docs: add http engine in-code docs

Also add algorithm how to add new engine in the `CONTRIBUTING.md` guide

CONTRIBUTING.md

@@ -35,6 +35,34 @@ The Android-specific library AAR is built with:
 
 (The `ANDROID_HOME` environment variable must be set appropriately.)
 
+## Adding a New Network Engine Implementation
+
+Currently, `ably-java` supports two different engines for network operations (HTTP calls and WebSocket connections):
+
+- **Default Engine**: Utilizes the built-in `HttpUrlConnection` for HTTP calls and the TooTallNate/Java-WebSocket library for WebSocket connections.
+- **OkHttp Engine**: Utilizes the OkHttp library for both HTTP and WebSocket connections.
+
+These engines are designed to be swappable. By default, the library comes with the default engine, but you can easily replace it with the OkHttp engine:
+
+```kotlin
+implementation("io.ably:ably-java:$ABLY_VERSION") {
+    exclude(group = "io.ably", module = "network-client-default")
+}
+runtimeOnly("io.ably:network-client-okhttp:$ABLY_VERSION")
+```
+
+### How to Add a New Network Engine
+
+To add a new network engine, follow these steps:
+
+1. **Implement the interfaces**:
+  - Implement the `HttpEngineFactory` and `WebSocketEngineFactory` interfaces for your custom engine.
+
+2. **Register the engine**:
+  - Modify the `getFirstAvailable()` methods in these interfaces to include your new implementation.
+
+Once done, your custom network engine will be available for use within `ably-java`.
+
 ### Code Standard
 
 #### Checkstyle

network-client-core/src/main/java/io/ably/lib/network/HttpCall.java

@@ -1,6 +1,18 @@
 package io.ably.lib.network;
 
+/**
+ * Cancelable Http request call
+ * <p/>
+ * Implementation should be thread-safe
+ */
 public interface HttpCall {
+    /**
+     * Synchronously execute Http request and return response from te server
+     */
     HttpResponse execute();
+
+    /**
+     * Cancel pending Http request
+     */
     void cancel();
 }

network-client-core/src/main/java/io/ably/lib/network/HttpEngine.java

@@ -1,6 +1,18 @@
 package io.ably.lib.network;
 
+/**
+ * An HTTP engine instance that can make cancelable HTTP requests.
+ * It contains some engine-wide configurations, such as proxy settings,
+ * if it operates under a corporate proxy.
+ */
 public interface HttpEngine {
+    /**
+     * @return cancelable Http request call
+     */
     HttpCall call(HttpRequest request);
+
+    /**
+     * @return <code>true</code> if it uses proxy, <code>false</code>  otherwise
+     */
     boolean isUsingProxy();
 }

network-client-core/src/main/java/io/ably/lib/network/HttpEngineFactory.java

@@ -2,11 +2,14 @@
 
 import java.lang.reflect.InvocationTargetException;
 
+/**
+ * The <code>HttpEngineFactory</code> is a utility class that produces a common HTTP Engine API
+ * for different implementations. Currently, it supports:
+ * - HttpURLConnection ({@link  EngineType#DEFAULT})
+ * - OkHttp ({@link  EngineType#OKHTTP})
+ */
 public interface HttpEngineFactory {
 
-    HttpEngine create(HttpEngineConfig config);
-    EngineType getEngineType();
-
     static HttpEngineFactory getFirstAvailable() {
         HttpEngineFactory okHttpFactory = tryGetOkHttpFactory();
         if (okHttpFactory != null) return okHttpFactory;
@@ -19,7 +22,8 @@ static HttpEngineFactory tryGetOkHttpFactory() {
         try {
             Class<?> okHttpFactoryClass = Class.forName("io.ably.lib.network.OkHttpEngineFactory");
             return (HttpEngineFactory) okHttpFactoryClass.getDeclaredConstructor().newInstance();
-        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | NoSuchMethodException | InvocationTargetException e) {
+        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | NoSuchMethodException |
+                 InvocationTargetException e) {
             return null;
         }
     }
@@ -28,8 +32,13 @@ static HttpEngineFactory tryGetDefaultFactory() {
         try {
             Class<?> defaultFactoryClass = Class.forName("io.ably.lib.network.DefaultHttpEngineFactory");
             return (HttpEngineFactory) defaultFactoryClass.getDeclaredConstructor().newInstance();
-        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | NoSuchMethodException | InvocationTargetException e) {
+        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | NoSuchMethodException |
+                 InvocationTargetException e) {
             return null;
         }
     }
+
+    HttpEngine create(HttpEngineConfig config);
+
+    EngineType getEngineType();
 }

network-client-core/src/main/java/io/ably/lib/network/WebSocketClient.java

@@ -1,7 +1,14 @@
 package io.ably.lib.network;
 
+/**
+ * WebSocketClient instance bind to the specified URI.
+ * The connection will be established once you call <var>connect</var>.
+ */
 public interface WebSocketClient {
 
+    /**
+     * Establish connection to the Websocket server
+     */
     void connect();
 
     /**
@@ -12,7 +19,7 @@ public interface WebSocketClient {
     /**
      * Sends the closing handshake. May be sent in response to any other handshake.
      *
-     * @param code    the closing code
+     * @param code   the closing code
      * @param reason the closing message
      */
     void close(int code, String reason);
@@ -21,13 +28,23 @@ public interface WebSocketClient {
      * This will close the connection immediately without a proper close handshake. The code and the
      * message therefore won't be transferred over the wire also they will be forwarded to `onClose`.
      *
-     * @param code    the closing code
+     * @param code   the closing code
      * @param reason the closing message
      **/
     void cancel(int code, String reason);
 
+    /**
+     * Sends binary <var>message</var> to the connected webSocket server.
+     *
+     * @param message The byte-Array of data to send to the WebSocket server.
+     */
     void send(byte[] message);
 
+    /**
+     * Sends <var>message</var> to the connected websocket server.
+     *
+     * @param message The string which will be transmitted.
+     */
     void send(String message);
 
 }

network-client-core/src/main/java/io/ably/lib/network/WebSocketEngine.java

@@ -1,5 +1,8 @@
 package io.ably.lib.network;
 
+/**
+ * Create WebSocket client bind to the specific URL
+ */
 public interface WebSocketEngine {
     WebSocketClient create(String url, WebSocketListener listener);
 }

network-client-core/src/main/java/io/ably/lib/network/WebSocketEngineFactory.java

@@ -2,10 +2,13 @@
 
 import java.lang.reflect.InvocationTargetException;
 
+/**
+ * The <code>WebSocketEngineFactory</code> is a utility class that produces a common WebSocket Engine API
+ * for different implementations. Currently, it supports:
+ * - TooTallNate/Java-WebSocket ({@link  EngineType#DEFAULT})
+ * - OkHttp ({@link  EngineType#OKHTTP})
+ */
 public interface WebSocketEngineFactory {
-    WebSocketEngine create(WebSocketEngineConfig config);
-    EngineType getEngineType();
-
     static WebSocketEngineFactory getFirstAvailable() {
         WebSocketEngineFactory okWebSocketFactory = tryGetOkWebSocketFactory();
         if (okWebSocketFactory != null) return okWebSocketFactory;
@@ -28,8 +31,13 @@ static WebSocketEngineFactory tryGetDefaultFactory() {
         try {
             Class<?> defaultFactoryClass = Class.forName("io.ably.lib.network.DefaultWebSocketEngineFactory");
             return (WebSocketEngineFactory) defaultFactoryClass.getDeclaredConstructor().newInstance();
-        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | NoSuchMethodException | InvocationTargetException e) {
+        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | NoSuchMethodException |
+                 InvocationTargetException e) {
             return null;
         }
     }
+
+    WebSocketEngine create(WebSocketEngineConfig config);
+
+    EngineType getEngineType();
 }

network-client-core/src/main/java/io/ably/lib/network/WebSocketListener.java

@@ -2,12 +2,56 @@
 
 import java.nio.ByteBuffer;
 
+/**
+ * WebSocket Listener
+ */
 public interface WebSocketListener {
+    /**
+     * Called after an opening handshake has been performed and the given websocket is ready to be
+     * written on.
+     */
     void onOpen();
+
+    /**
+     * Callback for binary messages received from the remote host
+     *
+     * @param blob The binary message that was received.
+     * @see #onMessage(String)
+     **/
     void onMessage(ByteBuffer blob);
+
+    /**
+     * Callback for string messages received from the remote host
+     *
+     * @param string The UTF-8 decoded message that was received.
+     * @see #onMessage(ByteBuffer)
+     **/
     void onMessage(String string);
+
+    /**
+     * Callback for receiving ping frame if it supported by websocket engine
+     */
     void onWebsocketPing();
+
+    /**
+     * Called after the websocket connection has been closed.
+     *
+     * @param reason Additional information string
+     **/
     void onClose(int code, String reason);
+
+    /**
+     * Called when errors occurs. If an error causes the websocket connection to fail {@link
+     * WebSocketListener#onClose(int, String)} will be called additionally.<br> This method will be called
+     * primarily because of IO or protocol errors.<br> If the given exception is an RuntimeException
+     * that probably means that you encountered a bug.<br>
+     *
+     * @param throwable The exception causing this error
+     **/
     void onError(Throwable throwable);
+
+    /**
+     * We invoke this callback when runtime is not able to use secure https algorithms (TLS 1.2 +)
+     */
     void onOldJavaVersionDetected(Throwable throwable);
 }