Java - URL Processing 評論   編輯

URL stands for Uniform Resource Locator and represents a resource on the World Wide Web, such as a Web page or FTP directory.

This section shows you how to write Java programs that communicate with a URL. A URL can be broken down into parts, as follows:

protocol://host:port/path?query#ref

Examples of protocols include HTTP, HTTPS, FTP, and File. The path is also referred to as the filename, and the host is also called the authority.

The following is a URL to a Web page whose protocol is HTTP:

http://www.amrood.com/index.html?language=en#j2se

Notice that this URL does not specify a port, in which case the default port for the protocol is used. With HTTP, the default port is 80.

URL Class Methods:

The java.net.URL class represents a URL and has complete set of methods to manipulate URL in Java.

The URL class has several constructors for creating URLs, including the following:

The URL class contains many methods for accessing the various parts of the URL being represented. Some of the methods in the URL class include the following:

Example:

The following URLDemo program demonstrates the various parts of a URL. A URL is entered on the command line, and the URLDemo program outputs each part of the given URL.

// File Name : URLDemo.java

import java.net.*;
import java.io.*;

public class URLDemo
{
   public static void main(String [] args)
   {
      try
      {
         URL url = new URL(args[0]);
         System.out.println("URL is " + url.toString());
         System.out.println("protocol is "
                                    + url.getProtocol());
         System.out.println("authority is "
                                    + url.getAuthority());
         System.out.println("file name is " + url.getFile());
         System.out.println("host is " + url.getHost());
         System.out.println("path is " + url.getPath());
         System.out.println("port is " + url.getPort());
         System.out.println("default port is "
                                   + url.getDefaultPort());
         System.out.println("query is " + url.getQuery());
         System.out.println("ref is " + url.getRef());
      }catch(IOException e)
      {
         e.printStackTrace();
      }
   }
}

A sample run of the thid program would produce following result:

$ java URLDemo http://www.amrood.com/index.html?language=en#j2se
URL is http://www.amrood.com/index.html?language=en#j2se
protocol is http
authority is www.amrood.com
file name is /index.html?language=en
host is www.amrood.com
path is /index.html
port is -1
default port is 80
query is language=en
ref is j2se

URLConnections Class Methods:

The openConnection() method returns a java.net.URLConnection, an abstract class whose subclasses represent the various types of URL connections.

For example:

• If you connect to a URL whose protocol is HTTP, the openConnection() method returns an HttpURLConnection object.

• If you connect to a URL that represents a JAR file, the openConnection() method returns a JarURLConnection object.

• etc...

The URLConnection class has many methods for setting or determining information about the connection, including the following:

Example:

The following URLConnectionDemo program connects to a URL entered from the command line.

If the URL represents an HTTP resource, the connection is cast to HttpURLConnection, and the data in the resource is read one line at a time.

// File Name : URLConnDemo.java

import java.net.*;
import java.io.*;
public class URLConnDemo
{
   public static void main(String [] args)
   {
      try
      {
         URL url = new URL(args[0]);
         URLConnection urlConnection = url.openConnection();
         HttpURLConnection connection = null;
         if(urlConnection instanceof HttpURLConnection)
         {
            connection = (HttpURLConnection) urlConnection;
         }
         else
         {
            System.out.println("Please enter an HTTP URL.");
            return;
         }
         BufferedReader in = new BufferedReader(
         new InputStreamReader(connection.getInputStream()));
         String urlString = "";
         String current;
         while((current = in.readLine()) != null)
         {
            urlString += current;
         }
         System.out.println(urlString);
      }catch(IOException e)
      {
         e.printStackTrace();
      }
   }
}

A sample run of the thid program would produce following result:

$ java URLConnDemo http://www.amrood.com

.....a complete HTML content of home page of amrood.com.....

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