[java] ApiClient library limits. New features.

Description

I faced with few critical(for me) limits of ApiClient generated by codegen.
I have 3 different retroft services in project, also I have another network modules like Fresco, Glide and custom raw http requests.

I want these features in generated code:

1. Ability of use one instance of HTTP client inside project.
   ApiClient creates new Okhttp client for every retrofit service creating. And I can't specify external instance.

public <S> S createService(Class<S> serviceClass) {
        return adapterBuilder
            .client(okBuilder.build())
            .build()
            .create(serviceClass);

    }
I want ability to set my own instance of OkHttpClicnet

2. Ability of use one instance of Gson in project;
   ApiClient creates gson wile builds retrofit adapter. I want to set my gson also.

 public void createDefaultAdapter() {
        Gson gson = new GsonBuilder()
                .setDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSZ")
                .registerTypeAdapter(DateTime.class, new DateTimeTypeAdapter())
                .registerTypeAdapter(LocalDate.class, new LocalDateTypeAdapter())
                .create();
    //creating of adapterBuilder

3. Using one retrofit instance for every service;
   Now ApiClient creates new instance of retrofit for every service build.

public <S> S createService(Class<S> serviceClass) {
        return adapterBuilder
            .client(okBuilder.build())
            .build()
            .create(serviceClass);

    }

Swagger-codegen version

2.2.1

Command line used for generation

java -jar swagger-codegen-cli.jar generate \
  -i api.yml \
  -l java \
  -o api_swagger \
  -c config.json 

{
  "invokerPackage": "com.***.***.api",
  "library": "retrofit2",
  "useRxJava": true
}

Steps to reproduce

generate any ApiClient with few retrofit interfaces.

Suggest a Fix

• Move creating of OkHttp and Gson to public functions. So user can create it outside of client and use in another cases.

fun buildDefaultOkHttp(): OkHttpClient {
//creating OkHttp following protocol

• Allow user to set custom OkHttpClient, Gson. Use default builders like buildDefaultOkHttp only when custom = null.
• Creating strategy of generating large objects like okhhtp, gson, retrofit

enum FactoryStrategy {
    singleton, //use one instance of class like okhttp client 
    newInstance // create new instance for every service call
}

Inspect the generated ApiClient, especially createService and createDefaultAdapter, to trace where OkHttp, Gson, and Retrofit instances are created. Define how custom instances and the singleton/newInstance strategy should be exposed, then verify generated services can reuse supplied instances without losing the current defaults.