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Java - URL Processing

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 −

https://www.amrood.com/index.htm?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.

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

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

public URL(String protocol, String host, int port, String file) throws MalformedURLException

Creates a URL by putting together the given parts.

public URL(String protocol, String host, String file) throws MalformedURLException

Identical to the previous constructor, except that the default port for the given protocol is used.

public URL(String url) throws MalformedURLException

Creates a URL from the given String.

public URL(URL context, String url) throws MalformedURLException

Creates a URL by parsing together the URL and String arguments.

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 −

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.io.IOException;
import java.net.URL;

public class URLDemo {

   public static void main(String [] args) {
      try {
         URL url = new URL("https://www.tutorialspoint.com/index.htm?language=en#j2se");
         
         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 this program will produce the following result −

URL is https://www.tutorialspoint.com/index.htm?language=en#j2se
protocol is https
authority is www.tutorialspoint.com
file name is /index.htm?language=en
host is www.tutorialspoint.com
path is /index.htm
port is -1
default port is 443
query is language=en
ref is j2se

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 −

void addRequestProperty(String key, String value)

Adds a general request property specified by a key-value pair.

boolean getAllowUserInteraction()

Returns the value of the allowUserInteraction field for this object.

int getConnectTimeout()

Returns setting for connect timeout.

Object getContent()

Retrieves the contents of this URL connection.

Object getContent(Class[] classes)

Retrieves the contents of this URL connection.

String getContentEncoding()

Returns the value of the content-encoding header field.

int getContentLength()

Returns the value of the content-length header field.

long getContentLengthLong()

Returns the value of the content-length header field as long.

String getContentType()

Returns the value of the content-type header field.

long getDate()

Returns the value of the date header field.

static boolean getDefaultAllowUserInteraction()

Returns the default value of the allowUserInteraction field.

boolean getDefaultUseCaches()

Returns the default value of a URLConnection's useCaches flag.

static boolean getDefaultUseCaches(String protocol)

Returns the default value of the useCaches flag for the given protocol.

boolean getDoInput()

Returns the value of this URLConnection's doInput flag.

boolean getDoOutput()

Returns the value of this URLConnection's doOutput flag.

long getExpiration()

Returns the value of the expires header field.

static FileNameMap getFileNameMap()

Loads filename map (a mimetable) from a data file.

String getHeaderField(int n)

Returns the value for the nth header field.

String getHeaderField(String name)

Returns the value of the named header field.

long getHeaderFieldDate(String name, long Default)

Returns the value of the named field parsed as date.

int getHeaderFieldInt(String name, int Default)

Returns the value of the named field parsed as a number.

String getHeaderFieldKey(int n)

Returns the key for the nth header field.

long getHeaderFieldLong(String name, long Default)

Returns the value of the named field parsed as a number.

Map<String,List<String>> getHeaderFields()

Returns an unmodifiable Map of the header fields.

long getIfModifiedSince()

Returns the value of this object's ifModifiedSince field.

InputStream getInputStream()

Returns an input stream that reads from this open connection.

int getLastModified()

Returns the value of the last-modified header field.

OutputStream getOutputStream()

Returns an output stream that writes to this connection.

Permission getPermission()

Returns a permission object representing the permission necessary to make the connection represented by this object.

int getReadTimeout()

Returns setting for read timeout. 0 return implies that the option is disabled (i.e., timeout of infinity).

Map<String,List<String>> getRequestProperties()

Returns an unmodifiable Map of general request properties for this connection.

String getRequestProperty(String key)

Returns the value of the named general request property for this connection.

URL getURL()

Returns the value of this URLConnection's URL field.

boolean getUseCaches()

Returns the value of this URLConnection's useCaches field.

static String guessContentTypeFromName(String fname)

Tries to determine the content type of an object, based on the specified "file" component of a URL.

static String guessContentTypeFromStream(InputStream is)

Tries to determine the type of an input stream based on the characters at the beginning of the input stream.

void setAllowUserInteraction(boolean allowuserinteraction)

Set the value of the allowUserInteraction field of this URLConnection.

void setConnectTimeout(int timeout)

Sets a specified timeout value, in milliseconds, to be used when opening a communications link to the resource referenced by this URLConnection.

static void setContentHandlerFactory(ContentHandlerFactory fac)

Sets the ContentHandlerFactory of an application.

static void setDefaultAllowUserInteraction(boolean defaultallowuserinteraction)

Sets the default value of the allowUserInteraction field for all future URLConnection objects to the specified value.

void setDefaultUseCaches(boolean defaultusecaches)

Sets the default value of the useCaches field to the specified value.

static void setDefaultUseCaches(String protocol, boolean defaultVal)

Sets the default value of the useCaches field for the named protocol to the given value.

void setDoInput(boolean doinput)

Sets the value of the doInput field for this URLConnection to the specified value.

void setDoOutput(boolean dooutput)

Sets the value of the doOutput field for this URLConnection to the specified value.

static void setFileNameMap(FileNameMap map)

Sets the FileNameMap.

void setIfModifiedSince(long ifmodifiedsince)

Sets the value of the ifModifiedSince field of this URLConnection to the specified value.

void setReadTimeout(int timeout)

Sets the read timeout to a specified timeout, in milliseconds.

void setRequestProperty(String key, String value)

Sets the general request property.

void setUseCaches(boolean usecaches)

Sets the value of the useCaches field of this URLConnection to the specified value.

String toString()

Returns a String representation of this URL connection.

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.

package com.tutorialspoint;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;

public class URLConnDemo {
   public static void main(String [] args) {
      try {
         URL url = new URL("https://www.tutorialspoint.com");
         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 this program will produce the following result −

$ java URLConnDemo

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

java_networking.htm

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