52.2. Implementing the Message Interface

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How to implement a custom message

Example 52.2, “Custom Message Implementation” outlines how to implement a message by extending the DefaultMessage class.

Example 52.2. Custom Message Implementation

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

public class CustomMessage extends DefaultMessage { 1

    public CustomMessage() { 2
        // Create message with default properties...

    public String toString() { 3
        // Return a stringified message...

    public CustomMessage newInstance() { 4
        return new CustomMessage( ... );

    protected Object createBody() { 5
        // Return message body (lazy creation).

    protected void populateInitialHeaders(Map<String, Object> map) { 6
        // Initialize headers from underlying message (lazy creation).

    protected void populateInitialAttachments(Map<String, DataHandler> map) { 7
        // 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.
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().
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