JavaFX Code Recipes - JavaFX: Developing Rich Internet Applications - page 309

javax.jms.MessageListener interface and registers itself with the JMS client
framework to be notified whenever a new message arrives. The MessageListener
defines a method that Subscriber must implement, public void onMessage(
Message msg) . When a message is received by the JMS client, it in turn calls the
onMessage() method in Subscriber .
ClockUpdater is a JavaFX class and it also extends javax.jms.MessageListener
and implements its own onMessage() function. The reason we cannot use this
directly with the JMS client framework is that the JMS client framework would
call this function using one of its threads, not the main JavaFX thread. Modifying
JavaFX objects on any other thread than the main JavaFX thread is not supported
and likely will cause a serious exception.

When the JMS client calls the Java Subscriber.onMessage() method, it will not
be on the JavaFX main thread. We need to move the message over to the JavaFX
main thread, and then push that message over to the JavaFX object. To do this,
we need to use the com.sun.javafx.runtime.Entry.deferAction() method
to invoke the JavaFX function on the main JavaFX thread.
The constructor for Subscriber takes two arguments: the first identifies the JMS
topic, and the second is the JavaFX class to notify when a message is detected from
JMS. Because the JavaFX class extends the Java interface MessageListener , we
can refer to this class directly. This is shown as Java code in Listing 12.1.

Listing 12.1 Subscriber.java

...
...

private MessageListener fxListener ;

public Subscriber(String topicName,
MessageListener fxListener ) throws JMSException {

this.fxListener = fxListener ;

...
...

When the JMS client calls the Subscriber.onMessage(Message msg) Java
method, we then use the Entry.deferAction() method to call the JavaFX
onMessage(msg:Message) function from the JavaFX main thread. Notice that
we need to change the message parameter to the Java onMessage() method to
final. This is so that it can be visible when the deferAction Runnable is
invoked. Listing 12.2 shows the Java onMessage(Message msg) implementation.