What Is a Protocol Handler?

The way the URL, URLStreamHandler, URLConnection, and URLStreamHandlerFactory classes work together can be confusing. Everything starts with a URL, which represents a pointer to a particular Internet resource. Each URL specifies the protocol used to access the resource; typical values for the protocol include mailto, http, and ftp. When you construct a URL object from the URL’s string representation, the constructor strips the protocol field and passes it to the URLStreamHandlerFactory . The factory’s job is to take the protocol, locate the right subclass of URLStreamHandler for the protocol, and create a new instance of that stream handler, which is stored as a field within the URL object. Each application has at most one URLStreamHandlerFactory; once the factory has been installed, attempting to install another will throw an Error.