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What are Message Converters?

JSON: Spring's Default

Beyond JSON: Why Customize?

The MessageConverter Interface

Building a Custom Converter Logic

public class CustomConverterLogic {

    // Simulates the 'toMessage' part: Object -> Bytes
    public static byte[] stringToBytes(String text) {
        System.out.println("Converting string to bytes (lower)...");
        return text.toLowerCase().getBytes();
    }

    // Simulates the 'fromMessage' part: Bytes -> Object
    public static String bytesToString(byte[] bytes) {
        System.out.println("Converting bytes to string (upper)...");
        return new String(bytes).toUpperCase();
    }

    public static void main(String[] args) {
        String originalMessage = "Hello CoddyKit!";
        System.out.println("Original: " + originalMessage);

        // Step 1: Convert to bytes for sending
        byte[] payloadBytes = stringToBytes(originalMessage);
        System.out.println("Payload Bytes: " + new String(payloadBytes));

        // Step 2: Convert bytes back to string for receiving
        String receivedMessage = bytesToString(payloadBytes);
        System.out.println("Received: " + receivedMessage);
    }
}

Integrating with Spring Config

import org.springframework.context.annotation.Configuration;
import org.springframework.messaging.converter.MessageConverter;
import org.springframework.messaging.converter.StringMessageConverter;
import org.springframework.messaging.simp.config.MessageBrokerRegistry;
import org.springframework.web.socket.config.annotation.EnableWebSocketMessageBroker;
import org.springframework.web.socket.config.annotation.StompEndpointRegistry;
import org.springframework.web.socket.config.annotation.WebSocketMessageBrokerConfigurer;

import java.util.List;

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

    @Override
    public void configureMessageBroker(MessageBrokerRegistry config) {
        config.enableSimpleBroker("/topic");
        config.setApplicationDestinationPrefixes("/app");
    }

    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/ws").withSockJS();
    }

    @Override
    public boolean configureMessageConverters(List<MessageConverter> messageConverters) {
        // Add your custom converter here
        // It's often good to add custom ones before default ones if they handle the same type
        messageConverters.add(new CustomStringReverseConverter()); // Your custom converter
        messageConverters.add(new StringMessageConverter()); // Example of adding a default
        // Return true to disable default converters, false to keep them
        return false; // Keep default converters as well
    }

    // Your custom converter class would implement MessageConverter
    // For brevity, the full implementation is omitted here.
    // It would contain logic similar to the previous scene's example.
    static class CustomStringReverseConverter implements MessageConverter {
        // ... implementation of supports, fromMessage, toMessage
        @Override
        public boolean supports(Class<?> payloadClass) {
            return String.class.equals(payloadClass);
        }
        @Override
        public Object fromMessage(org.springframework.messaging.Message<?> message, Class<?> targetClass) {
            // Example: Reverse incoming string
            String payload = new String((byte[]) message.getPayload());
            return new StringBuilder(payload).reverse().toString();
        }
        @Override
        public org.springframework.messaging.Message<?> toMessage(Object payload, org.springframework.messaging.MessageHeaders headers) {
            // Example: Convert outgoing string to bytes
            return new org.springframework.messaging.support.GenericMessage<>(((String) payload).getBytes());
        }
    }
}

Order of Converters Matters

Binary Formats: Protobuf & Avro

Client-Side Considerations

Check Your Understanding

Recap: Custom Converters