The use of an AtomicBoolean and no lock meant that it was possible
for a message to be queued and then never be flushed and sent to the
broker:
1. On t1, a message is received and isConnected is false. The message
will be queued.
2. On t2, CONNECTED is received from the broker. isConnected is set
to true, the queue is drained and the queued messages are forwarded
3. On t1, the message is added to the queue
To fix this, checking that isConnected is false (step 1 above) and the
queueing of a message (step 3 above) need to be performed as a unit
so that the flushing of the queued messages can't be interleaved. This
is achieved by synchronizing on a monitor and performing steps 1
and 3 and synchronizing on the same monitor while performing step 2.
The monitor is held while the messages are actually being forwarded
to the broker. An alternative would be to drain the queue into
a local variable, release the monitor, and then forward the messages.
The main advantage of this alternative is that the monitor is held for
less time. It also reduces the theoretical risk of deadlock by not
holding the monitor while making an alien call. The downside of the
alternative is that it may lead to messages being forwarded out of
order. For this reason the alternative approach was rejected.
When an annotated handler returns a Message from a @SubscribeEvent
or @MessageMapping method and it contains no destination in its
headers, use the received message's destination as the response
message's destination.
Without generics, extending AbstractPubSubChannelRegistry and using
a custom Message type requires some unpleasant casting and suppression
of warnings. By genericizing PubSubChannelRegistry and
AbstractPubSubChannelRegistry these problems can be avoided.
To improve compatibility between Spring's messaging classes and
Spring Integration, the type of Message that is created has been made
pluggable through the introduction of a factory abstraction;
MessageFactory.
By default a MessageFactory is provided that will create
org.springframework.messaging.GenericMessage instances, however this
can be replaced with an alternative implementation. For example,
Spring Integration can provide an implementation that creates
org.springframework.integration.message.GenericMessage instances.
This control over the type of Message that's created allows messages
to flow from Spring messaging code into Spring Integration code without
any need for conversion. In further support of this goal,
MessageChannel, MessageHandler, and SubscribableChannel have been
genericized to make the Message type that they deal with more
flexible.
org.springframework.web.stomp is now
org.springframework.web.messaging.stomp
Also classes in the ~.stomp.server and ~.stomp.adapter packages have
been renamed.
Allow WebSocketHandler methods to raise an exception.
By default we install ExceptionWebSocketHandlerDecorator, which logs
unhandled exceptions and closes the session. That decorator can be
extended or replaced.
Any exceptions that remain unhandled still (i.e. no exception handling
decorator), are caught in the lowest level before propagating to the
WebSocket engine or a SockJS transport handler and handled the same
way. That means default behavior is guaranteed but also fully
customizable.
Rossen Stoyanchev
Andy Wilkinson
Rossen Stoyanchev
Rob Winch
Juergen Hoeller