Java Storm 消费kafka数据即KafkaSpout的消费实例，还有为啥要input.getString(4)，这个4怎么来的。

使用kafka作为数据源，给storm的后续bolt提供数据的例子，还有为啥要input.getString(4)，这个4怎么来的。

先看main方法，步骤很清新的。

package com.lxk.storm;

import com.lxk.storm.bolt.OutInfoBolt;
import org.apache.kafka.common.utils.Utils;
import org.apache.storm.Config;
import org.apache.storm.LocalCluster;
import org.apache.storm.kafka.spout.KafkaSpout;
import org.apache.storm.kafka.spout.KafkaSpoutConfig;
import org.apache.storm.topology.TopologyBuilder;
import org.junit.Test;

/**
 * 就测试一下从kafka消费数据，然后在bolt里面可以看到数据。
 *
 * @author LiXuekai on 2020/10/13
 */
public class TestKafkaSpout {
    private static final String TOPOLOGY_NAME = "kafka-spout-topology";

    @Test
    public void test() {

        //1，创建了一个TopologyBuilder实例
        TopologyBuilder topologyBuilder = new TopologyBuilder();

        //1，实例化 kafka spout
        kafkaSpout(topologyBuilder);

        //3，bolt 消费数据
        outInfoBolt(topologyBuilder);

        //4，提交
        submitTopology(topologyBuilder);
    }

    private void submitTopology(TopologyBuilder builder) {
        //Config类是一个HashMap<String,Object>的子类，用来配置topology运行时的行为
        Config config = new Config();
        //设置worker数量
        //config.setNumWorkers(2);
        LocalCluster cluster = new LocalCluster();

        //本地提交
        cluster.submitTopology(TOPOLOGY_NAME, config, builder.createTopology());

        Utils.sleep(10000);
        cluster.killTopology(TOPOLOGY_NAME);
        cluster.shutdown();
    }

    private void outInfoBolt(TopologyBuilder topologyBuilder) {
        OutInfoBolt outInfoBolt = new OutInfoBolt();
        topologyBuilder.setBolt("out-info-bolt-id", outInfoBolt, 1).setNumTasks(1).shuffleGrouping("kafka-info-spout-id");

    }

    private void kafkaSpout(TopologyBuilder topologyBuilder) {
        KafkaSpout<String, String> kafkaSpout = new KafkaSpout<>(initKafkaSpoutConfig());
        topologyBuilder.setSpout("kafka-info-spout-id", kafkaSpout, 1);
    }

    private KafkaSpoutConfig<String, String> initKafkaSpoutConfig() {
        KafkaSpoutConfig.Builder<String, String> builder = KafkaSpoutConfig.builder("192.168.1.191:9092", "a_citic_test_lxk");
        builder.setProp("group.id", "KAFKA_STORM");
        return builder.build();
    }
}

注意的是初始化spout到topology的时候，使用的ID在后面设置bolt的时候要一致。然后是bolt的代码，就简单干一件事事儿，打印。

package com.lxk.storm.bolt;

import org.apache.storm.task.OutputCollector;
import org.apache.storm.task.TopologyContext;
import org.apache.storm.topology.OutputFieldsDeclarer;
import org.apache.storm.topology.base.BaseRichBolt;
import org.apache.storm.tuple.Tuple;

import java.util.Map;

/**
 * 接收传的信息，打印
 *
 * @author LiXuekai on 2020/10/13
 */
public class OutInfoBolt extends BaseRichBolt {

    @Override
    public void prepare(Map stormConf, TopologyContext context, OutputCollector collector) {

    }

    @Override
    public void execute(Tuple input) {
        try {
            // [topic, partition, offset, key, value]
            String string = input.getString(4);
            System.out.println(string);
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }

    }

    @Override
    public void declareOutputFields(OutputFieldsDeclarer declarer) {

    }
}

然后再放个debug的截图，就更能说明这个地方为啥要input.getString(4)，这个4是怎么来的了。