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Kotlin DSL

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25.12 Experimental

webforJ provides a Kotlin Domain Specific Language, or DSL, that lets you build UIs with concise, type-safe syntax. Instead of imperative Java code, you write declarative code that reads like a description of your UI structure.

Java
FlexLayout layout = new FlexLayout();
layout.setDirection(FlexDirection.COLUMN);
layout.setSpacing("10px");

TextField name = new TextField();
name.setLabel("Name");
name.setPlaceholder("Your name");
layout.add(name);

Button submit = new Button("Submit", ButtonTheme.PRIMARY);
submit.onClick(e -> handleSubmit());
layout.add(submit);
Kotlin DSL
flexLayout {
  direction = FlexDirection.COLUMN
  styles["gap"] = "10px"

  textField("Name", placeholder = "Your name")
  button("Submit", ButtonTheme.PRIMARY) {
    onClick { handleSubmit() }
  }
}

The DSL takes advantage of Kotlin extension functions, lambdas with receivers, and default parameters to create a natural builder syntax. Components nest inside each other, configuration happens in blocks, and the compiler catches structural mistakes before runtime.

Setup​

Experimental feature
This feature is experimental and may change or be removed in a future release.

No separate Kotlin installation is required. Maven handles compilation through the Kotlin Maven plugin, so any project that already builds with Maven can add Kotlin support with dependency and plugin configuration alone.

Quick start

To get a webforJ project using Kotlin up and running with all the needed configurations out of the box, see this section on using the webforJ Kotlin starter.

Dependencies​

Add the webforJ Kotlin DSL module and the Kotlin standard library to your pom.xml:

<dependency>
  <groupId>com.webforj.kotlin</groupId>
  <artifactId>webforj-kotlin</artifactId>
  <version>${webforj.version}</version>
</dependency>

<dependency>
  <groupId>org.jetbrains.kotlin</groupId>
  <artifactId>kotlin-stdlib-jdk8</artifactId>
  <version>${kotlin.version}</version>
</dependency>

If you plan to write tests in Kotlin, also add the Kotlin test dependency. It integrates with JUnit:

<dependency>
  <groupId>org.jetbrains.kotlin</groupId>
  <artifactId>kotlin-test</artifactId>
  <version>${kotlin.version}</version>
  <scope>test</scope>
</dependency>

Kotlin Maven plugin​

Add the Kotlin Maven plugin to compile both your Kotlin and Java sources. The sourceDirs configuration below allows Kotlin and Java files to coexist in the same project:

<plugin>
  <groupId>org.jetbrains.kotlin</groupId>
  <artifactId>kotlin-maven-plugin</artifactId>
  <version>${kotlin.version}</version>
  <executions>
    <execution>
      <id>compile</id>
      <phase>compile</phase>
      <goals>
        <goal>compile</goal>
      </goals>
      <configuration>
        <sourceDirs>
          <sourceDir>src/main/java</sourceDir>
          <sourceDir>target/generated-sources/annotations</sourceDir>
          <sourceDir>src/main/kotlin</sourceDir>
        </sourceDirs>
      </configuration>
    </execution>
    <execution>
      <id>test-compile</id>
      <phase>test-compile</phase>
      <goals>
        <goal>test-compile</goal>
      </goals>
      <configuration>
        <sourceDirs>
          <sourceDir>src/test/java</sourceDir>
          <sourceDir>target/generated-test-sources/test-annotations</sourceDir>
          <sourceDir>src/test/kotlin</sourceDir>
        </sourceDirs>
      </configuration>
    </execution>
  </executions>
  <configuration>
    <jvmTarget>${maven.compiler.target}</jvmTarget>
  </configuration>
</plugin>

With these additions, mvn compile compiles Kotlin sources alongside Java. Kotlin files can go in src/main/kotlin or src/main/java, and the plugin handles both.

Java interoperability

Kotlin compiles to JVM bytecode, so it works alongside existing Java code. You can use DSL-built Kotlin composites from Java classes, nest standard Java components inside DSL blocks with add(), and mix Kotlin and Java files in the same project.

Kotlin starter project​

If you'd rather skip the manual setup, the webforJ Kotlin Starter repository provides a ready-to-run project with all dependencies and plugin configuration already in place. Clone it and start building with the DSL right away.

Topics​

The following topics cover using the DSL, as well as extending it to any custom components or composites you create.