WebLogic Messaging API Class Library

IMessage Interface

The IMessage interface is the root interface of all JMS messages. It defines the message header and the Acknowledge method used for all messages.

Most message-oriented middleware (MOM) products treat messages as lightweight entities that consist of a header and a payload. The header contains fields used for message routing and identification; the payload contains the application data being sent.

JMS messages are composed of the following parts:

Message Bodies

The JMS API defines five types of message body:

Message Headers

The JMSCorrelationID header field is used for linking one message with another. It typically links a reply message with its requesting message. JMSCorrelationID can hold a provider-specific message ID, an application-specific String object, or a provider-native byte[] value.

Other message header fields include JMSMessageID - a unique message identifier, JMSDestination, JMSDeliveryMode, JMSPriority, JMSExpiration, JMSTimestamp, JMSRedelivered, JMSReplyTo, and JMSType.

Message Properties

An IMessage object contains a built-in facility for supporting application-defined property values. In effect, this provides a mechanism for adding application-specific header fields to a message.

Properties allow an application, via message selectors, to have JMS select, or filter, messages on its behalf using application-specific criteria.

Property names obey the rules for a message selector identifier. Property names must not be null, and must not be empty strings. If a property name is set and it is either null or an empty string, an ArgumentException will be thrown.

Property values can be boolean, sbyte, short, int, long, float, double, and String.

Property values are set prior to sending a message. When a client receives a message, its properties are in read-only mode. If a client attempts to set properties at this point, a MessageNotWriteableException is thrown. If ClearProperties is called, the properties can now be both read from and written to. Note that header fields are distinct from properties. Header fields are never in read-only mode.

A property value may duplicate a value in a message's body, or it may not. Although JMS does not define a policy for what should or should not be made a property, application developers should note that JMS will likely handle data in a message's body or header field more efficiently than data in a message's properties. For best performance, applications should use message properties only when they need to customize a message's header. The primary reason for doing this is to support customized message selection.

Message properties support the following conversion table. The marked cases are supported. The unmarked cases will throw a MessageException.

A value written as the row type can be read as the column type.


In addition to the type-specific set/get methods for properties, JMS provides the SetObjectProperty and GetObjectProperty methods. These support the same set of property types using the objectified primitive values. Their purpose is to allow the decision of property type to made at execution time rather than at compile time. They support the same property value conversions.

The SetObjectProperty method accepts values of class bool, sbyte, short, int, long, float, double, and String. An attempt to use any other class will throw a MessageFormatException.

The GetObjectProperty method only returns values of class bool, sbyte, short, int, long, float, double, and String.

The order of property values is not defined. To iterate through a message's property values, use GetPropertyNames to retrieve a property name enumeration and then use the various property get methods to retrieve their values.

A message's properties are deleted by the ClearProperties method. This leaves the message with an empty set of properties.

Getting a property value for a name which has not been set returns a null value. Only the GetStringProperty and GetObjectProperty methods can return a null value.

The JMS API reserves the JMSX property name prefix for optional JMS defined properties. The full set of these properties is defined in the Java Message Service specification. New JMS defined properties may be added in later versions of the JMS API. Support for these properties is optional. The String[] IConnectionMetaData.getJMSXPropertyNames method returns the names of the JMSX properties supported by a connection.

JMSX properties may be referenced in message selectors whether or not they are supported by a connection. If they are not present in a message, they are treated like any other absent property.

JMSX properties defined in the specification as "set by provider on send" are available to both the producer and the consumers of the message. JMSX properties defined in the specification as "set by provider on receive" are available only to the consumers.

The JMS API reserves the JMS_vendor_name property name prefix for provider-specific properties. Each provider defines its own value for vendor_name. This is the mechanism a JMS provider uses to make its special per-message services available to a JMS client.

Implementations of JMS Message Interfaces

The JMS API provides a set of message interfaces that define the JMS message model. It does not provide implementations of these interfaces.

JMS supplies a set of message factories with its ISession object for creating instances of messages.

Message Selectors

A JMS message selector allows a client to specify, by header field references and property references, the messages it is interested in. Only messages whose header and property values match the selector are delivered. What it means for a message not to be delivered depends on the IMessageConsumer being used.

Message selectors cannot reference message body values except via a WebLogic extension for XML text messages.

A message selector matches a message if the selector evaluates to true when the message's header field values and property values are substituted for their corresponding identifiers in the selector.

A message selector is a String whose syntax is based on a subset of the SQL92 conditional expression syntax. If the value of a message selector is an empty string, the value is treated as a null and indicates that there is no message selector for the message consumer.

The order of evaluation of a message selector is from left to right within precedence level. Parentheses can be used to change this order.

Predefined selector literals and operator names are shown here in uppercase; however, they are case insensitive.

A selector can contain:

JMS providers are required to verify the syntactic correctness of a message selector at the time it is presented. A method that provides a syntactically incorrect selector will result in a InvalidSelectorException. JMS providers may also optionally provide some semantic checking at the time the selector is presented. Not all semantic checking can be performed at the time a message selector is presented, because property types are not known.

The following message selector selects messages with a message type of car and color of blue and weight greater than 2500 pounds:

"JMSType = 'car' AND color = 'blue' AND weight > 2500"

Null Values

As noted above, property values may be NULL. The evaluation of selector expressions containing NULL values is defined by SQL92 NULL semantics. A brief description of these semantics is provided here.

SQL treats a NULL value as unknown. Comparison or arithmetic with an unknown value always yields an unknown value.

The IS NULL and IS NOT NULL operators convert an unknown value into the respective TRUE and FALSE values.

The boolean operators use three-valued logic as defined by the following tables:

The definition of the AND operator


The definition of the OR operator


The definition of the NOT operator


Special Notes

When used in a message selector, the JMSDeliveryMode header field is treated as having the values 'PERSISTENT' and 'NON_PERSISTENT'.

Date and time values use the standard Java long millisecond value which is the difference, measured in milliseconds, between a given time time and midnight, January 1, 1970 UTC. When a date or time literal is included in a message selector, it should be an integer literal for a millisecond value. For an example of how to convert between Java millisecond timestamps and .NET times, see the API doc for IMessage.JMSTimestamp.

Although SQL supports fixed decimal comparison and arithmetic, JMS message selectors do not. This is the reason for restricting exact numeric literals to those without a decimal (and the addition of numerics with a decimal as an alternate representation for approximate numeric values).

SQL comments are not supported.

For a list of all members of this type, see IMessage Members.




Namespace: WebLogic.Messaging

Assembly: WebLogic.Messaging (in WebLogic.Messaging.dll)

See Also

IMessage Members | WebLogic.Messaging Namespace | Receive() | Receive(long) | ReceiveNoWait() | MessageEventHandler | IBytesMessage | IMapMessage | IObjectMessage | IStreamMessage | ITextMessage