To generate client code with maven please include following plugin to your pom.xml:
<build>
<plugins>
<plugin>
<groupId>dev.guardrail</groupId>
<artifactId>guardrail-maven-plugin_2.12</artifactId>
<version>0.62.0</version>
<executions>
<execution>
<id>generate-app-server</id>
<goals>
<goal>generate-sources</goal>
</goals>
<configuration>
<language>java</language>
<framework>dropwizard</framework>
<kind>client</kind>
<specPath>spec.oas3.yaml</specPath>
<packageName>demowizard.client.generated</packageName>
</configuration>
</execution>
</executions>
</plugin>
(...)
</plugins>
</build>
For JDK9+ you also need to add javax.annotation:javax.annotation-api
dependency:
<dependencies>
<dependency>
<groupId>javax.annotation</groupId>
<artifactId>javax.annotation-api</artifactId>
<version>1.3.2</version>
</dependency>
</dependencies>
(See it in action: guardrail-dev/guardrail-sample-maven-dropwizard)