如何使用Flink CEP检测模式a+b+

import org.apache.flink.api.java.tuple.Tuple2;
import org.apache.flink.cep.CEP;
import org.apache.flink.cep.PatternSelectFunction;
import org.apache.flink.cep.PatternStream;
import org.apache.flink.cep.pattern.Pattern;
import org.apache.flink.streaming.api.datastream.DataStream;
import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;

public class FlinkCEPExample {
    public static void main(String[] args) throws Exception {
        // 创建执行环境
        StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

        // 创建数据流，假设事件流中的事件类型为Tuple2<String, Integer>
        DataStream<Tuple2<String, Integer>> input = env.fromElements(
                new Tuple2<>("a", 1),
                new Tuple2<>("b", 2),
                new Tuple2<>("a", 3),
                new Tuple2<>("b", 4),
                new Tuple2<>("b", 5)
        );

        // 定义模式
        Pattern<Tuple2<String, Integer>, ?> pattern = Pattern.<Tuple2<String, Integer>>begin("start")
                .where(event -> event.f0.equals("a"))
                .followedBy("middle")
                .where(event -> event.f0.equals("b"))
                .oneOrMore().greedy();

        // 应用模式检测
        PatternStream<Tuple2<String, Integer>> patternStream = CEP.pattern(input, pattern);

        // 处理匹配事件
        patternStream.select(new PatternSelectFunction<Tuple2<String, Integer>, String>() {
            @Override
            public String select(Map<String, List<Tuple2<String, Integer>>> pattern) throws Exception {
                StringBuilder result = new StringBuilder();
                for (Tuple2<String, Integer> event : pattern.get("middle")) {
                    result.append(event.f0).append(event.f1).append(" ");
                }
                return result.toString();
            }
        }).print();

        // 执行任务
        env.execute("Flink CEP Example");
    }
}