Implementing the Message Interface

How to implement a custom message

Example 10.2 outlines how to implement a message by extending the DefaultMessage class.

Example 10.2. Custom Message Implementation

import org.apache.camel.Exchange;
import org.apache.camel.impl.DefaultMessage;

public class CustomMessage extends DefaultMessage { 

    public CustomMessage() { 
        // Create message with default properties...
    }

    @Override
    public String toString() { 
        // Return a stringified message...
    }

    public CustomExchange getExchange() { 
        return (CustomExchange)super.getExchange();
    }

    @Override
    public CustomMessage newInstance() { 
        return new CustomMessage( ... );
    }

    @Override
    protected Object createBody() { 
        // Return message body (lazy creation).
    }

    @Override
    protected void populateInitialHeaders(Map<String, Object> map) { 
        // Initialize headers from underlying message (lazy creation).
    }

    @Override
    protected void populateInitialAttachments(Map<String, DataHandler> map) { 
        // Initialize attachments from underlying message (lazy creation).
    }
}

	Implements a custom message class, `CustomMessage`, by extending the `org.apache.camel.impl.DefaultMessage` class.
	Typically, you need a default constructor that creates a message with default properties.
	Override the `toString()` method to customize message stringification.
	(Optional) This is a convenient method that returns a reference to the parent exchange instance, casts it to the correct type.
	The `newInstance()` method is called from inside the `MessageSupport.copy()` method. Customization of the `newInstance()` method should focus on copying all of the custom properties of the current message instance into the new message instance. The `MessageSupport.copy()` method copies the generic message properties by calling `copyFrom()`.
	The `createBody()` method works in conjunction with the `MessageSupport.getBody()` method to implement lazy access to the message body. By default, the message body is `null`. It is only when the application code tries to access the body (by calling `getBody()`), that the body should be created. The `MessageSupport.getBody()` automatically calls `createBody()`, when the message body is accessed for the first time.
	The `populateInitialHeaders()` method works in conjunction with the header getter and setter methods to implement lazy access to the message headers. This method parses the message to extract any message headers and inserts them into the hash map, `map`. The `populateInitialHeaders()` method is automatically called when a user attempts to access a header (or headers) for the first time (by calling `getHeader()`, `getHeaders()`, `setHeader()`, or `setHeaders()`).
	The `populateInitialAttachments()` method works in conjunction with the attachment getter and setter methods to implement lazy access to the attachments. This method extracts the message attachments and inserts them into the hash map, `map`. The `populateInitialAttachments()` method is automatically called when a user attempts to access an attachment (or attachments) for the first time by calling `getAttachment()`, `getAttachments()`, `getAttachmentNames()`, or `addAttachment()`.