ahucoder · ahucoder · Jun 6, 2025 · Jun 6, 2025
diff --git a/pom.xml b/pom.xml
@@ -26,6 +26,11 @@
             <artifactId>spring-boot-starter-web</artifactId>
             <version>3.4.5</version>
         </dependency>
+        <dependency>
+            <groupId>com.google.guava</groupId>
+            <artifactId>guava</artifactId>
+            <version>33.4.0-jre</version>
+        </dependency>
     </dependencies>
 
 </project>
diff --git a/src/main/java/observer/App.java b/src/main/java/observer/App.java
@@ -0,0 +1,36 @@
+package observer;
+
+import observer.event.WeatherUpdateEvent;
+import observer.eventbus.EventBus;
+import observer.listener.User;
+import observer.publisher.WeatherStationPublisher;
+
+public class App {
+
+    public static void main(String[] args) throws InterruptedException {
+        EventBus eventBus = new EventBus();
+        WeatherStationPublisher weatherStation = new WeatherStationPublisher(eventBus);
+
+        // create users
+        User user1 = new User("Tom", System.out::println);
+        User user2 = new User("Jerry", System.out::println);
+
+        // subscribe to weather update events
+        eventBus.subscribe(WeatherUpdateEvent.class, user1);
+        eventBus.subscribe(WeatherUpdateEvent.class, user2);
+
+        // activate the weather station
+        weatherStation.start();
+        System.out.println("The weather broadcast system has been activated");
+
+        // stop after running for a period of time
+        Thread.sleep(6100);
+        eventBus.unsubscribe(WeatherUpdateEvent.class, user1);
+        System.out.println("user1 has been unsubscribed");
+        Thread.sleep(6100);
+        weatherStation.stop();
+
+        System.out.println("The weather broadcast system has been turned off");
+    }
+
+}
diff --git a/src/main/java/observer/event/BaseEvent.java b/src/main/java/observer/event/BaseEvent.java
@@ -0,0 +1,31 @@
+package observer.event;
+
+import java.text.SimpleDateFormat;
+import java.util.Date;
+import java.util.Objects;
+
+public abstract class BaseEvent<T> implements Event<T> {
+    private final long timestamp;
+    private final T source;
+    private static final SimpleDateFormat DATE_FORMAT =
+            new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
+
+    protected BaseEvent(T source) {
+        this.timestamp = System.currentTimeMillis();
+        this.source = Objects.requireNonNull(source);
+    }
+
+    @Override
+    public long timestamp() {
+        return timestamp;
+    }
+
+    @Override
+    public T source() {
+        return source;
+    }
+
+    public String formattedTimestamp() {
+        return DATE_FORMAT.format(new Date(timestamp));
+    }
+}
diff --git a/src/main/java/observer/event/Event.java b/src/main/java/observer/event/Event.java
@@ -0,0 +1,7 @@
+package observer.event;
+
+public interface Event<T> {
+    long timestamp();
+
+    T source();
+}
diff --git a/src/main/java/observer/event/WeatherUpdateEvent.java b/src/main/java/observer/event/WeatherUpdateEvent.java
@@ -0,0 +1,7 @@
+package observer.event;
+
+public class WeatherUpdateEvent extends BaseEvent<String> {
+    public WeatherUpdateEvent(String source) {
+        super(source);
+    }
+}
diff --git a/src/main/java/observer/eventbus/EventBus.java b/src/main/java/observer/eventbus/EventBus.java
@@ -0,0 +1,56 @@
+package observer.eventbus;
+
+import com.google.common.collect.Lists;
+import com.google.common.collect.Maps;
+import observer.event.Event;
+import observer.listener.EventListener;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Event bus, responsible for event publishing and subscription
+ */
+public class EventBus {
+
+    private final Map<Class<?>, List<EventListener<?>>> listenerMap = Maps.newConcurrentMap();
+
+    /**
+     * subscribe event
+     *
+     * @param eventClass    event type
+     * @param eventListener event listener
+     * @param <E>           event generics
+     */
+    public <E extends Event<?>> void subscribe(Class<E> eventClass, EventListener<E> eventListener) {
+        listenerMap.computeIfAbsent(eventClass, v -> Lists.newCopyOnWriteArrayList()).add(eventListener);
+    }
+
+    /**
+     * cancel
+     *
+     * @param eventClass    event type
+     * @param eventListener event listener
+     * @param <E>           event generics
+     */
+    public <E extends Event<?>> void unsubscribe(Class<E> eventClass, EventListener<E> eventListener) {
+        List<EventListener<?>> eventListeners = listenerMap.get(eventClass);
+        if (eventListeners != null) {
+            eventListeners.remove(eventListener);
+        }
+    }
+
+    /**
+     * publish event
+     *
+     * @param event event
+     * @param <E>   event generics
+     */
+    @SuppressWarnings("unchecked")
+    public <E extends Event<?>> void publish(E event) {
+        List<EventListener<?>> eventListeners = listenerMap.get(event.getClass());
+        if (eventListeners != null) {
+            eventListeners.forEach(eventListener -> ((EventListener<E>) eventListener).onEvent(event));
+        }
+    }
+}
diff --git a/src/main/java/observer/listener/EventListener.java b/src/main/java/observer/listener/EventListener.java
@@ -0,0 +1,8 @@
+package observer.listener;
+
+import observer.event.Event;
+
+@FunctionalInterface
+public interface EventListener<T extends Event<?>> {
+    void onEvent(T event);
+}
diff --git a/src/main/java/observer/listener/User.java b/src/main/java/observer/listener/User.java
@@ -0,0 +1,21 @@
+package observer.listener;
+
+import observer.event.WeatherUpdateEvent;
+
+import java.util.function.Consumer;
+
+public class User implements EventListener<WeatherUpdateEvent> {
+    private final String name;
+    private final Consumer<String> consumer;
+
+    public User(String name, Consumer<String> consumer) {
+        this.name = name;
+        this.consumer = consumer;
+    }
+
+    @Override
+    public void onEvent(WeatherUpdateEvent event) {
+        String message = String.format("[%s] received weather update event: %s, current time is %s", name, event.source(), event.formattedTimestamp());
+        consumer.accept(message);
+    }
+}
diff --git a/src/main/java/observer/publisher/Publisher.java b/src/main/java/observer/publisher/Publisher.java
@@ -0,0 +1,52 @@
+package observer.publisher;
+
+import observer.event.Event;
+import observer.eventbus.EventBus;
+
+import java.util.Objects;
+
+public abstract class Publisher<E extends Event<?>> {
+    private final EventBus eventBus;
+    private final int intervalMillis;
+    private volatile boolean running;
+    private Thread workerThread;
+
+    public Publisher(EventBus eventBus, int intervalMillis) {
+        this.eventBus = Objects.requireNonNull(eventBus);
+        this.intervalMillis = intervalMillis;
+    }
+
+    protected abstract E getEventInfo();
+
+    public final void start() {
+        if (running) {
+            return;
+        }
+        running = true;
+        workerThread = new Thread(() -> {
+            try {
+                while (running) {
+                    E event = getEventInfo();
+                    if (event != null) {
+                        eventBus.publish(event);
+                        Thread.sleep(intervalMillis);
+                    }
+                }
+            } catch (InterruptedException e) {
+                Thread.currentThread().interrupt();
+            }
+        });
+        workerThread.start();
+    }
+
+    public final void stop() {
+        running = false;
+        if (workerThread != null) {
+            workerThread.interrupt();
+        }
+    }
+
+    public final boolean isRunning() {
+        return running;
+    }
+}
diff --git a/src/main/java/observer/publisher/WeatherStationPublisher.java b/src/main/java/observer/publisher/WeatherStationPublisher.java
@@ -0,0 +1,20 @@
+package observer.publisher;
+
+import observer.event.WeatherUpdateEvent;
+import observer.eventbus.EventBus;
+
+import java.util.Random;
+
+public class WeatherStationPublisher extends Publisher<WeatherUpdateEvent> {
+    private final Random random = new Random();
+
+    public WeatherStationPublisher(EventBus eventBus) {
+        super(eventBus, 3000);
+    }
+
+    @Override
+    protected WeatherUpdateEvent getEventInfo() {
+        String info = random.nextBoolean() ? "Sunny Day" : "Rain Day";
+        return new WeatherUpdateEvent(info);
+    }
+}
diff --git a/src/main/java/org/example/Main.java b/src/main/java/org/example/Main.java