Working With Messages

This section describes how to use the Message Queue Java API to compose, send, receive, and process messages.

Message Structure

A message consists of the following parts:

• A header containing identifying and routing information

• Optional properties that can be used to convey additional identifying information beyond that contained in the header

• A body containing the actual content of the message

The following sections discuss each of these in greater detail.

Message Header

Every message must have a header containing identifying and routing information. The header consists of a set of standard fields, which are defined in the Java Message Service Specification and summarized in Table 2-4. Some of these are set automatically by Message Queue in the course of producing and delivering a message, some depend on settings specified when a message producer sends a message, and others are set by the client on a message-by-message basis.

Table 2-4 Message Header Fields

The JMS Message interface defines methods for setting the value of each header field: for instance,

outMsg.setJMSReplyTo(replyDest);

Table 2-5 lists all of the available header specification methods.

Table 2-5 Message Header Specification Methods

The message identifier (JMSMessageID) is a string value uniquely identifying the message, assigned and set by the message broker when the message is sent. Because generating an identifier for each message adds to both the size of the message and the overhead involved in sending it, and because some client applications may not use them, the JMS interface provides a way to suppress the generation of message identifiers, using the message producer method setDisableMessageID (see Sending Messages).

The JMSDestination header field holds a Destination object representing the destination to which the message is directed, set by the message broker when the message is sent. There is also a JMSReplyTo field that you can set to specify a destination to which reply messages should be directed. Clients sending such a reply message can set its JMSCorrelationID header field to refer to the message to which they are replying. Typically this field is set to the message identifier string of the message being replied to, but client applications are free to substitute their own correlation conventions instead, using either the setJMSCorrelationID method (if the field value is a string) or the more general setJMSCorrelationIDAsBytes (if it is not).

The delivery mode (JMSDeliveryMode) specifies whether the message broker should log the message to stable storage. There are two possible values, PERSISTENT and NON_PERSISTENT, both defined as static constants of the JMS interface DeliveryMode: for example,

outMsg.setJMSDeliveryMode(DeliveryMode.NON_PERSISTENT);

The default delivery mode is PERSISTENT, represented by the static constant Message.DEFAULT_DELIVERY_MODE.

The choice of delivery mode represents a tradeoff between performance and reliability:

• In persistent mode, the broker logs the message to stable storage, ensuring that it will not be lost in transit in the event of transmission failure; the message is guaranteed to be delivered exactly once.

• In nonpersistent mode, the message is not logged to stable storage; it will be delivered at most once, but may be lost in case of failure and not delivered at all. This mode does, however, improve performance by reducing the broker’s message-handling overhead. It may thus be appropriate for applications in which performance is at a premium and reliability is not.

The message’s priority level (JMSPriority) is expressed as an integer from 0 (lowest) to 9 (highest). Priorities from 0 to 4 are considered gradations of normal priority, those from 5 to 9 of expedited priority. The default priority level is 4, represented by the static constant Message.DEFAULT_PRIORITY.

The Message Queue client runtime sets the JMSTimestamp header field to the time it delivers the message to the broker, expressed as a long integer in standard Java format (milliseconds since midnight, January 1, 1970 UTC). The message’s lifetime, specified when the message is sent, is added to this value and the result is stored in the JMSExpiration header field. (The default lifetime value of 0, represented by the static constant Message.DEFAULT_TIME_TO_LIVE, denotes an unlimited lifetime. In this case, the expiration time is also set to 0 to indicate that the message never expires.) As with the message identifier, client applications that do not use a message’s time stamp can improve performance by suppressing its generation with the message producer method setDisableMessageTimestamp (see Sending Messages).

The header field JMSType can contain an optional message type identifier string supplied by the client when the message is sent. This field is intended for use with other JMS providers; Message Queue clients can simply ignore it.

When a message already delivered must be delivered again because of a failure, the broker indicates this by setting the JMSRedelivered flag in the message header to true. This can happen, for instance, when a session is recovered or a transaction is rolled back. The receiving client can check this flag to avoid duplicate processing of the same message (such as when the message has already been successfully received but the client’s acknowledgment was missed by the broker).

See the Java Message Service Specification for a more detailed discussion of all message header fields.

Message Properties

A message property consists of a name string and an associated value, which must be either a string or one of the standard Java primitive data types (int, byte, short, long, float, double, or boolean). The Message interface provides methods for setting properties of each type (see Table 2-6). There is also a setObjectProperty method that accepts a primitive value in objectified form, as a Java object of class Integer, Byte, Short, Long, Float , Double, Boolean, or String . The clearProperties method deletes all properties associated with a message; the message header and body are not affected.

Table 2-6 Message Property Specification Methods

The JMS specification defines certain standard properties, listed in Table 2-7 . By convention, the names of all such standard properties begin with the letters JMSX; names of this form are reserved and must not be used by a client application for its own custom message properties. Similarly, property names beginning with JMS_SUN are reserved for provider-specific properties defined by Message Queue itself; these are discussed in Chapter 3, Message Queue Clients: Design and Features

Table 2-7 Standard JMS Message Properties

Message Body

The actual content of a message is contained in the message body. JMS defines six classes (or types) of message, each with a different body format:

• A text message (interface TextMessage) contains a Java string.

• A stream message (interface StreamMessage) contains a stream of Java primitive values, written and read sequentially.

• A map message (interface MapMessage) contains a set of name-value pairs, where each name is a string and each value is a Java primitive value. The order of the entries is undefined; they can be accessed randomly by name or enumerated sequentially.

• An object message (interface ObjectMessage) contains a serialized Java object (which may in turn be a collection of other objects).

• A bytes message (interface BytesMessage) contains a stream of uninterpreted bytes.

• A null message (interface Message) consists of a header and properties only, with no message body.

Each of these is a subinterface of the generic Message interface, extended with additional methods specific to the particular message type.

Composing Messages

The JMS Session interface provides methods for creating each type of message, as shown in Table 2-8. For instance, you can create a text message with a statement such as

TextMessage  outMsg = mySession.createTextMessage();

In general, these methods create a message with an empty body; the interfaces for specific message types then provide additional methods for filling the body with content, as described in the sections that follow.

Table 2-8 Session Methods for Message Creation

TextMessage
   outMsg = mySession.createTextMessage("Hello, World!");

Once a message has been delivered to a message consumer, its body is considered read-only; any attempt by the consumer to modify the message body will cause an exception (MessageNotWriteableException) to be thrown. The consumer can, however, empty the message body and place it in a writeable state by calling the message method clearBody:

outMsg.clearBody();

This places the message in the same state as if it had been newly created, ready to fill its body with new content.

Composing Text Messages

You create a text message with the session method createTextMessage. You can either initialize the message body directly at creation time

TextMessage  outMsg = mySession.createTextMessage("Hello, World!");

or simply create an empty message and then use its setText method (see Table 2-9 ) to set its content:

TextMessage  outMsg = mySession.createTextMessage();
outMsg.setText("Hello, World!");

Table 2-9 Text Message Composition Method

Composing Stream Messages

The session method createStreamMessage returns a new, empty stream message. You can then use the methods shown in Table 2-10 to write primitive data values into the message body, similarly to writing to a data stream: for example,

StreamMessage  outMsg = mySession.createStreamMessage();
outMsg.writeString("The Meaning of Life");
outMsg.writeInt(42);

Table 2-10 Stream Message Composition Methods

As a convenience for handling values whose types are not known until execution time, the writeObject method accepts a string or an objectified primitive value of class Integer, Byte, Short, Long, Float, Double , Boolean, or Character and writes the corresponding string or primitive value to the message stream: for example, the statements

Integer  meaningOfLife = new Integer(42);
outMsg.writeObject(meaningOfLife);

are equivalent to

outMsg.writeInt(42);

This method will throw an exception (MessageFormatException) if the argument given to it is not of class String or one of the objectified primitive classes.

Once you’ve written the entire message contents to the stream, the reset method

outMsg.reset();

puts the message body in read-only mode and repositions the stream to the beginning, ready to read (see Processing Messages). When the message is in this state, any attempt to write to the message stream will throw the exception MessageNotWriteableException. A call to the clearBody method (inherited from the superinterface Message) deletes the entire message body and makes it writeable again.

Composing Map Messages

Table 2-11 shows the methods available in the MapMessage interface for adding content to the body of a map message. Each of these methods takes two arguments, a name string and a primitive or string value of the appropriate type, and adds the corresponding name-value pair to the message body: for example,

StreamMessage  outMsg = mySession.createMapMessage();
outMsg.setInt("The Meaning of Life", 42);

Table 2-11 Map Message Composition Methods

Like stream messages, map messages provide a convenience method (setObject) for dealing with values whose type is determined dynamically at execution time: for example, the statements

Integer  meaningOfLife = new Integer(42);
outMsg.setObject("The Meaning of Life", meaningOfLife);

are equivalent to

outMsg.setInt("The Meaning of Life", 42);

The object supplied must be either a string object (class String) or an objectified primitive value of class Integer, Byte , Short, Long, Float, Double, Boolean, or Character; otherwise an exception (MessageFormatException) will be thrown.

Composing Object Messages

The ObjectMessage interface provides just one method, setObject (Table 2-12 ), for setting the body of an object message:

ObjectMessage  outMsg = mySession.createObjectMessage();
outMsg.setObject(bodyObject);

The argument to this method can be any serializable object (that is, an instance of any class that implements the standard Java interface Serializable). If the object is not serializable, the exception MessageFormatException will be thrown.

Table 2-12 Object Message Composition Method

As an alternative, you can initialize the message body directly when you create the message, by passing an object to the session method createObjectMessage:

ObjectMessage  outMsg = mySession.createObjectMessage(bodyObject);

Again, an exception will be thrown if the object is not serializable.

Composing Bytes Messages

The body of a bytes message simply consists of a stream of uninterpreted bytes; its interpretation is entirely a matter of agreement between sender and receiver. This type of message is intended primarily for encoding message formats required by other existing message systems; Message Queue clients should generally use one of the other, more specific message types instead.

Composing a bytes message is similar to composing a stream message (see Composing Stream Messages). You create the message with the session method createBytesMessage, then use the methods shown in Table 2-13 to encode primitive values into the message’s byte stream: for example,

BytesMessage  outMsg = mySession.createBytesMessage();
outMsg.writeUTF("The Meaning of Life");
outMsg.writeInt(42);

Table 2-13 Bytes Message Composition Methods

As with stream and map messages, you can use the generic object-based method writeObject to handle values whose type is unknown at compilation time: for example, the statements

Integer  meaningOfLife = new Integer(42);
outMsg.writeObject(meaningOfLife);

are equivalent to

outMsg.writeInt(42);

The message’s reset method

outMsg.reset();

puts the message body in read-only mode and repositions the byte stream to the beginning, ready to read (see Processing Messages). Attempting to write further content to a message in this state will cause an exception (MessageNotWriteableException). The inherited Message method clearBody can be used to delete the entire message body and make it writeable again.

Sending Messages

In order to send messages to a message broker, you must create a message producer using the session method createProducer:

MessageProducer  myProducer = mySession.createProducer(myDest);

The scope of the message producer is limited to the session that created it and the connection to which that session belongs. Table 2-14 shows the methods defined in the MessageProducer interface.

Table 2-14 Message Producer Methods

The createProducer method takes a destination as an argument, which may be either a (point-to-point) queue or a (publish/subscribe) topic. The producer will then send all of its messages to the specified destination. If the destination is a queue, the producer is called a sender for that queue; if it is a topic, the producer is a publisher to that topic. The message producer’s getDestination method returns this destination.

You also have the option of leaving the destination unspecified when you create a producer

MessageProducer  myProducer = mySession.createProducer(null);

in which case you must specify an explicit destination for each message. This option is typically used for producers that must send messages to a variety of destinations, such as those designated in the JMSReplyTo header fields of incoming messages (see Message Header).

A producer has a default delivery mode (persistent or nonpersistent), priority level, and message lifetime, which it will apply to all messages it sends unless explicitly overridden for an individual message. You can set these properties with the message producer methods setDeliveryMode, setPriority, and setTimeToLive, and retrieve them with getDeliveryMode, getPriority, and getTimeToLive. If you don’t set them explicitly, they default to persistent delivery, priority level 4, and a lifetime value of 0, denoting an unlimited message lifetime.

The heart of the message producer interface is the send method, which is available in a variety of overloaded forms. The simplest of these just takes a message as its only argument:

myProducer.send(outMsg);

This sends the specified message to the producer’s default destination, using the producer’s default delivery mode, priority, and message lifetime. Alternatively, you can explicitly specify the destination

myProducer.send(myDest, outMsg);

or the delivery mode, priority, and lifetime in milliseconds

myProducer.send(outMsg, DeliveryMode.NON_PERSISTENT, 9, 1000);

or all of these at once:

myProducer.send(myDest, outMsg, DeliveryMode.NON_PERSISTENT, 9, 1000);

Recall that if you did not specify a destination when creating the message producer, you must provide an explicit destination for each message you send.

As discussed earlier under Message Header, client applications that have no need for the message identifier and time stamp fields in the message header can gain some performance improvement by suppressing the generation of these fields, using the message producer’s setDisableMessageID and setdisableMessageTimestamp methods. Note that a true value for either of these flags disables the generation of the corresponding header field, while a false value enables it. Both flags are set to false by default, meaning that the broker will generate the values of these header fields unless explicitly instructed otherwise.

When you are finished using a message producer, you should call its close method

myProducer.close();

allowing the broker and client runtime to release any resources they may have allocated on the producer’s behalf.

Receiving Messages

Messages are received by a message consumer, within the context of a connection and a session. Once you have created a consumer, you can use it to receive messages in either of two ways:

• In synchronous message consumption, you explicitly request the delivery of messages when you are ready to receive them.

• In asynchronous message consumption, you register a message listener for the consumer. The Message Queue client runtime then calls the listener whenever it has a message to deliver.

These two forms of message consumption are described in the sections Receiving Messages Synchronously and Receiving Messages Asynchronously.

Creating Message Consumers

The session method createConsumer creates a generic consumer that can be used to receive messages from either a (point-to-point) queue or a (publish/subscribe) topic:

MessageConsumer  myConsumer = mySession.createConsumer(myDest);

If the destination is a queue, the consumer is called a receiver for that queue; if it is a topic, the consumer is a subscriber to that topic.

A subscriber created for a topic destination with the createConsumer method is always nondurable, meaning that it will receive only messages that are sent (published)to the topic while the subscriber is active. If you want the broker to retain messages published to a topic while no subscriber is active and deliver them when one becomes active again, you must instead create a durable subscriber, as described in Durable Subscribers.

Table 2-15 shows the methods defined in the MessageConsumer interface, which are discussed in detail in the relevant sections below.

Table 2-15 Message Consumer Methods

Message Selectors

If appropriate, you can restrict the messages a consumer will receive from its destination by supplying a message selector as an argument when you create the consumer:

String  mySelector = "/* Text of selector here */";
MessageConsumer  myConsumer = mySession.createConsumer(myDest, mySelector);

The selector is a string whose syntax is based on a subset of the SQL92 conditional expression syntax, which allows you to filter the messages you receive based on the values of their properties (see Message Properties). See the Java Message Service Specification for a complete description of this syntax. The message consumer’s getMessageSelector method returns the consumer’s selector string (or null if no selector was specified when the consumer was created):

String  mySelector = myConsumer.getMessageSelector();

In some cases, the same connection may both publish and subscribe to the same topic destination. The createConsumer method accepts an optional boolean argument that suppresses the delivery of messages published by the consumer’s own connection:

String  mySelector = "/* Text of selector here */";
MessageConsumer
        myConsumer = mySession.createConsumer(myDest, mySelector, true);

The resulting consumer will receive only messages published by a different connection.

Durable Subscribers

To receive messages delivered to a publish/subscribe topic while no message consumer is active, you must ask the message broker to create a durable subscriber for that topic. All sessions that create such subscribers for a given topic must have the same client identifier (see Using Connections ). When you create a durable subscriber, you supply a subscriber name that must be unique for that client identifier:

MessageConsumer
        myConsumer = mySession.createDurableSubscriber(myDest, "mySub");

(The object returned by the createDurableSubscriber method is actually typed as TopicSubscriber, but since that is a subinterface of MessageConsumer, you can safely assign it to a MessageConsumer variable. Note, however, that the destination myDest must be a publish/subscribe topic and not a point-to-point queue.)

You can think of a durable subscriber as a “virtual message consumer” for the specified topic, identified by the unique combination of a client identifier and subscriber name. When a message arrives for the topic and no message consumer is currently active for it, the message will be retained for later delivery. Whenever you create a consumer with the given client identifier and subscriber name, it will be considered to represent this same durable subscriber and will receive all of the accumulated messages that have arrived for the topic in the subscriber’s absence. Each message is retained until it is delivered to (and acknowledged by) such a consumer or until it expires.

Like the createConsumer method described in the preceding section (which creates nondurable subscribers), createDurableSubscriber can accept an optional message selector string and a boolean argument telling whether to suppress the delivery of messages published by the subscriber’s own connection:

String  mySelector = "/* Text of selector here */";
MessageConsumer  myConsumer
                    = mySession.createDurableSubscriber(myDest, "mySub",
                                                        mySelector, true);

You can change the terms of a durable subscription by creating a new subscriber with the same client identifier and subscription name but with a different topic, selector, or both. The effect is as if the old subscription were destroyed and a new one created with the same name. When you no longer need a durable subscription, you can destroy it with the session method unsubscribe:

mySession.unsubscribe("mySub");

Receiving Messages Synchronously

Once you have created a message consumer for a session, using either the createConsumer orcreateDurableSubscriber method, you must start the session’s connection to begin the flow of incoming messages:

myConnection.start();

(Note that it is not necessary to start a connection in order to produce messages, only to consume them.) You can then use the consumer’s receive method to receive messages synchronously from the message broker:

Message  inMsg = myConsumer.receive();

This returns the next available message for this consumer. If no message is immediately available, the receive method blocks until one arrives. You can also provide a timeout interval in milliseconds:

Message  inMsg = myConsumer.receive(1000);

In this case, if no message arrives before the specified timeout interval (1 second in the example) expires, the method will return with a null result. An alternative method, receiveNoWait, returns a null result immediately if no message is currently available:

Message  inMsg = myConsumer.receiveNoWait();

Receiving Messages Asynchronously

If you want your message consumer to receive incoming messages asynchronously, you must create a message listener to process the messages. This is a Java object that implements the JMS MessageListener interface. The procedure is as follows:

To Set Up a Message Queue Java Client to Receive Messages Asynchronously

1. Define a message listener class implementing the MessageListener interface.

   The interface consists of the single method onMessage, which accepts a message as a parameter and processes it in whatever way is appropriate for your application:

   public class MyMessageListener implements MessageListener
     {
           public void onMessage (Message  inMsg)
         {
           /* Code here to process message */
         }
      }

2. Create a message consumer.

   You can use either the createConsumer or createDurableSubscriber method of the session in which the consumer will operate: for instance,

   MessageConsumer  myConsumer = mySession.createConsumer(myDest);

3. Create an instance of your message listener class.

   MyMessageListener  myListener = new MyMessageListener();

4. Associate the message listener with your message consumer.

   The message consumer method setMessageListener accepts a message listener object and associates it with the given consumer:

   myConsumer.setMessageListener(myListener);

5. Start the connection to which this consumer’s session belongs.

   The connection’s start method begins the flow of messages from the message broker to your message consumer:

   myConnection.start();

   Once the connection is started, the Message Queue client runtime will call your message listener’s onMessage method each time it has a message to deliver to this consumer.

   To ensure that no messages are lost before your consumer is ready to receive them, it is important not to start the connection until after you have created the message listener and associated it with the consumer. If the connection is already started, you should stop it before creating an asynchronous consumer, then start it again when the consumer is ready to begin processing.

   Setting a consumer’s message listener to null removes any message listener previously associated with it:

   myConsumer.setMessageListener(null);

   The consumer’s getMessageListener method returns its current message listener (or null if there is none):

   MyMessageListener  myListener = myConsumer.getMessageListener();

Acknowledging Messages

If you have specified client-acknowledge as your session’s acknowledgment mode (see Acknowledgment Modes), it is your client application’s responsibility to explicitly acknowledge each message it receives. If you have received the message synchronously, using a message consumer’s receive (or receiveNoWait) method, you should process the message first and then acknowledge it; if you have received it asynchronously, your message listener’s onMessage method should acknowledge the message after processing it. This ensures that the message broker will not delete the message from persistent storage until processing is complete.

Table 2-16 shows the methods available for acknowledging a message. The most general is acknowledge, defined in the standard JMS interface javax.jms.Message:

inMsg.acknowledge();

This acknowledges all unacknowledged messages consumed by the session up to the time of call. You can use this method to acknowledge each message individually as you receive it, or you can group several messages together and acknowledge them all at once by calling acknowledge on the last one in the group.

Table 2-16 Message Acknowledgment Methods

The Message Queue version of the Message interface, defined in the package com.sun.messaging.jms, adds two more methods that provide more flexible control over which messages you acknowledge. The acknowledgeThisMessage method just acknowledges the single message for which it is called, rather than all messages consumed by the session; acknowledgeUpThroughThisMessage acknowledges the message for which it is called and all previous messages; messages received after that message remain unacknowledged.

Browsing Messages

If the destination from which you are consuming messages is a point-to-point queue, you can use a queue browser to examine the messages in the queue without consuming them. The session method createBrowser creates a browser for a specified queue:

QueueBrowser  myBrowser = mySession.createBrowser(myDest);

The method will throw an exception (InvalidDestinationException) if you try to pass it a topic destination instead of a queue. You can also supply a selector string as an optional second argument:

String        mySelector = "/* Text of selector here */";
QueueBrowser  myBrowser  = mySession.createBrowser(myDest, mySelector);

Table 2-17 shows the methods defined in the QueueBrowser interface. The getQueue and getMessageSelector methods return the browser’s queue and selector string, respectively.

Table 2-17 Queue Browser Methods

The most important queue browser method is getEnumeration, which returns a Java enumeration object that you can use to iterate through the messages in the queue, as shown in Example 2-4.

Example 2-4 Browsing a Queue

Enumeration  queueMessages = myBrowser.getEnumeration();
Message      eachMessage;

while ( queueMessages.hasMoreElements() )
  { eachMessage = queueMessages.nextElement();
    /* Do something with the message */
  }

The browser’s close method closes it when you’re through with it:

myBrowser.close();

Closing a Consumer

As a matter of good programming practice, you should close a message consumer when you have no further need for it. Closing a session or connection automatically closes all consumers associated with it; to close a consumer without closing the session or connection to which it belongs, you can use its close method:

myConsumer.close();

For a consumer that is a nondurable topic subscriber, this terminates the flow of messages to the consumer. However, if the consumer is a queue receiver or a durable topic subscriber, messages will continue to be accumulated for the destination and will be delivered the next time a consumer for that destination becomes active. To terminate a durable subscription permanently, call its session’s unsubscribe method with the subscriber name as an argument:

mySession.unsubscribe("mySub");

Processing Messages

Processing a message after you have received it may entail examining its header fields, properties, and body. The following sections describe how this is done.

Retrieving Message Header Fields

The standard JMS message header fields are described in Table 2-4. Table 2-18 shows the methods provided by the JMS Message interface for retrieving the values of these fields: for instance, you can obtain a message’s reply destination with the statement

Destination  replyDest = inMsg.getJMSReplyTo();

Table 2-18 Message Header Retrieval Methods

Retrieving Message Properties

Table 2-19 lists the methods defined in the JMS Message interface for retrieving the values of a message’s properties (see Message Properties). There is a retrieval method for each of the possible primitive types that a property value can assume: for instance, you can obtain a message’s time stamp with the statement

long  timeStamp = inMsg.getLongProperty("JMSXRcvTimestamp");

Table 2-19 Message Property Retrieval Methods

There is also a generic getObjectProperty method that returns a property value in objectified form, as a Java object of class Integer, Byte, Short, Long, Float , Double, Boolean, or String . For example, another way to obtain a message’s time stamp, equivalent to that shown above, would be

Long  timeStampObject = (Long)inMsg.getObjectProperty("JMSXRcvTimestamp");
long  timeStamp = timeStampObject.longValue();

If the message has no property with the requested name, getObjectProperty will return null; the message method propertyExists tests whether this is the case.

The getPropertyNames method returns a Java enumeration object for iterating through all of the property names associated with a given message; you can then use the retrieval methods shown in the table to retrieve each of the properties by name, as shown in Example 2-5.

Example 2-5 Enumerating Message Properties

Enumeration  propNames = inMsg.getPropertyNames();
String       eachName;
Object       eachValue;

while ( propNames.hasMoreElements() )
  { eachName  = propNames.nextElement();
    eachValue = inMsg.getObjectProperty(eachName);
    /* Do something with the value */
  }
               

Processing the Message Body

The methods for retrieving the contents of a message’s body essentially parallel those for composing the body, as described earlier under Composing Messages. The following sections describe these methods for each of the possible message types (text, stream, map, object, and bytes).

Processing Text Messages

The text message method getText (Table 2-20 ) retrieves the contents of a text message’s body in the form of a string:

String  textBody = inMsg.getText();

Table 2-20 Text Message Access Method

Processing Stream Messages

Reading the contents of a stream message is similar to reading from a data stream, using the access methods shown in Table 2-21: for example, the statement

int  intVal = inMsg.readInt();

retrieves an integer value from the message stream.

Table 2-21 Stream Message Access Methods

The readObject method returns the next value from the message stream in objectified form, as a Java object of the class corresponding to the value’s primitive data type: for instance, if the value is of type int, readObject returns an object of class Integer. The following statements are equivalent to the one shown above:

Integer  intObject = (Integer) inMsg.readObject();
int      intVal    = intObject.intValue();

Processing Map Messages

The MapMessage interface provides the methods shown in Table 2-22 for reading the body of a map message. Each access method takes a name string as an argument and returns the value to which that name is mapped: for instance, under the example shown in Composing Map Messages, the statement

int  meaningOfLife = inMsg.getInt("The Meaning of Life");

would set the variable meaningOfLife to the value 42.

Table 2-22 Map Message Access Methods

Like stream messages, map messages provide an access method, getObject, that returns a value from the map in objectified form, as a Java object of the class corresponding to the value’s primitive data type: for instance, if the value is of type int, getObject returns an object of class Integer. The following statements are equivalent to the one shown above:

Integer  meaningObject = (Integer) inMsg.getObject("The Meaning of Life");
int      meaningOfLife = meaningObject.intValue();

The itemExists method returns a boolean value indicating whether the message map contains an association for a given name string:

if ( inMsg.itemExists("The Meaning of Life") )
  { /* Life is meaningful */
  }
else
  { /* Life is meaningless */
  }

The getMapNames method returns a Java enumeration object for iterating through all of the names defined in the map; you can then use getObject to retrieve the corresponding values, as shown in Example 2-6.

Example 2-6 Enumerating Map Message Values

Enumeration  mapNames = inMsg.getMapNames();
String       eachName;
Object       eachValue;

while ( mapNames.hasMoreElements() )
  { eachName  = mapNames.nextElement();
    eachValue = inMsg.getObject(eachName);
    /* Do something with the value */
  }
                  

Processing Object Messages

The ObjectMessage interface provides just one method, getObject (Table 2-23 ), for retrieving the serialized object that is the body of an object message:

Object  messageBody = inMsg.getObject();

You can then typecast the result to a more specific class and process it in whatever way is appropriate.

Table 2-23 Object Message Access Method

Processing Bytes Messages

The body of a bytes message simply consists of a stream of uninterpreted bytes; its interpretation is entirely a matter of agreement between sender and receiver. This type of message is intended primarily for decoding message formats used by other existing message systems; Message Queue clients should generally use one of the other, more specific message types instead.

Reading the body of a bytes message is similar to reading a stream message (see Processing Stream Messages): you use the methods shown in Table 2-24 to decode primitive values from the message’s byte stream. For example, the statement

int  intVal = inMsg.readInt();

retrieves an integer value from the byte stream. The getBodyLength method returns the length of the entire message body in bytes:

int  bodyLength = inMsg.getBodyLength();

Table 2-24 Bytes Message Access Methods