Kotlin data classes and mapstruct

[Pozo]

Related to #1298 and #782

A few months ago I came to the same conclusion with the all-open and the no-arg plugin like @thePhil and @nilotpal1981. However I really wanted to avoid using something like

data class Person(var firstName: String?, var lastName: String?, var phoneNumber: String?, var birthdate: LocalDate?) {
    // Necessary for MapStruct
    constructor() : this(null, null, null, null)
}

Luckily since mapstruct 1.3.0.Beta2 it's possible to use builders for immutable classes. According to the documentation we can implement our custom builder provider logic. And the holiday season just came :) and I could create a proof of concept which is focusing on Kotlin's data classes.

So you can write down something like this:

@KotlinBuilder
data class Person(var firstName: String, var lastName: String, var phoneNumber: String, var birthdate: LocalDate)

The main idea is generating a builder for each data class and pass these builders to mapstruct via a custom BuilderProvider.

As I mentioned this is a proof of concept repository but I want to develop it further. If you think that this might be a way to use mapstruct with kotlin please share your idea or request and create an Issue here:

https://github.com/Pozo/mapstruct-kotlin

[filiphr]

I had a brief look at the repository. The idea is OK. However, I think that implementing #73 is the correct way for implementing this. When MapStruct can use constructors for creating objects there won't be any difference between a normal Java class with constructor and Kotlin data classes.

What do you think about #73?

[ahulyk]

@filiphr any news on that?

[ahulyk]

@Pozo i like your approach. It looks ok for now. Do you have plans to release it?

[gildor]

However I really wanted to avoid using something like

Why not just add default params to primary constructor?

data class Person(
    var firstName: String? = null, // may be not only null
    var lastName: String? = null,
    var phoneNumber: String? = null,
    var birthdate: LocalDate? = null
)

In this case default constructor also will be created

[ahulyk]

@gildor
In most cases we want to avoid mutability (especially on DTO layer):

data class Person(
    val firstName: String,
    val lastName: String,
    val phoneNumber: String,
    val birthdate: LocalDate
)

[Pozo]

@filiphr Thank you for the information. I read through the #73 and I must agree with you. That would be a more generic way to achieve immutable class mapping, however my work was focusing on Kotlin’s data classes.

When I started the implementation I’ve found #847 PR which seemed abandoned, and a few comments #73 (comment) #73 (comment) about the difficulties of a generic solution, so I decided to go only for Kotlin’s data classes.

@ahulyk Thank you for your interest, however as I mentioned this is only a proof of concept right now. I just want to see how many people agree with this idea so I don’t plan to create any release yet.

@gildor Thank you four your suggestion, however in this case I'm agreeing with @ahulyk, When I’m using pure Kotlin I really want to avoid something like you just wrote. I understand that it’s working but I think we should use Kotlin in an idiomatic way when It’s possible and one of It’s strengths is the immutability by default.

[FearlessHyena]

Although @Pozo 's implementation is still PoC I still agree with @ahulyk that it's a cleaner and better approach than having to define a mutable data class
Any plans on releasing or maybe having something similar for Kotlin?

[Pozo]

@FearlessHyena I decided to push it a bit forward. I've made several adjustment on the repository today, so you can try it out with the help of jitpack.io. Don't forget to raise a ticket or open an issue if you have something to share.

[FearlessHyena]

@Pozo This is great!!! :D
Currently using this in my project and it works without any issues 👍
I'll raise a ticket if I notice any problems
It would be really nice if this or atleast something similar were merged into the main project. So much cleaner to use!

[Sam-Kruglov]

@Pozo works for me, thank you!
my configuration:

...
<dependency>
    <groupId>org.mapstruct</groupId>
    <artifactId>mapstruct</artifactId>
    <version>${mapstruct.version}</version>
</dependency>
<dependency>
    <groupId>com.github.pozo.mapstruct-kotlin</groupId>
    <artifactId>mapstruct-kotlin</artifactId>
    <version>${mapstruct.version}</version>
</dependency>
...
<executions>
    <execution>
        <id>compile</id>
        <goals>
            <goal>compile</goal>
        </goals>
    </execution>
    <execution>
        <id>kapt</id>
        <goals>
            <goal>kapt</goal>
        </goals>
        <configuration>
            <sourceDirs>
                <sourceDir>src/main/java</sourceDir>
            </sourceDirs>
            <annotationProcessorPaths>
                <annotationProcessorPath>
                    <groupId>com.github.pozo.mapstruct-kotlin</groupId>
                    <artifactId>mapstruct-kotlin-processor</artifactId>
                    <version>${mapstruct.version}</version>
                </annotationProcessorPath>
            </annotationProcessorPaths>
            <annotationProcessorArgs>
                <arg>mapstruct.defaultComponentModel=spring</arg>
                <arg>mapstruct.unmappedTargetPolicy=ERROR</arg>
            </annotationProcessorArgs>
        </configuration>
    </execution>
</executions>

Also, it creates an empty folder under target/generated-sources/kaptKotlin.

[FearlessHyena]

Hi @Pozo I've been using it for a few weeks now and haven't seen any issues
Any plans on removing that Beta tag and making this official? :)

[Sam-Kruglov]

Hey, @Pozo one more vote for production!

[lkjh654]

@Pozo +1

[Sam-Kruglov]

I would make 1 comment: instead of MyClassBuilder.builder().setField("value").create() I would prefer MyClassBuilder.builder().field("value").build(). Don't know if mapstruct will work with that though