The Structure of a JMS Message

To make a messaging scheme work, all participants must agree on just what a message is. The JMS API defines a basic message structure that is not only simple, but also very flexible, allowing messages to be created to match formats suitable for non-JMS applications on heterogeneous platforms.

As illustrated in Figure 15.4, a JMS message object is composed of Header fields, a set of application-specific properties, and a message body.

Figure 15.4. The structure of a JMS message.

Message Header Fields

A header contains values used by both clients and providers to identify and route messages, for example the destination, a reply-to destination, the message type, and the message expiration time. All messages support the same standard set of header fields as described in Table 15.2.

Table 15.2. JMS Supported Message Types

Header Field	Description	Defined By
JMSCorrelationID	A client can use the JMSCorrelationID header field to link one message with another. A typical use is to link a response message with its request message.	Client application
JMSDestination	Contains the destination (queue or topic) to which the message is to be delivered. When a message is sent, this value is ignored. After completion of the send, it holds the destination object specified by the sending method. When a message is received, its destination value must be equivalent to the value assigned when it was sent.	send() method
JMSDeliveryMode	Contains the delivery mode ( PERSISTENT or NON_PERSISTENT ) specified when the message was sent.	send() method
JMSDeliveryTime	Defines the earliest absolute time at which a message can be delivered to a consumer.	send() method
JMSExpiration	When a message is sent, its expiration time is calculated as the sum of the time-to-live value specified on the send method and the current GMT.	send() method
	If the time-to-live is specified as zero, expiration is set to zero, which indicates the message does not expire.
	When GMT is later than an undelivered message's expiration time, the message should be destroyed . WebLogic JMS removes expired messages from the system to prevent their delivery.
JMSMessageID	Contains a string value that uniquely identifies each message sent by a provider. When a message is sent, this value is ignored. When the message is received, it contains a provider-assigned value.	send() method
JMSPriority	Contains the message's priority. When a message is sent, this value is ignored. After completion of the send, it holds the value specified by the method sending the message. JMS defines a 10-level priority value with 0 as the lowest priority and 9 as the highest. In addition, clients should consider priorities 0 “4 as gradations of normal priority and priorities 5 “9 as gradations of expedited priority.	Message consumer
	JMS does not require that a provider strictly implement priority ordering of messages; however, it should do its best to deliver expedited messages ahead of normal messages.
JMSRedelivered	Specifies a flag set when a message is redelivered because no acknowledgement was received.	JMS provider
JMSReplyTo	The JMSReplyTo header field contains a queue or topic to which reply messages should be sent.	Client application
	This feature can be used with the JMSCorrelationID header field to coordinate request/response messages.
JMSTimestamp	Contains the time a message was handed off to a provider to be sent.	Message consumer
	It is not the time the message was actually transmitted because the actual send may occur later due to transactions or other client-side queuing of messages.
	(The value stored in this field is a Java millis time value.)
JMSType	Contains a message type identifier supplied by a client when a message is sent.	Client application

Message Property Fields (Optional)

These fields contain a set of application-defined name /value pairs that are accessible via the getter and setter methods on the Message object.

Although message property fields may be used for application-specific purposes, JMS provides them primarily for use in conjunction with message selectors for filtering messages by the consumer.

Message Body

A JMS message body contains the content being delivered from a producer to a consumer. JMS provides five types of message bodies that extend javax.jms.Message , which consists of message headers and properties, but no message body. The message types supported by WebLogic JMS are described in Table 15.3.

Table 15.3. JMS Supported Message Types

Message Type	Description
StreamMessage	A message containing a stream of Java primitives.
MapMessage	Similar to HashTable , contains name-value pairs.
TextMessage	Message containing a String; this includes XML strings.
ObjectMessage	Message containing a serialized Java object.
BytesMessage	Message containing a byte stream. Can be used to implement binary format messages for non-Java clients.
XMLMessage	Not part of the JMS standard. However, WebLogic includes this message type to allow an application to construct and deconstruct an XML message using a DOM tree. XMLMessage extends the TextMessage message type.