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9.3 Working with URLs

A URL points to an object on the Internet. It's a collection of information that identifies an item, tells you where to find it, and specifies a method for communicating with it or retrieving it from its source. A URL refers to any kind of information source. It might point to static data, such as a file on a local filesystem, a Web server, or an FTP archive; or it can point to a more dynamic object such as a news article on a news spool or a record in a WAIS database. URLs can even refer to less tangible resources such as Telnet sessions and mailing addresses.

A URL is usually presented as a string of text, like an address.[3] Since there are many different ways to locate an item on the Net, and different mediums and transports require different kinds of information, there are different formats for different kinds of URLs. The most common form specifies three things: a network host or server, the name of the item and its location on that host, and a protocol by which the host should communicate:

[3] The term URL was coined by the Uniform Resource Identifier (URI) working group of the IETF to distinguish URLs from the more general notion of Uniform Resource Names or URNs. URLs are really just static addresses, whereas URNs would be more persistent and abstract identifiers used to resolve the location of an object anywhere on the Net. URLs are defined in RFC 1738 and RFC 1808.

protocol://hostname/location/item

protocol is an identifier such as "http," "ftp," or "gopher"; hostname is an Internet hostname; and the location and item components form a path that identifies the object on that host. Variants of this form allow extra information to be packed into the URL, specifying things like port numbers for the communications protocol and fragment identifiers that reference parts inside the object.

We sometimes speak of a URL that is relative to a base URL. In that case we are using the base URL as a starting point and supplying additional information. For example, the base URL might point to a directory on a Web server; a relative URL might name a particular file in that directory.

The URL class

A URL is represented by an instance of the java.net.URL class. A URL object manages all information in a URL string and provides methods for retrieving the object it identifies. We can construct a URL object from a URL specification string or from its component parts:

try { 
    URL aDoc = new URL( "http://foo.bar.com/documents/homepage.html" ); 
    URL sameDoc = 
        new URL("http","foo.bar.com","documents/homepage.html"); 
}  
catch ( MalformedURLException e ) { } 

The two URL objects above point to the same network resource, the homepage.html document on the server foo.bar.com. Whether or not the resource actually exists and is available isn't known until we try to access it. At this point, the URL object just contains data about the object's location and how to access it. No connection to the server has been made. We can examine the URL's components with the getProtocol(), getHost(), and getFile() methods. We can also compare it to another URL with the sameFile() method. sameFile() determines if two URLs point to the same resource. It can be fooled, but sameFile does more than compare the URLs for equality; it takes into account the possibility that one server may have several names, and other factors.

When a URL is created, its specification is parsed to identify the protocol component. If the protocol doesn't make sense, or if Java can't find a protocol handler for it, the URL constructor throws a MalformedURLException. A protocol handler is a Java class that implements the communications protocol for accessing the URL resource. For example, given an "http" URL, Java prepares to use the HTTP protocol handler to retrieve documents from the specified server.

Stream Data

The most general way to get data back from URL is to ask for an InputStream from the URL by calling openStream(). If you're writing an applet that will be running under Netscape, this is about your only choice. In fact, it's a good choice if you want to receive continuous updates from a dynamic information source. The drawback is that you have to parse the contents of an object yourself. Not all types of URLs support the openStream() method; you'll get an UnknownServiceException if yours doesn't.

The following code reads a single line from an HTML file:

try { 
    URL url = new URL("http://server/index.html"); 
    DataInputStream dis = new DataInputStream( url.openStream() ); 
    String line = dis.readLine(); 

We ask for an InputStream with openStream(), and wrap it in a DataInputStream to read a line of text. Here, because we are specifying the "http" protocol in the URL, we still require the services of an HTTP protocol handler. As we'll discuss more in a bit, that brings up some questions about what handlers we have available to us and where. This example partially works around those issues because no content handler is involved; we read the data and interpret it as a content handler would. However, there are even more limitations on what applets can do right now. For the time being, if you construct URLs relative to the applet's codeBase(), you should be able to use them in applets as in the above example. This should guarantee that the needed protocol is available and accessible to the applet. Again, we'll discuss the more general issues a bit later.

Getting the Content as an Object

openStream() operates at a lower level than the more general content-handling mechanism implemented by the URL class. We showed it first because, until some things are settled, you'll be limited as to when you can use URLs in their more powerful role. When a proper content handler is available to Java (currently, only if you supply one with your standalone application), you'll be able to retrieve the object the URL addresses as a complete object, by calling the URL's getContent() method. getContent() initiates a connection to the host, fetches the data for you, determines the data's MIME type, and invokes a content handler to turn the data into a Java object.

For example: given the URL http://foo.bar.com/index.html, a call to getContent() uses the HTTP protocol handler to receive the data and the HTML content handler to turn the data into some kind of object. A URL that points to a plain-text file would use a text-content handler that might return a String object. A GIF file might be turned into an Image object for display, using a GIF content handler. If we accessed the GIF file using an "ftp" URL, Java would use the same content handler, but would use the FTP protocol handler to receive the data.

getContent() returns the output of the content handler. Now we're faced with a problem: exactly what did we get? Since the content handler can return almost anything, the return type of getContent() is Object. Before doing anything meaningful with this Object, we must cast it into some other data type that we can work with. For example, if we expect a String, we'll cast the result of getContent() to a String:

String content; 
 
try  
    content = (String)myURL.getContent(); 
catch ( Exception e ) { } 

Of course, we are presuming we will in fact get a String object back from this URL. If we're wrong, we'll get a ClassCastException. Since it's common for servers to be confused (or even lie) about the MIME types of the objects they serve, it's wise to catch that exception (it's a subclass of RuntimeException, so catching it is optional) or to check the type of the returned object with the instanceof operator:

if ( content instanceof String ) { 
    String s = (String)content; 
    ... 

Various kinds of errors can occur when trying to retrieve the data. For example, getContent() can throw an IOException if there is a communications error; IOException is not a type of RuntimeException, so we must catch it explicitly, or declare the method that calls getContent() can throw it. Other kinds of errors can happen at the application level: some knowledge of how the handlers deal with errors is necessary.

For example, consider a URL that refers to a nonexistent file on an HTTP server. When requested, the server probably returns a valid HTML document that consists of the familiar "404 Not Found" message. An appropriate HTML content handler is invoked to interpret this and return it as it would any other HTML object. At this point, there are several alternatives, depending entirely on the content handler's implementation. It might return a String containing the error message; it could also conceivably return some other kind of object or throw a specialized subclass of IOException. To find out that an error occurred, the application may have to look directly at the object returned from getContent(). After all, what is an error to the application may not be an error as far as the protocol or content handlers are concerned. "404 Not Found" isn't an error at this level; it's a perfectly valid document.

Another type of error occurs if a content handler that understands the data's MIME type isn't available. In this case, getContent() invokes a minimal content handler used for data with an unknown type and returns the data as a raw InputStream. A sophisticated application might specialize this behavior to try to decide what to do with the data on its own.

The openStream() and getContent() methods both implicitly create a connection to the remote URL object. For some applications, it may be necessary to use the openConnection() method of the URL to interact directly with the protocol handler. openConnection() returns a URLConnection object, which represents a single, active connection to the URL resource. We'll examine URLConnections further when we start writing protocol handlers.


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