Two implementations of the JmsTemplate class are provided. The class JmsTemplate uses the JMS 1.1 API and the subclass JmsTemplate102 uses the JMS 1.0.2 API.

Code that uses the JmsTemplate only needs to implement callback interfaces giving them a clearly defined contract. The MessageCreator callback interface creates a message given a Session provided by the calling code in JmsTemplate. In order to allow for more complex usage of the JMS API, the callback SessionCallback provides the user with the JMS session and the callback ProducerCallback exposes a Session and MessageProducer pair.

The JMS API exposes two types of send methods, one that takes delivery mode, priority, and time-to-live as quality of service (QOS) parameters and one that takes no QOS parameters which uses default values. Since there are many send methods in JmsTemplate, the setting of the QOS parameters have been exposed as bean properties to avoid duplication in the number of send methods. Similarly, the timeout value for synchronous receive calls is set using the property setReceiveTimeout.

Some JMS providers allow the setting of default QOS values administratively through the configuration of the ConnectionFactory. This has the effect that a call to MessageProducer's send method send(Destination destination, Message message) will use different QOS default values than those specified in the JMS specification. In order to provide consistent management of QOS values, the JmsTemplate must therefore be specifically enabled to use its own QOS values by setting the boolean property isExplicitQosEnabled to true.

The JmsTemplate requires a reference to a ConnectionFactory. The ConnectionFactory is part of the JMS specification and serves as the entry point for working with JMS. It is used by the client application as a factory to create connections with the JMS provider and encapsulates various configuration parameters, many of which are vendor specific such as SSL configuration options.

When using JMS inside an EJB, the vendor provides implementations of the JMS interfaces so that they can participate in declarative transaction management and perform pooling of connections and session. In order to use this implementation, Java EE containers typically require that you declare a JMS connection factory as a resource-ref inside the EJB or servlet deployment descriptors. To ensure the use of these features with the JmsTemplate inside an EJB, the client application should ensure that it references the managed implementation of the ConnectionFactory.

Spring provides an implementation of the ConnectionFactory interface, SingleConnectionFactory, that will return the same Connection on all createConnection calls and ignore calls to close. This is useful for testing and standalone environments so that the same connection can be used for multiple JmsTemplate calls that may span any number of transactions. SingleConnectionFactory takes a reference to a standard ConnectionFactory that would typically come from JNDI.

Destinations, like ConnectionFactories, are JMS administered objects that can be stored and retrieved in JNDI. When configuring a Spring application context you can use the JNDI factory class JndiObjectFactoryBean to perform dependency injection on your object's references to JMS destinations. However, often this strategy is cumbersome if there are a large number of destinations in the application or if there are advanced destination management features unique to the JMS provider. Examples of such advanced destination management would be the creation of dynamic destinations or support for a hierarchical namespace of destinations. The JmsTemplate delegates the resolution of a destination name to a JMS destination object to an implementation of the interface DestinationResolver. DynamicDestinationResolver is the default implementation used by JmsTemplate and accommodates resolving dynamic destinations. A JndiDestinationResolver is also provided that acts as a service locator for destinations contained in JNDI and optionally falls back to the behavior contained in DynamicDestinationResolver.

Quite often the destinations used in a JMS application are only known at runtime and therefore cannot be administratively created when the application is deployed. This is often because there is shared application logic between interacting system components that create destinations at runtime according to a well-known naming convention. Even though the creation of dynamic destinations are not part of the JMS specification, most vendors have provided this functionality. Dynamic destinations are created with a name defined by the user which differentiates them from temporary destinations and are often not registered in JNDI. The API used to create dynamic destinations varies from provider to provider since the properties associated with the destination are vendor specific. However, a simple implementation choice that is sometimes made by vendors is to disregard the warnings in the JMS specification and to use the TopicSession method createTopic(String topicName) or the QueueSession method createQueue(String queueName) to create a new destination with default destination properties. Depending on the vendor implementation, DynamicDestinationResolver may then also create a physical destination instead of only resolving one.

The boolean property pubSubDomain is used to configure the JmsTemplate with knowledge of what JMS domain is being used. By default the value of this property is false, indicating that the point-to-point domain, Queues, will be used. In the 1.0.2 implementation the value of this property determines if the JmsTemplate's send operations will send a message to a Queue or to a Topic. This flag has no effect on send operations for the 1.1 implementation. However, in both implementations, this property determines the behavior of dynamic destination resolution via implementations of the DestinationResolver interface.

You can also configure the JmsTemplate with a default destination via the property defaultDestination. The default destination will be used with send and receive operations that do not refer to a specific destination.

One of the most common uses of JMS messages in the EJB world is to drive message-driven beans (MDBs). Spring offers a solution to create message-driven POJOs (MDPs) in a way that does not tie a user to an EJB container. (See the section entitled Section 19.4.2, “Asynchronous Reception - Message-Driven POJOs” for detailed coverage of Spring's MDP support.)

A subclass of AbstractMessageListenerContainer is used to receive messages from a JMS message queue and drive the MDPs that are injected into it. The AbstractMessageListenerContainer is responsible for all threading of message reception and dispatch into the MDPs for processing. A message listener container is the intermediary between an MDP and a messaging provider, and takes care of registering to receive messages, participating in transactions, resource acquisition and release, exception conversion and suchlike. This allows you as an application developer to write the (posssibly complex) business logic associated with receiving a message (and possibly responding to it), and delegates boilerplate JMS infrastructure concerns to the framework.

There are three subclasses of AbstractMessageListenerContainer packaged with Spring, each with its specialised feature set.

Spring provides a JmsTransactionManager that manages transactions for a single JMS ConnectionFactory. This allows JMS applications to leverage the managed transaction features of Spring as described in Chapter 9, Transaction management. The JmsTransactionManager binds a Connection/Session pair from the specified ConnectionFactory to the thread. However, in a Java EE environment the ConnectionFactory will pool connections and sessions, so the instances that are bound to the thread depend on the pooling behavior. In a standalone environment, using Spring's SingleConnectionFactory will result in a using a single JMS Connection with each transaction having its own Session. The JmsTemplate can also be used with the JtaTransactionManager and an XA-capable JMS ConnectionFactory for performing distributed transactions.

Reusing code across a managed and unmanaged transactional environment can be confusing when using the JMS API to create a Session from a Connection. This is because the JMS API only has only one factory method to create a Session and it requires values for the transaction and acknowledgement modes. In a managed environment, setting these values is the responsibility of the environment's transactional infrastructure, so these values are ignored by the vendor's wrapper to the JMS Connection. When using the JmsTemplate in an unmanaged environment you can specify these values through the use of the properties SessionTransacted and SessionAcknowledgeMode. When using a PlatformTransactionManager with JmsTemplate, the template will always be given a transactional JMS Session.

In order to facilitate the sending of domain model objects, the JmsTemplate has various send methods that take a Java object as an argument for a message's data content. The overloaded methods convertAndSend and receiveAndConvert in JmsTemplate delegate the conversion process to an instance of the MessageConverter interface. This interface defines a simple contract to convert between Java objects and JMS messages. The default implementation SimpleMessageConverter supports conversion between String and TextMessage, byte[] and BytesMesssage, and java.util.Map and MapMessage. By using the converter, you and your application code can focus on the business object that is being sent or received via JMS and not be concerned with the details of how it is represented as a JMS message.

The sandbox currently includes a MapMessageConverter which uses reflection to convert between a JavaBean and a MapMessage. Other popular implementations choices you might implement yourself are Converters that use an existing XML marshalling package, such as JAXB, Castor, XMLBeans, or XStream, to create a TextMessage representing the object.

To accommodate the setting of a message's properties, headers, and body that can not be generically encapsulated inside a converter class, the MessagePostProcessor interface gives you access to the message after it has been converted, but before it is sent. The example below demonstrates how to modify a message header and a property after a java.util.Map is converted to a message.

public void sendWithConversion() {
  Map m = new HashMap();
  m.put("Name", "Mark");
  m.put("Age", new Integer(47));
  jmsTemplate.convertAndSend("testQueue", m, new MessagePostProcessor() {
    public Message postProcessMessage(Message message) throws JMSException {
      message.setIntProperty("AccountID", 1234);
      message.setJMSCorrelationID("123-00001");
      return message;
    }
  });
}

This results in a message of the form:

MapMessage={ 
  Header={ 
    ... standard headers ...
    CorrelationID={123-00001} 
  } 
  Properties={ 
    AccountID={Integer:1234}
  } 
  Fields={ 
    Name={String:Mark} 
    Age={Integer:47} 
  } 
}

While the send operations cover many common usage scenarios, there are cases when you want to perform multiple operations on a JMS Session or MessageProducer. The SessionCallback and ProducerCallback expose the JMS Session and Session / MessageProducer pair respectfully. The execute() methods on JmsTemplate execute these callback methods.

While JMS is typically associated with asynchronous processing, it is possible to consume messages synchronously. The overloaded receive(..) methods provide this functionality. During a synchronous receive, the calling thread blocks until a message becomes available. This can be a dangerous operation since the calling thread can potentially be blocked indefinitely. The property receiveTimeout specifies how long the receiver should wait before giving up waiting for a message.

In a fashion similar to a Message-Driven Bean (MDB) in the EJB world, the Message-Driven POJO (MDP) acts as a receiver for JMS messages. The one restriction (but see also below for the discussion of the MessageListenerAdapter class) on an MDP is that it must implement the javax.jms.MessageListener interface. Please also be aware that in the case where your POJO will be receiving messages on multiple threads, it is important to ensure that your implementation is thread-safe.

Below is a simple implementation of an MDP:

import javax.jms.JMSException;
import javax.jms.Message;
import javax.jms.MessageListener;
import javax.jms.TextMessage;

public class ExampleListener implements MessageListener {

  public void onMessage(Message message) {
    if (message instanceof TextMessage) {
      try {
        System.out.println(((TextMessage) message).getText());
      } catch (JMSException ex) {
        throw new RuntimeException(ex);
      }
    } else {
      throw new IllegalArgumentException("Message must be of type TextMessage");
    }
  }
}

Once you've implemented your MessageListener, it's time to create a message listener container.

Find below an example of how to define and configure one of the message listener containers that ships with Spring (in this case the DefaultMessageListenerContainer).

<!-- this is the Message Driven POJO (MDP) -->
<bean id="messageListener" class="jmsexample.ExampleListener" />

<!-- and this is the message listener container -->
<bean id="listenerContainer"
  class="org.springframework.jms.listener.DefaultMessageListenerContainer">
  <property name="concurrentConsumers" value="5"/>
  <property name="connectionFactory" ref="connectionFactory" />
  <property name="destination" ref="destination" />
  <property name="messageListener" ref="messageListener" />
</bean>

Please refer to the Spring Javadoc of the various message listener containers for a full description of the features supported by each implementation.

The SessionAwareMessageListener interface is a Spring-specific interface that provides a similar contract the JMS MessageListener interface, but also provides the message handling method with access to the JMS Session from which the Message was received.

package org.springframework.jms.listener;

public interface SessionAwareMessageListener {

    void onMessage(Message message, Session session) throws JMSException;
}

You can choose to have your MDPs implement this interface (in preference to the standard JMS MessageListener interface) if you want your MDPs to be able to respond to any received messages (using the Session supplied in the onMessage(Message, Session) method). All of the message listener container implementations that ship wth Spring have support for MDPs that implement either the MessageListener or SessionAwareMessageListener interface. Classes that implement the SessionAwareMessageListener come with the caveat that they are then tied to Spring through the interface. The choice of whether or not to use it is left entirely up to you as an application developer or architect.

Please note that the 'onMessage(..)' method of the SessionAwareMessageListener interface throws JMSException. In contrast to the standard JMS MessageListener interface, when using the SessionAwareMessageListener interface, it is the responsibility of the client code to handle any exceptions thrown.

The MessageListenerAdapter class is the final component in Spring's asynchronous messaging support: in a nutshell, it allows you to expose almost any class as a MDP (there are of course some constraints).

	Note
	If you are using the JMS 1.0.2 API, you will want to use the MessageListenerAdapter102 class which provides the exact same functionality and value add as the MessageListenerAdapter class, but for the JMS 1.0.2 API.

Consider the following interface definition. Notice that although the interface extends neither the MessageListener nor SessionAwareMessageListener interfaces, it can still be used as a MDP via the use of the MessageListenerAdapter class. Notice also how the various message handling methods are strongly typed according to the contents of the various Message types that they can receive and handle.

public interface MessageDelegate {

    void handleMessage(String message);

    void handleMessage(Map message);

    void handleMessage(byte[] message);

    void handleMessage(Serializable message);
}

public class DefaultMessageDelegate implements MessageDelegate {
    // implementation elided for clarity...
}

In particular, note how the above implementation of the MessageDelegate interface (the above DefaultMessageDelegate class) has no JMS dependencies at all. It truly is a POJO that we will make into an MDP via the following configuration.

<!-- this is the Message Driven POJO (MDP) -->
<bean id="messageListener" class="org.springframework.jms.listener.adapter.MessageListenerAdapter">
    <constructor-arg>
        <bean class="jmsexample.DefaultMessageDelegate"/>
    </constructor-arg>
</bean>

<!-- and this is the message listener container... -->
<bean id="listenerContainer"
  class="org.springframework.jms.listener.DefaultMessageListenerContainer">
  <property name="concurrentConsumers" value="5"/>
  <property name="connectionFactory" ref="connectionFactory" />
  <property name="destination" ref="destination" />
  <property name="messageListener" ref="messageListener" />
</bean>

Below is an example of another MDP that can only handle the receiving of JMS TextMessage messages. Notice how the message handling method is actually called 'receive' (the name of the message handling method in a MessageListenerAdapter defaults to 'handleMessage'), but it is configurable (as you will see below). Notice also how the 'receive(..)' method is strongly typed to receive and respond only to JMS TextMessage messages.

public interface TextMessageDelegate {

    void receive(TextMessage message);
}

public class DefaultTextMessageDelegate implements TextMessageDelegate {
    // implementation elided for clarity...
}

The configuration of the attendant MessageListenerAdapter would look like this:

<bean id="messageListener" class="org.springframework.jms.listener.adapter.MessageListenerAdapter">
    <constructor-arg>
        <bean class="jmsexample.DefaultTextMessageDelegate"/>
    </constructor-arg>
    <property name="defaultListenerMethod" value="receive"/>
    <!-- we don't want automatic message context extraction -->
    <property name="messageConverter">
        <null/>
    </property>
</bean>

Please note that if the above 'messageListener' receives a JMS Message of a type other than TextMessage, an IllegalStateException will be thrown (and subsequently swallowed).

Another of the capabilities of the MessageListenerAdapter class is the ability to automatically send back a response Message if a handler method returns a non-void value.

Consider the following interface and it's implementation:

public interface ResponsiveTextMessageDelegate {

    // notice the return type...
    String receive(TextMessage message);
}

public class DefaultResponsiveTextMessageDelegate implements ResponsiveTextMessageDelegate {
    // implementation elided for clarity...
}

If the above DefaultResponsiveTextMessageDelegate is used in conjunction with a MessageListenerAdapter then any non-null value that is returned from the execution of the 'receive(..)' method will (in the default configuration) be converted into a TextMessage. The resulting TextMessage will then be sent to the Destination (if one exists) defined in the JMS Reply-To property of the original Message, or the default Destination set on the MessageListenerAdapter (if one has been configured); if no Destination is found then an InvalidDestinationException will be thrown (and please note that this exception will not be swallowed and will propagate up the call stack).

Participation in transactions requires only a couple of minor changes. You will need to create a transaction manager and register that transaction manager with one of the subclasses that can participate in transactions (DefaultMessageListenerContainer or ServerSessionMessageListenerContainer).

To create the transaction manager, you'll want to create an instance of JmsTransactionManager and give it an XA transaction-capable connection factory.

<bean id="transactionManager" class="org.springframework.jms.connection.JmsTransactionManager">
  <property name="connectionFactory" ref="connectionFactory" />
</bean>

Then you just need to add it to our earlier container configuration. The container will take care of the rest.

<bean id="listenerContainer" class="org.springframework.jms.listener.DefaultMessageListenerContainer">
  <property name="concurrentConsumers" value="5" />
  <property name="connectionFactory" ref="connectionFactory" />
  <property name="destination" ref="destination" />
  <property name="messageListener" ref="messageListener" />
  <property name="transactionManager" ref="transactionManager" />
</bean>