This example uses the gson library to map java objects to json strings. The (de)serializers are generic, but they don't always need to be !

Serializer

Code

public class GsonSerializer<T> implements Serializer<T> {

    private Gson gson = new GsonBuilder().create();
    
    @Override
    public void configure(Map<String, ?> config, boolean isKey) {
        // this is called right after construction
        // use it for initialisation
    }
    
    @Override
    public byte[] serialize(String s, T t) {
        return gson.toJson(t).getBytes();
    }
    
    @Override
    public void close() {
        // this is called right before destruction
    }
}

Usage

Serializers are defined through the required key.serializer and value.serializer producer properties.

Assume we have a POJO class named SensorValue and that we want to produce messages without any key (keys set to null):

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
// ... other producer properties ... 
props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
props.put("value.serializer", GsonSerializer.class.getName());

Producer<String, SensorValue> producer = new KafkaProducer<>(properties);
// ... produce messages ... 
producer.close();

(key.serializer is a required configuration. Since we don't specify message keys, we keep the StringSerializer shipped with kafka, which is able to handle null).

---

deserializer

Code

public class GsonDeserializer<T> implements Deserializer<T> {

    public static final String CONFIG_VALUE_CLASS = "value.deserializer.class";
    public static final String CONFIG_KEY_CLASS = "key.deserializer.class";
    private Class<T> cls;

    private Gson gson = new GsonBuilder().create();


    @Override
    public void configure(Map<String, ?> config, boolean isKey) {
        String configKey = isKey ? CONFIG_KEY_CLASS : CONFIG_VALUE_CLASS;
        String clsName = String.valueOf(config.get(configKey));

        try {
            cls = (Class<T>) Class.forName(clsName);
        } catch (ClassNotFoundException e) {
            System.err.printf("Failed to configure GsonDeserializer. " +
                    "Did you forget to specify the '%s' property ?%n",
                    configKey);
        }
    }


    @Override
    public T deserialize(String topic, byte[] bytes) {
        return (T) gson.fromJson(new String(bytes), cls);
    }


    @Override
    public void close() {}
}

Usage

Deserializers are defined through the required key.deserializer and value.deserializer consumer properties.

Assume we have a POJO class named SensorValue and that we want to produce messages without any key (keys set to null):

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
// ... other consumer properties ... 
props.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
props.put("value.deserializer", GsonDeserializer.class.getName());
props.put(GsonDeserializer.CONFIG_VALUE_CLASS, SensorValue.class.getName());

try (KafkaConsumer<String, SensorValue> consumer = new KafkaConsumer<>(props)) {
    // ... consume messages ... 
}

Here, we add a custom property to the consumer configuration, namely CONFIG_VALUE_CLASS. The GsonDeserializer will use it in the configure() method to determine what POJO class it should handle (all properties added to props will be passed to the configure method in the form of a map).