The objects used to implement JMS messaging remain essentially the same across programming domains: connection factories, connections, sessions, producers, consumers, messages, and destinations. These objects are shown in Figure 2-5. The figure shows, from the top down, how objects are derived, starting with the connection factory object.
Two of the objects, connection factories and destinations, are shown to reside in an object store. This is to underline the fact that these objects are normally created, configured, and managed as administered objects. We assume that connection factories and destinations are created administratively (rather than programmatically) throughout this chapter.
Figure 2-5 JMS Programming Objects
Table 2-2 summarizes the steps required to send and receive messages. Note that steps 1 through 6 are the same for senders and receivers.
Table 2-2 Producing and Consuming Messages.
The following sections describe the objects used by producers and consumers: connections, sessions, messages, and destinations. We will then complete the discussion of JMS objects by describing the production and consumption of messages.
A client uses a connection factory object (ConnectionFactory) to create a connection. A connection object ( Connection) represents a client’s active connection to the broker. It uses the underlying Message Queue connection service that is either started by default or is explicitly started by the administrator for this client.
Both allocation of communication resources and authentication of the client take place when a connection is created. It is a relatively heavyweight object, and most clients do all their messaging with a single connection. Connections support concurrent use: any number of producers and consumers can share a connection.
When you create a connection factory, you can configure the behavior of all connections derived from it by setting its properties. For Message Queue, these specify the following information:
The name of the host on which the broker resides, the connection service desired, and the port through which the client is to access that service.
How automatic reconnection to the broker should be handled if the connection fails. This feature reconnects the client to the same (or, in a broker cluster, to a different broker) if a connection is lost.
The default name and password of any user attempting the connection. This information is used to authenticate the user and authorize operations if a password is not specified at connection time.
How to manage the flow of control and payload messages between the broker and the client runtime.
It is possible to override connection factory properties from the command line used to start the client application. It is also possible to override properties for any given connection by explicitly setting properties for that connection.
You can use a connection object to create session objects, to set up an exception listener, or to obtain JMS version and JMS provider information.
If the connection represents a communication channel between the client and the broker, a session marks a single conversation between them. Mainly, you use a session object to create messages, message producers, and message consumers. When you create a session, you configure reliable delivery through a number of acknowledgement options or through transactions. For more information, see Reliable Message Delivery.
According to the JMS specification, a session is a single-threaded context for producing and consuming messages. You can create multiple message producers and consumers for a single session, but you are restricted to using them serially. The threading implementation varies slightly for Java and C clients. Consult the appropriate developer’s guide for additional information about threading implementation and restrictions.
You can also use a session object to do the following:
Create and configure temporary topics and queues; these are used as part of the request-reply pattern. See The Request-Reply Pattern.
Support transaction processing.
Define a serial order for producing or consuming messages.
Serialize the execution of message listeners for asynchronous consumers (see Consuming a Message).
Define when messages are considered processed.
A header is required of every JMS message. The header contains ten predefined fields, which are listed and described in Table 2-3.
Table 2-3 JMS-Defined Message Header
As you can see from reading through this table, message header fields serve a variety of purposes: identifying a message, configuring the routing of messages, providing information about message handling, and so on.
Non-persistent messages are guaranteed to be delivered at most once. Non-persistent messages can be lost if the message service fails.
Some message header fields are set by the JMS provider (the Message Queue broker and/or client runtime) and others are set by the client. Message producers may need to configure header values to obtain certain messaging behaviors; message consumers may need to read header values in order to understand how the message was routed and what further processing it might need.
For all messages produced by a specific message producer.
For each message when it is produced.
If these fields are set at more than one level, values set when producing a message override those set for the message’s producer.
Names of constant used for message header fields vary with the language implementation. See Oracle GlassFish Server Message Queue 4.5 Developer’s Guide for Java Clients or Oracle GlassFish Server Message Queue 4.5 Developer’s Guide for C Clients for more information.
A message can also include optional header fields, called properties, specified as property name and property value pairs. Properties allow clients and providers to extend the message header and can contain any information that the client or the JMS provider finds useful to identify and process a message. Message properties allow a consuming client to ask that only those messages be delivered which fit a given criteria. For instance, a consuming client might indicate an interest for payroll messages concerning part-time employees located in New Jersey. The JMS provider will not deliver messages that do not meet the specified criteria.
The JMS specification defines nine standard properties. Some of these are set by the client and some by the JMS provider. Their names begin with the reserved characters “JMSX.” The client or the JMS provider can use these properties to determine who sent a message, the identity of the application sending a message, the state of the message, how often and when it was delivered, tansaction identification, and so forth. These properties are useful to the JMS provider in routing messages and in providing diagnostic information.
Message Queue defines a number of additional message properties. These properties are used to identify compressed messages and how messages should be handled if they cannot be delivered. For more information see Managing Message Size in Oracle GlassFish Server Message Queue 4.5 Developer’s Guide for Java Clients.
The message body contains the data that clients want to exchange.
The JMS message body type determines what the body may contain and how it should be processed by the consumer, as specified in Table 2-4. The Session object includes a create method for each type of message body.
Table 2-4 Message Body Types