[all] allow per-template overloading with Handlerbar (as with Mustache)

[rosbly]

Description

With swagger-codegen 2.0 & mustache templates, I could specify a --template-dir containing only the templates I wanted to overload. swagger-codegen would use my templates when available and otherwise fall back to the default ones.

With Handlerbar templates, swagger-codegen only reads --template-dir so I need to copy the whole original directory and make my changes.

This is very cumbersome when only a slight change in one or a few templates is needed.

Swagger-codegen version

3.0.x

Suggest a fix/enhancement

Some tweaking is needed in HandlebarTemplateEngine, using a handlebars CompositeTemplateLoader would be appropriate. Template path processing looks tricky which is why I don't have a pull request.

Thanks

[sarumont]

Possibly related to #8904, though that mentions Mustache and Spring specifically. I see this with Mustache templates and -l java -D models, so I believe it is indeed global.

Relevant stack, attempting to only override pojo.mustache:

Exception in thread "Thread-1" java.lang.RuntimeException: Could not generate model 'Connector'
        at io.swagger.codegen.v3.DefaultGenerator.generateModels(DefaultGenerator.java:405)
        at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:719)
        at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:342)
        at java.lang.Thread.run(Thread.java:748)
Caused by: java.io.FileNotFoundException: /Users/richard/dev/nf/connector/templates/java/model.mustache
        at com.github.jknack.handlebars.io.URLTemplateLoader.sourceAt(URLTemplateLoader.java:70)
        at com.github.jknack.handlebars.Handlebars.compile(Handlebars.java:357)
        at com.github.jknack.handlebars.Handlebars.compile(Handlebars.java:343)
        at io.swagger.codegen.v3.templates.HandlebarTemplateEngine.getHandlebars(HandlebarTemplateEngine.java:48)
        at io.swagger.codegen.v3.templates.HandlebarTemplateEngine.getRendered(HandlebarTemplateEngine.java:24)
        at io.swagger.codegen.v3.DefaultGenerator.processTemplateToFile(DefaultGenerator.java:735)
        at io.swagger.codegen.v3.DefaultGenerator.generateModels(DefaultGenerator.java:392)
        ... 3 more

[tibors13]

With HandleBarTemplateEngine.getHandlebars(String templateFile) there is a problem with following code

        if (this.config.additionalProperties().get("templateDir") != null) {
            templateLoader = new FileTemplateLoader(templateDir, ".mustache");
        } else {
            templateLoader = new ClassPathTemplateLoader("/" + templateDir, ".mustache");
        }

It relies on templateDir, so when -template-dir is used, it cannot load file from classpath, as it never goes there (without -t it works fine)

[frantuma]

@HugoMario maybe fixed by #11162?

[HugoMario]

Fixed by #11191