赏析Kafka Streams程序的优雅关闭

Kafka官网上的WordCountDemo

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 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
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package org.apache.kafka.streams.examples.wordcount;

import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.common.serialization.Serdes;
import org.apache.kafka.streams.KafkaStreams;
import org.apache.kafka.streams.StreamsBuilder;
import org.apache.kafka.streams.StreamsConfig;
import org.apache.kafka.streams.kstream.KStream;
import org.apache.kafka.streams.kstream.KTable;
import org.apache.kafka.streams.kstream.KeyValueMapper;
import org.apache.kafka.streams.kstream.Produced;
import org.apache.kafka.streams.kstream.ValueMapper;

import java.util.Arrays;
import java.util.Locale;
import java.util.Properties;
import java.util.concurrent.CountDownLatch;

/**
 * Demonstrates, using the high-level KStream DSL, how to implement the WordCount program
 * that computes a simple word occurrence histogram from an input text.
 *
 * In this example, the input stream reads from a topic named "streams-plaintext-input", where the values of messages
 * represent lines of text; and the histogram output is written to topic "streams-wordcount-output" where each record
 * is an updated count of a single word.
 *
 * Before running this example you must create the input topic and the output topic (e.g. via
 * bin/kafka-topics.sh --create ...), and write some data to the input topic (e.g. via
 * bin/kafka-console-producer.sh). Otherwise you won't see any data arriving in the output topic.
 */
public class WordCountDemo {

    public static void main(String[] args) throws Exception {
        Properties props = new Properties();
        props.put(StreamsConfig.APPLICATION_ID_CONFIG, "streams-wordcount");
        props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
        props.put(StreamsConfig.CACHE_MAX_BYTES_BUFFERING_CONFIG, 0);
        props.put(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());
        props.put(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());

        // setting offset reset to earliest so that we can re-run the demo code with the same pre-loaded data
        // Note: To re-run the demo, you need to use the offset reset tool:
        // https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Streams+Application+Reset+Tool
        props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");

        StreamsBuilder builder = new StreamsBuilder();

        KStream source = builder.stream("streams-plaintext-input");

        KTable counts = source
            .flatMapValues(new ValueMapper>() {
                @Override
                public Iterable apply(String value) {
                    return Arrays.asList(value.toLowerCase(Locale.getDefault()).split(" "));
                }
            })
            .groupBy(new KeyValueMapper() {
                @Override
                public String apply(String key, String value) {
                    return value;
                }
            })
            .count();

        // need to override value serde to Long type
        counts.toStream().to("streams-wordcount-output", Produced.with(Serdes.String(), Serdes.Long()));

        final KafkaStreams streams = new KafkaStreams(builder.build(), props);
        //构造一个latch，计数次数为1
        final CountDownLatch latch = new CountDownLatch(1);

        // attach shutdown handler to catch control-c
        // 注册一个关闭的钩子，在用户执行ctrl+c关闭程序的时候，执行线程中的逻辑
        Runtime.getRuntime().addShutdownHook(new Thread("streams-wordcount-shutdown-hook") {
            @Override
            public void run() {
                //关闭kafka streams程序中的一系列线程
                streams.close();
                //让latch减1，从而可以让下面的try块中的latch.wait()方法能够执行完毕
                latch.countDown();
            }
        });

        try {
            //该方法会启动一系列线程
            streams.start();
            //调用await方法，使得主线程阻塞在这儿，直到有的线程使得latch计数被减到0
            latch.await();
        } catch (Throwable e) {
            //遇到异常的时候，程序退出码为1
            System.exit(1);
        }
        //程序正常退出，退出码为0
        System.exit(0);
    }
}

这种经典的写法可以借鉴到自己的代码中。