Kafka 消息序列化和反序列化（上）

Kafka Producer在发送消息时必须配置的参数为：bootstrap.servers、key.serializer、value.serializer。序列化操作是在拦截器（Interceptor）执行之后并且在分配分区(partitions)之前执行的。

首先我们通过一段示例代码来看下普通情况下Kafka Producer如何编写：

public class ProducerJavaDemo {
    public static final String brokerList = "192.168.0.2:9092,192.168.0.3:9092,192.168.0.4:9092";
    public static final String topic = "hidden-topic";

    public static void main(String[] args) {
        Properties properties = new Properties();
        properties.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        properties.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        properties.put("client.id", "hidden-producer-client-id-1");
        properties.put("bootstrap.servers", brokerList);

        Producer<String,String> producer = new KafkaProducer<String,String>(properties);

        while (true) {
            String message = "kafka_message-" + new Date().getTime() + "-edited by hidden.zhu";
            ProducerRecord<String, String> producerRecord = new ProducerRecord<String, String>(topic,message);
            try {
                Future<RecordMetadata> future =  producer.send(producerRecord, new Callback() {
                    public void onCompletion(RecordMetadata metadata, Exception exception) {
                        System.out.print(metadata.offset()+"    ");
                        System.out.print(metadata.topic()+"    ");
                        System.out.println(metadata.partition());
                    }
                });
            } catch (Exception e) {
                e.printStackTrace();
            }
            try {
                TimeUnit.MILLISECONDS.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

这里采用的客户端不是0.8.x.x时代的Scala版本，而是Java编写的新Kafka Producer, 相应的Maven依赖如下：

<dependency>
    <groupId>org.apache.kafka</groupId>
    <artifactId>kafka-clients</artifactId>
    <version>1.0.0</version>
</dependency>

上面的程序中使用的是Kafka客户端自带的org.apache.kafka.common.serialization.StringSerializer，除了用于String类型的序列化器之外还有：ByteArray、ByteBuffer、Bytes、Double、Integer、Long这几种类型，它们都实现了org.apache.kafka.common.serialization.Serializer接口，此接口有三种方法：

public void configure(Map<String, ?> configs, boolean isKey)：用来配置当前类。
public byte[] serialize(String topic, T data)：用来执行序列化。
public void close()：用来关闭当前序列化器。一般情况下这个方法都是个空方法，如果实现了此方法，必须确保此方法的幂等性，因为这个方法很可能会被KafkaProducer调用多次。
下面我们来看看Kafka中org.apache.kafka.common.serialization.StringSerializer的具体实现，源码如下：

public class StringSerializer implements Serializer<String> {
    private String encoding = "UTF8";

    @Override
    public void configure(Map<String, ?> configs, boolean isKey) {
        String propertyName = isKey ? "key.serializer.encoding" : "value.serializer.encoding";
        Object encodingValue = configs.get(propertyName);
        if (encodingValue == null)
            encodingValue = configs.get("serializer.encoding");
        if (encodingValue != null && encodingValue instanceof String)
            encoding = (String) encodingValue;
    }

    @Override
    public byte[] serialize(String topic, String data) {
        try {
            if (data == null)
                return null;
            else
                return data.getBytes(encoding);
        } catch (UnsupportedEncodingException e) {
            throw new SerializationException("Error when serializing string to byte[] due to unsupported encoding " + encoding);
        }
    }

    @Override
    public void close() {
        // nothing to do
    }
}

首先看下StringSerializer中的configure(Map)

public class Company {
    private String name;
    private String address;
    //省略Getter, Setter, Constructor & toString方法
}

接下去我们来实现Company类型的Serializer，即下面代码示例中的DemoSerializer。

package com.hidden.client;

public class DemoSerializer implements Serializer<Company> {
    public void configure(Map<String, ?> configs, boolean isKey) {}
    public byte[] serialize(String topic, Company data) {
        if (data == null) {
            return null;
        }
        byte[] name, address;
        try {
            if (data.getName() != null) {
                name = data.getName().getBytes("UTF-8");
            } else {
                name = new byte[0];
            }
            if (data.getAddress() != null) {
                address = data.getAddress().getBytes("UTF-8");
            } else {
                address = new byte[0];
            }
            ByteBuffer buffer = ByteBuffer.allocate(4+4+name.length + address.length);
            buffer.putInt(name.length);
            buffer.put(name);
            buffer.putInt(address.length);
            buffer.put(address);
            return buffer.array();
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
        return new byte[0];
    }
    public void close() {}
}

使用时只需要在Kafka Producer的config中修改value.serializer属性即可，示例如下：

properties.put("value.serializer", "com.hidden.client.DemoSerializer");
//记得也要将相应的String类型改为Company类型，如：
//Producer<String,Company> producer = new KafkaProducer<String,Company>(properties);
//Company company = new Company();
//company.setName("hidden.cooperation-" + new Date().getTime());
//company.setAddress("Shanghai, China");
//ProducerRecord<String, Company> producerRecord = new ProducerRecord<String, Company>(topic,company);1234567

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