Java network programming
Network programming refers to writing programs that run on multiple devices (computers), which are all connected through a network.
The J2SE API in the java.net package contains classes and interfaces that provide low-level communication details. You can use these classes and interfaces directly to focus on solving problems rather than on communication details.
The java.net package provides support for two common network protocols:
TCP: TCP is the abbreviation of Transmission Control Protocol, It guarantees reliable communication between two applications. Commonly used for Internet protocols, known as TCP/IP.
UDP: UDP is the abbreviation of User Datagram Protocol, a connectionless protocol. Provides packets of data to be sent between applications.
This tutorial mainly explains the following two topics.
Socket Programming: This is the most widely used networking concept and it has been explained in great detail
URL Processing: This part will be discussed in another space. Click here to learn more about URL processing in the Java language.
Socket Programming
Sockets provide a communication mechanism between two computers using TCP. The client program creates a socket and attempts to connect to the server's socket.
When the connection is established, the server will create a Socket object. The client and server can now communicate by writing to and reading from the Socket object.
The java.net.Socket class represents a socket, and the java.net.ServerSocket class provides a mechanism for server programs to listen to clients and establish connections with them.
The following steps occur when establishing a TCP connection using sockets between two computers:
The server instantiates a ServerSocket object, which represents communication through a port on the server.
The server calls the accept() method of the ServerSocket class, which waits until the client connects to the given port on the server.
While the server is waiting, a client instantiates a Socket object, specifying the server name and port number to request a connection.
The constructor of the Socket class attempts to connect the client to the specified server and port number. If communication is established, a Socket object is created on the client to communicate with the server.
On the server side, the accept() method returns a new socket reference on the server that is connected to the client's socket.
TCP is a two-way communication protocol, so data can be sent through two data streams at the same time. The following is a complete set of useful methods provided by some classes to implement sockets.
Methods of the ServerSocket class
The server application obtains a port by using the java.net.ServerSocket class and listens for client requests.
The ServerSocket class has four construction methods:
| Serial number | Method description |
| 1 | public ServerSocket(int port) throws IOException Create a server socket bound to a specific port. |
| 2 | public ServerSocket(int port, int backlog) throws IOException Creates a server socket using the specified backlog and binds it to the specified local port number. |
| 3 | public ServerSocket(int port, int backlog, InetAddress address) throws IOException Creates a server with the specified port, listening backlog, and local IP address to bind to. |
| 4 | public ServerSocket() throws IOException Create an unbound server socket. |
Create an unbound server socket. If the ServerSocket construction method does not throw an exception, it means that your application has successfully bound to the specified port and is listening for client requests.
Here are some common methods of the ServerSocket class:
| Serial number | Method description |
| 1 | public int getLocalPort() Returns the port on which this socket is listening. |
| 2 | public Socket accept() throws IOException Listens for and accepts connections to this socket. |
| 3 | public void setSoTimeout(int timeout) Enable/disable SO_TIMEOUT by specifying the timeout value in milliseconds. |
| 4 | public void bind(SocketAddress host, int backlog) Bind a ServerSocket to a specific address (IP address and port number). |
Methods of the Socket class
The java.net.Socket class represents the socket used by both the client and the server to communicate with each other. The client obtains a Socket object through instantiation, and the server obtains a Socket object through the return value of the accept() method.
The Socket class has five construction methods.
| Serial number | Method description |
| 1 | public Socket(String host, int port) throws UnknownHostException, IOException. Creates a stream socket and connects it to the specified port number on the specified host. |
| 2 | public Socket(InetAddress host, int port) throws IOException Creates a stream socket and connects it to the specified port number at the specified IP address. |
| 3 | public Socket(String host, int port, InetAddress localAddress, int localPort) throws IOException. Creates a socket and connects it to the specified remote port on the specified remote host. |
| 4 | public Socket(InetAddress host, int port, InetAddress localAddress, int localPort) throws IOException. Creates a socket and connects it to the specified remote port on the specified remote address. |
| 5 | public Socket() Create an unconnected socket through the system default type SocketImpl |
When the Socket constructor returns, it does not simply instantiate a Socket object, it will actually try to connect to the specified server and port.
Some interesting methods are listed below. Note that both the client and the server have a Socket object, so both the client and the server can call these methods.
| Serial number | Method description |
| 1 | public void connect(SocketAddress host, int timeout) throws IOException Connect this socket to the server and specify a timeout value. |
| 2 | public InetAddress getInetAddress() Returns the address of the socket connection. |
| 3 | public int getPort() Returns the remote port to which this socket is connected. |
| 4 | public int getLocalPort() Returns the local port to which this socket is bound. |
| 5 | public SocketAddress getRemoteSocketAddress() Returns the address of the endpoint this socket is connected to, or null if not connected. |
| 6 | public InputStream getInputStream() throws IOException Returns the input stream for this socket. |
| 7 | public OutputStream getOutputStream() throws IOException Returns the output stream for this socket. |
| 8 | public void close() throws IOException Close this socket. |
Methods of the InetAddress class
This class represents the Internet Protocol (IP) address. The following is a list of the more useful methods for Socket programming:
| Serial number | Method description |
| 1 | static InetAddress getByAddress(byte[] addr) Returns an InetAddress object given the original IP address. |
| 2 | static InetAddress getByAddress(String host, byte[] addr) Creates an InetAddress based on the provided hostname and IP address. |
| 3 | static InetAddress getByName(String host) Determines the IP address of a host given a hostname. |
| 4 | String getHostAddress() Returns the IP address string as a text representation. |
| 5 | String getHostName() Get the hostname of this IP address. |
| 6 | static InetAddress getLocalHost() Return localhost. |
| 7 | String toString() Convert this IP address to a String. |
Socket Client Example
The following GreetingClient is a client program that connects to the server through socket and sends a request, and then Wait for a response.
// 文件名 GreetingClient.java
import java.net.*;
import java.io.*;
public class GreetingClient
{
public static void main(String [] args)
{
String serverName = args[0];
int port = Integer.parseInt(args[1]);
try
{
System.out.println("Connecting to " + serverName
+ " on port " + port);
Socket client = new Socket(serverName, port);
System.out.println("Just connected to "
+ client.getRemoteSocketAddress());
OutputStream outToServer = client.getOutputStream();
DataOutputStream out =
new DataOutputStream(outToServer);
out.writeUTF("Hello from "
+ client.getLocalSocketAddress());
InputStream inFromServer = client.getInputStream();
DataInputStream in =
new DataInputStream(inFromServer);
System.out.println("Server says " + in.readUTF());
client.close();
}catch(IOException e)
{
e.printStackTrace();
}
}
}Socket Server Example
The following GreetingServer program is a server-side application that uses Socket to listen to a specified port.
// 文件名 GreetingServer.java
import java.net.*;
import java.io.*;
public class GreetingServer extends Thread
{
private ServerSocket serverSocket;
public GreetingServer(int port) throws IOException
{
serverSocket = new ServerSocket(port);
serverSocket.setSoTimeout(10000);
}
public void run()
{
while(true)
{
try
{
System.out.println("Waiting for client on port " +
serverSocket.getLocalPort() + "...");
Socket server = serverSocket.accept();
System.out.println("Just connected to "
+ server.getRemoteSocketAddress());
DataInputStream in =
new DataInputStream(server.getInputStream());
System.out.println(in.readUTF());
DataOutputStream out =
new DataOutputStream(server.getOutputStream());
out.writeUTF("Thank you for connecting to "
+ server.getLocalSocketAddress() + "\nGoodbye!");
server.close();
}catch(SocketTimeoutException s)
{
System.out.println("Socket timed out!");
break;
}catch(IOException e)
{
e.printStackTrace();
break;
}
}
}
public static void main(String [] args)
{
int port = Integer.parseInt(args[0]);
try
{
Thread t = new GreetingServer(port);
t.start();
}catch(IOException e)
{
e.printStackTrace();
}
}
}Compile the above java code, and execute the following command to start the service, using the port number 6066:
$ java GreetingServer 6066 Waiting for client on port 6066...Start the client as follows:
$ java GreetingClient localhost 6066 Connecting to localhost on port 6066 Just connected to localhost/127.0.0.1:6066 Server says Thank you for connecting to /127.0.0.1:6066 Goodbye!
