I am writing a Java application using SWT widgets. I would like to update the state of certain widgets upon a certain event happening (for example, updating the data model's state).
Is there something in Java similar to Cocoa's NSNotificationCenter, where I can register an object to listen for notification events and respond to them, as well as have other objects "fire off" a notification?
Ok, suppose that for example, you want parts of your program to be notified when your Loader starts a scan, and when it finishes a scan (don't worry about what a Loader is, or what a scan is, these are examples from some code I have lying around from my last job). You define an interface, call it "ScanListener", like
public interface ScanListener
{
public void scanStarted();
public void scanCompleted();
}
Now the Loader defines a method for your other code to register for callbacks, like
public void addScanListener(ScanListener listener)
{
listeners.add(listener);
}
The Loader, when it starts a scan, executes the following code
for (ScanListener listener : listeners)
{
listener.scanStarted();
}
and when it finishes, it does the same thing with listener.scanCompleted();
The code that needs to be notified of these events implements that interface (either themselves, or in an internal class), and calls "loader.addScanListener(this)". Its scanStarted() and scanCompleted() methods are called at the appropriate times. You can even do this with callbacks that take arguments and/or return results. It's all up to you.
What sort of notifications are you looking for? If all you want is for one object to be able to tell anybody else "hey, I've changed, update accordingly", the easiest way is to use the existing Observer interface and Observable class. Or write your own with an interface that defines what you want to get called on the listeners from the one that's changed.
There's no pre-existing per-process service that dispatches events in java that's equivalent to the default NSNotificationCenter. In java, the type of the event is specified by the event object being a particular type (which also means that the notification method depends on that type) rather than using a string. Prior to generics, writing a general event dispatcher and receiver that is also typesafe isn't really possible (witness the proliferation of *Event classes and *EventListener interfaces in the AWT and Spring libraries).
There are some facilities for event dispatch. As Paul mentioned, there's java.util.Observable, which as you point out, requires subclassing. There's also java.beans.PropertyChangeSupport, which could be useful depending on your situation.
You could also write one yourself. The source for PropertyChangeSupport is likely available in the openjdk, and you could look at the abandoned Apache Commons Event project. Depending on your needs, you may have to worry about stuff like threading, seralization, memory leaks (ensuring deregistration or using weak references), and concurrent modification (iterate over a copy of your list of listeners, as a listener may decide to unregister itself in response to a change).
Now that generics exist in Java, a generic event dispatch library would be possible; however, I haven't come across any. Anyone?
There's actually a facility built in to Java that does exactly what you want, but it's not something you may have considered, and, to be honest, it is likely a bit heavyweight for what you want.
That said, however, it does exist.
It's JMX.
You create MBeans, and then others can register for events from those MBeans. The MBean can then send of a Notification.
I personally wouldn't consider using it for this case (I'd just pound out my own), but the facility is there and it well defined and documented.
Not Java, but the IPython project has a notification center written in Python here that you could use as a template for a Java version.
In Java this would be a provider firing notifications to its listeners. But Java does not offer the loose coupling you get with Cocoa's NSNotification because in Java providers and subscribers must have references to each other. Compare for this chapter 18 in "Learn Objective-C for Java Developers".
There is an implementation of IOS NSNotificationCenter in Java.
You can find sources code in :
This Github project
Related
Heard about java 'publish-subscribe' style communication between components without requiring the components to explicitly be aware of each other, which is Event bus.It seems that using event bus we can communicate between different classes very easily with less coding needed.I know that NSNotifications in iOS also do this. NSNotification is not a replacement here.Please let me know apart form delegation pattern what is a good solution in iOS which is a good replacement for EventBus for communication between classes.?
With Swift you can use SwiftEventBus. It's just a nice wrapper around NSNotificationCenter and DispatchQueue.
Register to an event:
SwiftEventBus.onMainThread(target, name: "someEventName") { result in
// UI thread
// Do something when the event occurr
}
Trigger an event:
SwiftEventBus.post("someEventName")
And if you need to customize it, the source code is short, clear and easy to understand.
I think you can use NSNotificationCenter for this, I read your comment about it is one-to-many and it's true by default but you can specify from which object do you want to receive messages like this:
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(someSelector:)
name:#"MyPersonalNotification"
object:someOtherObject];
Here you will receive the MyPersonalNotification in someSelector: only when someOtherObject post it. This made the communication one-to-one.
Also you can use the Key-Value Observing API but I personally found it somewhat uncomfortable.
Have a look at tolo.
The functionality is somewhat similar to an event bus and it has one big advantage over NSNotification as you don't have to unregister when deallocating (like in iOS9).
For a school project, I need to write a simple Server in Java that continuously listens on an incoming directory and moves files from this directory to some place else. The server needs to log info and error messages, so I thought I could use the Proxy pattern for this. Thus, I created the following ServerInterface:
public interface ServerInterface extends Runnable {
public void initialize(String repPath, ExecutorInterface executor, File propertiesFile) throws ServerInitException;
public void run();
public void terminate();
public void updateHTML();
public File[] scanIncomingDir();
public List<DatasetAttributes> moveIncomingFilesIfComplete(File[] incomingFiles);
}
Then I've created an implementation Server representing the real object and a class ProxyServer representing the proxy. The Server furthermore has a factory method that creates a ProxyServer object but returns it as a ServerInterface.
The run-method on the proxy-object looks like this:
#Override
public void run(){
log(LogLevels.INFO, "server is running ...");
while( !stopped ){
try {
File[] incomingContent = scanIncomingDir();
moveIncomingFilesIfComplete(incomingContent);
updateHTML();
pause();
} catch (Exception e) {
logger.logException(e, new Timestamp(timestampProvider.getTimestamp()));
pause();
}
}
log(LogLevels.INFO, "server stopped");
}
The functions that are called within the try statement simply log something and then propagate the call to the real object. So far, so good. But now that I've implemented the run-method this way in the proxy object, the run-method in the real object becomes obsolete and thus, is just empty (same goes for the terminate-method).
So I ask my-self: is that ok? Is that the way the proxy pattern should be implemented?
The way I see it, I'm mixing up "real" and "proxy"-behaviour ... Normally, the real-server should be "stuck" in the while-loop and not the proxy-server, right? I tried to avoid mixing this up, but neither approaches were satisfying:
I could implement the run-method in the real object and then hand over the proxy object to the real object in order to still be able to log during the while-loop. But then the real object would do some logging, which is I tried to avoid by writing a proxy in the first place.
I could say, only Proxy-Server is Runnable, thus deleting run and terminate from the Interface, but this would break up the Proxy pattern.
Should I may be use another design? Or I am seeing a problem where there is none?
You're definitely thinking in the right way. You've hit upon an interesting notion.
Features like logging, as you've described, are an example of what we call cross-cutting concerns in Aspect Oriented programming.
A cross-cutting concern is a requirement that will be used in many objects.
. . therefore, they have the tendency to break object oriented programming. What does this mean?
If you try to create a class that is all about moving files from place A to place B, and the implementation of a method to do that first talks about logging (and then transactions, and then security) then that isn't very OO is it? It breaks the single responsibility principle.
Enter Aspect Oriented Programming
This is the reason we have AOP - it exists to modularize and encapsulate these cross-cutting concerns. It works as follows:
Define all the places where we want the cross-cutting feature to be applied.
Use the intercept design pattern to "weave" in that feature.
Ways we can "weave" in a requirement with AOP
One way is to use a Java DynamicProxy as you've described. This is the default in for example the Spring Framework. This only works for interfaces.
Another way is to use a byte-code engineering library such as asm, cglib, Javassist - these intercept the classloader to provide a new sub-class at runtime.
A 3rd way is to use compile-time weaving - to change the code (or byte-code) at compile-time.
One more way is to use a java agent (an argument to the JVM).
The latter two options are supported in AspectJ.
In Conclusion:
It sounds as though you're moving towards Aspect Oriented Programming (AOP), so please check this out. Note also that the Spring Framework has a lot of features to simplify the application of AOP, though in your case, given this is a school assignment, its probably better to delve into the core concepts behind AOP itself.
NB: If you're building a production-grade server, logging may be a full-blown feature, and thus worth using AOP. . in other cases its probably simple enough to just in-line.
You should use Observer pattern in this case:
The observer pattern is a software design pattern in which an object,
called the subject, maintains a list of its dependents, called
observers, and notifies them automatically of any state changes,
usually by calling one of their methods.
Your Observable will observe changes in directory, by time pooling, or as already was suggested here, with WatchService. Changes of directory will notify Observer which will take action of moving files. Both Observable and Observer should log their actions.
You shold also know that Observer pattern became a part of Java JDK by implementing java.util.Observable and java.util.Observer.
You can make your proxy aware of the real object. Basically your proxy will delegate the call to run method to the real implementation.
Before the delegation, the proxy first logs the startup. After delegation, the proxy logs the "shutdown":
// Snapshot from what should look like the run method implementation
// in your proxy.
public ServerInterfaceProxy(ServerInterface target){
this.proxiedTarget = target;
}
public void run(){
log(LogLevels.INFO, "server is running ...");
this.proxiedTarget.run();
log(LogLevels.INFO, "server is running ...");
}
This implementation can also be perceived as a Decorator pattern implementation. IMHO, I believe that to some extent (when it comes to implementation) Proxy and Decorator are equivalent : Both intercept/capture behavior of a target.
Look at Java 7's WatchService class.
Using Proxy behaviour for this is almost certainly overkill.
I am building an a application that uses the event observer pattern to allow points in the system to be extended easily by 3rd party vendors who wish to add their own functionality.
This works well however it relies on 1 thing:
I must decide which points in the system vendors may wish to extend the system and trigger an event in that particular point in the flow.
It is entirely plausible that a vendor may wish to extend a different part of the system that I have not triggered an event from.
What are the alternatives here, to allow the vendor to extend the system from any point in the system they choose - or as close to that goal as possible - using something similar to event observer i.e. the can simply hook into the system where the choose to?
Okay, so you need to do two things. The first is to go out and find some real clients, and talk to them. Where do they need extensions? They know that much better than you do. The second thing is to err on the side of not providing hooks. You can always add them later, but you will have to support anything that you put out, even if it's not used.
As #Holger said, there is no way to provide arbitrary hooks to any point in your system, and it's really not desirable for you to do so. That's a maintenance nightmare.
Holger mentioned already that AOP may help you. You simply annotate your code with your custom annotations and let your customers define their own aspects (they will be required to recompile the entire code base). For example:
#Data
public class Records {
#Saving
public void add(Record rec) {
// your code
}
}
Later, your customer may say "I want to be triggered every time anyone is saving some data". He will have to define his own "aspect" with this custom "shortcut" (pseudo code):
#Aspect
public class OnSaving {
#Before("execution(* #Data #Saving(..))")
public void shortcut(JoinPoint pnt) {
// your customer's code
}
}
Your source code won't be changed, just recompiled. I assumed AspectJ usage.
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I think that you have heard of message/event buses, it's the single place when all events in the system flow. Similar architectures are found in computer's motherboards and LAN networks. It's a good approach for motherboards and networks as it reduces the number of wires, but is it good for software development? We don't have such restrictions as electronics does.
The simplest implementation of message bus/event bus can be like:
class EventBus {
void addListener(EventBusListener l}{...}
void fireEvent(Event e) {...}
}
Posting events is done with bus.fireEvent(event), receiving messages is enabled by bus.addListener(listener). Such architectures are sometimes used for software development, for example MVP4G implements similar message bus for GWT.
Active projects:
Google Guava EventBus
MBassador by Benjamin Diedrichsen
Mycila PubSub by Mathieu Carbou
mvp4g Event Bus
Simple Java Event Bus
Dormant/Dead projects:
Sun/Oracle JavaBeans InfoBus
https://eventbus.dev.java.net/ [Broken link]
It's just the popular Observer (Listener) pattern made 'globally' - each object in the system can listen to each message, and I think it's bad, it breaks the Encapsulation principle (each object knows about everything) and Single Responsibility principle (eg when some object needs to a new type of message, event bus often needs to be changed for example to add a new Listener class or a new method in the Listener class).
For these reasons I think, that for most software, Observer pattern is better than event bus. What do you think about event bus, does it make any good sense for typical applications?
EDIT: I'm not talking about 'big' enterprise solutions like ESB - they can be useful (what's more ESB offers much, much more than just an event bus). I'm asking about usefulness of using message bus in 'regular' Java code for object-to-object connection - some people do it, check the links above. Event bus is probably best solution for telephone-to-telephone communication or computer-to-computer communication because each telefone (or computer) in a network can typically talk to each other, and bus reduces the number of wires. But objects rarely talk to each other - how many collaborators one object can have - 3, 5?
I am considering using a In memory Event Bus for my regular java code and my rationale is as follows
Each object in the system can listen to each message, and I think it's
bad, it breaks the Encapsulation principle (each object knows about
everything)
I am not sure if this is really true, I class needs to register with the event bus to start with, similar to observer pattern, Once a class has registered with the Event Bus, only the methods which have the appropriate signature and annotation are notified.
and Single Responsibility principle (eg when some object needs to a
new type of message, event bus often needs to be changed for example
to add a new Listener class or a new method in the Listener class).
I totally disagree with
event bus often needs to be changed
The event bus is never changed
I agree with
add a new Listener class or a new method in the Listener class
How does this break SRP ?, I can have a BookEventListener which subscribes to all events pertaining to my Book Entity, and yes I can add methods to this class but still this class is cohesive ...
Why I plan to use it ? It helps me model the "when" of my domain ....
Usually we hear some thing like send a mail "when" book is purchased
we go write down
book.purchase();
sendEmail()
Then we are told add a audit log when a book is purchased , we go to the above snippet
book.purchase();
sendEmail();
**auditBook();**
Right there OCP violated
I Prefer
book.purchase();
EventBus.raiseEvent(bookPurchasedEvent);
Then keep adding handlers as needed Open for Extension Closed for Modification
Thanks
Some people like it because it is the embodiment of the Facade pattern or Mediator pattern. It centralizes cross-cutting activities like logging, alerting, monitoring, security, etc.
Some people don't like it because it is often a Singleton point of failure. Everyone has to know about it.
I use it heavily in JavaScript. There can be so many various widgets that all need to do some sort of action whenever something else happens -- there is no real hierarchy of ownership of objects. Instead of passing references of every object to every object, or just making every object global, when something significant happens inside a particular widget, I can just publish "/thisWidget/somethingHappened" -- instead of filling that widget with all kinds of code specific to the API of other widgets. The I have a single class that contains all the "wiring", or "plubming" as they like to call it in the Java Spring framework. This class contains references to all of my widgets, and has all of the code for what happens after each various event fires.
It is centralized, easy to access and maintain, and if one thing changes or I want a new process to occur on a specific event, I don't have to search through every single class/object/widget to try to find out where something is being handled. I can just go to my "operator" class -- the one that handles all the "wiring" when a particular event happens, and see every repercussion of that event. In this system, every individual widget is completely API agnostic of the other widgets. It simply publishes what has happened to it or what it is doing.
I'm having trouble understanding what you're really asking in your question. You give an example of a simple event bus which is actually just Observable with a different name, then you say;
For these reasons I think, that for
most software, Observer pattern is
better than event bus. What do you
think about event bus, does it make
any good sense for typical
applications?
..but given your example, they are the same. This makes me wonder if you have ever used something like a Enterprise Service Bus. At a base level an ESB logically does the same thing as the observer pattern, but commercial products add much, much more. Its like an event bus on steroids. They are complicated software products and offer;
Message pickup
Generate events by listening to various endpoints. The endpoint can be a listener (such as a HTTP server), a messaging system (such as JMS), a database or pretty much anything else you want.
Message routing
Take your event and send it to one/many endpoint. Routing can be pretty smart, the bus might route the message depending on the message type, the message contents or any other criteria. Routing can be intelligent and dynamic.
Message Transformation
Transforms your message into another format, this can be as simnple as from XML to JSON or from a row on a database table to a HTTP request. Transformation can occur within the data itself, for example swapping date formats.
Data Enrichment
Adds or modifies data in your message by calling services along the way. For example if a message has a postcode in it the bus might use a postcode lookup service to add in address data.
..and lots, lots more. When you start looking into the details you can really start to see why people use these things.
Because it can be an important step in the way to decouple the application modules to a service based architecture.
So in your case if you have not the intention to decouple the modules of your application into isolated services then the native implementation of the Observer pattern will make it a simpler solution.
But If you want to build lets say a micro-services architecture the event-bus will allow to get the benefits of this architecture style so you could for instance update and deploy just some part of your application without affect others, because they are just connected through the event-bus.
So the main question here is the desired level of application components decoupling.
Some references about it:
http://microservices.io/patterns/data/event-driven-architecture.html
http://tech.grammarly.com/blog/posts/Decoupling-A-Monolithic-Server-Application.html
A good analogy is that of a telephone exchange, where every handset can dial to every other handset. A compromised handset can tune into other conversations. Program control flows like wires(cyclomatic complexity anyone!) This is similar to the requirement of having a connection/physical medium between two end points. This is So for N handsets instead of having NC2 (Combinatorial logic) flows for every new handset we tend to get N flows.
A reduction in complexity implies easy to understand code. Lets start with the prominent points you have highlighted: 1. Global knowledge 2. Intrusive modifications.
Global Knowledge: Consider message event to be an envelop. From event handler/sender perspective there is no data being exposed, it is seeing an envelop (unless an derived class tries to do some inspection using 'instanceof' checks). In a good OOP design, this would never occur.
Intrusive modifications: Instead of having a event specific listener approach, one can use a global event handling approach. As such we have a global event type (on which data is piggy backed and down-casted). This is much like the PropertyBeanSupport model in Java. With a single event type we are required to have a single sender and listener types. This implies you need not modify the bus/listeners every time you see something new. The ugly down-casting can be soothened using Adapter pattern (Please don't start that another level of redirection quote!). Programmers can write assembly in any language. So need for commonsense and smartness can not be substituted. All I intend to state is it can be a effective tool.
The actual event receivers can use the listeners (composition/proxy) easily. In such a Java code base, listeners would look like stand alone inner class declarations (with unused warning being flagged in certain IDEs). This is akeen to two players on the beach playing a ball game, the players don't react until they see the ball.
'#duffymo' points out another interesting aspect: 'Single point of failure'. This would/can in theory effect any object residing in memory(RAM) and not specific to MessageHandlers.
As a practical example, our app sync's with a web service every x number of minutes, and if any new data is received, we need to update the GUI. Now, because the SyncAdapter runs on a background thread, you can't simply point to a textview and modify its properties, you have to bubble up an event. And the only way to make sure you catch that event is if you have a shared (static,singleton) object passing that event around for subscribers to handle.
I'm looking for something similar to the Proxy pattern or the Dynamic Proxy Classes, only that I don't want to intercept method calls before they are invoked on the real object, but rather I'd like to intercept properties that are being changed. I'd like the proxy to be able to represent multiple objects with different sets of properties. Something like the Proxy class in Action Script 3 would be fine.
Here's what I want to achieve in general:
I have a thread running with an object that manages a list of values (numbers, strings, objects) which were handed over by other threads in the program, so the class can take care of creating regular persistent snapshots on disk for the purpose of checkpointing the application. This persistor object manages a "dirty" flag that signifies whether the list of values has changed since the last checkpoint and needs to lock the list while it's busy writing it to disk.
The persistor and the other components identify a particular item via a common name, so that when recovering from a crash, the other components can first check if the persistor has their latest copy saved and continue working where they left off.
During normal operation, in order to work with the objects they handed over to the persistor, I want them to receive a reference to a proxy object that looks as if it were the original one, but whenever they change some value on it, the persistor notices and acts accordingly, for example by marking the item or the list as dirty before actually setting the real value.
Edit: Alternatively, are there generic setters (like in PHP 5) in Java, that is, a method that gets called if a property doesn't exist? Or is there a type of object that I can add properties to at runtime?
If with "properties" you mean JavaBean properties, i.e. represented bay a getter and/or a setter method, then you can use a dynamic proxy to intercept the set method.
If you mean instance variables, then no can do - not on the Java level. Perhaps something could be done by manipulations on the byte code level though.
Actually, the easiest way to do it is probably by using AspectJ and defining a set() pointcut (which will intercept the field access on the byte code level).
The design pattern you are looking for is: Differential Execution. I do believe.
How does differential execution work?
Is a question I answered that deals with this.
However, may I suggest that you use a callback instead? You will have to read about this, but the general idea is that you can implement interfaces (often called listeners) that active upon "something interesting" happening. Such as having a data structure be changed.
Obligitory links:
Wiki Differential execution
Wiki Callback
Alright, here is the answer as I see it. Differential Execution is O(N) time. This is really reasonable, but if that doesn't work for ya Callbacks will. Callbacks basically work by passing a method by parameter to your class that is changing the array. This method will take the value changed and the location of the item, pass it back by parameter to the "storage class" and change the value approipriately. So, yes, you have to back each change with a method call.
I realize now this is not what you want. What it appears that you want is a way that you can supply some kind of listener on each variable in an array that would be called when that item is changed. The listener would then change the corresponding array in your "backup" to refect this change.
Natively I can't think of a way to do this. You can, of course, create your own listeners and events, using an interface. This is basically the same idea as the callbacks, though nicer to look at.
Then there is reflection... Java has reflection, and I am positive you can write something using it to do this. However, reflection is notoriously slow. Not to mention a pain to code (in my opinion).
Hope that helps...
I don't want to intercept method calls before they are invoked on the real object, but
rather I'd like to intercept properties that are being changed
So in fact, the objects you want to monitor are no convenient beans but a resurgence of C structs. The only way that comes to my mind to do that is with the Field Access call in JVMTI.
I wanted to do the same thing myself. My solution was to use dynamic proxy wrappers using Javassist. I would generate a class that implements the same interface as the class of my target object, wrap my proxy class around original class, and delegate all method calls on proxy to the original, except setters which would also fire the PropertyChangeEvent.
Anyway I posted the full explanation and the code on my blog here:
http://clockwork-fig.blogspot.com/2010/11/javabean-property-change-listener-with.html