Implemented a custom DNS resolver, see #859

src/main/java/org/java_websocket/client/DnsResolver.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2010-2019 Nathan Rajlich
+ *
+ *  Permission is hereby granted, free of charge, to any person
+ *  obtaining a copy of this software and associated documentation
+ *  files (the "Software"), to deal in the Software without
+ *  restriction, including without limitation the rights to use,
+ *  copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  copies of the Software, and to permit persons to whom the
+ *  Software is furnished to do so, subject to the following
+ *  conditions:
+ *
+ *  The above copyright notice and this permission notice shall be
+ *  included in all copies or substantial portions of the Software.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ *  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+ *  OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ *  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ *  HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ *  WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ *  FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ *  OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package org.java_websocket.client;
+
+import java.net.InetAddress;
+import java.net.URI;
+import java.net.UnknownHostException;
+
+/**
+ * Users may implement this interface to override the default DNS lookup offered
+ * by the OS.
+ *
+ * @since 1.4.1
+ */
+public interface DnsResolver {
+
+	/**
+	 * Resolves the IP address for the given URI.
+	 *
+	 * This method should never return null. If it's not able to resolve the IP
+	 * address then it should throw an UnknownHostException
+	 *
+	 * @param uri The URI to be resolved
+	 *
+	 * @return The resolved IP address
+	 *
+	 * @throws UnknownHostException if no IP address for the <code>uri</code> could be found.
+	 */
+	InetAddress resolve(URI uri) throws UnknownHostException;
+
+}

src/main/java/org/java_websocket/client/WebSocketClient.java

@@ -28,10 +28,12 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.net.Proxy;
 import java.net.Socket;
 import java.net.URI;
+import java.net.UnknownHostException;
 import java.nio.ByteBuffer;
 import java.util.Collection;
 import java.util.Collections;
@@ -131,6 +133,14 @@ public abstract class WebSocketClient extends AbstractWebSocket implements Runna
 	 */
 	private int connectTimeout = 0;
 
+	/**
+	 * DNS resolver that translates a URI to an InetAddress
+	 *
+	 * @see InetAddress
+	 * @since 1.4.1
+	 */
+	private DnsResolver dnsResolver = null;
+
 	/**
 	 * Constructs a WebSocketClient instance and sets it to the connect to the
 	 * specified URI. The channel does not attampt to connect automatically. The connection
@@ -195,6 +205,12 @@ public WebSocketClient( URI serverUri , Draft protocolDraft , Map<String,String>
 		}
 		this.uri = serverUri;
 		this.draft = protocolDraft;
+		this.dnsResolver = new DnsResolver() {
+			@Override
+			public InetAddress resolve(URI uri) throws UnknownHostException {
+				return InetAddress.getByName(uri.getHost());
+			}
+		};
 		if(httpHeaders != null) {
 			headers = new TreeMap<String, String>(String.CASE_INSENSITIVE_ORDER);
 			headers.putAll(httpHeaders);
@@ -266,6 +282,17 @@ public void clearHeaders() {
 		headers = null;
 	}
 
+	/**
+	 * Sets a custom DNS resolver.
+	 *
+	 * @param dnsResolver The DnsResolver to use.
+	 *
+	 * @since 1.4.1
+	 */
+	public void setDnsResolver(DnsResolver dnsResolver) {
+		this.dnsResolver = dnsResolver;
+	}
+
 	/**
 	 * Reinitiates the websocket connection. This method does not block.
 	 * @since 1.3.8
@@ -431,8 +458,9 @@ public void run() {
 			socket.setTcpNoDelay( isTcpNoDelay() );
 			socket.setReuseAddress( isReuseAddr() );
 
-			if( !socket.isBound() ) {
-				socket.connect( new InetSocketAddress( uri.getHost(), getPort() ), connectTimeout );
+			if (!socket.isBound()) {
+				InetSocketAddress addr = new InetSocketAddress(dnsResolver.resolve(uri), this.getPort());
+				socket.connect(addr, connectTimeout);
 			}
 
 			// if the socket is set by others we don't apply any TLS wrapper
@@ -509,9 +537,9 @@ private void sendHandshake() throws InvalidHandshakeException {
 		if( part2 != null )
 			path += '?' + part2;
 		int port = getPort();
-		String host = uri.getHost() + ( 
+		String host = uri.getHost() + (
 			(port != WebSocketImpl.DEFAULT_PORT && port != WebSocketImpl.DEFAULT_WSS_PORT)
-			? ":" + port 
+			? ":" + port
 			: "" );
 
 		HandshakeImpl1Client handshake = new HandshakeImpl1Client();
@@ -844,7 +872,7 @@ public InetSocketAddress getLocalSocketAddress() {
 	public InetSocketAddress getRemoteSocketAddress() {
 		return engine.getRemoteSocketAddress();
 	}
-	
+
 	@Override
 	public String getResourceDescriptor() {
 		return uri.getPath();