I logged the following issue:
http://java.net/jira/browse/JMS_SPEC-33
This is a placeholder for this goal I proposed some time ago for JMS 2.0, and which received support from EG members:
"We should provide a modern, easier-to-use API, probably using annotations and CDI. There seemed to be a variety of
ideas of how we might do this, and we will need to explore these in detail. Whilst I think we mostly see this as a
feature for Java EE containers, there seems to be interest in offering features to those using a Java SE environment as
well."
There has been some discussion already about this, but I'd like to go back a few steps to discuss what we are trying to
achieve, and how we might achieve it, and where technologies like annotations and CDI might fit in.
One of the reasons I'm doing this is that although we have some Java language and CDI experts in this EG, I know from
talking to individuals that the level of knowledge in this group is variable. I hope the "experts" take this as a hint
that they should be prepared to help teach the others...
Also, I want to avoid us saying "CDI is the solution, now what's the problem" and, in particular, make sure that we
don't simply use CDI as a way to hide the complexities of the JMS API when we could be eliminating those complexities,
not hiding them.
I also think we need to discuss the extent to which any improvements could be offered to Java SE applications as well as
those running Java EE. Although it is tempting to instinctively define our goal as offering the same improvements to
both, I think there's much greater scope to offer improvements to Java EE applications, given that Java EE is by
definition intended to make application development simpler and already offers a significantly simpler experience of
using JMS than Java SE. At the every least, the different nature of the two environments will by definition mean the way
that JMS is used will always be different in each.
One thing we should bear in mind is that the JMS API wasn't really designed with Java EE in mind. Java EE already
imposes significant restrictions on how the API can be used, and we might want to consider whether this offers
opportunities to simplify the API for such users.
WHAT'S THE PROBLEM WITH THE JMS API?
------------------------------------
I'd like to start with discussion of the API and what is wrong with it, before we get on to possible solutions. What are
the main problems with the JMS API at present which make it more difficult to use than it should?
I think the starting point is that the JMS API is rather more cumbersome than it need to be, with a lot of boilerplate
code needed to send or receive a single message. Here is how the existing JMS spec expects you to write a method which
sends a String:
private void send(String messageText) throws NamingException, JMSException {
Properties props = new Properties();
InitialContext ic = new InitialContext(props);
ConnectionFactory connectionFactory = (ConnectionFactory) ic.lookup("myConnectionFactory");
Connection connection = connectionFactory.createConnection();
try {
Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
Destination destination = (Destination) ic.lookup("MyQueue");
MessageProducer producer = session.createProducer(destination);
Message message = session.createTextMessage(messageText);
producer.send(message);
} finally {
if (connection!=null) connection.close();
}
}
That is indeed a lot of boilerplate code (i.e. code that has to be included in many places with little or no
alteration). It also throws two Exceptions which need to be handled somehow. In addition the administrator is expected
to use provider-specific tools to bind suitable Connection and Destination objects in JNDI.
In a Java EE container, resource injection allows things to be simplified a little by removing the need to create an
InitialContext and replacing the JNDI lookups with @Resource annotations. This means that NamingException no longer
needs to be explicitly handled.
@Resource(name="myQueue") Destination destination;
@Resource(name="myConnectionFactory") ConnectionFactory connectionFactory;
private void send(String messageText) throws JMSException {
Connection connection = connectionFactory.createConnection();
try {
Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
MessageProducer producer = session.createProducer(destination);
Message message = session.createTextMessage(messageText);
producer.send(message);
} finally {
if (connection!=null) connection.close();
}
}
This still requires a JMSException to be handled, and the administrator is still expected to use provider-specific tools
to bind suitable Connection and Destination objects in JNDI.
Almost identical issues arise if you write code to synchronously consume a message. Let's leave asynchronously consuming
messages (via MessageListeners and MDBs until a little later).
So, what are the problems here?
PROBLEMS
--------
1. The need to create several intermediate objects which we might use only once and not use again. In the above example,
the Connection, Session, MessageProducer and TextMessage objects will never be used again after this method returns
(though in other scenarious they might).
2. The need to tidy up resources after use. In practice, this means calling Connection.close() after use, ideally in a
finally block. Whether this physically releases resources or simply returns the connection to a pool, if we don't call
this method we will eventually run out of either resources or connections in the pool.
3. The rather redundant arguments to connection.createSession(). In a Java SE container we have two arguments when one
would do, and in a Java EE EJB or web container they're ignored.
4. The dependence on JNDI as a means to isolate provider-specific connection factory and destination configuration from
the portable application (which is currently a fundamendal fature of JMS, so this dependence is currently by design).
EXISTING DIFFERENCES BETWEEN JMS API in SE and EE
-------------------------------------------------
Although it is desirable to keep the JMS API the same when in a Java SE and a Java EE environment, I think we should
recognise that the API within the Java EE EJB or web container is already significantly simpler than the full JMS API
and would be easier to simplify further. Here are some of the things that we might be able to simplify in a Java EE
environment.
(I should point out that I'm just trying to provoke discussion here and not trying to make specific proposals)
- Connections (as objects exposed to applications) are of limited use, other than to create a single Session. For
example, each Connection is limited to one Session, so there is no point in having separate Connection and Session
objects. The methods setClientID(), setExceptionListener() and stop() are forbidden. This leaves start() as just about
the only useful method remaining on a Connection (and then only when consuming messages). So do we really need to expose
Connections to the application at all? (Note that Java EE connection pooling means the application doesn't need to keep
instances around purely to avoid the cost of creating them).
- Sessions are also of limited use, other than to create producers and consumers. Transactions are managed by the
transaction manager, so we don't need commit() or rollback(). Client acknowledgement is forbidden, so we don't need
acknowledge(). This mainly leaves createMessage() and the other factory methods for messages, which have no particular
need to be on the Session object at all. So if we move these factory methods somewhere else, do we need to expose
Sessions to the application at all? Sessions have one further important function in JMS: the resources of a session may
only be used by a single thread at a time. Any change to hide the Session from the application should not break this
restriction or prevent the application observing it.
- MessageProducers are obviously useful for sending messages. But do we need to expose a MessageProducer object to the
application, other than to allow us to call send() once? The main purpose of a MessageProducer, other than representing
a tuple of <destination, deliveryMode, priority, timeToLive> is that it defines JMS message order. So we can't make this
object disappear, though we can perhaps make it invisible to the application by defining some rules such as when the
order of repeated calls to send() is honoured by the JMS provider.
- MessageConsumers represent the tuple <destination, messageSelector>, and for durable topic subscriptions
<subscriptionName, clientId> as well. For a queue there is no particular difference between one MessageConsumer instance
and another (for the same <destination, messageSelector>). However for a topic the object identity of the
MessageConsumer is significant in that each instance receives a copy of all the messages received on the topic, so we
probably need to continue to expose this object to the application, at least for synchronous message consumption (let's
talk about async message consumption separately).
- Messages (and their subclasses) are essentially wrappers for the message payload which allow the application to define
user-defined message properties which can be used by message selectors, and one or two header properties such as
JMSReplyTo and JMSCorrelationID. Whilst these continue to be needed, perhaps we could provide an simpler API which
allows a payload to to be passed directly for use by applications which don't need them.
Note that since most of the simplfications suggested above are only possible in a Java EE EJB or web container, we would
be introducing a different API in Java EE as for Java SE. However it could be argued that we *already* have a different
API in a Java EE EJB or web container (with resource injection, 1 session per connection, forbidden methods, connection
pooling, etc) but just don't explain this in the JMS spec and javadocs, much to the confusion of application developers.
ASYNC MESSAGE CONSUMPTION
-------------------------
Now let's talk about asynchronous message consumption. We need to consider this separately because this is the feature
which *already* has a completely different API in a Java SE environment and in a Java EE EJB or web container (even
though the JMS spec doesn't mention this at all).
In a Java SE environment (or the Java EE application client container), message may be consumed asynchronously by
creating a MessageConsumer as described above, and then calling MessageConsumer.setMessageListener() to register a
MessageListener with an onMessage() method.
In a Java EE EJB or web container, the use of MessageConsumer.setMessageListener() is explicitly forbidden (Java EE 6
Platform Spec section EE.6.7). Instead, the only means provided for asynchronous message consumption is the
message-driven bean (MDB). Since this was defined in the EJB spec rather than the JMS spec this is defined
declaratively, using either deployment descriptors or annotations. Here's a simple example using annotation:
@MessageDriven(mappedName = "jms/inboundQueue")
public class NewMessageBean implements MessageListener {
public void onMessage(Message message) {
System.out.println("Received a message");
}
}
Where "jms/inboundQueue" is the JNDI name where the queue object can be obtained. Additional information, such as the
message selector, can be specified using the activationConfig element of the MessageDriven annotation:
@MessageDriven(mappedName = "jms/inboundQueue", activationConfig = {
@ActivationConfigProperty(
propertyName = "messageSelector",
propertyValue = "JMSType = 'car' AND color = 'blue'"),
@ActivationConfigProperty(propertyName = "destinationType", propertyValue = "javax.jms.Queue")
})
The above examples are for GlassFish, and there does appear to be a problem in that the EJB 3.1 spec is distinctly vague
about how a MDB is defined. The relevant sections are 5.4.15, 5.4.16 and 5.4.17.1. The only activation config properties
defined are acknowledgeMode (only used when transactions are bean-managed, and which must be either Auto-acknowledge or
Dups-ok-acknowledge), messageSelector, destinationType (which must be must be either javax.jms.Queue or javax.jms.Topic)
and subscriptionDurability (which must be either Durable or NonDurable). But it doesn't specify how the destination is
defined or, when subscriptionDurability is Durable, how the subscription name and clientId are defined.
The JCA 1.6 spec has some additional guidance, at least for MDBs that use a resource adapter. Section B2 states thatr
providers are "strongly encouraged" to provide the properties mentioned above and also destination, subscriptionName and
clientId, with destination and destinationType as "required" properties.
Whatever else we do, there seems to be a clear need to make these mandatory for JMS MDBs, whether or not they use JCA,
so I've logged this separately as
http://java.net/jira/browse/JMS_SPEC-30 and we've alreay started discussing the
details separately in the thread for that issue.
But it seems to me that the existing MDB feature already addresses a lot of the API problems I listed in the section
"PROBLEMS" above.
1. There's no creation of intermediate objects which have no real purpose. Indeed no objects are exposed to the
application at all apart from the incoming message itself.
2. There's no need for the application to close or tidy up resources after use.
3. There's no need for the user to use the problematical createSession() method
4. The only potential complexity this leaves, of the four I listed above, is the need to define the queue or topic as an
administered object in JNDI - though strictly speaking since the configuration properties are not fully standardised it
might be possible to avoid even this, if we think it beneficial.
What other "problems" with MDBs does this leave? I can think of a few possible ones:
* Reliance on MessageListener interface. You can't simply annotate any arbitrary method as the callback.
* You can only have one callback per MDB class
* Other types of bean such as stateless session beans and singleton beans can't consume messages asynchronously
* MDBs can't be used in a Java SE environment (or the Java EE application client). In those environments you're back to
using the raw JMS API.
* Any others?
We can certainly think of how we might make MDBs easier to use or more flexible. We can also think of how the changes
currently being considered in the EJB 3.2 EG offer opportunities for generaising MDBs.
However it seems to me that MDBs already go a long way to offering a simple API to applications, and that our real
priority is to make it easier and simpler to *send* messages and to consume them *synchronously*.
CDI
---
So far I've not mentioned CDI (JSR 299). This is part of Java EE 6 and is already built-in to a Java EE container, so
long as you define a beans.xml file in your application.
What does this give us? As I write at the start I'm hoping that other EG members can help share their knowledge on this,
but as far as I can see it:
1. Provides a way to offer a simplified API which is simply a wrapper on top of the JMS API, effectively replacing
boilerplate factory code with annotations which inject objects.
2. Provides a way to define the "scope" of a object, so that close() can be called automatically when it falls out of
scope. Scopes might also allow us to define the circumstances when repeated calls to some new send method use the same
MessageProducer, and when they use different MessageProducers (important for message order).
3. Offers an internal async event mechanism which has some similarity with async messages.
CDI is not built into Java SE, and the existing CDI spec doesn't require implementations to support Java SE
environments, though this may change. However any CDI-based API in Java SE may need to be significantly different from a
CDI-based API in Java EE because Java SE does not support resource injection. Also, any API for use in Java SE will need
to reflect the additional features available in SE (local transactions, client ack, multiple sessions per connection etc).
ANNOTATIONS
-----------
We shouldn't forget that annotations don't have to use CDI. Just like with MDBs, if we want to pass information to the
container using annotations we can simply define the annotations we want.
WHAT NEXT?
---------
I'm not trying to make any proposals at this stage. The purpose of this email was to raise the issues that I think we
need to consider as part of doing that. In particular:
1. What do *you* think needs to be improved in the JMS API?
2. To what extent should we be trying to simplify the plain Java API, especially in a Java EE environment when a
significant proportion of it is redundant anyway?
3. How might the use of annotations (not necessarily CDI) make the JMS API simpler?
4. How might CDI make the JMS API simpler?
5. If we do try to hide complexities behind CDI, will users have to be aware of what is happening behind the scenes and
occasionally dip into using some of the hidden objects? If so, are we really making this simpler?
6. Is it possible to provide similar benefits to plain Java SE applications as to Java EE applications, or are the
environments just too different, bearing in mind the existing differences between JMS in Java EE and Java SE?
I look forward to the discussion.
Nigel