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How to Create a Network Application in Java

Writing code that executes on a certain device is very satisfying. But, writing code that executes on several devices that communicate with each other is simply life-affirming. This article will teach you how to connect and exchange...

Steps

  1. How to Create a Network Application in Java Picture 1How to Create a Network Application in Java Picture 1
    Create a class. Create a class and name it however you want. In this article, it will be named NetworkAppExample. 
    public class NetworkAppExample { }
  2. How to Create a Network Application in Java Picture 2How to Create a Network Application in Java Picture 2
    Create a main method. Create a main method and declare it might throw exceptions of Exception type and any subclass of it - all exceptions. This is considered a bad practice, but is acceptable for barebone examples. 
    public class NetworkAppExample { public static void main(String[] args) throws Exception { } }
  3. How to Create a Network Application in Java Picture 3How to Create a Network Application in Java Picture 3
    Declare server address. This example will use local host address and an arbitrary port number. Port number needs to be in a range from 0 to 65535 (inclusive). However, port numbers to avoid range from 0 to 1023 (inclusive) because they are reserved system ports. 
    public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; } }
  4. How to Create a Network Application in Java Picture 4How to Create a Network Application in Java Picture 4
    Create a server. Server is bound to the address and port and listens for incoming connections. In Java, ServerSocket represents server-side endpoint and its function is accepting new connections. ServerSocket does not have streams for reading and sending data because it does not represent connection between a server and a client. 
    import java.net.InetAddress; import java.net.ServerSocket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); } }
  5. How to Create a Network Application in Java Picture 5How to Create a Network Application in Java Picture 5
    Log server inception. For logging purposes, print to the console that server has been started. 
    import java.net.InetAddress; import java.net.ServerSocket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); } }
  6. How to Create a Network Application in Java Picture 6How to Create a Network Application in Java Picture 6
    Create a client. Client is bound to the address and port of a server and listens for packets (messages) after connection is established. In Java, Socket represents either a client-side endpoint connected to the server or a connection (from server) to client and is used to communicate with the party on the other end. 
    import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); } }
  7. How to Create a Network Application in Java Picture 7How to Create a Network Application in Java Picture 7
    Log connection attempt. For logging purposes, print to the console that connection has been attempted. 
    import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); } }
  8. How to Create a Network Application in Java Picture 8How to Create a Network Application in Java Picture 8
    Establish connection. Clients will never connect unless server listens for and accepts, in other words establishes, connections. In Java, connections are established using accept() method of ServerSocket class. The method will block execution until a client connects. 
    import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); } }
  9. How to Create a Network Application in Java Picture 9How to Create a Network Application in Java Picture 9
    Log established connection. For logging purposes, print to the console that connection between server and client has been established. 
    import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); System.out.println("Connection established."); } }
  10. How to Create a Network Application in Java Picture 10How to Create a Network Application in Java Picture 10
    Prepare communication streams. Communication is done over streams and, in this application, raw streams of (connection from) server (to client) and client need to be chained to either data or object streams. Remember, both parties need to use the same stream type.
    1. Data streams 
      import java.io.DataInputStream; import java.io.DataOutputStream; import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); System.out.println("Connection established."); DataOutputStream clientOut = new DataOutputStream(client.getOutputStream()); DataInputStream clientIn = new DataInputStream(client.getInputStream()); DataOutputStream serverOut = new DataOutputStream(connection.getOutputStream()); DataInputStream serverIn = new DataInputStream(connection.getInputStream()); } }
    2. Object streams
      When multiple object streams are used, input streams have to be initialized in the same order as output streams because ObjectOutputStream sends a header to the other party and ObjectInputStream blocks execution until it reads the header. 
      import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); System.out.println("Connection established."); ObjectOutputStream clientOut = new ObjectOutputStream(client.getOutputStream()); ObjectOutputStream serverOut = new ObjectOutputStream(connection.getOutputStream()); ObjectInputStream clientIn = new ObjectInputStream(client.getInputStream()); ObjectInputStream serverIn = new ObjectInputStream(connection.getInputStream()); } }

      Order as specified in the code above might be easier to remember - first initialize output streams then input streams in the same order. However, another order for initialization of object streams is the following:

      ObjectOutputStream clientOut = new ObjectOutputStream(client.getOutputStream()); ObjectInputStream serverIn = new ObjectInputStream(connection.getInputStream()); ObjectOutputStream serverOut = new ObjectOutputStream(connection.getOutputStream()); ObjectInputStream clientIn = new ObjectInputStream(client.getInputStream());
  11. How to Create a Network Application in Java Picture 11How to Create a Network Application in Java Picture 11
    Log that communication is ready. For logging purposes, print to the console that communication is ready. 
    // code omitted import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); System.out.println("Connection established."); // code omitted System.out.println("Communication is ready."); } }
  12. How to Create a Network Application in Java Picture 12How to Create a Network Application in Java Picture 12
    Create a message. In this application, Hello World text will be sent to the server either as byte[] or String. Declare a variable of the type that depends on the stream used. Use byte[] for data streams and String for object streams.
    1. Data streams
      Using data streams, serialization is done by converting objects into primitive data types or a String. In this case, String is converted to byte[] instead of written using writeBytes() method to show how it would be done with other objects, such as images or other files. 
      import java.io.DataInputStream; import java.io.DataOutputStream; import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); System.out.println("Connection established."); DataOutputStream clientOut = new DataOutputStream(client.getOutputStream()); DataInputStream clientIn = new DataInputStream(client.getInputStream()); DataOutputStream serverOut = new DataOutputStream(connection.getOutputStream()); DataInputStream serverIn = new DataInputStream(connection.getInputStream()); System.out.println("Communication is ready."); byte[] messageOut = "Hello World".getBytes(); } }
    2. Object streams 
      import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); System.out.println("Connection established."); ObjectOutputStream clientOut = new ObjectOutputStream(client.getOutputStream()); ObjectOutputStream serverOut = new ObjectOutputStream(connection.getOutputStream()); ObjectInputStream clientIn = new ObjectInputStream(client.getInputStream()); ObjectInputStream serverIn = new ObjectInputStream(connection.getInputStream()); System.out.println("Communication is ready."); String messageOut = "Hello World"; } }
  13. How to Create a Network Application in Java Picture 13How to Create a Network Application in Java Picture 13
    Send the message. Write data to the output stream and flush the stream to ensure the data has been written entirely.
    1. Data streams
      Length of a message needs to be sent first so the other party knows how many bytes it needs to read. After the length is sent as a primitive integer type, bytes can be sent. 
      import java.io.DataInputStream; import java.io.DataOutputStream; import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); System.out.println("Connection established."); DataOutputStream clientOut = new DataOutputStream(client.getOutputStream()); DataInputStream clientIn = new DataInputStream(client.getInputStream()); DataOutputStream serverOut = new DataOutputStream(connection.getOutputStream()); DataInputStream serverIn = new DataInputStream(connection.getInputStream()); System.out.println("Communication is ready."); byte[] messageOut = "Hello World".getBytes(); clientOut.writeInt(messageOut.length); clientOut.write(messageOut); clientOut.flush(); } }
    2. Object streams 
      import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); System.out.println("Connection established."); ObjectOutputStream clientOut = new ObjectOutputStream(client.getOutputStream()); ObjectOutputStream serverOut = new ObjectOutputStream(connection.getOutputStream()); ObjectInputStream clientIn = new ObjectInputStream(client.getInputStream()); ObjectInputStream serverIn = new ObjectInputStream(connection.getInputStream()); System.out.println("Communication is ready."); String messageOut = "Hello World"; clientOut.writeObject(messageOut); clientOut.flush(); } }
  14. How to Create a Network Application in Java Picture 14How to Create a Network Application in Java Picture 14
    Log sent message. For logging purposes, print to the console that message has been sent.
    1. Data streams 
      import java.io.DataInputStream; import java.io.DataOutputStream; import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); System.out.println("Connection established."); DataOutputStream clientOut = new DataOutputStream(client.getOutputStream()); DataInputStream clientIn = new DataInputStream(client.getInputStream()); DataOutputStream serverOut = new DataOutputStream(connection.getOutputStream()); DataInputStream serverIn = new DataInputStream(connection.getInputStream()); System.out.println("Communication is ready."); byte[] messageOut = "Hello World".getBytes(); clientOut.writeInt(messageOut.length); clientOut.write(messageOut); clientOut.flush(); System.out.println("Message sent to server: " + new String(messageOut)); } }
    2. Object streams 
      import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.net.InetAddress; import java.net.ServerSocket; import java.net.Socket; public class NetworkAppExample { public static void main(String[] args) throws Exception { String host = "localhost"; int port = 10430; ServerSocket server = new ServerSocket(port, 50, InetAddress.getByName(host)); System.out.println("Server started."); Socket client = new Socket(host, port); System.out.println("Connecting to server..."); Socket connection = server.accept(); System.out.println("Connection established."); ObjectOutputStream clientOut = new ObjectOutputStream(client.getOutputStream()); ObjectOutputStream serverOut
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