api-usage.java

import static net.ocheyedan.clapi.ClApi;

// class structure removed for brevity

void usageExample {

  using(ThriftVersion.2);
  callCodeDependingUponThriftVersion2();

  using(ThriftVersion.3);
  callCodeDependingUponThriftVersion3();

}

clapi.java

public final class ClApi extends URLClassLoader {

  private static final ClApi instance = new ClApi(); // or injected

  public static void using(Version version) {
    instance.using.set(version);
  }

  // created in init from some yaml file or something which links (auto-generated?) Version (i.e., ThriftVersion.2)
  // to the appropriate ClassLoader
  private final ImmutableMap<Version, ClassLoader> config; 

  // this class would become the system-classloader and it would contain all non-multi-versioned jars but then
  // delegate to those ClassLoader's in {@link #config} depending upon what's set in the ThreadLocal {@link #using} variable.

  private final ThreadLocal<Version> using;

  private ClApi() { }

}

public interface Version { }