（8）FlinkSQL自定义UDF

Flink提供了自定义函数的基础能力，在需要满足特殊业务场景需求时，根据自身需要按需定制自己的UDF 下面将简单演示一个UDF的定义和UDF的使用过程：
（1）定义一个UDF

package com.udf;

import org.apache.flink.table.functions.ScalarFunction;

/**
 * Created by lj on 2022-07-25.
 */
public class TestUDF extends ScalarFunction {
    public String eval(String value) {
        return value + "_udf";
    }
}

（2）使用UDF

public static void main(String[] args) throws Exception {

        StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
        env.setParallelism(1);
        StreamTableEnvironment tableEnv = StreamTableEnvironment.create(env);
        DataStreamSource streamSource = env.socketTextStream("127.0.0.1", 9999,"\n");
        SingleOutputStreamOperator waterDS = streamSource.map(new MapFunction() {
            @Override
            public WaterSensor map(String s) throws Exception {
                String[] split = s.split(",");
                return new WaterSensor(split[0], Long.parseLong(split[1]), Integer.parseInt(split[2]));
            }
        });

        // 将流转化为表
        Table table = tableEnv.fromDataStream(waterDS,
                $("id"),
                $("ts"),
                $("vc"),
                $("pt").proctime());

        tableEnv.createTemporaryView("EventTable", table);

/*
        // 1. 直接调用自定义udf 函数
        //        table.select(call(myFunction.class,$("id"))).execute().print();
        // 2. 先注册在使用
        tableEnv.createTemporarySystemFunction("MyLength",myFunction.class);
        //2.1 在使用注册的自定义函数 名称为MyLength
        //        table.select(call("MyLength",$("id"))).execute().print();
        // 2.2 采用sql 的方式进行使用自定义函数
            tableEnv.sqlQuery("select id, MyLength(id) from "+table).execute().print();
* */

        tableEnv.createTemporarySystemFunction("MyLength",TestUDF.class);
        Table result = tableEnv.sqlQuery(
                "SELECT " +
                        "id as componentname, " +                //window_start, window_end,
                        "COUNT(ts) as componentcount ,SUM(ts) as componentsum, " +
                        "MyLength(cast(COUNT(ts) as string)) as testudf " +
                        "FROM TABLE( " +
                        "TUMBLE( TABLE EventTable , " +
                        "DESCRIPTOR(pt), " +
                        "INTERVAL '10' SECOND)) " +
                        "GROUP BY id , window_start, window_end"
        );

        tableEnv.toRetractStream(result, Row.class).print("toRetractStream");       //缩进模式

        env.execute();
    }

（3）应用效果