我正在为一个 processElement apache flink 1.4中的函数：

public class ProcessFunctionClass extends ProcessFunction<Tuple2<String, String>, Tuple2<String, String>>{

    private ListState<String> listState;

    public void processElement(Tuple2<String, String> tuple2,  Context context, Collector<Tuple2<String, String>> collector) {

        // if the state is empty, start a timer
        if (listState.get().iterator().hasNext() == false)
            context.timerService().registerEventTimeTimer(10000);

        listState.add("someStringToBeStored");

        // ...
    }

}

当计时器过期时，我有这个函数：

public void onTimer(long timestamp, OnTimerContext ctx, Collector<Tuple2<String, String>> out) throws Exception {
    Iterable<String> strings = listState.get();
    int cnt = 0;
    int totalLength = 0;
    Iterator<String> it = strings.iterator();
    while (it.hasNext()) {
        cnt++;
        totalLength += it.next().length();
    }
    LOGGER.info("cnt is:" + cnt);
    LOGGER.info("totalLength is:" + totalLength);

    // clearing the state
    listState.clear();
}

但是每次运行应用程序时 cnt 始终为1，并且 totalLength 此时已处理的特定字符串的长度。看起来这个状态没有保存在系统中。从这个代码可以清楚地看出我做错了什么吗？