Working with URLs
Trail: Custom Networking

Lesson: Working with URLs

URL is the acronym for Uniform Resource Locator. It is a reference (an address) to a resource on the Internet. You provide URLs to your favorite Web browser so that it can locate files on the Internet in the same way that you provide addresses on letters so that the post office can locate your correspondents.

Java programs that interact with the Internet also may use URLs to find the resources on the Internet they wish to access. Java programs can use a class called URL in the package to represent a URL address.

Terminology Note: 

The term URL can be ambiguous. It can refer to an Internet address or a URL object in a Java program. Where the meaning of URL needs to be specific, this text uses "URL address" to mean an Internet address and "URL object" to refer to an instance of the URL class in a program.

What Is a URL?

A URL takes the form of a string that describes how to find a resource on the Internet. URLs have two main components: the protocol needed to access the resource and the location of the resource.

Creating a URL

Within your Java programs, you can create a URL object that represents a URL address. The URL object always refers to an absolute URL but can be constructed from an absolute URL, a relative URL, or from URL components.

Parsing a URL

Gone are the days of parsing a URL to find out the host name, filename, and other information. With a valid URL object you can call any of its accessor methods to get all of that information from the URL without doing any string parsing!

Reading Directly from a URL

This section shows how your Java programs can read from a URL using the openStream() method.

Connecting to a URL

If you want to do more than just read from a URL, you can connect to it by calling openConnection() on the URL. The openConnection() method returns a URLConnection object that you can use for more general communications with the URL, such as reading from it, writing to it, or querying it for content and other information.

Reading from and Writing to a URLConnection

Some URLs, such as many that are connected to cgi-bin scripts, allow you to (or even require you to) write information to the URL. For example, a search script may require detailed query data to be written to the URL before the search can be performed. This section shows you how to write to a URL and how to get results back.

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