 2017 at 13:47

Nikita Ryanov · Accepted Answer · 2017-07-06 13:51:49Z

4

So, it was my big fault. I didn't notice that JoinColumn in my realization isn't an annotation.

class JoinColumn()

Well, it fixed ^_^:

annotation class JoinColumn()

answered Jul 6, 2017 at 13:51

Nikita Ryanov

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guenhter · Accepted Answer · 2017-07-06 13:44:55Z

1

As in java, the values for annotations must be available at compile time. That means that val joinColumns: List<JoinColumn> is not possible if the JoinColumn is a usual class or data-class. If it's an enum class (enum class JoinColumn), than it's possible to use it.

See also the official kotlin documentation for allowed types https://kotlinlang.org/docs/reference/annotations.html

Allowed parameter types are:

types that correspond to Java primitive types (Int, Long etc.);

strings;

classes (Foo::class);

enums;

other annotations;

arrays of the types listed above.

Annotation parameters cannot have nullable types, because the JVM does not support storing null as a value of an annotation attribute.

edited Jul 6, 2017 at 13:44

answered Jul 6, 2017 at 13:39

guenhter

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1 Comment

Nikita Ryanov Over a year ago

Thank you for your response! I re-checked my annotations and found the mistake. I forgot mark JoinColumn as annotation

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