SOCKET PROGRAMMING
WITH MOBILE SOCKET CLIENT
DEARTMENT OF COMPUTER SCIENCE
IOWA STATE UNIVERSITY
BASICS OF
ANDROID
SOCKET
PROGRAMMING
 A socket is a common interface for performing
network communication. Underneath the
hood, Android’s HTTP client library is using
sockets to send and receive data.
 Android Sockets are the same as Java Sockets.
 The package java.net provides support for
sockets programming (and more).
JAVA SOCKET
PROGRAMMING
 Classes defined in this package are:
 InetAddress
 Socket
 ServerSocket
 DatagramSocket
 DatagramPacket
InetAddress CLASS
 Static methods you can use to create new InetAddress objects.
 getByName(String host)
 getAllByName(String host)
 getLocalHost()
 Example Use of the class:
Code:
try {
InetAddress a = InetAddress.getByName(hostname);
System.out.println(hostname + ":" a.getHostAddress());
} catch (UnknownHostException e) {
System.out.println("No address found for " + hostname);
}
Output: www.yahoo.com:209.131.36.158
Socket CLASS
 A Socket class defines active TCP sockets only
 client sockets
 socket returned by accept();
 Constructor and Methods
 Constructor: Creates a stream socket and connects it to the specified port number
on the named host. Following are different constructors:
 Socket(InetAddress server, int port);
 Socket(InetAddress server, int port,InetAddress local, int localport);
 Socket(String hostname, int port);
 close() :Closes the Socket connection and releases all associated resources.
 Thus a Client Socket is created as follows:
Socket socclient=new
Socket(server_ip_address,server_port_number);
SocketServer CLASS
 A ServerSocket class defines a Server Socket. Passive sockets are supported
by this class.
 Constructor and Methods:
 Constructor: Creates a server socket and binds it to the specified local port number,
with the specified backlog. The following are different constructors:
 ServerSocket(int port);
 ServerSocket(int port, int backlog);
 ServerSocket(int port, int backlog, InetAddress bindAddr);
 Socket Accept(): Listens for a connection to be made to this socket and accepts it. This
method blocks until a connection is made. This is a method of the SocketServer Class
however the object instance returned by this method is of the Socket class.
 close(): Closes the Socket connection and releases all associated resources.
 A Server Socket can be created as follows:
ServerSocket server_sock = new ServerSocket(server_port_number);
TCP Socket
Client Server
Communication
 UDP is Connectionless and unreliable service. There isn’t an
initial handshaking phase. Transmitted data may be received
out of order, or lost.
UDP Sockets
 Socket Programming with UDP
 No need for a welcoming socket. No streams are attached
to the sockets. The sending hosts creates “packets” by
attaching the IP destination address and port number to
each batch of bytes and the receiving process must unravel
to received packet to obtain the packet’s information bytes.
 DatagramSocket class is used to create a UDP socket.
 DatagramSocket();
 DatagramSocket(int port);
 DatagramSocket(int port, InetAddress a);
 DatagramPacket class needed to specify the payload.
 DatagramPacket( byte[] buf, int len, InetAddress a, int port);
UDP Socket
Client Server
Communication
import java.io.*;
import java.net.*;
class UDPClient {
public static void main(String args[]) throws Exception
{
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Example
UDP Client
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length,
IPAddress, 9876);
clientSocket.send(sendPacket);
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
Example
UDP Client
clientSocket.receive(receivePacket);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close();
}
}
import java.io.*;
import java.net.*;
class UDPServer {
public static void main(String args[]) throws Exception
{
Example
UDP Server
DatagramSocket serverSocket = new
DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
{
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
String sentence = new String(receivePacket.getData());
InetAddress IPAddress =
receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
Example
UDP Server
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length,
IPAddress, port);
serverSocket.send(sendPacket);
}
}
}
Socket I/O is based on the Java I/O support in the
package java.io
Socket I/O
Once a Socket is created data can be transmitted using
Inputstreams and Outputstreams. InputStream and
OutputStream are abstract classes.
 An InputStream is a stream of incoming byte data. An
InputStream can be obtained from a Socket by using the
getInputStream() method.
InputStream
 In order to read from a stream, you must create a byte
buffer to read in data.Each call to read on an InputStream
fills your buffer with data and returns the number of bytes
read.
Example use of InputStream: BufferedReader in=new
BufferedReader(new
InputStreamReader(socclient.getInputStream()));
String data=in.readLine();
 An OutputStream is a stream of outgoing byte data
 An OutputStream can be obtained from a Socket by using the
getOutputStream() method.
 You can write data to a stream by passing in a byte buffer of
data.
OutputStream
 You should use the flush() method if you want to make sure
that the data you have written has been output to disk or sent
to the other end of the socket.
Eg:
PrintWriter
out
=
PrintWriter(socclient.getOutputStream(), true);
new
SCENARIO
 Consider a Java Socket Server and a Mobile Application Client.
 Consider the Mobile Application to run an Android OS.
 Let us look at some very simple examples to create an
Android Application that works as a socket client.
 Example 1: Consider A simple Socket Server which receives a
text file’s name with path from the Socket client and replies
saying weather the file exists in the given path or not. If the
file exists, it retrieves the contents of the text file.
 Example 2: Consider A simple Socket server which receives a
string and replies with the count of the number of occurrences
of the letter ‘s’ in the string.
 The Complete Code of these examples is available as part of
the module.
 Implement a Java Socket Server irrespective of the android nature of the client.
EXAMPLE I
JAVA SOCKET
SERVER
 The Following code snippet creats a ServerSocket and calls the accept() blocking call to
wait for connection.
public class SockServer {
public static void main(String[] args) throws Exception {
System.out.println(“ The File Retrieval server is running.");
int clientNumber = 0;
ServerSocket listener = new ServerSocket(9898);
Create Socket
try {
while (true) {
new Finder(listener.accept(), clientNumber++).start(); Wait for Client
Connection
}
} finally {
and create a new
thread for each
client
listener.close();
}
}
..
 The following code snippet defines the Finder Class that stores the
information about Each Socket Client connection.
..
EXAMPLE I
JAVA SOCKET
SERVER
private static class Finder extends Thread {
private Socket socket;
private int clientNumber;
Extends Thread Class
public Finder(Socket socket, int clientNumber) {
this.socket = socket;
this.clientNumber = clientNumber;
log("New connection with client# " + clientNumber + " at " + socket);
}
..
EXAMPLE I JAVA SOCKET SERVER
public void run() {
try {
BufferedReader in = new BufferedReader(
new
InputStreamReader(socket.getInputStream()));
PrintWriter out = new
PrintWriter(socket.getOutputStream(), true);
while (true) {
String input = in.readLine();
if (input == null || input.equals(".")) {
break;
}
CheckforFile cf = new CheckforFile();
int exists = cf.fileExists(input);
Input Stream
Output Stream
Get file path from Socket
Instantiate CheckforFile object who’s fileExists()
function returns 1 if file exists and 0 otherwise
EXAMPLE I JAVA SOCKET SERVER
File dir = new File(input)
if(exists==1)
{
System.out.println("The file exists");
BufferedReader br = new BufferedReader(new
FileReader(dir));
try {
StringBuilder sb = new StringBuilder();
String line = br.readLine();
Send data to Client which specifies if the file exits, the contents of
while (line != null) {
the file.
sb.append(line);
sb.append(System.lineSeparator());
line = br.readLine();
}
String everything = sb.toString();
out.println("Hello, client #" + clientNumber + ". The file
"+input+" exists"+"The contents of the file are:"+everything);
} finally {
br.close();
}
}
else
{
out.println("The file "+input+" does not exist");
Send data to client specifying that the file does not exist.
System.out.println("The file does not exist");
}
EXAMPLE I JAVA SOCKET SERVER
}
} catch (IOException e) {
log("Error handling client# " + clientNumber + ": " +
Handle Exceptions
e);
} finally {
try {
socket.close();
} catch (IOException e) {
log("Couldn't close a socket, what's going on?");
}
log("Connection with client# " + clientNumber + "
closed");
}
}
private void log(String message) {
System.out.println(message);
}
}
}
Close Socket
Handle Exceptions
 The Android Client is a simple Java Client with additional handlers
for updating the User Interface.
EXAMPLE I
ANDROID
SOCKET
CLIENT
 In Android we use a PrintWriter to output data to the socket
server because it writes characters as opposed to a PrintStream
which writes bytes.
 Use a Handler to communicate with the UI thread. The Android
User Interface is updated using a UI thread and hence the Socket
thread which is a Background thread requires a Handler object to
update the UI.
 AsyncTask can be used instead of Threads as they require lesser
maintenance. The basic example given however uses Threads for
simplicity.
 Create an Android Application with a Blank
Activity. If you are new to Android
Programming refer to the Modules on
Basic Android Programming.
Layout:
EXAMPLE I
ANDROID
SOCKET
CLIENT
 Create a Simple Layout with a TextField
reading “Text File Path”, an EditText field
that takes the path input and a Button
“Find”.
 Add a ScrollView to the Layout by adding
the following code to activity_main.xml
under <Project_folder> -> res -> layout.
<ScrollView
android:id="@+id/scrollView1"
android:layout_width="fill_parent"
android:layout_height="200dp">
<LinearLayout
android:id="@+id/linearLayout2"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:orientation="vertical">
</LinearLayout>
</ScrollView>
Add a button on click Method inside the Main Activity Class of the Application.
public void searchFile(View v)
{
EXAMPLE I
ANDROID
SOCKET
CLIENT
this.editText= (EditText)findViewById(R.id.editText1);
Get the View by ID and get the
final String value = editText.getText().toString();
contents of the field
displayMessage("Searching for file: "+value);
this method updates the UI
tclient=new Thread(new Runnable(){
public void run()
{
create a thread
try {
displayMessage("Socket being Created.. ");
socclient=new Socket(B);
create socket
PrintWriter out = new PrintWriter(socclient.getOutputStream(), true);
if(value!=null)
{
out.println(value);
send the filepath to the server
Output Stream
Message servermsg=Message.obtain();
BufferedReader in=new BufferedReader(new
InputStreamReader(socclient.getInputStream()));
InputStream
String data=in.readLine();
Read response from server
servermsg.obj="Server Response: "+data;
handler.sendMessage(servermsg);
EXAMPLE I
ANDROID
SOCKET
CLIENT
Use the handler to update UI
in.close();
}
close input and output streams.
out.close();
} catch (UnknownHostException e) {
e.printStackTrace();
}catch (IOException e) {
e.printStackTrace();
}
}
});
tclient.start();
}
handle exceptions
public void displayMessage(String msg)
the string parameter as a textview item
{
into the scrollview of your application
this.scrollView = (ScrollView) findViewById(R.id.scrollView1);
this.linearLayout = (LinearLayout) findViewById(R.id.linearLayout2);
TextView tx = new TextView(this);
EXAMPLE I
ANDROID
SOCKET
CLIENT
tx.setLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT,
LayoutParams.WRAP_CONTENT));
tx.setText(msg);
this.linearLayout.addView(tx);
}
Handler handler=new Handler() {
public void handleMessage(Message msg) override the handMessage method
{
displayMessage(msg.obj.toString());
}
};
EXAMPLE I
Output
 This is a Screenshot of the
Application running in the
Android Device with the
response from the server
containing the contents of the
text file.
EXAMPLE II
Output
 Screenshot of the
Application running in the
Android Device with the
response from the server
containing the number of
occurrences of the letter ‘s’
in the entered string.
THANK YOU!