Message (JBoss Application Server: Build 7.1.2.Final API)

The Message 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.

Within this general form, the definition of a message varies significantly across products. It would be quite difficult for the JMS API to support all of these message models.

With this in mind, the JMS message model has the following goals:

• Provide a single, unified message API
• Provide an API suitable for creating messages that match the format used by provider-native messaging applications
• Support the development of heterogeneous applications that span operating systems, machine architectures, and computer languages
• Support messages containing objects in the Java programming language ("Java objects")
• Support messages containing Extensible Markup Language (XML) pages

JMS messages are composed of the following parts:

• Header - All messages support the same set of header fields. Header fields contain values used by both clients and providers to identify and route messages.
• Properties - Each message contains a built-in facility for supporting application-defined property values. Properties provide an efficient mechanism for supporting application-defined message filtering.
• Body - The JMS API defines several types of message body, which cover the majority of messaging styles currently in use.

Message Bodies

The JMS API defines five types of message body:

• Stream - A StreamMessage object's message body contains a stream of primitive values in the Java programming language ("Java primitives"). It is filled and read sequentially.
• Map - A MapMessage object's message body contains a set of name-value pairs, where names are String objects, and values are Java primitives. The entries can be accessed sequentially or randomly by name. The order of the entries is undefined.
• Text - A TextMessage object's message body contains a java.lang.String object. This message type can be used to transport plain-text messages, and XML messages.
• Object - An ObjectMessage object's message body contains a Serializable Java object.
• Bytes - A BytesMessage object's message body contains a stream of uninterpreted bytes. This message type is for literally encoding a body to match an existing message format. In many cases, it is possible to use one of the other body types, which are easier to use. Although the JMS API allows the use of message properties with byte messages, they are typically not used, since the inclusion of properties may affect the format.

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.

Message Properties

A Message 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 a JMS provider select, or filter, messages on its behalf using application-specific criteria.

Property names must 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 IllegalArgumentException must be thrown.

Property values can be boolean, byte, 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 providers will likely handle data in a message's body 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 must be supported. The unmarked cases must throw a JMSException. The String-to-primitive conversions may throw a runtime exception if the primitive's valueOf method does not accept the String as a valid representation of the primitive.

A value written as the row type can be read as the column type. | | boolean byte short int long float double String |---------------------------------------------------------- |boolean | X X |byte | X X X X X |short | X X X X |int | X X X |long | X X |float | X X X |double | X X |String | X X X X X X X X |----------------------------------------------------------

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 Boolean, Byte, Short, Integer, Long, Float, Double, and String. An attempt to use any other class must throw a JMSException.

The getObjectProperty method only returns values of class Boolean, Byte, Short, Integer, 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. Attempting to read a null value as a primitive type must be treated as calling the primitive's corresponding valueOf(String) conversion method with a null value.

The JMS API reserves the JMSX property name prefix for 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[] ConnectionMetaData.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.

JMSXGroupID and JMSXGroupSeq are standard properties that clients should use if they want to group messages. All providers must support them. Unless specifically noted, the values and semantics of the JMSX properties are undefined.

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.

The purpose of provider-specific properties is to provide special features needed to integrate JMS clients with provider-native clients in a single JMS application. They should not be used for messaging between JMS clients.

Provider 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.

Each JMS provider supplies a set of message factories with its Session object for creating instances of messages. This allows a provider to use message implementations tailored to its specific needs.

A provider must be prepared to accept message implementations that are not its own. They may not be handled as efficiently as its own implementation; however, they must be handled.

Note the following exception case when a provider is handling a foreign message implementation. If the foreign message implementation contains a JMSReplyTo header field that is set to a foreign destination implementation, the provider is not required to handle or preserve the value of this header field.

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 MessageConsumer being used (see QueueReceiver and TopicSubscriber).

Message selectors cannot reference message body values.

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:

• Literals:
  • A string literal is enclosed in single quotes, with a single quote represented by doubled single quote; for example, 'literal' and 'literal''s'. Like string literals in the Java programming language, these use the Unicode character encoding.
  • An exact numeric literal is a numeric value without a decimal point, such as 57, -957, and +62; numbers in the range of long are supported. Exact numeric literals use the integer literal syntax of the Java programming language.
  • An approximate numeric literal is a numeric value in scientific notation, such as 7E3 and -57.9E2, or a numeric value with a decimal, such as 7., -95.7, and +6.2; numbers in the range of double are supported. Approximate literals use the floating-point literal syntax of the Java programming language.
  • The boolean literals TRUE and FALSE.
• Identifiers:
  • An identifier is an unlimited-length sequence of letters and digits, the first of which must be a letter. A letter is any character for which the method Character.isJavaLetter returns true. This includes '_' and '$'. A letter or digit is any character for which the method Character.isJavaLetterOrDigit returns true.
  • Identifiers cannot be the names NULL, TRUE, and FALSE.
  • Identifiers cannot be NOT, AND, OR, BETWEEN, LIKE, IN, IS, or ESCAPE.
  • Identifiers are either header field references or property references. The type of a property value in a message selector corresponds to the type used to set the property. If a property that does not exist in a message is referenced, its value is NULL.
  • The conversions that apply to the get methods for properties do not apply when a property is used in a message selector expression. For example, suppose you set a property as a string value, as in the following: myMessage.setStringProperty("NumberOfOrders", "2"); The following expression in a message selector would evaluate to false, because a string cannot be used in an arithmetic expression: "NumberOfOrders > 1"
  • Identifiers are case-sensitive.
  • Message header field references are restricted to JMSDeliveryMode, JMSPriority, JMSMessageID, JMSTimestamp, JMSCorrelationID, and JMSType. JMSMessageID, JMSCorrelationID, and JMSType values may be null and if so are treated as a NULL value.
  • Any name beginning with 'JMSX' is a JMS defined property name.
  • Any name beginning with 'JMS_' is a provider-specific property name.
  • Any name that does not begin with 'JMS' is an application-specific property name.
• White space is the same as that defined for the Java programming language: space, horizontal tab, form feed, and line terminator.
• Expressions:
  • A selector is a conditional expression; a selector that evaluates to true matches; a selector that evaluates to false or unknown does not match.
  • Arithmetic expressions are composed of themselves, arithmetic operations, identifiers (whose value is treated as a numeric literal), and numeric literals.
  • Conditional expressions are composed of themselves, comparison operations, and logical operations.
• Standard bracketing () for ordering expression evaluation is supported.
• Logical operators in precedence order: NOT, AND, OR
• Comparison operators: =, >, >=,