code: @Test public void decode(){
    RoutingSupport routing=mock(RoutingSupport.class);
    String route="route";
    SessionIdentifierCodec codec=new SimpleSessionIdentifierCodec(routing,route);
    String result=codec.decode(null);
    assertNull(result);
    String sessionId="session";
    when(routing.parse(sessionId)).thenReturn(new SimpleImmutableEntry<String,String>(sessionId,null));
    result=codec.decode(sessionId);
    assertSame(sessionId,result);
    String encodedSessionId="session.route";
    when(routing.parse(encodedSessionId)).thenReturn(new SimpleImmutableEntry<>(sessionId,route));
    result=codec.decode(encodedSessionId);
    assertSame(sessionId,result);
  }
  