Experiments in Streaming Content in Java ME(二)----Creating an RTSP Protocol Handler

Recall that RTSP is the actual protocol over which streaming commands are initiated, through which the RTP packets are received. The RTSP protocol is like a command initiator, a bit like HTTP. For a really good explanation of a typical RTSP session, please see these specifications for a simple RTSP client. For the purposes of this article, I am going to oversimplify the protocol implementation. Figure 1 shows the typical RTSP session between a client and a streaming server.

Figure 1. A typical RTSP session between a RTSP client and a streaming server (click for full-size image).

In a nutshell, an RTSP client initiates a session by sending a DESCRIBE request to the streaming server which means that the client wants more information about a media file. An example DESCRIBE request may look like this:

DESCRIBE rtsp://localhost:554/media.3gp rtsp/1.0
CSeq: 1

The URL for the media file is followed by the RTSP version that the client is following, and a carriage return/line feed (CRLF). The next line contains the sequence number of this request and increments for each subsequent request sent to the server. The command is terminated by a single line on its own (as are all RTSP commands).

All client commands that are successful receive a response that starts with RTSP/1.0 200 OK. For the DESCRIBE request, the server responds with several parameters, and if the file is present and streamable, this response contains any information for any tracks in special control strings that start with a a=control:trackID= String. The trackID is important and is used to create the next requests to the server.

Once described, the media file's separate tracks are set up for streaming using the SETUP command, and these commands should indicate the transport properties for the subsequent RTP packets. This is shown here:

SETUP rtsp://localhost:554/media.3gp/trackID=3 rtsp/1.0
CSeq: 2
TRANSPORT: UDP;unicast;client_port=8080-8081

The previous command indicates to the server to set up to stream trackID 3 of the media.3gp file, to send the packets via UDP, and to send them to port 8080 on the client (8081 is for RTCP commands). The response to the first SETUP command (if it is okay) will contain the session information for subsequent commands and must be included as shown here:

SETUP rtsp://localhost:554/media.3gp/trackID=3 rtsp/1.0
CSeq: 3
Session: 556372992204
TRANSPORT: UDP;unicast;client_port=8080-8081

An OK response from the server indicates that you can send the PLAY command, which will make the server start sending the RTP packets:

PLAY rtsp://localhost:554/media.3gp rtsp/1.0
CSeq: 3
Session: 556372992204

Notice that the PLAY command is issued only on the main media file, and not on any individual tracks. The same is true for the PAUSE and TEARDOWN commands, which are identical to the PLAY command, except for the command itself.

The following listing contains the RTSPProtocolHandler class. The comments in the code and the brief information so far should help with understanding how this protocol handler works:

importjava.util.Vector;
importjava.io.InputStream;
importjava.io.IOException;
importjava.io.OutputStream;

publicclassRTSPProtocolHandler{

//theaddressofthemediafileasanrtsp://...String
privateStringaddress;

//theinputstreamtoreceiveresponsefromtheserver
privateInputStreamis;

//theoutputstreamtowritetotheserver
privateOutputStreamos;

//theincrementingsequencenumberforeachrequest
//sentbytheclient
privatestaticintCSeq=1;

//thesessionidsentbytheserverafteraninitialsetup
privateStringsessionId;

//thenumberoftracksinamediafile
privateVectortracks=newVector(2);

//flagstoindicatethestatusofasession
privatebooleandescribed,setup,playing;
privateBooleanstopped=true;

//constants
privatestaticfinalStringCRLF=" ";
privatestaticfinalStringVERSION="rtsp/1.0";
privatestaticfinalStringTRACK_LINE="a=control:trackID=";
privatestaticfinalStringTRANSPORT_DATA=
"TRANSPORT:UDP;unicast;client_port=8080-8081";
privatestaticfinalStringRTSP_OK="RTSP/1.0200OK";

//baseconstructor,takesthemediaaddress,inputandoutputstreams
publicRTSPProtocolHandler(
Stringaddress,InputStreamis,OutputStreamOs){

this.address=address;
this.is=is;
this.os=Os;
}

//creates,sendsandparsesaDESCRIBEclientrequest
publicvoiddoDescribe()throwsIOException{

//ifalreadydescribed,return
if(described)return;

//createthebasecommand
StringbaseCommand=getBaseCommand("DESCRIBE"+address);

//executeitandreadtheresponse
Stringresponse=doCommand(baseCommand);

//theresponsewillcontaintrackinformation,amongstotherthings
parseTrackInformation(response);

//setflag
described=true;
}

//creates,sendsandparsesaSETUPclientrequest
publicvoiddoSetup()throwsIOException{

//ifnotdescribed
if(!described)thrownewIOException("NotDescribed!");

//createthebasecommandforthefirstSETUPtrack
StringbaseCommand=
getBaseCommand(
"SETUP"+address+"/trackID="+tracks.elementAt(0));

//addthestatictransportdata
baseCommand+=CRLF+TRANSPORT_DATA;

//readresponse
Stringresponse=doCommand(baseCommand);

//parseitforsessioninformation
parseSessionInfo(response);

//ifsessioninformationcannotbeparsed,itisanerror
if(sessionId==null)
thrownewIOException("Couldnotfindsessioninfo");

//now,sendSETUPcommandsforeachofthetracks
intcntOfTracks=tracks.size();
for(inti=1;i<cntOfTracks;i++){
baseCommand=
getBaseCommand(
"SETUP"+address+"/trackID="+tracks.elementAt(i));
baseCommand+=CRLF+"Session:"+sessionId+CRLF+TRANSPORT_DATA;
doCommand(baseCommand);
}

//thisisnowsetup
setup=true;
}

//issuesaPLAYcommand
publicvoiddoPlay()throwsIOException{

//mustbefirstsetup
if(!setup)thrownewIOException("NotSetup!");

//createbasecommand
StringbaseCommand=getBaseCommand("PLAY"+address);

//addsessioninformation
baseCommand+=CRLF+"Session:"+sessionId;

//executeit
doCommand(baseCommand);

//setflags
playing=true;
stopped=false;
}

//issuesaPAUSEcommand
publicvoiddoPause()throwsIOException{

//ifitisnotplaying,donothing
if(!playing)return;

//createbasecommand
StringbaseCommand=getBaseCommand("PAUSE"+address);

//addsessioninformation
baseCommand+=CRLF+"Session:"+sessionId;

//executeit
doCommand(baseCommand);

//setflags
stopped=true;
playing=false;
}

//issuesaTEARDOWNcommand
publicvoiddoTeardown()throwsIOException{

//ifnotsetup,nothingtoteardown
if(!setup)return;

//createbasecommand
StringbaseCommand=getBaseCommand("TEARDOWN"+address);

//addsessioninformation
baseCommand+=CRLF+"Session:"+sessionId;

//executeit
doCommand(baseCommand);

//setflags
described=setup=playing=false;
stopped=true;
}

//thismethodisaconveniencemethodtoputaRTSPcommandtogether
privateStringgetBaseCommand(Stringcommand){

return(
command+
""+
VERSION+//version
CRLF+
"CSeq:"+(CSeq++)//incrementingsequence
);
}

//executesacommandandreceivesresponsefromserver
privateStringdoCommand(StringfullCommand)throwsIOException{

//toreadtheresponsefromtheserver
byte[]buffer=newbyte[2048];

//debug
System.err.println("======CLIENTREQUEST======");
System.err.println(fullCommand+CRLF+CRLF);
System.err.println("============================");

//sendacommand
os.write((fullCommand+CRLF+CRLF).getBytes());

//readresponse
intlength=is.read(buffer);

Stringresponse=newString(buffer,0,length);

//emptythebuffer
buffer=null;

//iftheresponsedoesn'tstartwithanallclear
if(!response.startsWith(RTSP_OK))
thrownewIOException("Serverreturnedinvalidcode:"+response);

//debug
System.err.println("======SERVERRESPONSE======");
System.err.println(response.trim());
System.err.println("=============================");

returnresponse;
}

//conveniencemethodtoparseaserverresponsetoDESCRIBEcommand
//fortrackinformation
privatevoidparseTrackInformation(Stringresponse){

StringlocalRef=response;
StringtrackId="";
intindex=localRef.indexOf(TRACK_LINE);

//iteratethroughtheresponsetofindallinstancesofthe
//TRACK_LINE,whichindicatesallthetracks.Addallthe
//trackid'stothetracksvector
while(index!=-1){
intbaseIdx=index+TRACK_LINE.length();
trackId=localRef.substring(baseIdx,baseIdx+1);
localRef=localRef.substring(baseIdx+1,localRef.length());
index=localRef.indexOf(TRACK_LINE);
tracks.addElement(trackId);
}

}

//findoutthesessioninformationfromthefirstSETUPcommand
privatevoidparseSessionInfo(Stringresponse){

sessionId=
response.substring(
response.indexOf("Session:")+"Session:".length(),
response.indexOf("Date:")).trim();

}

}