Java Message Service (JMS) is the messaging standard to provide the exchanging of messages. Essentially, JMS are a set interfaces that provides a JMS client the ability to facilitate for the enterprise messaging. This also includes the extension to support the operations and features for Advanced Queuing (AQ)

The JMS Text message is sent to an AQ queue. There are other types of JMS messages ('bytes_messages', 'map_message', 'stream_message' etc) though this blog focuses on the text message, which is called SYS.AQ$_JMS_TEXT_MESSAGE.

• JMS provider <-- Provides the access to the JMS interfaces
• JMS client <-- Can be considered as application that creates the message. Also, it can be the application that receives the message
• JMS producer/publisher <-- Who ever creates and sends the message
• JMS consumer/subscriber <-- Receives the JMS message
• JMS message <-- The actual message being transferred
• JMS queue <-- Once the message has been delivered, this is where the message will reside until a JMS consumer/subscriber 'consumes' the message
• JMS topic <-- Used for publishing the message to multiple subscribers

The code below goes through a similar example though here's a basic JMS TEXT MESSAGE.

An example of the JMS_TEXT_MESSAGE is as follows:

SYS.AQ$_JMS_TEXT_MESSAGE(
 SYS.AQ$_JMS_HEADER(NULL,NULL,NULL,NULL,NULL,NULL,NULL),
 17,
 'text',
 NULL
)

Essentially, the SYS.AQ$_JMS_TEXT_MESSAGE as 4 components, which are highlighted below. the type of JMS_TEXT_MESSAGE components are as follows:

• HEADER - Header properties in the SYS.AQ$_JMS_HEADER datatype. (Explained in more detail below)
• TEXT_LEN - It's the size of the message, which it is set automatically. The example above is 29
• TEXT_VC - The message payload in VARCHAR2 format, if the payload is equal to or less than 4000 bytes. Because the example above is TEXT_LEN is less than 4000 (hence 29), this is populated and not the TEXT_LOB. If it's greater than 4000, this remains null and the TEXT_LOB is populated.
• TEXT_LOB - The message payload in CLOB (hence the 'LOB' name) format. This is populated if the TEXT_VS is greater than 4000, otherwise it's null

The HEADER value, which is known as the SYS.AQ$_JMS_HEADER datatype, contains the following attributes.

• REPLYTO - A Destination supplied by a client when a message is sent
• TYPE - The type of the message
• USERID - The identity of the user sending the message
• APPID - The identity of the application sending the message
• GROUPID - The identity of the message group of which this message is a part; set by the client
• GROUPSEQ - The sequence number of the message within the group
• PROPERTIES - Additional message properties in the SYS.AQ$_JMS_USERPROPARRAY datatype

So you have the header information, the size of the message, then either a VC or a LOB (basically a varchar or a BLOB). To populate the TEXT_LOB, you need to ensure the message exceeds 4000 characters. The example below shows an example on how an xml file is populated within the buffer reader and eventually as a message

static final String xmlFile = "/Users/danielredfern/Documents/identification.xml";

//Gets connection to DB
QueueConnection QCon = getConnection();
        
//Creates a session
Session session = QCon.createQueueSession(false, Session.CLIENT_ACKNOWLEDGE);
        
//Initiate the QueueConnection
QCon.start();
        
//Get queue
Queue queue = ((AQjmsSession) session).getQueue(user, queueName);
        
//Create Producer
MessageProducer producer = session.createProducer(queue);
        
        
BufferedReader br = new BufferedReader(new FileReader(new File(xmlFile)));
String line;
StringBuilder sb = new StringBuilder();

while((line=br.readLine())!= null){
	sb.append(line.trim());
}

String payload = sb.toString();
	  	
//Creates a textMessage and assigned it to the session
TextMessage tMsg = session.createTextMessage(payload);
        
producer.send(tMsg);
        
System.out.println("Sent message = " + tMsg.getText());

The identification.xml file contains characters that exceeds 4000 characters, so the VC becomes a BLOB. This is demonstrated when I query queue table, by using the following command:

select msgid, enq_time, enq_uid, q.user_data from queuetable1 q where q_name = 'queuename1'

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