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import net.engio.mbassy.dispatch.HandlerInvocation;
import net.engio.mbassy.listener.*;
import net.engio.mbassy.subscription.MessageEnvelope;
import net.engio.mbassy.subscription.SubscriptionContext;
import java.io.File;
import java.lang.annotation.*;
/**
* These examples show how to configure listeners and handlers based on the available configuration options.
*
* NOTE: The presented handler configurations compose very well because they are implemented using decorator pattern.
*
*/
public class ListenerDefinition {
/**
* By default any listener will be stored using weak references {@link java.lang.ref.WeakReference}.
* This implies that at any give point in time, listeners might be GC'ed if no other alive object holds a reference to it.
*
* In managed environments where object lifecycle is controlled
* by a framework (Spring, Guice etc.) this is a very handy feature because no attentions needs
* to be paid to proper unsubscription of listeners that have ended their life (session scoped beans etc.)
*
* NOTE: There is no dedicated maintenance task running to take care of GC'ed listeners.
* Automatic cleanup of orphaned weak references is an embedded process done during message publication.
*
*/
static class WeaklyReferencedListener{
// Handler definitions go here
}
/**
* In case that there is no other mechanism managing references to the listeners and they should
* just stick around until explicitly unsubscribed, listener classes need to be annotated accordingly.
*/
@Listener(references = References.Strong)
static class StronglyReferencedListener{
// This listener will stay subscribed until explicitly unsubscribed
}
/**
* This listeners demonstrates the very basic use cases of synchronous and asynchronous handler definitions.
*
*/
static class SyncAsyncListener{
/**
* Any published message will be delivered to this handler (as it consumes any object of type Object.class)
* Delivery is done using synchronous invocation, i.e. the handler is called from the thread running the message
* publication.
*
*/
@Handler
public void synchronousHandler(Object message) {
// do something
}
/**
* According to the handler configuration, this handler is invoked asynchronously, meaning that each handler
* invocation runs in a thread different from the one that runs the initial message publication.
*
* This feature is useful for computationally expensive or IO-bound tasks.
*
*/
@Handler(delivery = Invoke.Asynchronously)
public void asynchronousHandler(File message) {
// do something more expensive here
}
}
static class FilteringListener{
/**
* This handler consumes only strings (as there are no subtypes of final String.class).
* Furthermore, each string is passed through the list of defined filters and the handler is
* invoked only if all filters pass. In this case, only strings starting with 'http' will be handled.
*
*/
@Handler(delivery = Invoke.Synchronously,
filters = {@Filter(Urlfilter.class)})
public void httpUrlsOnly(String message) {
}
/**
* Another way of controlling which messages are delivered to handlers is by using JUEL expressions.
* These can be specified as conditions (no type checking etc.) and will be evaluated on the msg.
* This particular condition will filter out all empty strings
*/
@Handler(condition = "!msg.isEmpty()")
public void handleNonEmptyStrings(String msg) {
}
/**
*
*/
@Handler(delivery = Invoke.Synchronously, rejectSubtypes = true)
@Enveloped(messages = {Object.class, String.class})
public void handleUnrelatedMessageTypes(MessageEnvelope envelope) {
// the envelope will contain either an instance of Object.class or String.class
// if rejectSubtypes were set to 'false' (default) also subtypes of TestMessage or TestMessage2 would be allowed
}
static class Urlfilter implements IMessageFilter<String>{
public boolean accepts(String message, SubscriptionContext context){
return message.startsWith("http");
}
}
}
/**
* Listeners can use custom code to invoke their handlers. Custom invocation logic is defined on a per-handler
* basis (as the signature requires knowledge about concrete handler and message type).
*/
@Listener(references = References.Strong)
static class CustomInvocationListener {
@Handler(invocation = TimingInvocation.class)
public void handle(File message) {
// do timed operation here
}
public static class TimingInvocation extends HandlerInvocation<CustomInvocationListener, File> {
public TimingInvocation(SubscriptionContext context) {
super(context);
}
@Override
public void invoke(CustomInvocationListener listener, File message) {
long start = System.currentTimeMillis();
listener.handle(message);
long duration = System.currentTimeMillis() - start;
System.out.println("Time takes for handler invocation: " + duration + " ms");
}
}
}
/**
* Handler annotation that adds a condition checking for positive integers only
*/
@Retention(value = RetentionPolicy.RUNTIME)
@Inherited
@Handler(condition = "msg.getClass() == Integer.class && msg > 0")
@Synchronized
@Target(value = { ElementType.METHOD, ElementType.ANNOTATION_TYPE })
@interface SynchronizedPositiveIntegers{}
static class ListenerWithCustomAnnotation{
@SynchronizedPositiveIntegers
public void handlePositiveIntegers(Integer msg){
}
}
}
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