Establishing a Socket Communication Link

This section describes how to establish a socket communication link. A server application runs a socket in listen mode and a client application connects a client socket to the server socket.

In the previous section, we only discussed about how socket works with a communication link that has been established already. Now, let's see how two application programs can establish a communication link between the local socket and the remote socket.

To establish a communication link, one application program must act as a server, create a server socket with a given port number, and set the server socket in the listen mode waiting for a connection request from other program.

With one program running as a server listening for a connection request at a specific port number, the other program can now create a client socket with a given local port number, the Internet address of the computer system where the first program is running, and the port number where the server socket is listening. At this time, a connect request will be send over to the server socket. The server socket should then accept the connect request and instantiate a socket object to complete communication link.

J2SDK offers two main classes to support socket communication:

1. java.net.ServerSocket with methods:

2. java.net.Socket with methods:

The following diagram shows steps involved in establishing a two-way communication link using the methods provided by ServerSocket and Socket classes:

       Client System            Server System
       Internet Address #a      Internet Address #b
Step   Available Port #p        Available Port #q

1                               ss = new ServerSocket()
2                               ss.bind(#b+#q) 
3                               Socket cs = ss.accept()
4      cs = new Socket()        (waiting)
5      cs.bind(#a+#p)           (waiting)
6      cs.connect(#b+#q)        (receiving request)
7      (estalishing the link)   (establishing the link)
8      (cs is ready)            (cs is ready)

Last update: 2014.

Table of Contents

 About This JDK Tutorial Book

 Downloading and Installing JDK 1.8.0 on Windows

 Downloading and Installing JDK 1.7.0 on Windows

 Downloading and Installing JDK 1.6.2 on Windows

 Java Date-Time API

 Date, Time and Calendar Classes

 Date and Time Object and String Conversion

 Number Object and Numeric String Conversion

 Locales, Localization Methods and Resource Bundles

 Calling and Importing Classes Defined in Unnamed Packages

 HashSet, Vector, HashMap and Collection Classes

 Character Set Encoding Classes and Methods

 Character Set Encoding Maps

 Encoding Conversion Programs for Encoded Text Files

Socket Network Communication

 What Is a Socket?

Establishing a Socket Communication Link

 ReverseEchoer.java - A Simple Server Socket Application

 SocketClient.java - A Simple Client Socket Application

 ReverseEchoServer.java - A Multi-Connection Socket Server

 Binding Sockets to Specific Ports

 Datagram Network Communication

 DOM (Document Object Model) - API for XML Files

 SAX (Simple API for XML)

 DTD (Document Type Definition) - XML Validation

 XSD (XML Schema Definition) - XML Validation

 XSL (Extensible Stylesheet Language)

 Message Digest Algorithm Implementations in JDK

 Private key and Public Key Pair Generation

 PKCS#8/X.509 Private/Public Encoding Standards

 Digital Signature Algorithm and Sample Program

 "keytool" Commands and "keystore" Files

 KeyStore and Certificate Classes

 Secret Key Generation and Management

 Cipher - Secret Key Encryption and Decryption

 The SSL (Secure Socket Layer) Protocol

 SSL Socket Communication Testing Programs

 SSL Client Authentication

 HTTPS (Hypertext Transfer Protocol Secure)

 Outdated Tutorials

 References

 PDF Printing Version