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);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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