Does not compile with Java 9

Description

This is a friendly reminder that swagger-codegen fails on Java 9.
Everything compiles as expected with Java 8.

Swagger-codegen version

all version 2.x and 3.x

Steps to reproduce

Install Java 9

java --version
java 9.0.4
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

mvn clean package
-------------------------------------------------------------------------------
Test set: TestSuite
-------------------------------------------------------------------------------
Tests run: 582, Failures: 128, Errors: 0, Skipped: 3, Time elapsed: 7.065 sec <<< FAILURE! - in TestSuite
checkOptionsHelp(io.swagger.codegen.lagomScalaApi.LagomScalaApiOptionsTest)  Time elapsed: 0.03 sec  <<< FAILURE!
java.lang.NoClassDefFoundError: javax/annotation/PostConstruct
Caused by: java.lang.ClassNotFoundException: javax.annotation.PostConstruct

....

This might be related to:
https://stackoverflow.com/questions/34947994/neon-how-to-run-on-jdk9

Suggest a fix/enhancement

Right now it's fine to install Java 8 as instructed in the README.
This issue hopefully helps other people when searching for a solution.
But this should be fixed some day, too! 😉

In example, on my Mac I did the following:

brew cask install caskroom/versions/java8

export JAVA_HOME=`/usr/libexec/java_home -v 1.8`
export PATH=${JAVA_HOME}/bin:$PATH

java -version