Provider Architecture: Packet Extensions and Custom IQ's

Introduction

The Smack provider architecture is a system for plugging in custom XML parsing of packet extensions and IQ packets. The standard Smack Extensions are built using the provider architecture. There are two types of providers:

- `IQProvider` -- parses IQ requests into Java objects.
- `Extension Provider` -- parses XML sub-documents attached to packets into PacketExtension instances.

By default, Smack only knows how to process a few standard packets and sub-packets that are in a few namespaces such as:

- jabber:iq:auth
- jabber:iq:roster
- jabber:iq:register

There are many more IQ types and extensions that are part of XMPP standards, and of course an endless number that can be added as custom extensions. To support this, an extensible parsing mechanism is provided via Smack and user build providers.

Whenever a packet extension is found in a packet, parsing will be passed to the correct provider. Each provider can either implement the PacketExtensionProvider interface or be a standard Java Bean. In the former case, each extension provider is responsible for parsing the raw XML stream, via the XML Pull Parser, to contruct an object. In the latter case, bean introspection is used to try to automatically set the properties of the class using the values in the packet extension sub-element.

When no extension provider is registered for an element name and namespace combination, Smack will store all top-level elements of the sub-packet in the DefaultPacketExtension object and then attach it to the packet.

Management of these providers is accomplished via the ProviderManager class. There are multiple ways to add providers to the manager.

- Call addXXProvider methods - You can call the appropriate add methods directly.

          ProviderManager.getInstance().addIQProvider("element", "namespace", new MyIQProvider());
          ProviderManager.getInstance().addExtensionProvider("element", "namespace", new MyExtProvider());

- Add a loader - You can add a ProviderLoader which will inject a means of loading multiple providers (both types) into the manager. This is the mechanism used by Smack to load from the Smack specific file format (via ProviderFileLoader). Implementers can provide the means to load providers from any source they wish, or simply reuse the ProviderFileLoader to load from their own provider files.

          ProviderManager.getInstance().addLoader(new ProviderFileLoader(FileUtils.getStreamForUrl("classpath:com/myco/provider/myco_custom.providers", null)));

- VM Argument - You can add a provider file via the VM argument smack.provider.file. This will load the file at the specified URL during startup when Smack initializes. This also assumes the default configuration, since it requires that the VmArgInitializer was part of the startup configuration.

          -Dsmack.provider.file=classpath:com/myco/provider/myco_custom.providers
          or
          -Dsmack.provider.file=file:///c:/myco/provider/myco_custom.providers

IQ Providers

The IQ provider class can either implement the IQProvider interface, or extend the IQ class. In the former case, each IQProvider is responsible for parsing the raw XML stream to create an IQ instance. In the latter case, bean introspection is used to try to automatically set properties of the IQ instance using the values found in the IQ packet XML. For example, an XMPP time packet resembles the following:

Introspection

Time Packet

        <iq type='result' to='joe@example.com' from='mary@example.com' id='time_1'>
            <query xmlns='jabber:iq:time'>
                <utc>20020910T17:58:35</utc>
                <tz>MDT</tz>
                <display>Tue Sep 10 12:58:35 2002</display>
            </query>
        </iq>

Time IQ Class

        class Time extends IQ {
            private Date utc;
            private TimeZone timeZone;
            private String display;

            @Override
            public String getChildElementXML() {
                return null;
            }

            public void setUtc(String utcString) {
                try {
                    utc = StringUtils.parseDate(utcString);
                } catch (ParseException e) {
                }
            }

            public void setTimeZone(String zone) {
                timeZone = TimeZone.getTimeZone(zone);
            }

            public void setDisplay(String timeDisplay) {
                display = timeDisplay;
            }
        }

The introspection service will automatically try to convert the String value from the XML into a boolean, int, long, float, double, or Class depending on the type the IQ instance expects.

IQProvider Implementation

Disco Items Packet

        <iq type='result' from='shakespeare.lit' to='romeo@montague.net/orchard' id='items1'>
           <query xmlns='http://jabber.org/protocol/disco#items'>
               <item jid='people.shakespeare.lit' name='Directory of Characters'/>
               <item jid='plays.shakespeare.lit' name='Play-Specific Chatrooms'/>
               <item jid='mim.shakespeare.lit' name='Gateway to Marlowe IM'/>
               <item jid='words.shakespeare.lit' name='Shakespearean Lexicon'/>
               <item jid='globe.shakespeare.lit' name='Calendar of Performances'/>
               <item jid='headlines.shakespeare.lit' name='Latest Shakespearean News'/>
               <item jid='catalog.shakespeare.lit' name='Buy Shakespeare Stuff!'/>
               <item jid='en2fr.shakespeare.lit' name='French Translation Service'/>
           </query>
        </iq>

Disco Items IQProvider

        public class DiscoverItemsProvider implements IQProvider {

            public IQ parseIQ(XmlPullParser parser) throws Exception {
                DiscoverItems discoverItems = new DiscoverItems();
                boolean done = false;
                DiscoverItems.Item item;
                String jid = "";
                String name = "";
                String action = "";
                String node = "";
                discoverItems.setNode(parser.getAttributeValue("", "node"));
                while (!done) {
                    int eventType = parser.next();

                    if (eventType == XmlPullParser.START_TAG && "item".equals(parser.getName())) {
                        // Initialize the variables from the parsed XML
                        jid = parser.getAttributeValue("", "jid");
                        name = parser.getAttributeValue("", "name");
                        node = parser.getAttributeValue("", "node");
                        action = parser.getAttributeValue("", "action");
                    }
                    else if (eventType == XmlPullParser.END_TAG && "item".equals(parser.getName())) {
                        // Create a new Item and add it to DiscoverItems.
                        item = new DiscoverItems.Item(jid);
                        item.setName(name);
                        item.setNode(node);
                        item.setAction(action);
                        discoverItems.addItem(item);
                    }
                    else if (eventType == XmlPullParser.END_TAG && "query".equals(parser.getName())) {
                        done = true;
                    }
                }
                return discoverItems;
            }
        }

Extension Providers

Packet extension providers are responsible for parsing packet extensions, which are child elements in a custom namespace of IQ, message and presence packets.

Pubsub Subscription Packet

       <iq type='result' from='pubsub.shakespeare.lit' to='francisco@denmark.lit/barracks' id='sub1'>
           <pubsub xmlns='http://jabber.org/protocol/pubsub'>
               <subscription node='princely_musings' jid='francisco@denmark.lit' subscription='unconfigured'>
                   <subscribe-options>
                       <required/>
                   </subscribe-options>
               </subscription>
           </pubsub>
        </iq>

Subscription PacketExtensionProvider Implementation

        public class SubscriptionProvider implements PacketExtensionProvider {
            public PacketExtension parseExtension(XmlPullParser parser) throws Exception {
                String jid = parser.getAttributeValue(null, "jid");
                String nodeId = parser.getAttributeValue(null, "node");
                String subId = parser.getAttributeValue(null, "subid");
                String state = parser.getAttributeValue(null, "subscription");
                boolean isRequired = false;

                int tag = parser.next();

                if ((tag == XmlPullParser.START_TAG) && parser.getName().equals("subscribe-options")) {
                    tag = parser.next();

                    if ((tag == XmlPullParser.START_TAG) && parser.getName().equals("required"))
                        isRequired = true;

                    while (parser.next() != XmlPullParser.END_TAG && parser.getName() != "subscribe-options");
                }
                while (parser.getEventType() != XmlPullParser.END_TAG) parser.next();
                return new Subscription(jid, nodeId, subId, (state == null ? null : Subscription.State.valueOf(state)), isRequired);
            }
        }

Provider file format

This is the format for a provider file which can be parsed by the ProviderFileLoader.

     <?xml version="1.0"?>
     <smackProviders>
         <iqProvider>
             <elementName>query</elementName>
             <namespace>jabber:iq:time</namespace>
             <className>org.jivesoftware.smack.packet.Time</className>
         </iqProvider>

         <iqProvider>
             <elementName>query</elementName>
             <namespace>http://jabber.org/protocol/disco#items</namespace>
             <className>org.jivesoftware.smackx.provider.DiscoverItemsProvider</className>
         </iqProvider>

        <extensionProvider>
            <elementName>subscription</elementName>
            <namespace>http://jabber.org/protocol/pubsub</namespace>
            <className>org.jivesoftware.smackx.pubsub.provider.SubscriptionProvider</className>
        </extensionProvider>
     </smackProviders>

Each provider is associated with an element name and a namespace. If multiple provider entries attempt to register to handle the same namespace, the last entry added to the ProviderManager will overwrite any other that was loaded before it.