1. Secure Sockets
SSL (Secure Sockets Layer) is a standard security technology for establishing an encrypted link between a server and a client—typically a web server (website) and a browser, or a mail server and a mail client.
To get a certificate, you must create a Certificate Signing Request (CSR) on your server. This process creates a private key and public key on your server. The CSR data file that you send to the SSL Certificate issuer (called a Certificate Authority or CA) contains the public key. The CA uses the CSR data file to create a data structure to match your private key. The CA never sees the private key.
Once you receive the SSL Certificate, you install it on your server. You also install an intermediate certificate that establishes the credibility of your SSL Certificate by tying it to your CA’s root certificate. The instructions for installing and testing your certificate will be different depending on your server.

2. Secure Sockets
When a browser attempts to access a website that is secured by SSL, the browser and the web server establish an SSL connection using a process called an “SSL Handshake” (see diagram below). Note that the SSL Handshake is invisible to the user and happens instantaneously.
Essentially, three keys are used to set up the SSL connection: the public, private, and session keys. Anything encrypted with the public key can only be decrypted with the private key, and vice versa.
Because encrypting and decrypting with private and public key takes a lot of processing power, they are only used during the SSL Handshake to create a symmetric session key. After the secure connection is made, the session key is used to encrypt all transmitted data.

3. Secure Sockets

4. Secure Sockets
Browser connects to a web server (website) secured with SSL (https). Browser requests that the server identify itself.
Server sends a copy of its SSL Certificate, including the server’s public key.
Browser checks the certificate root against a list of trusted CAs and that the certificate is unexpired, unrevoked, and that its common name is valid for the website that it is connecting to. If the browser trusts the certificate, it creates, encrypts, and sends back a symmetric session key using the server’s public key.
Server decrypts the symmetric session key using its private key and sends back an acknowledgement encrypted with the session key to start the encrypted session.
Server and Browser now encrypt all transmitted data with the session key.

5. Java URL
The Java URL class represents an URL. URL is an acronym for Uniform Resource Locator. It points to a resource on the World Wide Web. For example:
A URL contains many information:
Protocol: In this case, http is the protocol.
Server name or IP Address: In this case, is the server name.
Port Number: It is an optional attribute. If we write http//ww.javatpoint.com:80/sonoojaiswal/ , 80 is the port number. If port number is not mentioned in the URL, it returns -1.
File Name or directory name: In this case, index.jsp is the file name.

6. B.E COMPUTER SCIENCE AND ENGINEERING(UG)
Commonly used methods of Java URL class The java.net.URL class provides many methods. The important methods of URL class are given below.
Method
Description
public String getProtocol()
it returns the protocol of the URL.
public String getHost()
it returns the host name of the URL.
public String getPort()
it returns the Port Number of the URL.
public String getFile()
it returns the file name of the URL.
public URLConnection openConnection()
it returns the instance of URLConnection i.e. associated with this URL.
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7. Java URL //URLDemo.java Output: Protocol: http
import java.io.*;  
import java.net.*;  
public class URLDemo{  
public static void main(String[] args){  
try{  
URL url=new URL("
System.out.println("Protocol: "+url.getProtocol());  
System.out.println("Host Name: "+url.getHost());  
System.out.println("Port Number: "+url.getPort());  
System.out.println("File Name: "+url.getFile());  
}catch(Exception e){System.out.println(e);}  }  }  
Output:
Protocol: http
Host Name:
Port Number: -1
File Name: /java-tutorial

8. Java URLConnection class
The Java URLConnection class represents a communication link between the URL and the application. This class can be used to read and write data to the specified resource referred by the URL.
How to get the object of URLConnection class
The openConnection() method of URL class returns the object of URLConnection class. Syntax:
public URLConnection openConnection()throws IOException{}  
Displaying source code of a webpage by URLConnecton class
The URLConnection class provides many methods, we can display all the data of a webpage by using the getInputStream() method. The getInputStream() method returns all the data of the specified URL in the stream that can be read and displayed
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9. Eg.Java URLConnection class
import java.io.*; import java.net.*; public class URLConnectionExample { public static void main(String[] args){ try{ URL url=new URL(" URLConnection urlcon=url.openConnection(); InputStream stream=urlcon.getInputStream(); int i; while((i=stream.read())!=-1){ System.out.print((char)i); } }catch(Exception e){System.out.println(e);} } } Output: Displays Source Code of the Web page
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10. Java HttpURLConnection class
The Java HttpURLConnection class is http specific URLConnection. It works for HTTP protocol only.
By the help of HttpURLConnection class, you can information of any HTTP URL such as header information, status code, response code etc.
The java.net.HttpURLConnection is subclass of URLConnection class.
How to get the object of HttpURLConnection class
The openConnection() method of URL class returns the object of URLConnection class. Syntax:
public URLConnection openConnection()throws IOException{}  
You can typecast it to HttpURLConnection type as given below.
URL url=new URL("
HttpURLConnection huc=(HttpURLConnection)url.openConnection();  
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Eg.
import java.io.*; import java.net.*; public class HttpURLConnectionDemo{ public static void main(String[] args){ try{ URL url=new URL(" HttpURLConnection huc=(HttpURLConnection)url.openConnection(); for(int i=1;i<=8;i++){ System.out.println(huc.getHeaderFieldKey(i)+" = "+huc.getHeaderField(i)); } huc.disconnect(); }catch(Exception e){System.out.println(e);} } } OUTPUT: Date = Wed, 08 Feb :00:48 GMT Server = Apache Set-Cookie = JSESSIONID=4429C9793A4D5C4B5717C9DC53C30D77; Path=/ Content-Type = text/html Cache-Control = max-age= Expires = Fri, 10 Mar :00:48 GMT Vary = Accept-Encoding,User-Agent Keep-Alive = timeout=5, max=500
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Inet Addresses
Java InetAddress class represents an IP address. The java.net.InetAddress class provides methods to get the IP of any host name for example  etc.
Method
Description
public static InetAddress getByName(String host) throws UnknownHostException
it returns the instance of InetAddress containing LocalHost IP and name.
public static InetAddress getLocalHost() throws UnknownHostException
it returns the instance of InetAdddress containing local host name and address.
public String getHostName()
it returns the host name of the IP address.
public String getHostAddress()
it returns the IP address in string format.
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Eg.
import java.io.*; import java.net.*; public class InetDemo{ public static void main(String[] args){ try{ InetAddress ip=InetAddress.getByName(" System.out.println("Host Name: "+ip.getHostName()); System.out.println("IP Address: "+ip.getHostAddress()); }catch(Exception e){System.out.println(e);} } } Output: Host Name: IP Address:
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UDP Datagram
The UDP provides unguaranteed connectionless transmission of data across an IP Network.By contrast TCP provides reliable Connection-Oriented Transmission of data
UDP Classes:
Java.net.DatagramSocket
Java.net.DatagramPacket
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15. Java DatagramSocket and DatagramPacket
Java DatagramSocket and DatagramPacket classes are used for connection-less socket programming.
Java DatagramSocket class
Java DatagramSocket class represents a connection-less socket for sending and receiving datagram packets.
A datagram is basically information but there is no guarantee of its content, arrival or arrival time.
Commonly used Constructors of DatagramSocket class
DatagramSocket() throws SocketException: it creates a datagram socket and binds it with the available Port Number on the localhost machine.
DatagramSocket(int port) throws SocketEeption: it creates a datagram socket and binds it with the given Port Number.
DatagramSocket(int port, InetAddress address) throws SocketException: it creates a datagram socket and binds it with the specified port number and host address

16. Java DatagramSocket and DatagramPacket
Java DatagramPacket class
Java DatagramPacket is a message that can be sent or received. If you send multiple packet, it may arrive in any order. Additionally, packet delivery is not guaranteed.
Commonly used Constructors of DatagramPacket class
DatagramPacket(byte[] barr, int length): it creates a datagram packet. This constructor is used to receive the packets.
DatagramPacket(byte[] barr, int length, InetAddress address, int port): it creates a datagram packet. This constructor is used to send the packets.

17. Example of Sending DatagramPacket by DatagramSocket
//DSender.java  
import java.net.*;  
public class DSender{  
 public static void main(String[] args) throws Exception {  
 DatagramSocket ds = new DatagramSocket();  
 String str = "Welcome java";  
 InetAddress ip = InetAddress.getByName(" ");  
  DatagramPacket dp = new DatagramPacket(str.getBytes(), str.length(), ip, 3000);  
    ds.send(dp);  
    ds.close();  
  }  
}  
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18. Example of Sending DatagramPacket by DatagramSocket
//DReceiver.java  
import java.net.*;  
public class DReceiver{  
  public static void main(String[] args) throws Exception {  
    DatagramSocket ds = new DatagramSocket(3000);  
    byte[] buf = new byte[1024];  
    DatagramPacket dp = new DatagramPacket(buf, 1024);  
    ds.receive(dp);  
    String str = new String(dp.getData(), 0, dp.getLength());  
    System.out.println(str);  
    ds.close();  
  }  
}  
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