swagger-maven-example | Example of using swagger-maven-plugin | Plugin library

You have several flaws in your build script.

You should not depend on built things in build script dependencies. This is a hen and egg problem. You need to execute the build to get the classes that are necessary to execute the build. This cannot work reliably, if at all.

Instead you should declare them as dependencies outside the buildscript block. The buildscript block is only for dependencies that are needed by your build script to run itself, like Gradle Tasks and Gradle Plugins or classes you use during the build, like the swagger-maven-plugin stuff which is correct in the buildscript block.

Besides that, you execute part of your swagger stuff (the instanciation, execution and printing) during the configuration phase instead of during the execution phase. Everything you do in a task closure, but outside any doFirst or doLast blocks is run during the configuration phase, when the build is configured and thus always, no matter what tasks you actually want to execute and no matter whether the task may already be up-to-date or not. For the up-to-date check to work and save your time you need to declare all inputs like files and properties that might change between executions and all outputs that you generate, then Gradle can do its magic to only execute the task when actually necessary.

Also you should not use println in build scripts. That is like using System.out.println in Java programs. Instead you should use the provided logging facility directly, e. g. doing logger.info 'Swagger GenDoc task is completed'.