Internet Programming with Java Course

1.6 Достъп до отдалечени ресурси чрез URL

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 java.net 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?

If you've been surfing the Web, you have undoubtedly heard the term URL and have used URLs to access HTML pages from the Web.

It's often easiest, although not entirely accurate, to think of a URL as the name of a file on the World Wide Web because most URLs refer to a file on some machine on the network. However, remember that URLs also can point to other resources on the network, such as database queries and command output.

Definition:  URL is an acronym for Uniform Resource Locator and is a reference (an address) to a resource on the Internet.

The following is an example of a URL which addresses the Java Web site hosted by Sun Microsystems:

As in the previous diagram, a URL has two main components:

· Protocol identifier

· Resource name

Note that the protocol identifier and the resource name are separated by a colon and two forward slashes. The protocol identifier indicates the name of the protocol to be used to fetch the resource. The example uses the Hypertext Transfer Protocol (HTTP), which is typically used to serve up hypertext documents. HTTP is just one of many different protocols used to access different types of resources on the net. Other protocols include File Transfer Protocol (FTP), Gopher, File, and News.

The resource name is the complete address to the resource. The format of the resource name depends entirely on the protocol used, but for many protocols, including HTTP, the resource name contains one or more of the components listed in the following table:

Host Name	The name of the machine on which the resource lives.
Filename	The pathname to the file on the machine.
Port Number	The port number to which to connect (typically optional).
Reference	A reference to a named anchor within a resource that usually identifies a specific location within a file (typically optional).

For many protocols, the host name and the filename are required, while the port number and reference are optional. For example, the resource name for an HTTP URL must specify a server on the network (Host Name) and the path to the document on that machine (Filename); it also can specify a port number and a reference. In the URL for the Java Web site java.sun.com is the host name and the trailing slash is shorthand for the file named /index.html.

Creating a URL

The easiest way to create a URL object is from a String that represents the human-readable form of the URL address. This is typically the form that another person will use for a URL. For example, the URL for the Gamelan site, which is a directory of Java resources, takes the following form:

 
http://www.gamelan.com/

In your Java program, you can use a String containing this text to create a URL object:

 
URL gamelan = new URL("http://www.gamelan.com/");

The URL object created above represents an absolute URL. An absolute URL contains all of the information necessary to reach the resource in question. You can also create URL objects from a relative URL address.

Creating a URL Relative to Another

A relative URL contains only enough information to reach the resource relative to (or in the context of) another URL.

Relative URL specifications are often used within HTML files. For example, suppose you write an HTML file called JoesHomePage.html. Within this page, are links to other pages, PicturesOfMe.html and MyKids.html, that are on the same machine and in the same directory as JoesHomePage.html. The links to PicturesOfMe.html and MyKids.html from JoesHomePage.html could be specified just as filenames, like this:

 
<a href="PicturesOfMe.html">Pictures of Me</a>
<a href="MyKids.html">Pictures of My Kids</a>

These URL addresses are relative URLs. That is, the URLs are specified relative to the file in which they are contained – JoesHomePage.html.

In your Java programs, you can create a URL object from a relative URL specification. For example, suppose you know two URLs at the Gamelan site:

 
http://www.gamelan.com/pages/Gamelan.game.html
http://www.gamelan.com/pages/Gamelan.net.html

You can create URL objects for these pages relative to their common base URL:

http://www.gamelan.com/page/

like this:

 
URL gamelan = new URL("http://www.gamelan.com/pages/");
URL gamelanGames = new URL(gamelan, "Gamelan.game.html");
URL gamelanNetwork = new URL(gamelan, "Gamelan.net.html");

This code snippet uses the URL constructor that lets you create a URL object from another URL object (the base) and a relative URL specification. The general form of this constructor is:

 
URL(URL baseURL, String relativeURL)

The first argument is a URL object that specifies the base of the new URL. The second argument is a String that specifies the rest of the resource name relative to the base. If baseURL is null, then this constructor treats relativeURL like an absolute URL specification. Conversely, if relativeURL is an absolute URL specification, then the constructor ignores baseURL.

This constructor is also useful for creating URL objects for named anchors (also called references) within a file. For example, suppose the Gamelan.network.html file has a named anchor called BOTTOM at the bottom of the file. You can use the relative URL constructor to create a URL object for it like this:

 
URL gamelanNetworkBottom = new URL(gamelanNetwork, "#BOTTOM");

Other URL Constructors

The URL class provides two additional constructors for creating a URL object. These constructors are useful when you are working with URLs, such as HTTP URLs, that have host name, filename, port number, and reference components in the resource name portion of the URL. These two constructors are useful when you do not have a String containing the complete URL specification, but you do know various components of the URL.

For example, suppose you design a network browsing panel similar to a file browsing panel that allows users to choose the protocol, host name, port number, and filename. You can construct a URL from the panel's components. The first constructor creates a URL object from a protocol, host name, and filename. The following code snippet creates a URL to the Gamelan.net.html file at the Gamelan site:

 
new URL("http", "www.gamelan.com", "/pages/Gamelan.net.html");

This is equivalent to

 
new URL("http://www.gamelan.com/pages/Gamelan.net.html");

The first argument is the protocol, the second is the host name, and the last is the pathname of the file. Note that the filename contains a forward slash at the beginning. This indicates that the filename is specified from the root of the host.

The final URL constructor adds the port number to the list of arguments used in the previous constructor:

 
URL gamelan = new URL( "http", "www.gamelan.com", 80,
 "pages/Gamelan.network.html" );

This creates a URL object for the following URL:

 
http://www.gamelan.com:80/pages/Gamelan.network.html

If you construct a URL object using one of these constructors, you can get a String containing the complete URL address by using the URL object's toString method or the equivalent toExternalForm method.

MalformedURLException

Each of the four URL constructors throws a MalformedURLException if the arguments to the constructor refer to a null or unknown protocol. Typically, you want to catch and handle this exception by embedding your URL constructor statements in a try/catch pair, like this:

 
try {
 URL myURL = new URL(. . .)
} catch (MalformedURLException e) {
 . . .
 // exception handler code here
 . . .
}

Note: URLs are "write-once" objects. Once you've created a URL object, you cannot change any of its attributes (protocol, host name, filename, or port number).

Parsing a URL

The URL class provides several methods that let you query URL objects. You can get the protocol, host name, port number, and filename from a URL using these accessor methods:

getProtocol – Returns the protocol identifier component of the URL.

getHost – Returns the host name component of the URL.

getPort – Returns the port number component of the URL. The getPort method returns an integer that is the port number. If the port is not set, getPort returns -1.

getFile – Returns the filename component of the URL.

getRef – Returns the reference component of the URL.

Note:  Remember that not all URL addresses contain these components. The URL class provides these methods because HTTP URLs do contain these components and are perhaps the most commonly used URLs. The URL class is somewhat HTTP-centric.

You can use these getXXX methods to get information about the URL regardless of the constructor that you used to create the URL object.

The URL class, along with these accessor methods, frees you from ever having to parse URLs again! Given any string specification of a URL, just create a new URL object and call any of the accessor methods for the information you need. This small example program creates a URL from a string specification and then uses the URL object's accessor methods to parse the URL:

 
import java.net.*;
import java.io.*;
 
public class ParseURL {
 public static void main(String[] args) throws Exception {
 URL aURL = new URL("http://java.sun.com:80/docs/books/"
 + "tutorial/index.html#DOWNLOADING");
 System.out.println("protocol = " + aURL.getProtocol());
 System.out.println("host = " + aURL.getHost());
 System.out.println("filename = " + aURL.getFile());
 System.out.println("port = " + aURL.getPort());
 System.out.println("ref = " + aURL.getRef());
 }
}

Here's the output displayed by the program:

 
protocol = http
host = java.sun.com
filename = /docs/books/tutorial/index.html
port = 80
ref = DOWNLOADING
 

Reading Directly from a URL

After you've successfully created a URL, you can call the URL's openStream() method to get a stream from which you can read the contents of the URL. The openStream() method returns a java.io.InputStream object, so reading from a URL is as easy as reading from an input stream.

The following small Java program uses openStream() to get an input stream on the URL http://www.yahoo.com/. It then opens a BufferedReader on the input stream and reads from the BufferedReader thereby reading from the URL. Everything read is copied to the standard output stream:

import java.net.*;

import java.io.*;

public class URLReader {

public static void main(String[] args) throws Exception {

URL yahoo = new URL("http://www.yahoo.com/");

BufferedReader in = new BufferedReader(

new InputStreamReader(yahoo.openStream()));

String inputLine;

while ((inputLine = in.readLine()) != null)

System.out.println(inputLine);

in.close();

}

}

When you run the program, you should see, scrolling by in your command window, the HTML commands and textual content from the HTML file located at http://www.yahoo.com/. Alternatively, the program might hang or you might see an exception stack trace.

Connecting to a URL

After you've successfully created a URL object, you can call the URL object's openConnection method to connect to it. When you connect to a URL, you are initializing a communication link between your Java program and the URL over the network. For example, you can open a connection to the Yahoo site with the following code:

 
try {
 URL yahoo = new URL("http://www.yahoo.com/");
 URLConnection yahooConnection = yahoo.openConnection();
} catch (MalformedURLException e) { // new URL() failed
 . . .
} catch (IOException e) { // openConnection() failed
 . . .
}

If possible, the openConnection method creates a new URLConnection (if an appropriate one does not already exist), initializes it, connects to the URL, and returns the URLConnection object. If something goes wrong--for example, the Yahoo server is down--then the openConnection method throws an IOException.

Now that you've successfully connected to your URL, you can use the URLConnection object to perform actions such as reading from or writing to the connection. The next section shows you how.

Reading from and Writing to a URLConnection

If you've successfully used openConnection to initiate communications with a URL, then you have a reference to a URLConnection object. The URLConnection class contains many methods that let you communicate with the URL over the network. URLConnection is an HTTP-centric class; that is, many of its methods are useful only when you are working with HTTP URLs. However, most URL protocols allow you to read from and write to the connection. This section describes both functions.

Reading from a URLConnection

The following program performs the same function as the URLReader program shown in Reading Directly from a URL.

However, rather than getting an input stream directly from the URL, this program explicitly opens a connection to a URL and gets an input stream from the connection. Then, like URLReader, this program creates a BufferedReader on the input stream and reads from it. The bold statements highlight the differences between this example and the previous.

 
import java.net.*;
import java.io.*;
 
public class URLConnectionReader {
 public static void main(String[] args) throws Exception {
 URL yahoo = new URL("http://www.yahoo.com/");
 URLConnection yc = yahoo.openConnection();
 BufferedReader in = new BufferedReader(
 new InputStreamReader(
 yc.getInputStream()));
 String inputLine;
 
 while ((inputLine = in.readLine()) != null) 
 System.out.println(inputLine);
 in.close();
 }
<