# Delegated property vs JPA field annotation

**URL:** <https://discuss.kotlinlang.org/t/delegated-property-vs-jpa-field-annotation/8654>\
**Category:** Language Design\
**Created:** [July 19, 2018, 2:59pm UTC](https://discuss.kotlinlang.org/t/delegated-property-vs-jpa-field-annotation/8654 "2018-07-19T14:59:36Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![snorbi](https://avatars.discourse-cdn.com/v4/letter/s/94ad74/32.png) [@snorbi](https://discuss.kotlinlang.org/u/snorbi)\
**Post date:** [July 19, 2018, 2:59pm UTC](https://discuss.kotlinlang.org/t/delegated-property-vs-jpa-field-annotation/8654/1 "2018-07-19T14:59:36Z")

</div>

I’m experimenting with property delegates + JPA.

I implemented a very simple property delegate:

```
class MutableNonSynchronizedLazy<T>(private val init: () -> T) : ReadWriteProperty<Any?, T> {

    private var value: T? = null

    override fun getValue(thisRef: Any?, property: KProperty<*>): T {
        if (value == null) {
            value = init()
        }
        return value!!
    }

    override fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
        this.value = value
    }
}

fun <T> mutableNonSynchronizedLazy(init: () -> T) = MutableNonSynchronizedLazy(init)

```

My problem is that I don’t find a way to add an annotation to a field delegated by `MutableNonSynchronizedLazy`:

```
@Entity
class SomeEntity {
    @field:ElementCollection(fetch = FetchType.EAGER)
    var map: Map<String, String?> by mutableNonSynchronizedLazy { HashMap<String, String?>() }
}

```

I get the error:

> ‘@field:’ annotations could be applied only to properties with backing fields

Maybe is there a way to use the “original” backing field of the property in the delegate’s `getValue()` and `setValue()` methods?

(I know that it is possible to define JPA annotations on getters instead of fields - but I prefer the latter.)

---

<div class="post-metadata">

**Author:** ![Wasabi375](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/wasabi375/32/4741_2.png) [@Wasabi375](https://discuss.kotlinlang.org/u/Wasabi375)\
**Post date:** [July 19, 2018, 3:56pm UTC](https://discuss.kotlinlang.org/t/delegated-property-vs-jpa-field-annotation/8654/2 "2018-07-19T15:56:19Z")

</div>

By “original” backing field I expect you mean the backing field used by the delegate class? It’s not possible.

```kotlin
var value by mutableNonSynchronizedLazy { ... }

```

basically get’s translated to the following code

```kotlin
invisible val delegateObject = mutableNonSynchronizedLazy{...}
var value
    get() = delegateObject.getValue()
    set(v) { delegateObject.setValue(v) }

```

So what you are asking for is that the annotation you apply to a value gets somehow applied to a value in a different class, which might be from a library or some external source. Also what would happen if you apply a different annotation somewhere else? Do all variables using this delegate type share the annotations?  
I think you see why you can’t apply annotations to the field of delegated variables.

---

<div class="post-metadata">

**Author:** ![snorbi](https://avatars.discourse-cdn.com/v4/letter/s/94ad74/32.png) [@snorbi](https://discuss.kotlinlang.org/u/snorbi)\
**Post date:** [July 19, 2018, 4:20pm UTC](https://discuss.kotlinlang.org/t/delegated-property-vs-jpa-field-annotation/8654/3 "2018-07-19T16:20:23Z")

</div>

Thanks for the quick reply, it sounds logical 🙂  
I think I will use simply this version:

```
@Entity
class SomeEntity {
    @ElementCollection(fetch = FetchType.EAGER)
    private var map: Map<String, String>? = null

    val mapAccessor: Map<String, String>
        get() {
            if (map == null) {
                map = HashMap()
            }
            return map ?: throw IllegalStateException()
        }
}

```