java.net.MalformedURLException: no protocol on URL based on a string modified with URLEncoder

You need to encode your parameter's values before concatenating them to URL.
Backslash \ is special character which have to be escaped as %5C

Escaping example:

String paramValue = "param\\with\\backslash";
String yourURLStr = "http://host.com?param=" + java.net.URLEncoder.encode(paramValue, "UTF-8");
java.net.URL url = new java.net.URL(yourURLStr);

The result is http://host.com?param=param%5Cwith%5Cbackslash which is properly formatted url string.

I have the same problem, i read the url with an properties file:

String configFile = System.getenv("system.Environment");
        if (configFile == null || "".equalsIgnoreCase(configFile.trim())) {
            configFile = "dev.properties";
        }
        // Load properties 
        Properties properties = new Properties();
        properties.load(getClass().getResourceAsStream("/" + configFile));
       //read url from file
        apiUrl = properties.getProperty("url").trim();
            URL url = new URL(apiUrl);
            //throw exception here
    URLConnection conn = url.openConnection();

dev.properties

url = "https://myDevServer.com/dev/api/gate"

it should be

dev.properties

url = https://myDevServer.com/dev/api/gate

without "" and my problem is solved.

According to oracle documentation

• Thrown to indicate that a malformed URL has occurred. Either no legal protocol could be found in a specification string or the string could not be parsed.

So it means it is not parsed inside the string.