@Override
  public SSLSession getSession() {
    return sessionToReturn != null ? sessionToReturn : delegateEngine.getSession();
  }
@param delegateEngine An {@link SSLEngine} to delegate to in most cases.
@param sessionToReturn If not {@code null}, return this certificate in {@link #getSession()} instead of delegating.
ambry-test-utils/src/main/java/com/github/ambry/rest/MockSSLEngine.java