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：

1. public void configure(Map<String, ?> configs, boolean isKey)：用来配置当前类。
2. public byte[] serialize(String topic, T data)：用来执行序列化。
3. 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);

示例中只修改了value.serializer,而key.serializer和value.serializer没有什么区别，如果有真实需要，修改以下也未尝不可。

接下一篇：Kafka消息序列化和反序列化（下）

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