A new approach to events in Java.

Java Signals is a typed, compact API that allows events to be implemented quickly and efficiently.

• A Signal (not to be confused with the JVM's native signal handling) is essentially a self contained event dispatching mechanism.
• Each Signal corresponds to a unique event, which contains its own list of listeners and event data types.
• Only a Signal instance is needed for each event, no extra constants or subclasses are needed.
• Event listeners register directly to the Signal, without need of an event manager and string based representation of the event type.
• Interfaces allow for easy encapsulation of Signal functionality where needed.
• The Java Signal's API is an event model that can be used outside of Applet development (AWT/Swing).

Examples

Basic Signal Example

// using the Java Signals event model in a simple banking example
import jsignal.Signal;
import jsignal.SignalException;

public class BankApp {

	public static void main(String args[]) {
		BankApp app = new BankApp();
		ATM atm = new ATM();
		// add the app as a listener to the ATM transactionComplete signal
		atm.transactionComplete.add(app, "handleNewBalance");
		atm.doTransaction();
	}

	public void handleNewBalance(String name, double balance) {
		System.out.println("New balance recieved, name:"+name+" balance:"+balance);
	}
}

class ATM {
	protected final Signal transactionComplete;
	
	public ATM() {
		// any number of parameter class types can be passed to the constructor
		transactionComplete = new Signal(String.class, double.class);
	}
	
	public void doTransaction() {
		// do something
		try {
			// dispatch the event; to enforce strict-typing of event data, the data types of the arguments must match up with formal parameters
			transactionComplete.dispatch("Paul", 17.06);
		}
		catch (SignalException e) {
			// SignalException is unchecked to reduce code-noise, but can still be caught if needed
			e.printStackTrace();
		}
	}
}

PrioritySignal Example

// using the Java Signals event model for priority based event dispatching
//...
// we can specify generically a comparable type to use as priority, in this case we will use Integers
PrioritySignal<Integer> transactionComplete = new PrioritySignal<Integer>(String.class, double.class);
//...

// add the high and low priority handlers
// note that Integers are compared by their natural ordering, so a lower number has a higher priority in this case
atm.transactionComplete.add(lowPriorityApp, "handleNewBalance", 1);
atm.transactionComplete.add(highPriorityApp, "handleNewBalance", 0);

WeakSignal Example

// using WeakSignals for memory sensitive code
//...
WeakSignal signal = new WeakSignal();
Listener listener = new Listener();
signal.add(listener, "callback");
listener = null;
//... after garbage collection

signal.dispatch(); // listener was garbaged collected and automatically removed as a listener from the WeakSignal instance

Thread Saftey

Any signal can be synchronized with the Signals.synchronizedSignal method.

PrioritySignal<Integer> signal = new PrioritySignal<Integer>(String.class, double.class);
// wrap the existing signal in a synchronized version, still functions as a PrioritySignal
ISignal safeSignal = Signals.synchronizedSignal(signal);

Note: Because native AWT events haven't yet been wrapped by Java Signals, there is no need to post a side by side comparison of the two methods. You can find Oracle's tutorial on events here

Links

• "Robert Penner's original Signals API for AS3":https://github.com/robertpenner/as3-signals