Use Kotlin SDKs generated in Postman

This guide provides instructions for using Kotlin SDK generated by the Postman SDK Generator. It covers how to import the SDK into your Kotlin project, run example code, and best practices for using the SDK effectively in your applications.

Postman SDK Generator generates Java SDKs that are fully compatible with Kotlin. Since Kotlin runs on the JVM and has seamless Java interoperability, you can use the generated Java SDK directly in your Kotlin projects. The SDK structure, build configuration, and publishing process are identical to the Java SDK. This guide provides Kotlin-specific code examples for all SDK features.

SDK structure

The Java SDK includes the following key files:

• [SdkName].java — Main SDK client, instantiated with [SdkName]Config.
• services/ — Service classes with both sync and async methods.
• models/ — Data models (POJOs with Lombok builders, enums, typed exceptions) with Jackson serialization.
• config/ — Configuration classes for auth, retry, and SDK client settings.
• pom.xml — Specifies dependencies (OkHttp, Jackson, Lombok).

The following is an example of the typical structure of a generated Java SDK:

Example usage

Each generated SDK includes an examples/ directory with a working project demonstrating SDK usage.

The example includes the following features:

• SDK initialization with config builder pattern
• Synchronous API calls
• Working with model builders
• Exception handling

Import the SDK locally

You can import the generated SDK into your Kotlin project using Gradle, Maven, or project file path. Below are instructions for each method.

Import using Gradle

To import the generated SDK using Gradle, first install it to your local Maven repository:

$
cd path/to/your-sdk
$
mvn clean install

Then, add the dependency to your build file:

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repositories {
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    mavenLocal()
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    mavenCentral()
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}
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dependencies {
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    implementation("com.example:your-sdk:1.0.0")
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}

Alternatively, if you want to reference the SDK as a JAR file, first build the fat JAR:

$
cd path/to/your-sdk
$
mvn compile assembly:single

Then, add the JAR to your build file:

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dependencies {
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    implementation(files("../path/to/your-sdk/target/your-sdk-1.0.0-jar-with-dependencies.jar"))
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}

Import using a Maven local repository

To import the generated SDK using a local Maven repository, do the following:

1. Install SDK to local Maven repository.

   $
   cd path/to/your-sdk
   $
   mvn clean install

2. Add dependency to your project’s pom.xml.

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   <dependency>
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       <groupId>com.example</groupId>
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       <artifactId>your-sdk</artifactId>
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       <version>1.0.0</version>
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   </dependency>

3. Use the SDK in your code.

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import com.example.sdk.Sdk
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import com.example.sdk.config.SdkConfig
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val config = SdkConfig.builder().build()
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val sdk = Sdk(config)

Import using the project file path

To import the generated SDK using the project file path, do the following:

1. Add SDK as a module in your project’s pom.xml.

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   <modules>
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       <module>../path/to/your-sdk</module>
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       <module>your-app</module>
   4
   </modules>

2. Reference as dependency.

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   <dependency>
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       <groupId>com.example</groupId>
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       <artifactId>your-sdk</artifactId>
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       <version>${project.version}</version>
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   </dependency>

Publish to Maven Central

To share your generated Java SDK with the community, you can publish it to Maven Central. This allows other developers to easily add it as a dependency in their Maven or Gradle projects. Follow the steps below to publish your SDK to Maven Central.

Prerequisites

To publish your SDK to Maven Central, you need the following:

• Sonatype Central Portal account (central.sonatype.com)
• GPG key for signing artifacts
• Namespace (group ID) verified on the Central Portal

Initial setup

To set up your environment for publishing to Maven Central, do the following:

1. Create account and verify your namespace:

   • Register at central.sonatype.com.
   • Verify your namespace (group ID, for example, com.example) through DNS or GitHub verification.
   • Verification is typically automated and completes within minutes.

2. Generate a GPG key.

   $
   gpg --gen-key
   $
   gpg --list-keys
   $
   gpg --keyserver keys.openpgp.org --send-keys YOUR_KEY_ID

3. Configure Maven settings.xml. Add the following to ~/.m2/settings.xml:

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   <settings>
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   <servers>
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       <server>
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       <id>central</id>
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       <username>your-central-portal-token-username</username>
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       <password>your-central-portal-token-password</password>
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       </server>
   8
   </servers>
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   <profiles>
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       <profile>
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       <id>central</id>
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       <activation>
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           <activeByDefault>true</activeByDefault>
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       </activation>
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       <properties>
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           <gpg.executable>gpg</gpg.executable>
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           <gpg.passphrase>your-gpg-passphrase</gpg.passphrase>
   18
       </properties>
   19
       </profile>
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   </profiles>
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   </settings>

Configure pom.xml

Add the following to your SDK’s pom.xml:

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<project>
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  <!-- Basic information -->
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  <groupId>com.example</groupId>
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  <artifactId>your-sdk</artifactId>
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  <version>1.0.0</version>
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  <packaging>jar</packaging>
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  <name>Your SDK</name>
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  <description>SDK for Your API</description>
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  <url>https://github.com/your-org/your-sdk</url>
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  <!-- License -->
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  <licenses>
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    <license>
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      <name>MIT License</name>
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      <url>https://opensource.org/licenses/MIT</url>
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    </license>
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  </licenses>
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  <!-- Developers -->
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  <developers>
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    <developer>
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      <name>Your Name</name>
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      <email>your@email.com</email>
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      <organization>Your Organization</organization>
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    </developer>
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  </developers>
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  <!-- SCM -->
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  <scm>
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    <connection>scm:git:git://github.com/your-org/your-sdk.git</connection>
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    <developerConnection>scm:git:ssh://github.com/your-org/your-sdk.git</developerConnection>
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    <url>https://github.com/your-org/your-sdk</url>
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  </scm>
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  <build>
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    <plugins>
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      <!-- Maven Source Plugin -->
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      <plugin>
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        <groupId>org.apache.maven.plugins</groupId>
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        <artifactId>maven-source-plugin</artifactId>
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        <version>3.3.1</version>
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        <executions>
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          <execution>
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            <id>attach-sources</id>
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            <goals>
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              <goal>jar-no-fork</goal>
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            </goals>
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          </execution>
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        </executions>
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      </plugin>
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      <!-- Maven Javadoc Plugin -->
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      <plugin>
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        <groupId>org.apache.maven.plugins</groupId>
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        <artifactId>maven-javadoc-plugin</artifactId>
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        <version>3.6.3</version>
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        <executions>
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          <execution>
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            <id>attach-javadocs</id>
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            <goals>
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              <goal>jar</goal>
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            </goals>
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          </execution>
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        </executions>
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      </plugin>
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      <!-- Maven GPG Plugin -->
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      <plugin>
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        <groupId>org.apache.maven.plugins</groupId>
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        <artifactId>maven-gpg-plugin</artifactId>
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        <version>3.2.7</version>
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        <executions>
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          <execution>
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            <id>sign-artifacts</id>
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            <phase>verify</phase>
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            <goals>
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              <goal>sign</goal>
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            </goals>
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          </execution>
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        </executions>
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      </plugin>
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      <!-- Central Publishing Maven Plugin -->
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      <plugin>
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        <groupId>org.sonatype.central</groupId>
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        <artifactId>central-publishing-maven-plugin</artifactId>
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        <version>0.6.0</version>
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        <extensions>true</extensions>
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        <configuration>
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          <publishingServerId>central</publishingServerId>
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          <autoPublish>true</autoPublish>
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        </configuration>
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      </plugin>
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    </plugins>
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  </build>
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</project>

Publish your Java SDK to Maven Central

1. Deploy to Maven Central.

   $
   mvn clean deploy

   With autoPublish set to true, artifacts are automatically published after validation. If set to false, you can manually publish from the Central Portal UI at central.sonatype.com.

2. Verify the publication. After deployment, verify that your artifacts are available in the staging repository on the Central Portal, for example, https://repo1.maven.org/maven2/com/example/your-sdk/. Once verified, they’re released to Maven Central.

Publish updates

To publish updates to your SDK, do the following:

$
# Update version in pom.xml
$
mvn versions:set -DnewVersion=1.0.1
$
mvn versions:commit
$
$
# Deploy
$
mvn clean deploy

Best practices for publishing Java SDKs

Use the following best practices when publishing your Java SDK to Maven Central:

• Use semantic versioning.
• Sign all artifacts with GPG.
• Include source and javadoc JARs.
• Provide comprehensive documentation.
• Tag releases in Git.
• Consider using the Maven Release Plugin for automated releases.

Advanced usage

You can further customize the SDK by modifying the generated code, adding new features, or integrating it with your existing codebase. Here are some advanced topics to explore.

Authentication

The SDK supports various authentication methods. You can configure authentication when initializing the SDK client.

The SDK includes authentication support based on the security schemes defined in your API specification. The following examples show the possible authentication methods.

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val config = SdkConfig.builder()
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    .apiKeyAuthConfig(
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        ApiKeyAuthConfig.builder()
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            .apiKey("your-api-key")
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            .build()
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    )
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    .build()
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val sdk = Sdk(config)

To update the credentials after initialization, run one of the following. The setApiKey, setAccessToken, and setBasicAuthCredentials methods are each only generated when the corresponding auth type is in the specification.

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sdk.setApiKey("new-api-key")
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sdk.setAccessToken("new-token")
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sdk.setBasicAuthCredentials("new-username", "new-password")

Synchronous and asynchronous calls

Every service method has a sync variant (methodName) and an async variant (methodNameAsync). Sync methods return the type directly (for example, Pet), while async methods return CompletableFuture<T> (for example, CompletableFuture<Pet>). For void methods, async returns CompletableFuture<Void>.

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import java.util.concurrent.CompletableFuture
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// Synchronous — blocks until the response is returned
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val pet = sdk.petService.getPetById(123)
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println("Pet: ${pet.name}")
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// Asynchronous — returns a CompletableFuture
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val futurePet: CompletableFuture<Pet> = sdk.petService.getPetByIdAsync(123)
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// Option 1: Non-blocking with callbacks
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futurePet
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    .thenAccept { p -> println("Pet: ${p.name}") }
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    .exceptionally { e ->
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        System.err.println("Error: ${e.message}")
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        null
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    }
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// Option 2: Blocking wait when needed
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val result = futurePet.get()

If you use Kotlin coroutines, you can convert CompletableFuture to a suspending call with the kotlinx-coroutines-jdk8 library:

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import kotlinx.coroutines.future.await
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suspend fun fetchPet() {
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    val pet = sdk.petService.getPetByIdAsync(123).await()
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    println("Pet: ${pet.name}")
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}

Environment management

The SDK supports multiple environments (for example, production, staging) with different base URLs.

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// Use environment enum
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sdk.setEnvironment(Environment.PRODUCTION)
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// Or set custom base URL
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sdk.setBaseUrl("https://custom.api.example.com")

Hierarchical configuration

Java supports all four levels of configuration: SDK (through SdkConfig or sdk.setEnvironment()/sdk.setBaseUrl()), and service, method, and request levels (through RequestConfig). Each level overrides the previous, with request being most specific.

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import com.example.sdk.Sdk
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import com.example.sdk.config.SdkConfig
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import com.example.sdk.config.RequestConfig
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import com.example.sdk.http.Environment
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// 1. SDK level — default for everything
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val config = SdkConfig.builder()
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    .baseUrl("https://api.example.com")
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    .timeout(10000)
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    .build()
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val sdk = Sdk(config)
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// 2. Service level — override for all methods in usersService
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sdk.usersService.setConfig(
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    RequestConfig.builder()
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        .timeout(15000L)
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        .build()
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)
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// 3. Method level — override for getUser only
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sdk.usersService.setGetUserConfig(
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    RequestConfig.builder()
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        .baseUrl("https://legacy-api.example.com")
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        .build()
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)
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// 4. Request level — override for this single call
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val user = sdk.usersService.getUser(
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    "123",
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    RequestConfig.builder()
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        .timeout(30000L)
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        .build()
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)

Error handling

The SDK raises custom exceptions for API errors, which include details about the error message, status code, and response body. You can catch these exceptions to handle errors gracefully in your application.

To handle errors, do the following:

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import com.example.sdk.exceptions.ApiError
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try {
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    val user = sdk.usersService.getUser("123")
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    println(user)
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} catch (e: ApiError) {
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    System.err.println("API error: ${e.message}")
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    System.err.println("Status code: ${e.status}")
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    System.err.println("Response: ${e.response}")
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} catch (e: Exception) {
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    System.err.println("Unexpected error: ${e.message}")
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}

To handle specific error codes, do the following:

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try {
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    sdk.usersService.deleteUser("123")
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} catch (e: ApiError) {
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    when (e.status) {
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        404 -> println("User not found")
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        403 -> println("Permission denied")
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        else -> println("API error: ${e.message}")
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    }
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}

When the API specification defines error response models, the SDK generates typed exception classes (for example, NotFoundErrorException) that extend ApiError and include the deserialized error model. The exception’s getMessage() and getStatus() work directly, but to access specification-defined error fields, use getError().

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import com.example.sdk.models.NotFoundErrorException
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try {
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    sdk.usersService.getUser("123")
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} catch (e: NotFoundErrorException) {
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    // message and status available directly from ApiError
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    println("Status: ${e.status}")
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    println("Message: ${e.message}")
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    // spec-defined error model fields require getError()
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    val error = e.error
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    println("Detail: ${error.detail}")
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    println("Request ID: ${error.requestId}")
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} catch (e: ApiError) {
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    // fallback for unmapped error responses
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    println("API error: ${e.status} ${e.message}")
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}

Timeouts and retries

To set a global timeout, do the following:

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val config = SdkConfig.builder()
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    .timeout(10000) // 10 seconds in milliseconds (default: 10000)
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    .build()
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val sdk = Sdk(config)

To pass the retry configuration through SdkConfig, do the following:

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val retryConfig = RetryConfig.builder()
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    .maxRetries(3)
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    .initialDelay(150)       // milliseconds between retries (default: 150)
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    .maxDelay(5000)          // max delay cap in milliseconds (default: 5000)
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    .backoffFactor(2.0)      // exponential backoff multiplier
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    .jitter(50)              // random jitter in milliseconds (default: 50)
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    .build()
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val config = SdkConfig.builder()
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    .retryConfig(retryConfig)
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    .build()
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val sdk = Sdk(config)

Validation

The SDK generates validators that enforce OpenAPI constraints at runtime. These include minLength, maxLength, pattern, min, max, uniqueItems, and more.

Validation happens automatically when making API calls. The SDK validates parameters before sending the request. If validation fails, a ValidationException is thrown with a list of Violation objects describing what went wrong. You don’t need to call validators manually. They’re built into the service methods.

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import com.example.sdk.validation.exceptions.ValidationException
2
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try {
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    // If the spec defines constraints (e.g. minLength: 3 for username),
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    // the SDK validates before sending the request
6
    sdk.usersService.createUser(user)
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} catch (e: ValidationException) {
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    // e.message returns a formatted summary of all violations
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    System.err.println(e.message)
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    // Output: "Validation failed with the following violations:
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    //          username: must be at least 3 characters long
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    //          age: must be greater than or equal to 0"
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    // Access individual violations for programmatic handling
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    for (violation in e.violations) {
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        System.err.println("${violation.path}: ${violation.message}")
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    }
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} catch (e: ApiError) {
19
    System.err.println("API error: ${e.message}")
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}

The list of supported constraints can also be grouped by type like this:

• String — minLength, maxLength, pattern (regex)
• Numeric — min, max (inclusive or exclusive)
• List/array — minItems, maxItems, uniqueItems
• Required fields — Validated automatically, based on the specification.

Optional and nullable field handling

The SDK uses JsonNullable<T> wrappers for optional fields, providing clear distinction between undefined, null, and present values. This is important for users building request models. Omitting an optional field versus explicitly setting it to null produces different JSON payloads.

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// Example 1: Creating an object with all field types
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val model = ModelWithOptionalNullableDefaultValues.builder()
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    .requiredString("requiredString")           // Required Non-Nullable
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    .requiredInteger(8L)                        // Required Non-Nullable
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    .requiredNullableString("not null")         // Required Nullable
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    .requiredNullableInteger(null)              // Required Nullable (can be null)
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    .optionalString("optionalString")           // Optional Non-Nullable
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    .optionalInteger(9L)                        // Optional Non-Nullable
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    .optionalNullableString("optional")         // Optional Nullable
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    .optionalNullableInteger(null)              // Optional Nullable (can be null)
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    .build()
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// Example 2: Optional fields omitted entirely
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val minimalModel = ModelWithOptionalNullableDefaultValues.builder()
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    .requiredString("requiredString")
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    .requiredInteger(8L)
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    .requiredNullableString(null)               // Still required, but can be null
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    .requiredNullableInteger(null)              // Still required, but can be null
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    // Optional fields omitted - they will be omitted from JSON when serialized and sent to API
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    .build()
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// Example 3: Optional fields with mixed values
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val mixedModel = ModelWithOptionalNullableDefaultValues.builder()
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    .requiredString("requiredString")
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    .requiredInteger(8L)
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    .requiredNullableString("not null")
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    .requiredNullableInteger(null)
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    .optionalString("present")                  // Optional non-nullable with value
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    .optionalNullableString(null)               // Optional nullable set to null
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    // optionalInteger and optionalNullableInteger omitted
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    .build()
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// What the server receives for Example 1 (all fields set):
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// {
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//   "requiredString": "requiredString",
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//   "requiredInteger": 8,
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//   "requiredNullableString": "not null",
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//   "requiredNullableInteger": null,
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//   "optionalString": "optionalString",
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//   "optionalInteger": 9,
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//   "optionalNullableString": "optional",
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//   "optionalNullableInteger": null
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// }
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// What the server receives for Example 2 (minimal model):
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// {
47
//   "requiredString": "requiredString",
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//   "requiredInteger": 8,
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//   "requiredNullableString": null,
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//   "requiredNullableInteger": null
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//   // Note: optional fields are completely omitted from JSON
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// }
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// What the server receives for Example 3 (mixed model):
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// {
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//   "requiredString": "requiredString",
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//   "requiredInteger": 8,
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//   "requiredNullableString": "not null",
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//   "requiredNullableInteger": null,
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//   "optionalString": "present",
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//   "optionalNullableString": null
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//   // Note: optionalInteger and optionalNullableInteger are omitted
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// }

Additional resources

Consider the following resources for using and customizing your Java SDK from Kotlin:

• SDK documentation — Check the documentation/ directory in your SDK.
• Javadoc — Generate with mvn javadoc:javadoc.
• Example usage — Refer to the examples/ directory for a working code sample demonstrating basic SDK usage.
• Kotlin-Java interop — Refer to the Kotlin documentation on calling Java from Kotlin for details on how Kotlin interacts with Java libraries.
• Coroutines integration — Use kotlinx-coroutines-jdk8 to convert CompletableFuture to Kotlin coroutines with .await().
• Dependencies — The SDK uses the following dependencies:
  • OkHttp: An HTTP client.
  • Jackson: JSON serialization and jackson-databind-nullable, a dependency used for optional nullable field handling.
  • Lombok: Reduce boilerplate code.