Allow tracking uptime of the internal web server similar to tracking API availability

DynmapCore/src/main/java/org/dynmap/DynmapCore.java

@@ -679,6 +679,7 @@ public class DynmapCore implements DynmapCommonAPI {
                 Log.warning("Using external server, but " + JsonFileClientUpdateComponent.class.toString() + " is DISABLED!");
                 webserverCompConfigWarn = true;
             }
+            DynmapWebserverStateListener.stateWebServerDisabled();
         }
         if (webserverCompConfigWarn) {
             Log.warning("If the website is missing files or not loading/updating, this might be why.");
@@ -1075,6 +1076,7 @@ public class DynmapCore implements DynmapCommonAPI {
         try {
             if(webServer != null) {
                 webServer.start();
+                DynmapWebserverStateListener.stateWebserverStarted();
                 Log.info("Web server started on address " + webhostname + ":" + webport);
             }
         } catch (Exception e) {
@@ -1100,6 +1102,7 @@ public class DynmapCore implements DynmapCommonAPI {
                 Log.severe("Failed to stop WebServer!", e);
             }
             webServer = null;
+            DynmapWebserverStateListener.stateWebserverStopped();
         }
 
         if (componentManager != null) {

DynmapCoreAPI/src/main/java/org/dynmap/DynmapWebserverStateListener.java

@@ -0,0 +1,87 @@
+package org.dynmap;
+
+import java.util.concurrent.CopyOnWriteArrayList;
+
+/**
+ * Listener class for the Dynmap integrated web server lifecycle, will only work if the internal webserver is actually being used.
+ */
+public abstract class DynmapWebserverStateListener
+{
+    /**
+     * Status flag indicating if the web server is online or offline.
+     */
+    private static boolean webserverStarted = false;
+
+    private static final CopyOnWriteArrayList<DynmapWebserverStateListener> listeners = new CopyOnWriteArrayList<>();
+
+    /**
+     * Called when API becomes enabled, or during call to register listener if API is already enabled
+     */
+    public abstract void webserverStarted();
+
+    /**
+     * Called when API becomes enabled, or during call to register listener if API is already enabled
+     */
+    public abstract void webserverStopped();
+
+    /**
+     * This method is fired to indicate the listener that the internal web server is not enabled, thus indicating that {@link DynmapWebserverStateListener#webserverStarted()} and {@link DynmapWebserverStateListener#webserverStopped()} will not fire.
+     */
+    public void webserverDisabled() {}
+
+    /**
+     * Register listener instance
+     *
+     * @param listener - listener to register
+     */
+    public static void register(DynmapWebserverStateListener listener)
+    {
+        listeners.add(listener);
+        if (webserverStarted)
+        {
+            listener.webserverStarted();
+        }
+    }
+
+    /**
+     * Unregister listener instance
+     *
+     * @param listener - listener to unregister
+     */
+    public static void unregister(DynmapWebserverStateListener listener) {
+        listeners.remove(listener);
+    }
+
+    // Internal call - MODS/PLUGINS MUST NOT USE
+    public static void stateWebserverStarted()
+    {
+        if (webserverStarted)
+        {
+            stateWebserverStopped();
+        }
+        webserverStarted = true;
+        for (DynmapWebserverStateListener l : listeners)
+        {
+            l.webserverStarted();
+        }
+    }
+
+    // Internal call - MODS/PLUGINS MUST NOT USE
+    public static void stateWebserverStopped()
+    {
+        for (DynmapWebserverStateListener l : listeners)
+        {
+            l.webserverStopped();
+        }
+        webserverStarted = false;
+    }
+
+    // Internal call - MODS/PLUGINS MUST NOT USE
+    public static void stateWebServerDisabled()
+    {
+        for (DynmapWebserverStateListener l : listeners)
+        {
+            l.webserverDisabled();
+        }
+    }
+}