[JAVA][Template] Creating and using a simple custom template

Hi everyone,

I'm trying to use openapi-generator in order to generate some classes. However, I'm not entirely sure I can do what I want using openapi-generator. I'd want to define my own classes using the mustache templating language, but render them using openapi-generator to make use of it parsing the API's docs.

I have also tried following this wiki. However, when I run mvn package I get the following exception:

--------------------------------------------------------------------
Test. set: com.my.company.service.My_codegenGeneratorTest
-------------------------------------------------------------------------------
Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.257 sec <<< FAILURE!
launchCodeGenerator(com.my.company.service.My_codegenGeneratorTest)  Time elapsed: 0.005 sec  <<< ERROR!
java.lang.NullPointerException
	at java.base/java.util.HashSet.<init>(HashSet.java:119)
	at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:464)
	at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:507)
	at com.my.company.service.My_codegenGeneratorTest.launchCodeGenerator(My_codegenGeneratorTest.java:32)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:567)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
	at org.apache.maven.surefire.junitcore.ClassDemarcatingRunner.run(ClassDemarcatingRunner.java:58)
	at org.junit.runners.Suite.runChild(Suite.java:128)
	at org.junit.runners.Suite.runChild(Suite.java:24)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:157)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:136)
	at org.apache.maven.surefire.junitcore.JUnitCoreWrapper.execute(JUnitCoreWrapper.java:62)
	at org.apache.maven.surefire.junitcore.JUnitCoreProvider.invoke(JUnitCoreProvider.java:139)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:567)
	at org.apache.maven.surefire.util.ReflectionUtils.invokeMethodWithArray(ReflectionUtils.java:189)
	at org.apache.maven.surefire.booter.ProviderFactory$ProviderProxy.invoke(ProviderFactory.java:165)
	at org.apache.maven.surefire.booter.ProviderFactory.invokeProvider(ProviderFactory.java:85)
	at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:103)
	at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:74)

After deleting the test directory from the template, it worked. But when doing java -cp ..., I get a message saying I'm missing a class:

Error: Could not find or load main class org.openapitools.codegen.OpenAPIGenerator

(I'm using openapi-generator 4.3.1 on macOS Mojave)

All in all, is there any easy way to create mustache template files and generate the files using openapi-generator? For instance, I'd like to do something simple like a file containing a list of all the available paths or definitions provided by the API.
Are there any specific resources or tutorials for creating something like this from scratch?

Thanks!

Start by reproducing the reported mvn package failure in My_codegenGeneratorTest, then inspect CodegenConfigurator.toContext at line 464 and the OpenAPIGenerator classpath invocation. Done means establishing whether custom Mustache templates and the requested generated file are supported, and documenting or correcting the failing workflow with a reproducible test.