[JAVA] RxJava2 support

[ber4444]

Description

When using the configuration settings,

  "library": "retrofit2",
  "useRxJava": true,

we are getting these dependencies:

ext {
    retrofit_version = "2.0.2"
    rx_java_version = "1.1.3"
}
dependencies {
    compile "com.squareup.retrofit2:retrofit:$retrofit_version"
    compile "com.squareup.retrofit2:converter-gson:$retrofit_version"
    compile "com.squareup.retrofit2:adapter-rxjava:$retrofit_version"
    compile "io.reactivex:rxjava:$rx_java_version"

However, we would like to add a useRxJava2 setting which would result in:

ext {
    retrofit_version = "2.0.2"
    rx_java_version = "2.0.1"
}
dependencies {
    compile "com.squareup.retrofit2:retrofit:$retrofitVersion"
    compile "com.squareup.retrofit2:converter-gson:$retrofitVersion"
    compile "com.jakewharton.retrofit:retrofit2-rxjava2-adapter:1.0.0"
    compile "io.reactivex.rxjava2:rxjava:$rx_java_version"

Related issues

Similar to this PR:
https://github.com/swagger-api/swagger-codegen/pull/1928/files

[cbornet]

Would you do a PR for that ?

[wing328]

Another option is to customize the template:

swagger-codegen/modules/swagger-codegen/src/main/resources/Java/libraries/retrofit2/build.gradle.mustache

Line 102 in 1dd9ee3

rx_java_version = "1.1.3"

and then use the -t option with the customized templates.

[ber4444]

I'll do the PR. Not as simple as @wing328 suggests because the class and package names changed as well.

[wing328]

@ber4444 thanks! I think we'll need to provide a CLI option, e.g. useRxJava2, so that users still have the option to use RxJava 1.x. Let me know if you need help on that.

[wing328]

@ber4444 I'll review your PR and let you know if I've any question.

[ber4444]

I updated the PR based on your feedback.

[wing328]

@ber4444 I've found 2 minor issues: #4708. Let me know if you need help fixing it.