Implement BookStoreExternalUrlService for URL checks

Signed-off-by: Pekka Helenius <fincer89@hotmail.com>

bookstore/src/main/java/com/fjordtek/bookstore/service/session/BookStoreExternalUrlService.java

@@ -0,0 +1,19 @@
+//Pekka Helenius <fincer89@hotmail.com>, Fjordtek 2020
+
+package com.fjordtek.bookstore.service.session;
+
+import java.io.IOException;
+
+/**
+*
+* This interface defines methods of BookStoreExternalUrlServiceImpl class.
+*
+* @author Pekka Helenius
+*
+*/
+
+public interface BookStoreExternalUrlService {
+
+	boolean getUrl(String url) throws IOException;
+
+}

bookstore/src/main/java/com/fjordtek/bookstore/service/session/BookStoreExternalUrlServiceImpl.java

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+//Pekka Helenius <fincer89@hotmail.com>, Fjordtek 2020
+
+package com.fjordtek.bookstore.service.session;
+
+import java.io.IOException;
+import java.net.HttpURLConnection;
+import java.net.URL;
+import java.net.URLConnection;
+import java.net.UnknownHostException;
+
+import org.springframework.stereotype.Component;
+
+/**
+ *
+ * This class implements BookStoreExternalUrlService interface,
+ * considering a situation where DNS name resolution may fail and
+ * we are unable to resolve external host IPs by their domain names.
+ * <p>
+ * Main motivation is to prevent server-side Thymeleaf process loops or any other
+ * unintended behavior caused by such events.
+ * <p>
+ * Reasons for name resolution failures vary: they can occur due to a faulty
+ * DNS server operational states or invalid client's DNS cache, broken websites,
+ * changed domain IPs, etc.
+ *
+ * @author Pekka Helenius
+ *
+ */
+
+@Component("ExternalUrl")
+public class BookStoreExternalUrlServiceImpl implements BookStoreExternalUrlService {
+
+	@Override
+	public boolean getUrl(String urlString) throws IOException {
+
+		try {
+			URL url = new URL(urlString);
+
+			URLConnection connection = url.openConnection();
+			HttpURLConnection httpConnection = (HttpURLConnection) connection;
+			httpConnection.setRequestMethod("HEAD");
+			try {
+				int responseCode = httpConnection.getResponseCode();
+
+				if (responseCode == HttpURLConnection.HTTP_OK) {
+					httpConnection.disconnect();
+					return true;
+				} else if (responseCode == HttpURLConnection.HTTP_NOT_FOUND) {
+					return false;
+				}
+			} catch (UnknownHostException eu) {
+				//eu.printStackTrace();
+				//throw eu;
+				return false;
+			}
+
+			httpConnection.disconnect();
+			return false;
+		} catch (IOException ei) {
+			//ei.printStackTrace();
+			//throw ei;
+			return false;
+		}
+	}
+
+}