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使用场景： 标量函数即 UDF，⽤于进⼀条数据出⼀条数据的场景。

开发流程：

• 实现 org.apache.flink.table.functions.ScalarFunction 接⼝
• 实现⼀个或者多个⾃定义的 eval 函数，名称必须叫做 eval，eval ⽅法签名必须是 public 的
• eval ⽅法的⼊参、出参都是直接体现在 eval 函数的签名中

开发案例：

import org.apache.flink.api.common.functions.MapFunction;
import org.apache.flink.api.java.tuple.Tuple2;
import org.apache.flink.streaming.api.datastream.DataStreamSource;
import org.apache.flink.streaming.api.datastream.SingleOutputStreamOperator;
import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
import org.apache.flink.table.annotation.DataTypeHint;
import org.apache.flink.table.annotation.InputGroup;
import org.apache.flink.table.api.*;
import org.apache.flink.table.api.bridge.java.StreamTableEnvironment;
import org.apache.flink.table.functions.ScalarFunction;
import static org.apache.flink.table.api.Expressions.*;/*** 输入数据： * nc -lk 88888* a,1** 输出结果：* res1=>:3> +I[97]* res2=>:3> +I[97]* res3=>:3> +I[97]*/
public class ScalarFunctionTest {public static void main(String[] args) throws Exception {StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();EnvironmentSettings settings = EnvironmentSettings.newInstance().useBlinkPlanner().inStreamingMode().build();StreamTableEnvironment tEnv = StreamTableEnvironment.create(env, settings);DataStreamSource<String> source = env.socketTextStream("localhost", 8888);SingleOutputStreamOperator<Tuple2<String, String>> tpStream = source.map(new MapFunction<String, Tuple2<String, String>>() {@Overridepublic Tuple2<String, String> map(String input) throws Exception {return new Tuple2<>(input.split(",")[0], input.split(",")[1]);}});Table table = tEnv.fromDataStream(tpStream, "id,name");tEnv.createTemporaryView("SourceTable",table);// 在 Table API ⾥不经注册直接调⽤函数Table res1 = tEnv.from("SourceTable").select(call(HashFunction.class, $("id")));// 注册函数tEnv.createTemporarySystemFunction("HashFunction", HashFunction.class);// 在 Table API ⾥调⽤注册好的函数Table res2 = tEnv.from("SourceTable").select(call("HashFunction", $("id")));// 在 SQL ⾥调⽤注册好的函数Table res3 = tEnv.sqlQuery("SELECT HashFunction(id) FROM SourceTable");tEnv.toDataStream(res1).print("res1=>");tEnv.toDataStream(res2).print("res2=>");tEnv.toDataStream(res3).print("res3=>");env.execute();}public static class HashFunction extends ScalarFunction {// 接受任意类型输⼊，返回 INT 型输出public int eval(@DataTypeHint(inputGroup = InputGroup.ANY) Object o) {return o.hashCode();}}
}

测试结果：