Smack provides an easy mechanism for attaching arbitrary properties to packets. Each property has a String name, and a value that is a Java primitive (int, long, float, double, boolean) or any Serializable object (a Java object is Serializable when it implements the Serializable interface).

Using the API

All major objects have property support, such as Message objects. The following code demonstrates how to set properties:

Message message = chat.createMessage();
// Add a Color object as a property.
message.setProperty("favoriteColor", new Color(0, 0, 255));
// Add an int as a property.
message.setProperty("favoriteNumber", 4);
chat.sendMessage(message);

Getting those same properties would use the following code:

Message message = chat.nextMessage();
// Get a Color object property.
Color favoriteColor = (Color)message.getProperty("favoriteColor");
// Get an int property. Note that properties are always returned as
// Objects, so we must cast the value to an Integer, then convert
// it to an int.
int favoriteNumber = ((Integer)message.getProperty("favoriteNumber")).intValue();

Objects as Properties

Using objects as property values is a very powerful and easy way to exchange data. However, you should keep the following in mind:

• Packet extensions are the more standard way to add extra data to XMPP stanzas. Using properties may be more convenient in some cases, however, since Smack will do the work of handling the XML.
• When you send a Java object as a property, only clients running Java will be able to interpret the data. So, consider using a series of primitive values to transfer data instead.
• Objects sent as property values must implement Serialiable. Additionally, both the sender and receiver must have identical versions of the class, or a serialization exception will occur when de-serializing the object.
• Serialized objects can potentially be quite large, which will use more bandwidth and server resources.

XML Format

The current XML format used to send property data is not a standard, so will likely not be recognized by clients not using Smack. The XML looks like the following (comments added for clarity):

<!-- All properties are in a x block. --> 
<properties xmlns="http://www.jivesoftware.com/xmlns/xmpp/properties">
    <!-- First, a property named "prop1" that's an integer. --> 
    <property>
        <name>prop1</name>
        <value type="integer">123</value>
    <property>
    <!-- Next, a Java object that's been serialized and then converted
         from binary data to base-64 encoded text. -->  
    <property>
        <name>blah2</name>
        <value type="java-object">adf612fna9nab</value>
    <property>
</properties> 

The currently supported types are: integer, long, float, double, boolean, string, and java-object.