Caused by: java.io.NotSerializableException: org.apache.kafka.common.metrics.MetricConfig

• flink 自定义数据源的开发过程中遇到这样一个错误。
• 错误原因：在于KafkaConsumer的初始化放在了定义处。
• 解决： 应该放在open方法内初始化。

•  DataStreamSource dataStreamSource = env.addSource(new KafkaSourceFunction());
          //获取数据 kafka消费数据获取
          DataStreamSource data = env.addSource(new RichSourceFunction() {
              KafkaConsumer kafkaConsumer;
  //            = new KafkaConsumer<>(prop);
              //new StringDeserializer(), new StringDeserializer());
  
              @Override
              public void open(Configuration parameters) throws Exception {
                  super.open(parameters);
                  String topic = "xxx";
                  Properties prop = new Properties();
                  prop.setProperty("bootstrap.servers","xxx:0000");
                  prop.setProperty("group.id","con1");
                  prop.put(ConsumerConfig.KEY_DESERIAL