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Programming WebLogic JMS |
The following sections describe WebLogic JMS components and features:
Note: For more information on the JMS classes described in this section, access the JMS Specification and Javadoc supplied on the Sun Microsystems' Java web site at the following location:
http://java.sun.com/products/jms/docs.html
JMS supports two messaging models: point-to-point (PTP) and publish/subscribe (Pub/sub). The messaging models are very similar, except for the following differences:
Each model is implemented with classes that extend common base classes. For example, the PTP class javax.jms.Queue and the Pub/sub class javax.jms.Topic both extend the class javax.jms.Destination.
Each message model is described in detail in the following sections.
Note: The terms producer and consumer are used as generic descriptions of applications that send and receive messages, respectively, in either messaging model. For each specific messaging model, however, unique terms specific to that model are used when referring to producers and consumers.
The point-to-point (PTP) messaging model enables one application to send a message to another. PTP messaging applications send and receive messages using named queues. A queue sender (producer) sends a message to a specific queue. A queue receiver (consumer) receives messages from a specific queue.
The following figure illustrates PTP messaging.
Figure 2-1 Point-to-Point (PTP) Messaging
Multiple queue senders and queue receivers can be associated with a single queue, but an individual message can be delivered to only one queue receiver.
If multiple queue receivers are listening for messages on a queue, WebLogic JMS determines which one will receive the next message on a first come, first serve basis. If no queue receivers are listening on the queue, messages remain in the queue until a queue receiver attaches to the queue.
The publish/subscribe (Pub/sub) messaging model enables an application to send a message to multiple applications. Pub/sub messaging applications send and receive messages by subscribing to a topic. A topic publisher (producer) sends messages to a specific topic. A topic subscriber (consumer) retrieves messages from a specific topic.
The following figure illustrates Pub/sub messaging.
Figure 2-2 Publish/Subscribe (Pub/Sub) Messaging
Unlike with the PTP messaging model, the Pub/sub messaging model allows multiple topic subscribers to receive the same message. JMS retains the message until all topic subscribers have received it.
The Pub/sub messaging model supports durable subscribers, allowing you to assign a name to a topic subscriber and associate it with a user or application. For more information about durable subscribers, see Setting Up Durable Subscriptions.
As per the "Message Delivery Mode" section of the JMS Specification, messages can be specified as persistent or non-persistent:
To create a JMS applications, use the javax.jms API. The API allows you to create the class objects necessary to connect to the JMS, and send and receive messages. JMS class interfaces are created as subclasses to provide queue- and topic-specific versions of the common parent classes.
The following table lists the JMS classes described in more detail in subsequent sections. For a complete description of all JMS classes, see the javax.jms, weblogic.jms.ServerSessionPoolFactory, or weblogic.jms.extensions Javadoc.
For information about configuring JMS objects, see Managing WebLogic JMS. The procedure for setting up a JMS application is presented in Setting Up a JMS Application.
A ConnectionFactory object encapsulates connection configuration information, and enables JMS applications to create a Connection. A connection factory supports concurrent use, enabling multiple threads to access the object simultaneously. You can use the preconfigured default connection factory provided by WebLogic JMS, or a system administrator can configure one or more connection factories to create connections with predefined attributes that suit your application.
The default connection factory can be looked up using the JNDI name, weblogic.jms.ConnectionFactory. You only need to define a connection factory if the default provided by WebLogic JMS is not suitable for your application. All default factory configuration attributes are set to the same default values as a user-defined connection factory. For more information about the default values for the connection factory attributes, see "JMS Connection Factory" in the Administration Console Online Help.
Another distinction when using the default connection factory is that you have no control over targeting the WebLogic Server instances where the connection factory may be deployed. However, you can enable and/or disable the default connection factory on a per-server basis. For more information on enabling or disabling the default connection factory, see "Server --> Services --> JMS" in the Administration Console Online Help.
If the default connection factory is not suitable for your application, a system administrator can define and configure one or more connection factories to create connections with predefined attributes — as long as each connection factory is uniquely named — and WebLogic Server adds them to the JNDI space during startup. The application then retrieves a connection factory using WebLogic JNDI. For information on configuring and deploying user-defined connection factories, see "Managing JMS" in the Administration Guide.
A system administrator can also establish cluster-wide, transparent access to JMS destinations from any server in the cluster, either by enabling the default connection factory for each server instance in the cluster, or by configuring one or more connection factories and targeting them to one or more server instances in the cluster. This way, each connection factory can be deployed on multiple WebLogic Servers. For more information on JMS clustering, refer to Configuring WebLogic JMS Clustering.
Notes: For backwards compatibility, WebLogic JMS still supports two deprecated default connection factories. The JNDI names for these factories are: javax.jms.QueueConnectionFactory and javax.jms.TopicConnectionFactory.
For information on migrating to a new default or user-defined connection factory from a deprecated connection factory, refer to Porting WebLogic JMS Applications.
The ConnectionFactory class does not define methods; however, its subclasses define methods for the respective messaging models. A connection factory supports concurrent use, enabling multiple threads to access the object simultaneously.
The following table describes the ConnectionFactory subclasses.
To learn how to use the ConnectionFactory class within an application, see Developing a WebLogic JMS Application, or the javax.jms.ConnectionFactory Javadoc.
A Connection object represents an open communication channel between an application and the messaging system, and is used to create a Session for producing and consuming messages. A connection creates server-side and client-side objects that manage the messaging activity between an application and JMS. A connection may also provide user authentication.
A Connection is created by a ConnectionFactory, obtained through a JNDI lookup.
Due to the resource overhead associated with authenticating users and setting up communications, most applications establish a single connection for all messaging. In the WebLogic Server, JMS traffic is multiplexed with other WebLogic services on the client connection to the server. No additional TCP/IP connections are created for JMS. Servlets and other server-side objects may also obtain JMS Connections.
By default, a connection is created in stopped mode. For information about how and when to start a stopped connection, see Starting, Stopping, and Closing a Connection.
Connections support concurrent use, enabling multiple threads to access the object simultaneously.
The following table describes the Connection subclasses.
To learn how to use the Connection class within an application, see Developing a WebLogic JMS Application, or the javax.jms.Connection Javadoc.
A Session object defines a serial order for the messages produced and consumed, and can create multiple message producers and message consumers. The same thread can be used for producing and consuming messages. If an application wants to have a separate thread for producing and consuming messages, the application should create a separate session for each function.
A Session is created by the Connection.
Note: A session and its message producers and consumers can only be accessed by one thread at a time. Their behavior is undefined if multiple threads access them simultaneously.
The following table describes the Session subclasses.
Producing and consuming messages for a JMS PTP provider. Created by QueueConnection. |
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Producing and consuming messages for a JMS Pub/sub provider. Created by TopicConnection. |
To learn how to use the Session class within an application, see Developing a WebLogic JMS Application, or the javax.jms.Session and weblogic.jms.extensions.WLSession javadocs.
In a non-transacted session, the application creating the session selects one of the five acknowledge modes defined in the following table.
The Session object acknowledges receipt of a message once the receiving application method has returned from processing it. |
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The Session object relies on the application to call an acknowledge method on a received message. Once the method is called, the session acknowledges all messages received since the last acknowledge. This mode allows an application to receive, process, and acknowledge a batch of messages with one call. Note: In the Administration Console, if the Acknowledge Policy attribute on the connection factory is set to Previous, but you want to acknowledge all received messages for a given session, then use the last message to invoke the acknowledge method. For more information on the Acknowledge Policy attribute, see "JMS Connection Factories" in the Administration Console Online Help. |
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The Session object acknowledges receipt of a message once the receiving application method has returned from processing it; duplicate acknowledges are permitted. This mode is most efficient in terms of resource usage. Note: You should avoid using this mode if your application cannot handle duplicate messages. Duplicate messages may be sent if an initial attempt to deliver a message fails. |
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No acknowledge is required. Messages sent to a NO_ACKNOWLEDGE session are immediately deleted from the server. Messages received in this mode are not recovered, and as a result messages may be lost and/or duplicate message may be delivered if an initial attempt to deliver a message fails. This mode is supported for applications that do not require the quality of service provided by session acknowledge, and that do not want to incur the associated overhead. Note: You should avoid using this mode if your application cannot handle lost or duplicate messages. Duplicate messages may be sent if an initial attempt to deliver a message fails. |
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Multicast mode with no acknowledge required. Messages sent to a MULTICAST_NO_ACKNOWLEDGE session share the same characteristics as NO_ACKNOWLEDGE mode, described previously. This mode is supported for applications that want to support multicasting, and that do not require the quality of service provided by session acknowledge. For more information on multicasting, see Using Multicasting. Note: You should avoid using this mode if your application cannot handle lost or duplicate messages. Duplicate messages may be sent if an initial attempt to deliver a message fails. |
In a transacted session, only one transaction is active at any given time. Any messages sent or received during a transaction are treated as an atomic unit.
When you create a transacted session, the acknowledge mode is ignored. When an application commits a transaction, all the messages that the application received during the transaction are acknowledged by the messaging system and messages it sent are accepted for delivery. If an application rolls back a transaction, the messages that the application received during the transaction are not acknowledged and messages it sent are discarded.
JMS can participate in distributed transactions with other Java services, such as EJB, that use the Java Transaction API (JTA). Transacted sessions do not support this capability as the transaction is restricted to accessing the messages associated with that session. For more information about using JMS with JTA, see Using JTA User Transactions.
A Destination object can be either a queue or topic, encapsulating the address syntax for a specific provider. The JMS specification does not define a standard address syntax due to the variations in syntax between providers.
Similar to a connection factory, an administrator defines and configures the destination and the WebLogic Server adds it to the JNDI space during startup. Applications can also create temporary destinations that exist only for the duration of the JMS connection in which they are created.
Note: Administrators can also configure multiple physical destinations as members of a single distributed destination set within a server cluster. For more information, see Distributed Destination.
On the client side, Queue and Topic objects are handles to the object on the server. Their methods only return their names. To access them for messaging, you create message producers and consumers that attach to them.
A destination supports concurrent use, enabling multiple threads to access the object simultaneously. JMS Queues and Topics extend javax.jms.Destination. The following table describes the Destination subclasses.
Note: An application has the option of browsing queues by creating a QueueBrowser object in its queue session. This object produces a snapshot of the messages in the queue at the time the queue browser is created. The application can view the messages in the queue, but the messages are not considered read and are not removed from the queue. For more information about browsing queues, see Browsing Header and Property Fields.
To learn how to use the Destination class within an application, see Developing a WebLogic JMS Application, or the javax.jms.Destination Javadoc.
Administrators can configure multiple physical destinations as members of a single distributed destination set within a WebLogic Server cluster. Once properly configured, your producers and consumers are able to send and receive to the distributed destination. WebLogic JMS then distributes the messaging load across all available destination members within the distributed destination.
MessageProducer and MessageConsumer
A MessageProducer object sends messages to a queue or topic. A MessageConsumer object receives messages from a queue or topic. Message producers and consumers operate independently of one another. Message producers generate and send messages regardless of whether a message consumer has been created and is waiting for a message, and vice versa.
A Session creates the MessageProducers and MessageConsumers that are attached to queues and topics.
The message sender and receiver objects are created as subclasses of the MessageProducer and MessageConsumer classes. The following table describes the MessageProducer and MessageConsumer subclasses.
The PTP model, as shown in the figure Point-to-Point (PTP) Messaging, allows multiple sessions to receive messages from the same queue. However, a message can only be delivered to one queue receiver. When there are multiple queue receivers, WebLogic JMS defines the next queue receiver that will receive a message on a first-come, first-serve basis.
The Pub/sub model, as shown in the figure Publish/Subscribe (Pub/Sub) Messaging, allows messages to be delivered to multiple topic subscribers. Topic subscribers can be durable or non-durable, as described in Setting Up Durable Subscriptions.
An application can use the same JMS connection to both publish and subscribe to a single topic. Because topic messages are delivered to all subscribers, an application can receive messages it has published itself. To prevent clients from receiving messages that they publish, a JMS application can set a noLocal attribute on the topic subscriber, as described in Step 5: Create Message Producers and Message Consumers Using the Session and Destinations.
To learn how to use the MessageProducer and MessageConsumer classes within an application, see Setting Up a JMS Application, or the javax.jms.MessageProducer and javax.jms.MessageConsumer javadocs.
A Message object encapsulates the information exchanged by applications. This information includes three components: a set of standard header fields, a set of application-specific properties, and a message body. The following sections describe these components.
Every JMS message contains a standard set of header fields that is included by default and available to message consumers. Some fields can be set by the message producers.
For information about setting message header fields, see Setting and Browsing Message Header and Property Fields, or to the javax.jms.Message Javadoc.
The following table describes the fields in the message headers and shows how values are defined for each field.
Specifies one of the following: a WebLogic JMSMessageID (described later in this table), an application-specific string, or a byte[] array. The JMSCorrelationID is used to correlate messages. There are two common applications for this field. The first application is to link messages by setting up a request/response scheme, as follows: 2. When an application receives the message, it copies the JMSMessageID into the JMSCorrelationID field of a response message that it sends back to the sending application. The second application is to use the JMSCorrelationID field to carry any String you choose, enabling a series of messages to be linked with some application-determined value. All JMSMessageIDs start with an ID: prefix. If you use the JMSCorrelationID for some other application-specific string, it must not begin with the ID: prefix. |
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Specifies PERSISTENT or NON_PERSISTENT messaging. When a persistent message is sent, WebLogic JMS stores it in the JMS file or JDBC database. The send() operation is not considered successful until delivery of the message can be guaranteed. A persistent message is guaranteed to be delivered at least once. WebLogic JMS does not store non-persistent messages in the JMS database. This mode of operation provides the lowest overhead. They are guaranteed to be delivered at least once unless there is a system failure, in which case messages may be lost. If a connection is closed or recovered, all non-persistent messages that have not yet been acknowledged will be redelivered. Once a non-persistent message is acknowledged, it will not be redelivered. When a message is sent, this value is ignored. When the message is received, it contains the delivery mode specified by the sending method. |
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Defines the earliest absolute time at which a message can be delivered to a consumer. This field can be used to sort messages in a destination and to select messages. For purposes of data type conversion, the JMSDeliveryTime is a long integer. |
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Specifies the destination (queue or topic) to which the message is to be delivered. The application's message producer sets the value of this field when the message is sent. When a message is sent, this value is ignored. When a message is received, its destination value must be equivalent to the value assigned when it was sent. |
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Specifies the expiration, or time-to-live value, for a message. WebLogic JMS calculates the JMSExpiration value as the sum of the application's time-to-live and the current GMT. If the application specifies time-to-live as 0, JMSExpiration is set to 0, which means the message never expires. WebLogic JMS removes expired messages from the system to prevent their delivery. |
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Contains a string value that uniquely identifies each message sent by a JMS Provider. All JMSMessageIDs start with an ID: prefix. When a message is sent, this value is ignored. When the message is received, it contains a provider-assigned value. |
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Specifies the priority level. This field is set before a message is sent. JMS defines ten priority levels, 0 to 9, 0 being the lowest priority. Levels 0-4 indicate gradations of normal priority, and level 5-9 indicate gradations of expedited priority. When the message is received, it contains the value specified by the method sending the message. You can sort destinations by priority by configuring a destination key, as described in Managing JMS in the Administration Guide. |
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Specifies a flag set when a message is redelivered because no acknowledge was received. This flag is of interest to a receiving application only. If set, the flag indicates that JMS may have delivered the message previously because one of the following is true:
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Specifies a queue or topic to which reply messages should be sent. This field is set by the sending application before the message is sent. This feature can be used with the JMSCorrelationID header field to coordinate request/response messages. Simply setting the JMSReplyTo field does not guarantee a response; it enables the receiving application to respond, if it so chooses. You may set the JMSReplyTo to null, which may have a semantic meaning to the receiving application, such as a notification event. |
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Contains the time at which the message was sent. WebLogic JMS writes the timestamp in the message when it accepts the message for delivery, not when the application sends the message. |
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Specifies the message type identifier (String) set by the sending application. The JMS specification allows some flexibility with this field in order to accommodate diverse JMS providers. Some messaging systems allow application-specific message types to be used. For such systems, the JMSType field could be used to hold a message type ID that provides access to the stored type definitions. |
The property fields of a message contain header fields added by the sending application. The properties are standard Java name/value pairs. Property names must conform to the message selector syntax specifications defined in the javax.jms.Message Javadoc. The following values are valid: boolean, byte, double, float, int, long, short, and String.
Although message property fields may be used for application-specific purposes, JMS provides them primarily for use in message selectors. For more information about message selectors, see Filtering Messages.
For information about setting message property fields, see Setting and Browsing Message Header and Property Fields, or to the javax.jms.Message Javadoc.
A message body contains the content being delivered from producer to consumer.
The following table describes the types of messages defined by JMS. All message types extend javax.jms.Message, which consists of message headers and properties, but no message body.
For more information, see the javax.jms.Message Javadoc. For more information about the access methods and, if applicable, the conversion charts associated with a particular message type, see the Javadoc for that message type.
A server session pool is a WebLogic-specific JMS feature that enables you to process messages concurrently. A server session pool factory is used to create a server-side ServerSessionPool.
WebLogic JMS defines one ServerSessionPoolFactory object, by default: weblogic.jms.ServerSessionPoolFactory:<name>, where <name> specifies the name of the JMS server to which the session pool is created. The WebLogic Server adds the default server session pool factory to the JNDI space during startup and the application subsequently retrieves the server session pool factory using WebLogic JNDI.
To learn how to use the server session pool factory within an application, see Defining Server Session Pools, or the weblogic.jms.ServerSessionPoolFactory Javadoc.
A ServerSessionPool application server object provides a pool of server sessions that connection consumers can retrieve in order to process messages concurrently.
A ServerSessionPool is created by the ServerSessionPoolFactory object obtained through a JNDI lookup.
To learn how to use the server session pool within an application, see Defining Server Session Pools, or the javax.jms.ServerSessionPool Javadoc.
A ServerSession application server object enables you to associate a thread with a JMS session by providing a context for creating, sending, and receiving messages.
A ServerSession is created by a ServerSessionPool object.
To learn how to use the server session within an application, see Defining Server Session Pools, or the javax.jms.ServerSession Javadoc.
A ConnectionConsumer object uses a server session to process received messages. If message traffic is heavy, the connection consumer can load each server session with multiple messages to minimize thread context switching.
A ConnectionConsumer is created by a Connection object.
To learn how to use the connection consumers within an application, see Defining Server Session Pools, or the javax.jms.ConnectionConsumer Javadoc.
Note: Connection consumer listeners run on the same JVM as the server.