all-open

Learn the all-open compiler plugin.

The all-open plugin

The all-open plugin makes Kotlin classes with certain annotations open at compile time. This is needed for framework proxies.

Spring's @Configuration, @Service and similar annotations require a CGLIB proxy, which needs the class to be open. In Kotlin classes are final by default. The all-open plugin is added with the open-for-annotation dependency and you declare in Gradle which annotations should be opened. You no longer mark classes open by hand. This is a standard setup in Spring and Micronaut projects.

An all-open example

In this lesson we show by hand what the all-open plugin does. Normally the plugin adds open automatically; here we model it.

Example: open class Entity(val id: Int) { open fun describe(): String = "Entity #$id" }. Then class UserEntity(id: Int) : Entity(id) { override fun describe(): String = "User #$id" }. Here Entity is open; UserEntity extends it and overrides describe. The assignment val e: Entity = UserEntity(42) gives polymorphism; e.describe() calls the subclass's version.

Exercise Write Entity + UserEntity

Define open class Entity(val id: Int) with open fun describe(): String = "Entity #${'$'}id". Define class UserEntity(id: Int) : Entity(id) with override fun describe(): String = "User #${'$'}id". In main, write val e: Entity = UserEntity(42) and println(e.describe()).

Show example solution

open class Entity(val id: Int) {
    open fun describe(): String = "Entity #$id"
}
class UserEntity(id: Int) : Entity(id) {
    override fun describe(): String = "User #$id"
}
fun main() {
    val e: Entity = UserEntity(42)
    println(e.describe())
}

Test yourself

Why is all-open used?
- To make a class final
- To add an annotation
- To open a class for a CGLIB proxy
- To delete the IR
Show answer

To open a class for a CGLIB proxy — Frameworks like Spring need a class to be open for a proxy.

Add the open keyword

____ class Entity(val id: Int) {
    open fun describe(): String = "Entity #$id"
}

Show answer

open class Entity(val id: Int) {
    open fun describe(): String = "Entity #$id"
}

Match all-open parts

open

Enables inheritance
override

Overrides a method
@Configuration

Spring annotation
CGLIB

Proxy library

Next lesson: no-arg

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