How we use Kotlin with Exposed at TouK

Why Kotlin? At TouK, we try to early adopt technologies. We don’t have a starter project skeleton that is reused in every new project, we want to try something that fits the project needs, even if it’s not that popular yet. We tried Kotlin first it mid 2016, right after reaching 1.0.2 version

Why Kotlin?

At TouK, we try to early adopt technologies. We don’t have a starter project skeleton reused in every new project; we want to try something that fits the project’s needs, even if it’s not that popular yet. We tried Kotlin first in mid-2016, right after reaching the 1.0.2 version. It was getting really popular in Android development, but almost nobody used it on the backend, especially — with production deployment. After reading some “hello world” examples, including this great article by Sebastien Deleuze we decided to try Kotlin as main language for a new MVNO project. The project was mainly a backend for a mobile app, with some integrations with external services (chat, sms, payments) and background tasks regarding customer subscriptions. We felt that Kotlin would be something fresh and more pleasant for developers, but we also liked the “not reinventing the wheel” approach — reusing large parts of the Java/JVM ecosystem we were happy with for existing projects (Spring, Gradle, JUnit, Mockito).

Why Exposed?

We initially felt that Kotlin + JPA/Hibernate is not a perfect match. Kotlin’s functional nature with first-class immutability support was not something that could seemly integrate with full-blown ORM started in the pre-Java8 era. But Sebastien’s article led us to try Exposed — a SQL access library maintained by JetBrains. From the beginning, we really liked the main assumptions of Exposed:

  • not trying to be full ORM framework
  • two flavors — typesafe SQL DSL and DAO/ActiveRecord style
  • lightweight, no reflection
  • no code generation
  • Spring integration
  • no annotations on your domain classes (in SQL DSL flavor)
  • open for extension (e.g. PostGIS and new DB dialects)

TL;DR

If you want to see how we use Kotlin + Exposed duo in our projects, check out this Github repo. It’s a Spring Boot app exposing REST API with the implementation of Medium clone as specified in http://realworld.io (“The mother of all demo apps”).

Another nice example is this repo by Seb Schmidt.

SQL DSL

In our projects we decided to try the “typesafe SQL DSL” flavor of Exposed. In this approach you don’t have to add anything to your domain classes, just need to write a simple schema mapping using Kotlin in configuration-as-code manner:

data class User(
  val username: Username,
  val password: String,
  val email: String
)

object UserTable : Table("users") {
    val username = text("username")
    val email = text("email")
    val password = text("password")
}

And then you can write type/null-safe queries with direct mapping to your domain classes:

UserTable.select { UserTable.username eq username }?.toUser()

// or 

UserTable.select { UserTable.username like username }.map { it.toUser() }

fun ResultRow.toUser() = User(
       username = this[UserTable.username],
       email = this[UserTable.email],
       password = this[UserTable.password]
)

RefIds

We like type-safe RefIds in our domain code. This is particularly useful in DDD-ish architectures, where you can keep those RefIds in a shared domain and use them to communicate between contexts.

So we wrap plan ids (longs, strings) into simple wrapper classes (e.g. UserId, ArticleId, Username, Slug). Exposed allows to easily register your own column types or even generic WrapperColumnType implementation that you can find in our repo.

Using this technique you can rewrite this mapping to something like this:

sealed class UserId : RefId<Long>() {
  object New : UserId() {
    override val value: Long by IdNotPersistedDelegate<Long>()
  }
  data class Persisted(override val value: Long) : UserId() {
    override fun toString() = "UserId(value=$value)"
  }
}

data class User(
   val id: UserId = UserId.New,
   //...
)

fun Table.userId(name: String) = longWrapper<UserId>(name, UserId::Persisted, UserId::value)

object UserTable : Table("users") {
  val id = userId("id").primaryKey().autoIncrement()
//...
}

And now we can query by type-safe RefIds:

override fun findBy(userId: UserId) =
  UserTable.select { UserTable.id eq userId }?.toUser()

Relationship mapping

One of the most significant selling points of ORMs is how easy it is to deal with relations. You just annotate the related field/collection with OneToOne or OneToMany and then can fetch the whole graph of objects at once. In theory — quite a nice idea, but in practice, things often go wrong. I’m not going to dig into details, instead, I recommend you read e.g. these fragments of “Opinionated JPA with Querydsl” book:

In Exposed SQL DSL approach you have to do relationship mapping by yourself — if you need to. Let’s consider Article and Tag case from our project’s domain. We have a many-to-many relation here, so we need additional “article_tags” table:

object ArticleTagTable : Table("article_tags") {
   val tagId = tagId("tag_id").references(TagTable.id)
   val articleId = articleId("article_id").references(ArticleTable.id)
}

When creating an Article, we have to attach all the associated Tags by populating Article’s generated id into ArticleTabTable entries:

override fun create(article: Article): Article {
   val savedArticle = ArticleTable.insert { it.from(article) }
           .getOrThrow(ArticleTable.id)
           .let { article.copy(id = it) }
   savedArticle.tags.forEach { tag ->
       ArticleTagTable.insert {
           it[ArticleTagTable.tagId] = tag.id
           it[ArticleTagTable.articleId] = savedArticle.id
       }
   }
   return savedArticle
}

The funny part is the mapping of Article with Tags in query methods — in API specification Tags are always returned with the Article — so we need to eagerly fetch tags by using leftJoin:

val ArticleWithTags = (ArticleTable leftJoin ArticleTagTable leftJoin TagTable)

override fun findBy(articleId: ArticleId) =
  ArticleWithTags
    .select { ArticleTable.id eq articleId }
    .toArticles()
    .singleOrNull()

After joining, we have then one ResultRow per one Article-Tag pair, so we have to group them by ArticleId, and build the correct Article object by adding Tags for each matching resultRow:

fun Iterable<ResultRow>.toArticles(): List<ResultRow> {
   return fold(mutableMapOf<ArticleId, Article>()) { map, resultRow ->
       val article = resultRow.toArticle()
       val tagId = resultRow.tryGet(ArticleTagTable.tagId)
       val tag = tagId?.let { resultRow.toTag() }
       val current = map.getOrDefault(article.id, article)
       map[article.id] = current.copy(tags = current.tags + listOfNotNull(tag))
       map
   }.values.toList()
}

This implementation allows us to solve all the possible cases:

  • no articles (fold just returns empty map)
  • articles with no tags (tag is null, so listOfNotNull(tag) is empty)
  • articles with many tags (an article with a single tag is inserted into the map, then other tags are added in copy method)

However, consider when you need to fetch the dependent structure with the root object? For tags it makes sense since you always want the tags with the article, and the count of tags for any article should not be that huge. What about the comments? You definitely don’t want all the comments each time you fetch the article, instead you’ll need some kind of paging or even making a parent-child hierarchy for comments for the article. That’s why we recommend having this relationship mapped indirectly — every Comment should have ArticleId property, and the CommentRepository could have methods like:

fun findAllBy(articleId: ArticleId): List<Comment>
// or
fun findAllByPaged(articleId: ArticleId, pageRequest: PageRequest): Page<Comment>

Extendibility

Exposed is by design open for extension, making it even easier with Kotlin’s support for extension methods. You can define your own column type or expressions, e.g. for PostGIS point type support as Sebastian showed in his article. We used similar PostGIS extension in our project too. We were also able to implement a simple support for Java8 DateTime column type — for now, Exposed has Joda-time support, a generic approach for various date/time libraries is planned in the roadmap.

The bigger thing was Oracle DB dialect — we were forced to migrate to Oracle at some time in our project. We submitted a pull-request with foundations of Oracle 12 support, being tested in production for a while (then, we moved back to PostgreSQL…). The implementation was rather straightforward, with DataType- and FunctionProvider interfaces to provide and just a few tweaks in batch insert support.

Final thoughts

Our developer experience with Kotlin+Exposed duo was really pleasant. If you don’t plan to map many relations directly, just use simple data classes, connected by RefIds, it works really well. The Exposed library itself may need more exhaustive documentation and removing some annoying details (e.g. transaction management via thread-local — which is already on the roadmap), but we definitely recommend you give it a try in your project!

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How to automate tests with Groovy 2.0, Spock and Gradle

This is the launch of the 1st blog in my life, so cheers and have a nice reading!

y u no test?

Couple of years ago I wasn't a big fan of unit testing. It was obvious to me that well prepared unit tests are crucial though. I didn't known why exactly crucial yet then. I just felt they are important. My disliking to write automation tests was mostly related to the effort necessary to prepare them. Also a spaghetti code was easily spotted in test sources.

Some goodies at hand

Now I know! Test are crucial to get a better design and a confidence. Confidence to improve without a hesitation. Moreover, now I have the tool to make test automation easy as Sunday morning... I'm talking about the Spock Framework. If you got here probably already know what the Spock is, so I won't introduce it. Enough to say that Spock is an awesome unit testing tool which, thanks to Groovy AST Transformation, simplifies creation of tests greatly.

An obstacle

The point is, since a new major version of Groovy has been released (2.0), there is no matching version of Spock available yet.

What now?

Well, in a matter of fact there is such a version. It's still under development though. It can be obtained from this Maven repository. We can of course use the Maven to build a project and run tests. But why not to go even more "groovy" way? XML is not for humans, is it? Lets use Gradle.

The build file

Update: at the end of the post is updated version of the build file.
apply plugin: 'groovy'
apply plugin: 'idea'

def langLevel = 1.7

sourceCompatibility = langLevel
targetCompatibility = langLevel

group = 'com.tamashumi.example.testwithspock'
version = '0.1'

repositories {
mavenLocal()
mavenCentral()
maven { url 'http://oss.sonatype.org/content/repositories/snapshots/' }
}

dependencies {
groovy 'org.codehaus.groovy:groovy-all:2.0.1'
testCompile 'org.spockframework:spock-core:0.7-groovy-2.0-SNAPSHOT'
}

idea {
project {
jdkName = langLevel
languageLevel = langLevel
}
}
As you can see the build.gradle file is almost self-explanatory. Groovy plugin is applied to compile groovy code. It needs groovy-all.jar - declared in version 2.0 at dependencies block just next to Spock in version 0.7. What's most important, mentioned Maven repository URL is added at repositories block.

Project structure and execution

Gradle's default project directory structure is similar to Maven's one. Unfortunately there is no 'create project' task and you have to create it by hand. It's not a big obstacle though. The structure you will create will more or less look as follows:
<project root>

├── build.gradle
└── src
├── main
│ ├── groovy
└── test
└── groovy
To build a project now you can type command gradle build or gradle test to only run tests.

How about Java?

You can test native Java code with Spock. Just add src/main/java directory and a following line to the build.gradle:
apply plugin: 'java'
This way if you don't want or just can't deploy Groovy compiled stuff into your production JVM for any reason, still whole goodness of testing with Spock and Groovy is at your hand.

A silly-simple example

Just to show that it works, here you go with a basic example.

Java simple example class:

public class SimpleJavaClass {

public int sumAll(int... args) {

int sum = 0;

for (int arg : args){
sum += arg;
}

return sum;
}
}

Groovy simple example class:

class SimpleGroovyClass {

String concatenateAll(char separator, String... args) {

args.join(separator as String)
}
}

The test, uhm... I mean the Specification:

class JustASpecification extends Specification {

@Unroll('Sums integers #integers into: #expectedResult')
def "Can sum different amount of integers"() {

given:
def instance = new SimpleJavaClass()

when:
def result = instance.sumAll(* integers)

then:
result == expectedResult

where:
expectedResult | integers
11 | [3, 3, 5]
8 | [3, 5]
254 | [2, 4, 8, 16, 32, 64, 128]
22 | [7, 5, 6, 2, 2]
}

@Unroll('Concatenates strings #strings with separator "#separator" into: #expectedResult')
def "Can concatenate different amount of integers with a specified separator"() {

given:
def instance = new SimpleGroovyClass()

when:
def result = instance.concatenateAll(separator, * strings)

then:
result == expectedResult

where:
expectedResult | separator | strings
'Whasup dude?' | ' ' as char | ['Whasup', 'dude?']
'2012/09/15' | '/' as char | ['2012', '09', '15']
'nice-to-meet-you' | '-' as char | ['nice', 'to', 'meet', 'you']
}
}
To run tests with Gradle simply execute command gradle test. Test reports can be found at <project root>/build/reports/tests/index.html and look kind a like this.


Please note that, thanks to @Unroll annotation, test is executed once per each parameters row in the 'table' at specification's where: block. This isn't a Java label, but a AST transformation magic.

IDE integration

Gradle's plugin for Iintellij Idea

I've added also Intellij Idea plugin for IDE project generation and some configuration for it (IDE's JDK name). To generate Idea's project files just run command: gradle idea There are available Eclipse and Netbeans plugins too, however I haven't tested them. Idea's one works well.

Intellij Idea's plugins for Gradle

Idea itself has a light Gradle support built-in on its own. To not get confused: Gradle has plugin for Idea and Idea has plugin for Gradle. To get even more 'pluginated', there is also JetGradle plugin within Idea. However I haven't found good reason for it's existence - well, maybe excluding one. It shows dependency tree. There is a bug though - JetGradle work's fine only for lang level 1.6. Strangely all the plugins together do not conflict each other. They even give complementary, quite useful tool set.

Running tests under IDE

Jest to add something sweet this is how Specification looks when run with jUnit  runner under Intellij Idea (right mouse button on JustASpecification class or whole folder of specification extending classes and select "Run ...". You'll see a nice view like this.

Building web application

If you need to build Java web application and bundle it as war archive just add plugin by typing the line
apply plugin: 'war'
in the build.gradle file and create a directory src/main/webapp.

Want to know more?

If you haven't heard about Spock or Gradle before or just curious, check the following links:

What next?

The last thing left is to write the real production code you are about to test. No matter will it be Groovy or Java, I leave this to your need and invention. Of course, you are welcome to post a comments here. I'll answer or even write some more posts about the subject.

Important update

Spock version 0.7 has been released, so the above build file doesn't work anymore. It's easy to fix it though. Just remove last dash and a word SNAPSHOT from Spock dependency declaration. Other important thing is that now spock-core depends on groovy-all-2.0.5, so to avoid dependency conflict groovy dependency should be changed from version 2.0.1 to 2.0.5.
Besides oss.sonata.org snapshots maven repository can be removed. No obstacles any more and the build file now looks as follows:
apply plugin: 'groovy'
apply plugin: 'idea'

def langLevel = 1.7

sourceCompatibility = langLevel
targetCompatibility = langLevel

group = 'com.tamashumi.example.testwithspock'
version = '0.1'

repositories {
mavenLocal()
mavenCentral()
}

dependencies {
groovy 'org.codehaus.groovy:groovy-all:2.0.5'
testCompile 'org.spockframework:spock-core:0.7-groovy-2.0'
}

idea {
project {
jdkName = langLevel
languageLevel = langLevel
}
}