When should we use Observer and Observable?

java

event-driven-architecture

thread-safety

memory-leaks

byAnton Shumikhin·Mar 12, 2025

Initialize Observer pattern for decoupling: a subject informs observers about its changes. With Java 9+, think of java.beans.PropertyChangeListener as your primary choice, Do not hold onto deprecated Observer/Observable. Reactive Streams offer more advanced options—RxJava being a top pick.

Example:

import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;

public class DataModel {
    private String data;
    private PropertyChangeSupport pcs = new PropertyChangeSupport(this);

    public void addListener(PropertyChangeListener listener) {
        pcs.addPropertyChangeListener(listener);  // Welcome aboard, listener!
    }
    
    public void setData(String newData) {
        String oldData = this.data;
        this.data = newData;
        pcs.firePropertyChange("data", oldData, newData);  // Attention listeners, we've got news!
    }
}

// Usage:
DataModel model = new DataModel();
model.addListener((evt) -> System.out.println("Data changed to: " + evt.getNewValue()));  // Ears open for new information.
model.setData("UpdatedData");  // Just another day making changes.

Choose PropertyChangeSupport for a simplified observer pattern — it's like having a personal assistant for handling listener registration and notification on property updates.

Observer and Observable in dynamic systems

Dynamic systems are where Observer pattern shines, aiding objects to react to state changes. Think of it as following a friend's updates on an Instagram feed. It holds a particular prominence for event-driven architectures, assuring system consistency by synchronizing all observers with the subject's state.

Scaling applications - The Observer way

When scaling applications, alterations become ticklish. Observer pattern comes to rescue by allowing addition or removal of observers with minimum disruption, courtesy addObserver() and deleteObserver(). This pattern endorses plug-n-play models and maintainability, without a deep dive into the observed object.

Real-time updates application

Living examples like passport authority updates or an active message board notifications, truly captures the essence of Observer pattern. It efficiently communicates updates. A clear flag post-notification helps in making sure updates are communicated properly while avoiding repeated or missed notifications.

Implementing Observer and Observable

Use the Observer pattern not only for updating UI or registering user inputs. Consider scenarios like environment sensors tracking changes, stock market price monitors, or progress report systems. With loose coupling, every observer can concentrate on its specific concept, responding to changes as dictated by the "something" they observe.

System consistency - Observing the observers

Maintaining system consistency is the topmost priority in sophisticated applications. The Observer pattern ensures all application facets are updated in real-time, mirroring state changes across all its components. This avoids data inconsistency caused by outdated states.

Notifications made flexible

Observers can modify their reaction to notifications. Depending upon the case, an observer may decide to immediately react, stack updates for batch processing, or ignore a few updates. This flexibility broadens the application of the pattern accommodating a wide range of situations.

Merits and pitfalls of Observer pattern

While the Observer pattern is a powerful tool, don't misuse it. A common issue faced is over-notification, where observers get notified too often, leading to performance drawbacks. To prevent this, ensure notifications are dispatched only when there is an actual change in state which can be validated through the setChanged() method.

Maintaining thread safety

Thread safety is crucial in implementing this pattern. While dealing with concurrent updates and notifications, potential hazards such as race conditions or deadlocks could arise. A thorough synchronization of the notification mechanism can prevent these situations.

Preventing memory leaks

Be cautious about memory leaks. Unmanaged observers might not get garbage collected, leading to memory issues. Ensure you have a mechanism to properly detach observers when they are no longer needed, by implementing deleteObserver().

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