黑暗之魂3鬼切老断在哪:introduction-jain-sip (1)

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:

                         java -jar textclient.jar  

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:

                         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:

                         public class SipLayerimplements SipListener {

The methods of this interface are:

                         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.

                         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.

                         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) throwsPeerUnavailableException, 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:

                         ...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 ListeningPoints, and a SipProvider. These objects will be needed in the upcoming methods to send and receive messages.

An Introduction to the JAIN SIP API
Pages: 1, 2, 3

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:

                         public void sendMessage(String to, String message) throwsParseException, 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:

                         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.

                         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:

                         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:

                         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.

                         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.

                         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:

                         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:

                         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 OKresponse = messageFactory.createResponse(200, req);ToHeader toHeader = (ToHeader)response.getHeader(ToHeader.NAME);toHeader.setTag("888"); //Identifier, specific to your applicationServerTransaction 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:

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

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

                         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 ListeningPoints and SipProviders. 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.