Adding a HealthCheckRegistryListener

metrics-healthchecks/src/main/java/com/codahale/metrics/health/HealthCheckRegistry.java

@@ -15,12 +15,39 @@ public class HealthCheckRegistry {
     private static final Logger LOGGER = LoggerFactory.getLogger(HealthCheckRegistry.class);
 
     private final ConcurrentMap<String, HealthCheck> healthChecks;
+    private final List<HealthCheckRegistryListener> listeners;
 
     /**
      * Creates a new {@link HealthCheckRegistry}.
      */
     public HealthCheckRegistry() {
         this.healthChecks = new ConcurrentHashMap<String, HealthCheck>();
+        this.listeners = new CopyOnWriteArrayList<HealthCheckRegistryListener>();
+    }
+
+    /**
+     * Adds a {@link HealthCheckRegistryListener} to a collection of listeners
+     * that will be notified on health check registration. Listeners will be
+     * notified in the order in which they are added.
+     * The listener will be notified of all existing health checks when it first registers.
+     * 
+     * @param listener listener to add
+     */
+    public void addListener(HealthCheckRegistryListener listener) {
+        listeners.add(listener);
+        for (Map.Entry<String, HealthCheck> entry : healthChecks.entrySet()) {
+            listener.onHealthCheckAdded(entry.getKey(), entry.getValue());
+        }
+    }
+
+    /**
+     * Removes a {@link HealthCheckRegistryListener} from this registry's
+     * collection of listeners.
+     * 
+     * @param listener listener to remove
+     */
+    public void removeListener(HealthCheckRegistryListener listener) {
+        listeners.remove(listener);
     }
 
     /**
@@ -30,7 +57,10 @@ public HealthCheckRegistry() {
      * @param healthCheck the {@link HealthCheck} instance
      */
     public void register(String name, HealthCheck healthCheck) {
-        healthChecks.putIfAbsent(name, healthCheck);
+        HealthCheck existing = healthChecks.putIfAbsent(name, healthCheck);
+        if (existing == null) {
+            onHealthCheckAdded(name, healthCheck);
+        }
     }
 
     /**
@@ -40,6 +70,7 @@ public void register(String name, HealthCheck healthCheck) {
      */
     public void unregister(String name) {
         healthChecks.remove(name);
+        onHealthCheckRemoved(name);
     }
 
     /**
@@ -106,4 +137,17 @@ public Result call() throws Exception {
         }
         return Collections.unmodifiableSortedMap(results);
     }
+
+    private void onHealthCheckAdded(String name, HealthCheck healthCheck) {
+        for (HealthCheckRegistryListener listener : listeners) {
+            listener.onHealthCheckAdded(name, healthCheck);
+        }
+    }
+
+    private void onHealthCheckRemoved(String name) {
+        for (HealthCheckRegistryListener listener : listeners) {
+            listener.onHealthCheckRemoved(name);
+        }
+    }
+
 }

metrics-healthchecks/src/main/java/com/codahale/metrics/health/HealthCheckRegistryListener.java

@@ -0,0 +1,23 @@
+package com.codahale.metrics.health;
+
+import java.util.EventListener;
+
+/**
+ * A listener contract for {@link HealthCheckRegistry} events.
+ */
+public interface HealthCheckRegistryListener extends EventListener {
+
+    /**
+     * Called when a new {@link HealthCheck} is added to the registry.
+     * @param name the name of the health check
+     * @param healthCheck the health check
+     */
+    void onHealthCheckAdded(String name, HealthCheck healthCheck);
+
+    /**
+     * Called when a {@link HealthCheck} is removed from the registry.
+     * @param name the name of the health check
+     */
+    void onHealthCheckRemoved(String name);
+
+}

metrics-healthchecks/src/test/java/com/codahale/metrics/health/HealthCheckRegistryTest.java

@@ -10,11 +10,11 @@
 import java.util.concurrent.TimeUnit;
 
 import static org.assertj.core.api.Assertions.*;
-import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.when;
+import static org.mockito.Mockito.*;
 
 public class HealthCheckRegistryTest {
     private final HealthCheckRegistry registry = new HealthCheckRegistry();
+    private final HealthCheckRegistryListener listener = mock(HealthCheckRegistryListener.class);
 
     private final HealthCheck hc1 = mock(HealthCheck.class);
     private final HealthCheck hc2 = mock(HealthCheck.class);
@@ -24,6 +24,8 @@ public class HealthCheckRegistryTest {
 
     @Before
     public void setUp() throws Exception {
+        registry.addListener(listener);
+
         when(hc1.execute()).thenReturn(r1);
 
         when(hc2.execute()).thenReturn(r2);
@@ -32,6 +34,45 @@ public void setUp() throws Exception {
         registry.register("hc2", hc2);
     }
 
+    @Test
+    public void registeringHealthCheckTriggersNotification() {
+        verify(listener).onHealthCheckAdded("hc1", hc1);
+        verify(listener).onHealthCheckAdded("hc2", hc2);
+    }
+
+    @Test
+    public void removingHealthCheckTriggersNotification() {
+        registry.unregister("hc1");
+        registry.unregister("hc2");
+
+        verify(listener).onHealthCheckRemoved("hc1");
+        verify(listener).onHealthCheckRemoved("hc2");
+    }
+
+    @Test
+    public void addingListenerCatchesExistingHealthChecks() {
+        HealthCheckRegistryListener listener = mock(HealthCheckRegistryListener.class);
+        HealthCheckRegistry registry = new HealthCheckRegistry();
+        registry.register("hc1", hc1);
+        registry.register("hc2", hc2);
+        registry.addListener(listener);
+
+        verify(listener).onHealthCheckAdded("hc1", hc1);
+        verify(listener).onHealthCheckAdded("hc2", hc2);
+    }
+
+    @Test
+    public void removedListenerDoesNotReceiveUpdates() {
+        HealthCheckRegistryListener listener = mock(HealthCheckRegistryListener.class);
+        HealthCheckRegistry registry = new HealthCheckRegistry();
+        registry.addListener(listener);
+        registry.register("hc1", hc1);
+        registry.removeListener(listener);
+        registry.register("hc2", hc2);
+
+        verify(listener).onHealthCheckAdded("hc1", hc1);
+    }
+
     @Test
     public void runsRegisteredHealthChecks() throws Exception {
         final Map<String, HealthCheck.Result> results = registry.runHealthChecks();