storm中declareOutputFields方法的使用规则及与fieldsGrouping方法的联系

declareOutputFields方法中声明了该bolt/spout输出的字段个数，供下游使用，在该bolt中的execute方法中，emit发射的字段个数必须和声明的相同，否则报错：Tuple created with wrong number of fields. Expected 2 fields but got 1 fields。

        fieldsGrouping方法设定从上一个bolt中获取哪些fields。fieldsGrouping("2", new Fields("ahah","bb"))，Fields中的参数名字必须和上一个bolt中声明的字段名字相同，否则报错：“Topology submission exception. (topology name='HelloStorm') #”

测试Spout代码如下：

public class Produce extends BaseRichSpout {
    private SpoutOutputCollector collector;
    @Override
    public void nextTuple() {
        String[] str1=new String[]{"aaa","bbb","ccc","ddd"};
        String[] str2=new String[]{"ee","ff","gg","kk"};
        Random random=new Random();
        String word1=str1[random.nextInt(str1.length)];
        String word2=str2[random.nextInt(str2.length)];
        collector.emit(new Values(word1,word2));
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void open(Map arg0, TopologyContext arg1, SpoutOutputCollector arg2) {
        this.collector=arg2;
    }
    @Override
    public void declareOutputFields(OutputFieldsDeclarer arg0) {
        arg0.declare(new Fields("ppee","ttt"));
    }
}

测试Output代码如下：

public class Output extends BaseBasicBolt{
    @Override
    public void execute(Tuple tuple, BasicOutputCollector arg1) {
        String str=tuple.getString(0);
        String str1=tuple.getString(1);
        arg1.emit(new Values(str,str1));
        /*此处发射的字段个数必须要和declareOutputFields中声明的字段个数相同，否则报错：
         *  Tuple created with wrong number of fields. Expected 2 fields but got 1 fields
         */
    }

    @Override
    public void declareOutputFields(OutputFieldsDeclarer arg0) {
        arg0.declare(new Fields("ahah","bb"));
    }
}

测试writer代码如下：

public class Writer extends BaseBasicBolt{

    @Override
    public void execute(Tuple arg0, BasicOutputCollector arg1) {
        // TODO Auto-generated method stub
        String str=arg0.getString(0);
        String str1=arg0.getString(1);
        System.out.println(str+"   "+str1+"  "+this);
    }

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

    @Override
    public void declareOutputFields(OutputFieldsDeclarer arg0) {
        arg0.declare(new Fields("hehe"));
    }
}

测试Main类如下：

public class main {

    public static void main(String[] args) throws Exception {
        TopologyBuilder builder=new TopologyBuilder();
        builder.setSpout("1", new Produce(),1);
        builder.setBolt("2", new Output(), 4).shuffleGrouping("1");
        builder.setBolt("3", new Writer(), 8).fieldsGrouping("2", new Fields("ahah","bb"));
        /*设定从上一个bolt中获取"ahah"字段，这个“ahah”必须要和上一个bolt中的declare声明的字段相同，
         * 否则报错，“Topology submission exception. (topology name='HelloStorm')
         * #
         * stream: [default] of component [2] with non-existent fields: #{"ahah1"})>”
         *
         */

        Config con=new Config();
        con.setDebug(false);
        con.setNumWorkers(5);
//      StormSubmitter.submitTopology("first", con, builder.createTopology());
        LocalCluster cluster = new LocalCluster();
        cluster.submitTopology("HelloStorm", con, builder.createTopology());
    }
}

    在运行测试的过程中发现一个有趣的现象，在builder.setBolt("3", new Writer(), 8).fieldsGrouping("2", new Fields("ahah","bb"))中的Fields中如果以第二个字段"bb"分组，则只打印出一个对象的哈希值，如果以第一个字段''ahah''或者两个字段则正常打印出多个对象的哈希值。《后续：经leader指导，是因为发射数据量的问题，发射数据改成"word+System.currentTimeMillis()",则按正常设置生成相应的Task对象》

    fieldsGrouping分组的含义是同id的tuple同时只能在一个Task上。