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URLConnection Class

What is the URL?

URL stands for Uniform Resource Locator, is used to specify addresses on the World Wide Web. A URL relates to the identification of any resource connected to the web.

URL syntax:

Protocol://hostname/other_information(files or directories)

The protocol indicates how the information is transferred from the link. The protocol follows by a colon, two slashes, and then the domain name. The domain name is the computer name on which the resource is located. The particular files or sub-directories are specified after the domain name. The single forward slash is used to separate the directory names.

What is the URLConnection class?

Using the URL, we will get to know about the resources on the internet. To make a connection and send or retrieve the information from those resources, we are going to use a URLConnection class.

Features:

  • URLConnection class is an abstract class whose subclasses makes the connection between the user java application and URL resource on the web.
  • By using this class, we can read and write to and from any resource referenced by a URL object.
  • Once a connection is established and has a URLConnection object, we can use it to read or write or get further information about content length, etc.

To get the object of the URLConnection class:

Since it is an abstract class; hence, we cannot create an object of URLConnection, but we can create a reference for URLConnection by using the URL class:

Syntax:

URL url=new URL(http://www.youtube.com);
URLConnection ucon=url.openConnection(); 

The constructor of the class:

Constructor Description
URLConnection(URL url) It constructs a URLConnection to the specified url.

Method of the class:

Method Description
protected boolean allowUserInteraction(void1) The URL examines if the user interaction is allowed or not, such as popping up an authentication dialogue.
 protected  boolean connected() It specifies whether a communication link to a URL is established or not; true indicates it can be established; false indicates it cannot be established.
protected  boolean doInput() It specifies whether we can read from the specified URL resource or not, true indicates we can read (default value), false indicates we cannot read.
 protected boolean doOutput() It examines whether we can write to a URL resource or not; true indicates we can write. If false-we cannot write.
protected URL url() It specifies the URL object from which the  URLConnection object has been established.
protected  boolean useCaches() Caching means temporary storage of web documents in the local system. It indicates whether caching is allowed or not; true indicates it is allowed and false indicates the protocol always tries to get a fresh copy of a resource.
public Map<String,List<String>>getHeaderFields()   It returns the unmodifiable map of the header field. The key in the map represents a response header field name. The value represents the list of field values.
public InputStream getInputStream()   It returns the InputStream to read to the open connection.
public OutputStream getOutputStream()   It returns the OutputStream to write to the open connection.
public long getLastModified()   It returns the time of the last-modified header field.

Note: As we have seen, URLConnection fields are protected, so they can’t be accessed directly unless our class is in the same package or subclass of URLConnection class.

There are the setter and getter methods to interact with the class fields.

The getter methods are:

Method Description
 public  boolean  getAllowUserInteraction() It returns the value of AllowUserInteraction.  
public  Object getContent() It retrieves the content of the URL .
 public boolean  getDoInput() It returns the value of URLConnection  doInput field.
 public boolean getDoOutput() It returns the value of URLConnection doOutput field.
public URL getURL() It retrieves the URL object to which this URLConnection is connected.
public  boolean  getUseCaches() It returns the value of this URLConnection usecaches  field.
public long getIfModifiedSince() It returns the value of the ifModifiedSince field.
 public String getContentEncoding(void) It returns the value of the content-encoding header field. It is used to compress the media type.
  public Object getContent() It retrieves the contents of the URLConnection.
  public int getContentLength() It returns the size of content associated with resourcea.
  public int getLastModified() It returns the time and date of the last-modification of the resource.
  public String getContentType() It returns a type of content found in the resource.
public long getDate() It returns the time and date of the response.
  public long getExpiration() It returns the expiry time and date of the resource.
public URL getURL() It returns the URL to which URLConnection connects.

The setter methods are:

Method Description
public void  setAllowUserInteraction(boolean allowuserinteraction) It sets the Allowuserinteraction field.
public void setIfModifiedsince(long ifmodifiedsince) It sets the IfModifiedSince field.
public void setUseCaches(Boolean usecaches) It sets the value of the URLConnection usecaches  field.
public void setDoInput(boolean doinput) It sets the value of URLConnection doInput field.
public void setDoOutput(boolean  dooutput) It sets the value of URLConnection doOutput field.

Example 1: Using the getInputStream() to read  from the  open connection.

import java.io.*;
 import java.net.*;
 class URLConn
 {
 public static void main(String s4[])throws Exception
 {
 URL u1=new URL("https://www.tutorialandexample.com/java-tutorial");
 URLConnection u2=u1.openConnection(); 
 //getInputStream() returns the InputStream to read from the connection.
 InputStream is=u2.getInputStream();
 int i;
 while((i=is.read())!=-1)
 {
 System.out.print((char)i);
 }
 }
 } 

The output of the above program:

Using the getInputStream()

Example 2: Using BufferedReader with InputStreamReader to increase the speed to read from the open connection.

import java.net.*;
import java.io.*;
import java.net.URL;
import java.net.URLConnection;
public class URLConn {
public static void main(String s1[]) throws Exception {
URL  url1 = new URL("http://www.google.com/");
URLConnection yc =url1.openConnection(); 
BufferedReader in = new BufferedReader(new InputStreamReader(yc.getInputStream()));
String inputLine;
while ((inputLine = in.readLine()) != null) 
System.out.println(inputLine);
in.close(); 
}
} 

The output of the above program:

Using BufferedReader

We get all the data of a webpage using the getInputStream() method, so the data is not shown  completely here.

Example 3: Using the various methods:

import java.net.URLConnection;
 import java.net.URL;
 import java.io.*;
 import java.util.*;
 public class URLConn
 {
 public static void main(String s[])
 {
 try 
 {
 URL url=new URL("http://google.com");
 URLConnection urlcon=url.openConnection();
 InputStreamReader insr=new InputStreamReader((InputStream)urlcon.getContent());
 BufferedReader br=new BufferedReader(insr);
 String str;
 while((str=br.readLine())!=null)
 {
 System.out.println(str);
 } 
 br.close();
 //returns the value of allowuserinteraction to specify  user interaction  allow or not .
 System.out.println("getAllowUserInteraction()"+urlcon.getAllowUserInteraction());
 //returns the value of doinput field to specify whether we can read from specified URL or not.
 System.out.println("getDoInput()"+urlcon.getDoInput());
 //returns the value of the doutput field to specify whether we can write to a URL resource or not.
 System.out.println("getDoOutput()"+urlcon.getDoOutput());
 //returns the value of the ifmodifiedsince field. 
 System.out.println("getIfModifiedSince()"+urlcon.getIfModifiedSince());
 //returns the value of the usecaches  to indicates caching allow or not.
 System.out.println("getUseCaches()"+urlcon.getUseCaches());
 //returns the URL of the connection 
 System.out.println("getURL()"+urlcon.getURL());
 //It returns the value of the content encoding header field. The content encoding entity header use to compress the media-type.
 System.out.println("getContentEncoding()"+urlcon.getContentEncoding()); 
 //returns the date and time of last modified  resource 
 System.out.println("getLastModified()"+urlcon.getLastModified());
 /*it returns key and value pair of unmodifiable map in which key represents response header fields name and value represents the list of field values.*/
 Map> map=urlcon.getHeaderFields();
 for(Map.Entry> me:map.entrySet())
 { 
 String key=me.getKey();
 List valueList=me.getValue();
 System.out.println("\n key:  "+key);
 for(String value:valueList)
 {
 System.out.println(" value :"+value+" "); 
 }
 }
 }
 catch(Exception e)
 {
 e.printStackTrace(); 
 }
 }
 } 

The output of the above program:

getInputStream() method

URLConnection has two subclasses

1.HttpURLConnection

2.JarURLConnection

HttpURLConnection class:

It is the http specific URLConnection class, works for http protocol that relates to the application-level protocol for distributed, collaborative, hypermedia information systems. The http is a generic and stateless protocol.

syntax:

URL url=new URL(“https://www.tutorialandexample.com/java-tutorial”);
HttpURLConnection hurl=(HttpURLConnection)url.openConnection(); //Typecasting to HttpURLConnection. 

Example: The code given below returns the key and value for the nth header field.

import java.io.*; 
import java.net.*; 
public class URLConn{ 
public static void main(String s1[]){ 
try{ 
URL url=new URL("https://www.tutorialandexample.com/java-tutorial"); 
HttpURLConnection hurl=(HttpURLConnection)url.openConnection(); 
for(int i=1;i<=8;i++){  
//getHeaderField returns the value for the nth header field.
//getHeaderFieldKey returns the key for the nth header field.
System.out.println(hurl.getHeaderFieldKey(i)+" = "+hurl.getHeaderField(i)); 
} 
hurl.disconnect();  
}catch(Exception e){System.out.println(e);
} 
} 
}

The output of the above program:

HttpURLConnection class

JarURLConnection class:

 JAR bundles all the files in one package. Java has the mechanisms to get the information from a jar file in which the JarURLConnection class is mostly used for that purpose. It is a URLConnection to a JAR file, an entry, or directory in a JAR file.

syntax:

jar:!/{entry}

Explanation:

Jar URL starts with a general URL that points to the location of the jar archive. Then “jar:” protocol is prefixed, and finally, “!/” and path to the file inside the jar archive is suffixed at the end of this URL.

Example:

import java.io.IOException; 
import java.net.JarURLConnection; 
import java.net.MalformedURLException; 
import java.net.URL; 
import java.security.cert.Certificate; 
import java.util.Arrays; 
import java.util.Map.Entry;  
import java.util.jar.Attributes; 
import java.util.jar.JarEntry; 
import java.util.jar.JarFile; 
import java.util.jar.Manifest; 
public class URLConn 
{  
private final static String JAR ="jar:file:/C:/Users/ABCD/Desktop/Jar" + "/first.jar!/first.class";
public static void main(String s5[]) throws Exception 
{ 
try { 
//creates a URL relates to a jar file.
URL url = new URL(JAR); 
//creates a jar URL connection object.
JarURLConnection jurl = (JarURLConnection) url.openConnection();
//It returns the URL to jar file for this connection. 
System.out.println("The Jar file URL is : " + jurl.getJarFileURL());  
//It returns the jar file for the connection.
JarFile jarFile = jurl.getJarFile(); 
System.out.println("The Jar Entry is : " + jurl.getJarEntry());
//It returns the manifest for this connection. Manifest specifies the special file that has the information of the files packaged in a JAR file.
Manifest man = jarFile.getManifest(); 
System.out.println("Manifest :" + man.toString()); 
//It returns the JAR entry object for the connection. Entry point is usually the class having the method public static void main(String s[]) 
JarEntry jentry = jurl.getJarEntry(); 
System.out.println("Jar Entry : " + jentry.toString()); 
//It returns attributes for this connection if URL points to jar entry file.
Attributes attr = jurl.getAttributes(); 
System.out.println("Attributes : " + attr); 
//It returns the main attributes of the connection.
Attributes attr1 = jurl.getMainAttributes(); 
for (Entry e : attr1.entrySet())  
{ 
System.out.println(e.getKey() + " " + e.getValue()); 
} 
} catch (MalformedURLException e) { 
e.printStackTrace(); 
} catch (IOException e) { 
e.printStackTrace(); 
} 
} 
}

The output of the program:

JarURLConnection class



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