Java: Observer

Observer

Observer is a behavioral design pattern that allows one objects to notify other objects about changes in their state.

The Observer pattern provides a way to subscribe and unsubscribe to and from these events for any object that implements a subscriber interface.

More about Observer

Application of the pattern in Java

Complexity: Average

Popularity: High

Usage examples: The Observer is pretty common in Java code, especially in the GUI components. It provides a way to react to events happening in other objects without coupling to their classes.

Here are some examples of the pattern in core Java libraries:

Identification: The pattern can be recognized by subscription methods, that store objects in a list and by calls to the update method issued to objects in that list.

Example: Event subscription

In this example, the Observer pattern establishes indirect collaboration between objects of a text editor. Each time the Editor object changes, it notifies its subscribers. EmailNotificationListener and LogOpenListener react to these notifications by executing their primary behaviors.

Subscriber classes are not coupled to the editor class and can be reused in other apps if needed. The Editor class depends only on the abstract subscriber interface. This allows adding new subscriber types without changing editor's code.

publisher

publisher/EventManager.java: Basic publisher

package refactoring_guru.observer.example.publisher;

import refactoring_guru.observer.example.listeners.EventListener;

import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class EventManager {
    Map<String, List<EventListener>> listeners = new HashMap<>();

    public EventManager(String... operations) {
        for (String operation : operations) {
            this.listeners.put(operation, new ArrayList<>());
        }
    }

    public void subscribe(String eventType, EventListener listener) {
        List<EventListener> users = listeners.get(eventType);
        users.add(listener);
    }

    public void unsubscribe(String eventType, EventListener listener) {
        List<EventListener> users = listeners.get(eventType);
        int index = users.indexOf(listener);
        users.remove(index);
    }

    public void notify(String eventType, File file) {
        List<EventListener> users = listeners.get(eventType);
        for (EventListener listener : users) {
            listener.update(eventType, file);
        }
    }
}

editor

editor/Editor.java: Concrete publisher, tracked by other objects

package refactoring_guru.observer.example.editor;

import refactoring_guru.observer.example.publisher.EventManager;

import java.io.File;

public class Editor {
    public EventManager events;
    private File file;

    public Editor() {
        this.events = new EventManager("open", "save");
    }

    public void openFile(String filePath) {
        this.file = new File(filePath);
        events.notify("open", file);
    }

    public void saveFile() throws Exception {
        if (this.file != null) {
            events.notify("save", file);
        } else {
            throw new Exception("Please open a file first.");
        }
    }
}

listeners

listeners/EventListener.java: Common observer interface

package refactoring_guru.observer.example.listeners;

import java.io.File;

public interface EventListener {
    public void update(String eventType, File file);
}

listeners/EmailNotificationListener.java: Sends emails upon receiving notification

package refactoring_guru.observer.example.listeners;

import java.io.File;

public class EmailNotificationListener implements EventListener {
    private String email;

    public EmailNotificationListener(String email) {
        this.email = email;
    }

    @Override
    public void update(String eventType, File file) {
        System.out.println("Email to " + email + ": Someone has performed " + eventType + " operation with the following file: " + file.getName());
    }
}

listeners/LogOpenListener.java: Writes a message to a log upon receiving notification

package refactoring_guru.observer.example.listeners;

import java.io.File;

public class LogOpenListener implements EventListener {
    private File log;

    public LogOpenListener(String fileName) {
        this.log = new File(fileName);
    }

    @Override
    public void update(String eventType, File file) {
        System.out.println("Save to log " + log + ": Someone has performed " + eventType + " operation with the following file: " + file.getName());
    }
}

Demo.java: Initialization code

package refactoring_guru.observer.example;

import refactoring_guru.observer.example.listeners.EmailNotificationListener;
import refactoring_guru.observer.example.listeners.LogOpenListener;
import refactoring_guru.observer.example.editor.Editor;

public class Demo {
    public static void main(String[] args) {
        Editor editor = new Editor();
        editor.events.subscribe("open", new LogOpenListener("/path/to/log/file.txt"));
        editor.events.subscribe("save", new EmailNotificationListener("admin@example.com"));

        try {
            editor.openFile("test.txt");
            editor.saveFile();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

OutputDemo.txt: Execution result

Save to log \path\to\log\file.txt: Someone has performed open operation with the following file: test.txt
Email to admin@example.com: Someone has performed save operation with the following file: test.txt