An Introduction to the JAIN SIP API

by Emmanuel Proulx

10/17/2007

Abstract

This article shows how to develop client-side
applications using the Session Initiation Protocol (SIP) on Java SE. It
presents the JAIN SIP API, a powerful "SIP stack." A simple IM
application is shown and then dissected to explore this technology.

About JAIN SIP API

The Java APIs for Integrated Networks (JAIN) is a JCP work group managing telecommunication standards. Session Initiation Protocol (SIP)
is a standard communication protocol, discussed in a previous article.
Put Java and SIP together and you get the JAIN SIP API, a standard and
powerful API for telecommunications. This idea started in 1999 with JSR 32. The reference implementation
is open source, very stable, and very widely used. If you're writing a
SIP application with Java, using this SIP stack makes perfect sense.
This API is typically used for client-side application development. Other container-based technologies, like SIP Servlet API
(see BEA WebLogic SIP Server for example), are better suited for
server-side development. A previous article focuses on the SIP Servlet
API. Now we're going to turn to the client API stack.

Prerequisites

This article requires a good understanding of Java.
Also, I suggest you familiarize yourself with SIP, as using the JAIN
SIP API requires a good understanding of this protocol. SIP signaling,
especially messages and headers, is particularly important. See the
References section at the end of the article for links to relevant
information.

Libraries

To get the JAIN SIP API libraries, go to the jain-sip project home page. Click the link to go to the download page. You'll need to get these files:

JainSipApi1.2.jar

: SIP interfaces and main classes

JainSipRi1.2.jar

: SIP reference implementation

log4j-1.2.8.jar

(Available inside the file

jain-sip-1.2.jar

under the folder

jain-sip/lib

): Logging service

concurrent.jar

(Available inside the file

jain-sip-1.2.jar

under the folder

jain-sip/lib

): Concurrency utilities

Other files are not needed in this example. Include these libraries in your project.
I suggest you have a look at the first section of the Appendix
for a list of the various classes and interfaces provided in the JAIN
SIP API. These classes and interfaces are standard and provided to you
free of charge, and soon you will see how to use them as part of our

SipLayer

example class.

The TextClient Sample Application

As an example of a JAIN SIP API program, let's
dissect an application that you may be familiar with if you read the
article on the topic of SIP Servlets I wrote previously. The TextClient
is an instant messaging application that can send and receive text
messages over the SIP protocol. One instance of this application can
send messages to another instance, but in theory, this client can be
used to send messages to other kinds of SIP instant messaging clients,
and even SIP server applications. Figure 1 shows a screen capture.



Figure 1. The TextClient returns
To run this application, first you need to download
the provided source code. Second, you have to build it using the
provided Ant script. This will produce a JAR file. Finally, you run the
application, using this command:

[code]java -jar textclient.jar <username> <port>

You can run multiple instances of this client (make
sure they use different ports) and send each other messages. The rest
of the article explores the code of this sample application.

TextClient Code Overview

A couple of classes and an interface make up the whole TextClient code. This table introduces them:

Class / Interface	Description
TextClient	Main class, Swing window that contains the widgets of the application. See Figure 1.
SipLayer	Class that takes care of all the SIP communication. It is instantiated by the TextClient class, and calls it back through the MessageProcessor interface.
MessageProcessor	Callback interface (observer pattern), which serves to decouple the SipLayer from its container.

In the upcoming sections, I'll introduce the

MessageProcessor

and then spend most of the time looking at

SipLayer

. I won't talk about the

TextClient

class at all because it simply contains user interface Swing code and
is not relevant to the topic of this article. For more information,
please refer to the source code provided with this article.

Message Processor

Before I jump into the

SipLayer

class, I'll briefly cover the

MessageProcessor

interface. To decouple the SIP layer from the GUI layer, you use a
callback interface that allows sending information from the former
without having to know the signature of the latter. The interface is
shown below:

[code]public interface MessageProcessor

{

    public void processMessage(String sender, String message);

    public void processError(String errorMessage);

    public void processInfo(String infoMessage);

}

The

SipLayer

constructor will take an implementation of this interface (that is, the

TextClient

object) as a parameter and will hold on to it. Later you'll be able to use this object to send information back to the GUI.

SIP Stack Preparation

Let's start writing the

SipLayer

class.

TextClient

must be able to receive asynchronous messages coming from other SIP end
points. The observer pattern is used for this: The class implements the

SipListener

interface to process incoming messages:

[code]public class SipLayer

	implements SipListener {

The methods of this interface are:

[code]    void processRequest(RequestEvent evt);

    void processResponse(ResponseEvent evt);

    void processTimeout(TimeoutEvent evt);

    void processIOException(IOExceptionEvent evt);

    void processTransactionTerminated(TransactionTerminatedEvent evt);

    void processDialogTerminated(DialogTerminatedEvent evt);

In this example, the most important methods evidently are

processRequest()

and

processResponse()

for processing incoming messages. I'll look at those a bit later.
Next are two fields to store objects needed later.
These are not directly related to the SIP API, but you'll need them for
the example. The first is a

MessageProcessor

object as
discussed before. You also need to keep the username handy. These two
fields have getters and setters which, for brevity, I'm not showing in
this article.

[code]private MessageProcessor messageProcessor;

private String username;

Next is the constructor. A typical way to start a
JAIN SIP API application—and TextClient follows this pattern—is to set
up a bunch of objects that will be useful later on. I'm talking about a
number of factories, and a single SIP stack instance, initialized.

[code]private SipStack sipStack;

private SipFactory sipFactory;

private AddressFactory addressFactory;

private HeaderFactory headerFactory;

private MessageFactory messageFactory;

private SipProvider sipProvider;

public SipLayer(String username, String ip, int port) throws

        PeerUnavailableException, TransportNotSupportedException,

        InvalidArgumentException, ObjectInUseException,

        TooManyListenersException {

  setUsername(username);

  sipFactory = SipFactory.getInstance();

  sipFactory.setPathName("gov.nist");

  Properties properties = new Properties();

  properties.setProperty("javax.sip.STACK_NAME",

          "TextClient");

  properties.setProperty("javax.sip.IP_ADDRESS",

          ip);

  sipStack = sipFactory.createSipStack(properties);

  headerFactory = sipFactory.createHeaderFactory();

  addressFactory = sipFactory.createAddressFactory();

  messageFactory = sipFactory.createMessageFactory();

  ...

The SIP factory is used to instantiate a

SipStack

implementation, but since there could be more than one implementation, you must name the one you want via the

setPathName()

method. The name "

gov.nist

" denotes the SIP stack you've got.
The

SipStack

object takes in a number
of properties. At a minimum, you must set the stack name. All other
properties are optional. Here I'm setting an IP address to use by the
stack, for cases where a single computer has more than one IP address.
Note that there are standard properties, which all SIP API
implementations must support, and non-standard ones that are dependent
on the implementation. See the References section for links to these
properties.
The next step is to create a pair of

ListeningPoint

and

SipProvider

objects. These objects provide the communication functionality of
sending and receiving messages. There's one set of these for TCP and
one set for UDP. This is also where you select the

SipLayer

(this) as a listener of incoming SIP messages:

[code]...

  ListeningPoint tcp = sipStack.createListeningPoint(port, "tcp");

  ListeningPoint udp = sipStack.createListeningPoint(port, "udp");

  sipProvider = sipStack.createSipProvider(tcp);

  sipProvider.addSipListener(this);

  sipProvider = sipStack.createSipProvider(udp);

  sipProvider.addSipListener(this);

}

And this is how the constructor ends. You've just used the JAIN SIP API to create a

SipStack

instance, a bunch of factories, two

ListeningPoint

s, and a

SipProvider

. These objects will be needed in the upcoming methods to send and receive messages.

Sending a SIP Request

Let's now write a method to send a SIP message with the JAIN SIP API.
In the prerequisites I suggested that you must know
SIP fairly well before you can start using the SIP API. You'll now see
what I mean by that! The SIP API is quite a low-level abstraction and,
in most cases, doesn't use default values or hide headers, request
URIs, or contents of SIP messages. The advantage of this design is that
you have complete control over what SIP messages contain.
The following method is a bit lengthy. It prepares and sends a SIP request. It can roughly be split into four subsections:

Create main elements

Create message

Complete message

Send message

The following main SIP elements are minimally needed to construct a message using the JAIN SIP API:

Request URI

Method

Call-ID header

CSeq header

From header

An array of Via headers

Max-forwards header

For information about these elements, please see An
Introduction to SIP, Part 1 (Dev2Dev, 2006). The following code snippet
creates all of these elements:

[code]public void sendMessage(String to, String message) throws
	    ParseException, InvalidArgumentException, SipException {

	SipURI from = addressFactory.createSipURI(getUsername(),
	        getHost() + ":" + getPort());
    Address fromNameAddress = addressFactory.createAddress(from);
	fromNameAddress.setDisplayName(getUsername());
	FromHeader fromHeader =
		headerFactory.createFromHeader(fromNameAddress,
		        "textclientv1.0");

	String username = to.substring(to.indexOf(":")+1, to.indexOf("@"));
	String address = to.substring(to.indexOf("@")+1);

	SipURI toAddress =
		addressFactory.createSipURI(username, address);
	Address toNameAddress = addressFactory.createAddress(toAddress);
	toNameAddress.setDisplayName(username);
	ToHeader toHeader =
		headerFactory.createToHeader(toNameAddress, null);

	SipURI requestURI =
		addressFactory.createSipURI(username, address);
	requestURI.setTransportParam("udp");

	ArrayList viaHeaders = new ArrayList();
	ViaHeader viaHeader =
		headerFactory.createViaHeader(
			getHost(),
			getPort(),
			"udp",
			null);
	viaHeaders.add(viaHeader);

	CallIdHeader callIdHeader = sipProvider.getNewCallId();

	CSeqHeader cSeqHeader =
		headerFactory.createCSeqHeader(1, Request.MESSAGE);

	MaxForwardsHeader maxForwards =
		headerFactory.createMaxForwardsHeader(70);
	...

I'm using factories that were created in the constructor,

HeaderFactory

and

AddressFactory

, to instantiate these elements.
Next let's instantiate the actual SIP message itself, passing in all the elements created earlier:

[code]       Request request =  messageFactory.createRequest(
  	requestURI, Request.MESSAGE, callIdHeader, cSeqHeader,
  	fromHeader, toHeader, viaHeaders,  	maxForwards);
...

Note the use of

MessageFactory

for this step.
Then let's add other elements to the message: a
contact header and the contents of the message (payload). It's possible
to add custom headers too at this point.

[code]	SipURI contactURI = addressFactory.createSipURI(getUsername(),
	        getHost());
	contactURI.setPort(getPort());
	Address contactAddress = addressFactory.createAddress(contactURI);
	contactAddress.setDisplayName(getUsername());
	ContactHeader contactHeader =
		headerFactory.createContactHeader(contactAddress);
	request.addHeader(contactHeader);

	ContentTypeHeader contentTypeHeader =
		headerFactory.createContentTypeHeader("text", "plain");
	request.setContent(message, contentTypeHeader);
	...

For more information on how to further massage the request, there's a description of the

Request

interface in appendix.
Lastly, you send the message using the

SipProvider

instance:

[code]	sipProvider.sendRequest(request);
}

Sending Messages Inside a Dialog

You're sending our message outside a dialog. That
means messages are not related to each other. This works well for a
simple instant-messaging application like the TextClient.
An alternative would be to create a dialog
(sometimes called a session) using an INVITE message, and then send
messages inside this dialog. The TextClient doesn't use this technique.
However, I think it's something worth learning. So as a compromise,
this subsection describes how it's done.
Sending a message inside a dialog requires the creation of

Dialog

and

Transaction

objects. On the initial message (that is, the message that creates the
dialog), instead of using the provider to send out the message, you
instantiate a

Transaction

and then get the

Dialog

from it. You keep the

Dialog

reference for later. You then use the

Transaction

to send the message:

[code]ClientTransaction trans = sipProvider.getNewClientTransaction(invite);
dialog = trans.getDialog();
trans.sendRequest();

Later when you wish to send a new message inside the same dialog, you use the

Dialog

object from before to create a new request. You can then massage the request and, lastly, use the

Transaction

to send out the message.

[code]request = dialog.createRequest(Request.MESSAGE);

request.setHeader(contactHeader);
request.setContent(message, contentTypeHeader);

ClientTransaction trans = sipProvider.getNewClientTransaction(request);
trans.sendRequest();

Essentially, you're skipping the "create main
elements" step when sending a message inside an existing dialog. When
you use an INVITE to create a dialog, don't forget to clean it up by
sending an in-dialog BYE message at the end. This technique is also
used to refresh registrations and subscriptions.
Previously, you've seen the

SipListener

interface, which contains the

processDialogTerminated()

and

processTransactionTerminated()

methods. These are called automatically at the end of a dialog and
transaction, respectively. Typically, you implement these methods to
clean things up (for example, discard the

Dialog

and

Transaction

instances). You'll leave these two methods empty as you don't need them in TextClient.

Receiving a Response

Earlier, you registered a listener of incoming messages. The listener interface,

SipListener

, contains the method

processResponse()

, which is called by the SIP stack when a SIP response message arrives.

processResponse()

takes a single parameter of type

ResponseEvent

, which encapsulates a

Response

object. Let's implement this method now.

[code]public void processResponse(ResponseEvent evt) {
	Response response = evt.getResponse();
	int status = response.getStatusCode();

	if( (status >= 200) && (status < 300) ) { //Success!
		messageProcessor.processInfo("--Sent");
		return;
	}

	messageProcessor.processError("Previous message not sent: " +
			status);
}

In this method you check if the response of an
earlier MESSAGE message represents a success (2xx range of status
codes) or an error (otherwise). You then relay this information back to
the user through the callback interface.
Typically, you read only the

Response

object in the

processResponse()

method. The only exception is for a success response to an INVITE
message; in this case, you must send an ACK request right back, like
this:

[code]Dialog dialog = evt.getClientTransaction().getDialog()
Request ack =  dialog.createAck()
dialog.sendAck( ack );

For a description of the

Response

interface, refer to the Appendix.

Receiving a Request

Receiving a SIP request message is just as easy as receiving a response. You just implement another method of the

SipListener

interface,

processRequest()

, and the SIP stack will call it automatically. The single parameter of this method is a

RequestEvent

object, which contains (you guessed it) a

Request

object. This is the same type that you've seen before, and it has the
same methods. However, you shouldn't set any fields on an incoming
request as it doesn't make much sense.
A typical implementation of

processRequest()

analyzes the request, and then creates and sends back an appropriate response. This is what you'll do now:

[code]public void processRequest(RequestEvent evt) {
	Request req = evt.getRequest();

	String method = req.getMethod();
	if( ! method.equals("MESSAGE")) { //bad request type.
		messageProcessor.processError("Bad request type: " + method);
		return;
	}

	FromHeader from = (FromHeader)req.getHeader("From");
	messageProcessor.processMessage(
			from.getAddress().toString(),
			new String(req.getRawContent()));
	Response response=null;
	try { //Reply with OK
		response = messageFactory.createResponse(200, req);
		ToHeader toHeader = (ToHeader)response.getHeader(ToHeader.NAME);
		toHeader.setTag("888"); //Identifier, specific to your application
		ServerTransaction st = sipProvider.getNewServerTransaction(req);
		st.sendResponse(response);
	} catch (Throwable e) {
		e.printStackTrace();
		messageProcessor.processError("Can't send OK reply.");
	}
}

In this case, you always reply with a success
response (200), but you could also send back any of the error responses
(typically 4xx range). Here's a useful list of SIP status codes.

Dealing With Error Conditions

There are other methods in the

SipListener

interface that you haven't implemented yet. They are called by the SIP
stack when a request cannot be sent for specific reasons. For example,
the

processTimeout()

is called when the end point
receiving the message doesn't answer in time. This is a special
situation for which there is no response, so no

Response

object is available. The

TimeoutEvent

parameter contains, among other things, the

ClientTransaction

of the request that timed out, and you can use this to link back to the
original request if you want to. In this implementation you simply
inform the user using the callback interface:

[code]public void processTimeout(TimeoutEvent evt) {
	messageProcessor.processError("Previous message not sent: " +
			"timeout");
}

Similarily, Input/Output (IO) errors are processed using the following method:

[code]public void processIOException(IOExceptionEvent evt) {
	messageProcessor.processError("Previous message not sent: " +
			"I/O Exception");
}

Point-to-Point vs. Client/Server

SIP client applications can be used standalone
(point to point) or together with a server to provide extra
functionality like proxying or call routing.
I suggest you have a look at my Aarticle on SIP
Servlets. It contains a neat SIP Server application that can work with
TextClient to provide a chat room-type service. This illustrates how
you can use the TextClient together with the BEA WebLogic SIP Server
and double its usefulness.

Download

Download the TextClient source code here.

Summary

This article provides an overview of the JAIN SIP
API, and I've shown how to write a simple application to use this
technology. Right now, you should have a pretty good idea of the APIs
available, and know how to write your own IM client with SIP.
Nevertheless, why stop here? I could add many more
features to this application. And as I said before, if the client talks
with a server application, you can double its usefulness. If you need
suggestions, consider the following:

Automatic text answers, store-and-forward
(for example, "John is offline right now, but he will receive your
messages as soon as he logs back in")

A neat networked checkers video game

A location-based service for your laptop

A media-sharing client

An RSS-like client

The possibilities are almost limitless.

References

A concise tutorial,
including SIP message formats. Be sure to read the section "Relation
among Call, Dialog, Transaction & Message" (external tutorial)

Standard SIP stack properties

Non-standard SIP stack properties of the reference implementation

SIP status codes

RTP stack

SIP Communicator (java.net project)

Appendix

This section is a reference to the various classes and interfaces available in the JAIN SIP API.

API overview

Here's an overview of the main classes and interfaces found in the JAIN SIP API reference implementation.

Class / Interface	Description
SipFactory / AddressFactory / HeaderFactory / MessageFactory	Factory classes to create the various objects of the system. They return objects that implement standard interfaces.
SipStack	The first interface you'll need, used to create ListeningPoint s and SipProvider s.
ListeningPoint	This interface encapsulates a transport/port pair (for example, UDP/5060).
SipProvider	This interface is used to send SIP messages. You can also register a listener for incoming SIP messages using this interface. See SipListener below.
SipListener	You must implement this interface to allow receiving incoming SIP messages.
RequestEvent / ResponseEvent	Represent an incoming SIP request, response. Passed to your SipListener for processing. Contains a Request or Response object, respectively.
TimeoutEvent	Represents a failure condition when there's no reply to an outgoing request. Passed to your SipListener for processing.
IOExceptionEvent	Represents a failure condition when there's an Input/Output problem sending an outgoing request. Passed to your SipListener for processing.
Request / Response	Represent a SIP request, response. Both are sub-interfaces of the Message interface. They provide access to headers, content, and other parts of SIP messages.
Dialog	An object of this interface encapsulates a SIP dialog. (Reminder: In a dialog all messages are related to the same call; a dialog often starts with an INVITE and ends with a BYE.)
ClientTransaction / ServerTransaction	Encapsulate SIP transactions. (Reminder: A transaction starts with a request and ends with a final response. Transactions often live within a dialog.)

Message

Interface

The

Message

interface is the base interface for SIP messages. For your reference, here's an overview of available methods.

Method	Description
void addHeader(Header) void setHeader(Header)	Sets header fields to the SIP message. The first method can be used for headers that are repeatable or can have multiple values, like the Contact header. The second method removes existing headers of this type and then adds a single header value.
void removeHeader(Header)	Removes existing headers of this type.
ListIterator getHeaderNames()	Returns all header names.
ListIterator getUnrecognizedHeaders()	Returns header names for non-standard header types.
Header getHeader(String) ListIterator getHeaders(String)	Getters for specific headers. The second form returns all values of repeatable headers, or headers with multiple values, like the Contact header.
void setContent(Object, ContentTypeHeader)	Sets the payload of the message, as well as the Content-Type header. Content-Length is also set if the type is a string, else use void setContentLength(ContentLengthHeader) .
byte [] getRawContent() Object getContent()	Retrieves the payload of the message.
void removeContent()	Empties the payload.
void setContentLength(ContentLengthHeader) ContentLengthHeader getContentLength() void setContentLanguage(ContentLanguageHeader) ContentLanguageHeader getContentLanguage() void setContentEncoding(ContentEncodingHeader) ContentEncodingHeader getContentEncoding() void setContentDisposition(ContentDispositionHeader) ContentDispositionHeader getContentDisposition()	Special payload-related header accessors. Rarely used.
void setExpires(ExpiresHeader) ExpiresHeader getExpires()	Manages the Expires header.
void setSipVersion(String) String getSipVersion()	Accessors for the SIP version element. Rarely used, defaults to SIP/2.0.
Object clone()	Creates a copy of the message. Rarely used.

Request

interface

Now let's glance through the

Request

interface (sub-interface of

Message

above):

Method	Description
String getMethod() void setMethod(String)	Accessors for the method element. Can be any SIP method, including those in the constants of the Request interface: ACK, BYE, CANCEL, INVITE, OPTIONS, REGISTER, NOTIFY, SUBSCRIBE, MESSAGE, REFER, INFO, PRACK, and UPDATE.
URI getRequestURI() void setRequestURI(URI)	Accessors for the request URI, which is the first line of a SIP request. Typically, this is an instance of SipURI .

Response

interface

The

Response

interface also extends the

Message

interface:

Method	Description
void setStatusCode() int getStatusCode()	Accessors for the status code. This can be any SIP status code, including those in the constant members of the Response interface. Here are a few of them: RINGING (180), OK (200), BAD_REQUEST (400), and so on.
void setReasonPhrase(String) String getReasonPhrase()	Accessors for the human-readable explanation of the status code.

Emmanuel Proulx is an expert in J2EE and SIP. He is a
certified WebLogic Server engineer.