Chapter 26

Java Socket Programming

by Stephen Ingram

CONTENTS

• An Introduction to Sockets
• Java Connection-Oriented Classes
• Java Datagram Classes
• An HTTP Server Application
• Summary

For full Java client/server applet connectivity, an applet server is required. This chapter initiates the development of a Java HTTP server. Before beginning the development of the server, however, you need some background knowledge of socket programming. This chapter begins with a socket overview followed by an exploration of Java's socket classes. The remainder of the chapter focuses on the construction of a Java HTTP Web server.

After reading this chapter, you should be able to do the following:

• Understand the socket abstraction
• Know the different modes of socket operation
• Have a working knowledge of the HTTP protocol
• Be capable of applying the Java socket classes
• Understand applet socket use and limitations
• Comprehend the HTTP Java server

An Introduction to Sockets

The computers on the Internet are connected by the TCP/IP protocol. In the 1980s, the Advanced Research Projects Agency (ARPA) of the U.S. government funded the University of California at Berkeley to provide a UNIX implementation of the TCP/IP protocol suite. What was developed was termed the socket interface (although you may hear it called the Berkeley-socket interface or just Berkeley sockets). Today, the socket interface is the most widely used method for accessing a TCP/IP network.

A socket is nothing more than a convenient abstraction. It represents a connection point into a TCP/IP network, much like the electrical sockets in your home provide a connection point for your appliances. When two computers want to converse, each uses a socket. One computer is termed the server-it opens a socket and listens for connections. The other computer is termed the client-it calls the server socket to start the connection. To establish a connection, all that's needed is a server's destination address and port number.

Each computer in a TCP/IP network has a unique address. Ports represent individual connections within that address. This is analogous to corporate mail-each person within a company shares the same address, but a letter is routed within the company by the person's name. Each port within a computer shares the same address, but data is routed within each computer by the port number. When a socket is created, it must be associated with a specific port-this process is known as binding to a port.

Socket Transmission Modes

Sockets have two major modes of operation: connection-oriented and connectionless modes. Connection-oriented sockets operate like a telephone: they must establish a connection and then hang up. Everything that flows between these two events arrives in the same order it was sent. Connectionless sockets operate like the mail: delivery is not guaranteed, and multiple pieces of mail may arrive in a different order than they were sent.

The mode you use is determined by an application's needs. If reliability is important, connection-oriented operation is better. File servers must have all their data arrive correctly and in sequence. If some data is lost, the server's usefulness is invalidated. Some applications-time servers, for example-send discrete chunks of data at regular intervals. If data were to get lost, the server would not want the network to retry because by the time the resent data arrived, it would be too old to have any accuracy. When you need reliability, be aware that it does come with a price. Ensuring data sequence and correctness requires extra processing and memory usage; this extra overhead can slow down the response times of a server.

Connectionless operation uses the User Datagram Protocol (UDP). A datagram is a self-contained unit that has all the information needed to attempt its delivery. Think of it as an envelope-it has a destination and return address on the outside and contains the data to be sent on the inside. A socket in this mode does not have to connect to a destination socket; it simply sends the datagram. The UDP protocol promises only to make a best-effort delivery attempt. Connectionless operation is fast and efficient, but not guaranteed.

Connection-oriented operation uses the Transport Control Protocol (TCP). A socket in this mode must connect to the destination before sending data. Once connected, the sockets are accessed using a streams interface: open-read-write-close. Everything sent by one socket is received by the other end of the connection in exactly the same order it was sent. Connection-oriented operation is less efficient than connectionless operation, but it's guaranteed.

Sun Microsystems has always been a proponent of internetworking, so it isn't surprising to find rich support for sockets in the Java class hierarchy. In fact, the Java classes have significantly reduced the skill needed to create a sockets program. Each transmission mode is implemented in a separate set of Java classes. This chapter discusses the connection-oriented classes first.

Java Connection-Oriented Classes

The connection-oriented classes within Java have both a client and a server representative. The client half tends to be the simplest to set up, so we cover it first.

Listing 26.1 shows a simple client application. It requests an HTML document from a server and displays the response to the console.

Listing 26.1. A simple socket client.

import java.io.*;
import java.net.*;

/**
 * An application that opens a connection to a Web server and reads
 * a single Web page from the connection.
*/
public class SimpleWebClient {
    public static void main(String args[])
    {
        try
        {
            // Open a client socket connection
            Socket clientSocket1 = new Socket("www.javasoft.com", 80);
            System.out.println("Client1: " + clientSocket1);

            // Get a Web page
            getPage(clientSocket1);
        }
        catch (UnknownHostException uhe)
        {
            System.out.println("UnknownHostException: " + uhe);
        }
        catch (IOException ioe)
        {
            System.err.println("IOException: " + ioe);
        }
    }

    /**
     * Request a Web page using the passed client socket.
     * Display the reply and close the client socket.
     */
    public static void getPage(Socket clientSocket)
    {
        try
        {
            // Acquire the input and output streams
            DataOutputStream outbound = new DataOutputStream(
                clientSocket.getOutputStream() );
            DataInputStream inbound = new DataInputStream(
                clientSocket.getInputStream() );

            // Write the HTTP request to the server
            outbound.writeBytes("GET / HTTP/1.0\r\n\r\n");

            // Read the response
            String responseLine;
            while ((responseLine = inbound.readLine()) != null)
            {
                // Display each line to the console
                System.out.println(responseLine);

                // This code checks for EOF.  There is a bug in the
                // socket close code under Win 95.  readLine() will
                // not return null when the client socket is closed
                // by the server.
                if ( responseLine.indexOf("</HTML>") != -1 )
                    break;
            }

            // Clean up
            outbound.close();
            inbound.close();
            clientSocket.close();
        }
        catch (IOException ioe)
        {
            System.out.println("IOException: " + ioe);
        }
    }
}

Recall that a client socket issues a connect call to a listening server socket. Client sockets are created and connected by using a constructor from the Socket class. The following line creates a client socket and connects it to a host:

Socket clientSocket = new Socket("merlin", 80);

The first parameter is the name of the host you want to connect to; the second parameter is the port number. A host name specifies only the destination computer. The port number is required to complete the transaction and allow an individual application to receive the call. In this case, port number 80 was specified, the well-known port number for the HTTP protocol. Other well-known port numbers are shown in Table 26.1. Port numbers are not mandated by any governing body, but are assigned by convention-this is why they are said to be "well known."

Because the Socket class is connection oriented, it provides a streams interface for reads and writes. Classes from the java.io package should be used to access a connected socket:

DataOutputStream outbound = new DataOutputStream( clientSocket.getOutputStream() );
DataInputStream inbound = new DataInputStream( clientSocket.getInputStream() );

Once the streams are created, normal stream operations can be performed. The following code snippet requests a Web page and echoes the response to the screen:

outbound.writeBytes("GET / HTTP/1.0\r\n\r\n);
String responseLine;
while ( (responseLine = inbound.readLine()) != null)
{
    System.out.println(responseLine);
}

When the program is done using the socket, the connection must be closed:

outbound.close();
inbound.close();
clientSocket.close();

Notice that the socket streams are closed first. All socket streams should be closed before the socket is closed. This application is relatively simple, but all client programs follow the same basic script:

1. Create the client socket connection.
2. Acquire read and write streams to the socket.
3. Use the streams according to the server's protocol.
4. Close the streams.
5. Close the socket.

Using a server socket is only slightly more complicated than using a client socket, as explained in the following section.

Server Sockets

Listing 26.2 is a partial listing of a simple server application. The complete server
example can be found on the CD-ROM that accompanies this book in SimpleWebServer.java.

Listing 26.2. A simple server application.

/**
 * An application that listens for connections and serves a simple
 * HTML document.
 */
class SimpleWebServer {
    public static void main(String args[])
    {
        ServerSocket serverSocket = null;
        Socket clientSocket = null;
        int connects = 0;
        try
        {
            // Create the server socket
            serverSocket = new ServerSocket(80, 5);

            while (connects < 5)
            {
                // Wait for a connection
                clientSocket = serverSocket.accept();

                //Service the connection
                ServiceClient(clientSocket);
                connects++;
            }
            serverSocket.close();
        }
        catch (IOException ioe)
        {
            System.out.println("Error in SimpleWebServer: " + ioe);
        }
    }

    public static void ServiceClient(Socket client)
        throws IOException
    {
        DataInputStream inbound = null;
        DataOutputStream outbound = null;
        try
        {
            // Acquire the streams for IO
            inbound = new DataInputStream( client.getInputStream());
            outbound = new DataOutputStream( client.getOutputStream());

            // Format the output (response header and tiny HTML document)
            StringBuffer buffer = PrepareOutput();

            String inputLine;
            while ((inputLine = inbound.readLine()) != null)
            {
                // If end of HTTP request, send the response
                if ( inputLine.equals("") )
                {
                    outbound.writeBytes(buffer.toString());
                    break;
                }
            }
        }
        finally
        {
            // Clean up
            System.out.println("Cleaning up connection: " + client);
            outbound.close();
            inbound.close();
            client.close();
            client.close();
        }
    }

Servers do not actively create connections. Instead, they passively listen for a client connect request and then provide their services. Servers are created with a constructor from the ServerSocket class. The following line creates a server socket and binds it to port 80:

ServerSocket serverSocket = new ServerSocket(80, 5);

The first parameter is the port number on which the server should listen. The second parameter is optional. The API documentation indicates that this second parameter is a listen time, but in traditional sockets programming, the listen function's second parameter is the listen stack depth. As it turns out, this is also true for the second constructor parameter. A server can receive connect requests from many clients at the same time, but each call must be processed one at a time. The listen stack is a queue of unanswered connect requests. The preceding code instructs the socket driver to maintain the last five connect requests. If the constructor omits the listen stack depth, a default value of 50 is used.

Once the socket is created and listening for connections, incoming connections are created and placed on the listen stack. The accept() method is called to lift individual connections off the stack:

Socket clientSocket = serverSocket.accept();

This method returns a connected client socket used to converse with the caller. No conversations are ever conducted over the server socket itself. Instead, the server socket spawns a new socket in the accept() method. The server socket is still open and queuing new connection requests.

As you do with the client socket, the next step is to create an input and output stream:

DataInputStream inbound = new DataInputStream( clientSocket.getInputStream() );
DataOutputStream outbound = new DataOutputStream( clientSocket.getOutputStream() );

Normal I/O operations can now be performed by using the newly created streams. This server waits for the client to send a blank line before sending its response. When the conversation is finished, the server closes the streams and the client socket. At this point, the server tries to accept more calls. What happens when there are no calls waiting in the queue? The method waits for one to arrive. This behavior is known as blocking. The accept() method blocks the server thread from performing any other tasks until a new call arrives. When five connects have been serviced, the server exits by closing its server socket. Any queued calls are canceled.

All servers follow the same basic script:

1. Create the server socket and begin listening.
2. Call the accept() method to get new connections.
3. Create input and output streams for the returned socket.
4. Conduct the conversation based on the agreed protocol.
5. Close the client streams and socket.
6. Go back to step 2 or continue to step 7.
7. Close the server socket.

Figure 26.1 summarizes the steps needed for client/server connection-oriented applications.

Figure 26.1 : Client and server connection-oriented applications.

Iterative and Concurrent Servers

The client/server application just presented is known as an iterative server because the code accepts a client connection and completely processes it before it accepts another connection. More complex servers are concurrent servers: Instead of accepting connections and immediately processing them, a concurrent server spawns a new thread to process each new request, so it seems as though the server is processing many requests simultaneously. All commercial Web servers are concurrent servers.

Java Datagram Classes

Unlike the client and server portions of connection-oriented classes, the datagram versions of the client and server behave in nearly identical manners-the only difference occurs in implementation. For the datagram model, the same class is used for both client and server halves. The following lines create client and server datagram sockets:

DatagramSocket serverSocket = new DatagramSocket( 4545 );
DatagramSocket clientSocket = new DatagramSocket();

The server specifies its port using the lone constructor parameter 4545. Because the client calls the server, the client can use any available port. The omitted constructor parameter in the second call instructs the operating system to assign the next available port number. The client could have requested a specific port, but the call would fail if some other socket had already bound itself to that port. It's better not to specify a port unless the intent is to be a server.

Because streams can't be acquired for communication, how do you talk to a DatagramSocket object? The answer lies in the DatagramPacket class.

Receiving Datagrams

The DatagramPacket class is used to receive and send data over DatagramSocket classes. The packet class contains connection information as well as the data. As was explained earlier, datagrams are self-contained transmission units. The DatagramPacket class encapsulates these units. The following lines receive data from a datagram socket:

DatagramPacket packet = new DatagramPacket(new byte[512], 512);
clientSocket.receive(packet);

The constructor for the packet must know where to place the received data. A 512-byte buffer is created and passed to the constructor as the first parameter. The second constructor parameter is the size of the buffer. Like the accept() method in the ServerSocket class, the receive() method blocks until data is available.

Sending Datagrams

Sending datagrams is really very simple; all that's needed is a complete address. Addresses are created and tracked using the InetAddress class. This class has no public constructors, but it does contain several static methods that can be used to create an instance of the class. The following list shows the public methods that create InetAddress class instances:

Getting the local host is useful for informational purposes, but only the first two methods are actually used to send packets. Both getByName() and getAllByName() require the name of the destination host. The first method merely returns the first match it finds. The second method is needed because a computer can have more than one address. When this occurs, the computer is said to be multi-homed. The computer has one name, but multiple ways to reach it.

All the creation methods are marked as static. They must be called as follows:

InetAddress addr1 = InetAddress.getByName("localhost");
InetAddress addr2[] = InetAddress.getAllByName("localhost");
InetAddress addr3 = InetAddress.getLocalHost();

Any of these calls can throw an UnknownHostException. If a computer is not connected to a Domain Name Server (DNS), or if the host is really not found, an exception is thrown. If a computer does not have an active TCP/IP configuration, then getLocalHost() is likely to fail with this exception as well.

Once an address is determined, datagrams can be sent. The following lines transmit a string to a destination socket:

String toSend = "This is the data to send!";
byte[] sendbuf = new byte[ toSend.length() ];
toSend.getBytes( 0, toSend.length(), sendbuf, 0 );
DatagramPacket sendPacket = new DatagramPacket( sendbuf, sendbuf.length, addr, 
port);
clientSocket.send( sendPacket );

First, the string must be converted to a byte array. The getBytes() method takes care of the conversion. Then a new DatagramPacket instance must be created. Notice the two extra parameters at the end of the constructor. Because this will be a send packet, the address and port of the destination must also be placed into the packet. An applet may know the address of its server, but how does a server know the address of its client? Remember that a datagram is like an envelope-it has a return address. When any packet is received, the return address can be extracted from the packet by using getAddress() and getPort(). This is how a server would respond to a client packet:

DatagramPacket sendPacket = new DatagramPacket( sendbuf, sendbuf.length,
    recvPacket.getAddress(), recvPacket.getPort() );
serverSocket.send( sendPacket );

Unlike connection-oriented operation, datagram servers are actually less complicated than the datagram client.

Datagram Servers

The basic script for datagram servers is as follows:

1. Create the datagram socket on a specific port.
2. Call receive() to wait for incoming packets.
3. Respond to received packets according to the agreed protocol.
4. Go back to step 2 or continue to step 5.
5. Close the datagram socket.

Listing 26.3 shows a simple datagram echo server. This server echoes back any packets it
receives.

Listing 26.3. A simple datagram echo server.

import java.io.*;
import java.net.*;

public class SimpleDatagramServer
{
    public static void main(String[] args)
    {
        DatagramSocket socket = null;
        DatagramPacket recvPacket, sendPacket;
        try
        {
            socket = new DatagramSocket(4545);
            while (socket != null)
            {
                recvPacket= new DatagramPacket(new byte[512], 512);
                socket.receive(recvPacket);
                sendPacket = new DatagramPacket(
                    recvPacket.getData(), recvPacket.getLength(),
                    recvPacket.getAddress(), recvPacket.getPort() );
                socket.send( sendPacket );
            }
        }
        catch (SocketException se)
        {
            System.out.println("Error in SimpleDatagramServer: " + se);
        }
        catch (IOException ioe)
        {
            System.out.println("Error in SimpleDatagramServer: " + ioe);
        }
    }
}

Datagram Clients

The corresponding client uses the same process with one exception: A client must initiate the conversation. The basic recipe for datagram clients is as follows:

1. Create the datagram socket on any available port.
2. Create the address to send to.
3. Send the data according to the server's protocol.
4. Wait for incoming data.
5. Go to step 3 (send more data), step 4 (wait for incoming data), or step 6 (exit).
6. Close the datagram socket.

Figure 26.2 summarizes the steps needed for client/server datagram applications. The symmetry between client and server is evident in this figure; compare Figure 26.2 with Figure 26.1.

Figure 26.2 : Client and server datagram applications.

Listing 26.4 shows a simple datagram client. It reads user input strings and sends them to the echo server from Listing 26.3. The echo server sends the data right back, and the client prints the response to the console.

Listing 26.4. A simple datagram client.

import java.io.*;
import java.net.*;

public class SimpleDatagramClient
{
    private DatagramSocket socket = null;
    private DatagramPacket recvPacket, sendPacket;
    private int hostPort;

    public static void main(String[] args)
    {
        DatagramSocket socket = null;
        DatagramPacket recvPacket, sendPacket;
        try
        {
            socket = new DatagramSocket();
            InetAddress hostAddress = InetAddress.getByName("localhost");
            DataInputStream userData = new DataInputStream( System.in );
            while (socket != null)
            {
                String userString = userData.readLine();
                if (userString == null || userString.equals(""))
                    return;
                byte sendbuf[] = new byte[ userString.length() ];
                userString.getBytes(0, userString.length(), sendbuf, 0);
                sendPacket = new DatagramPacket(
                    sendbuf, sendbuf.length, hostAddress, 4545 );
                socket.send( sendPacket );
                recvPacket= new DatagramPacket(new byte[512], 512);
                socket.receive(recvPacket);
                System.out.write(recvPacket.getData(), 0,
                    recvPacket.getLength());
                System.out.print("\n");
            }
        }
        catch (SocketException se)
        {
            System.out.println("Error in SimpleDatagramClient: " + se);
        }
        catch (IOException ioe)
        {
            System.out.println("Error in SimpleDatagramClient: " + ioe);
        }
    }
}

All the examples so far have been Java applications. Running these in an applet presents an extra complication: security.

An HTTP Server Application

Client applets need an HTTP Web server so that they can open sockets. If an applet is loaded into a browser from a hard drive, no socket activity is allowed to take place. This presents a significant hurdle to Java client applet development and testing. A simple solution is to write an HTTP server application. Once written, additional server threads can be added to provide all types of back-end connectivity.

HTTP Primer

Before diving into the project, you need some background information about the HTTP protocol. The Hypertext Transfer Protocol (HTTP) has been in use on the World Wide Web since 1990. All applet-bearing Web pages are sent over the Net with HTTP. Our server will support a subset of HTTP version 1.0 in that only file requests will be handled. As long as browser page requests can be fulfilled, the server will have accomplished its goal.

HTTP uses a stream-oriented (TCP) socket connection. Typically, port 80 is used, but other port numbers can be substituted. All the protocol is sent in plain-text format. An example of a conversation was demonstrated in Listings 26.1 and 26.2. The server listens on port 80 for a client request, which takes this format:

GET FILE HTTP/1.0

The first word is referred to as the "method" of the request. Table 26.2 lists all the request methods for HTTP/1.0.

The second parameter of a request is a file path. Each of the following URLs is followed by the request that will be formulated and sent:

HTTP://www.qnet.com/
GET / HTTP/1.0

HTTP://www.qnet.com/index.html
GET /index.html HTTP/1.0

HTTP://www.qnet.com/classes/applet.html
GET /classes/applet.html HTTP/1.0

The request does not end until a blank line containing only a carriage return (\r) and a line feed (\n) is received. After the method line, a number of optional lines can be sent. Netscape Navigator 2.0 produces the following request:

GET / HTTP/1.0
Connection: Keep-Alive
User-Agent: Mozilla/2.0 (Win95; I)
Host: merlin
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, */*

Responses use a header similar to the request:

HTTP/1.0 200 OK
Content-type: text/html
Content-Length: 128

Like the request, the response header is not complete until a blank line is sent containing only a carriage return and a line feed. The first line contains a version identification string followed by a status code indicating the results of the request. Table 26.3 lists all the defined status codes. The server sends only two of these: 200 and 404. The text that follows the status code is optional. It may be omitted; if it is present, it might not match the definitions given in the table.

Immediately after the response header, the requested file is sent. When the file is completely transmitted, the socket connection is closed. Each request-response pair consumes a new socket connection.

That's enough information for you to construct a basic Web server. Full information on the HTTP protocol can be retrieved from this URL:

HTTP://www.w3.org/

A Basic Web Server

The basic Web server follows the construction of the SimpleWebServer from Listing 26.2. Many improvements will be made to method and response handling. The simple server does not parse or store the request header as it arrives. The new Web server will have to parse and store the requests for later processing. To do this, you need a class to contain an HTTP request.

The HTTPrequest Class

Listing 26.5 shows the complete HTTPrequest class. The class must contain all the information that could be conveyed in a request header.

Listing 26.5. The HTTPrequest class.

import java.io.*;
import java.util.*;
import java.net.*;
import NameValue;

/**
 * This class maintains all the information from an HTTP request
 */
public class HTTPrequest
{
    public String version;
    public String method;
    public String file;
    public Socket clientSocket;
    public DataInputStream inbound;
    public NameValue headerpairs[];

    /**
     * Create an instance of this class
     */
    public HTTPrequest()
    {
        version = null;
        method = null;
        file = null;
        clientSocket = null;
        inbound = null;
        headerpairs = new NameValue[0];
    }

    /**
     * Add a name/value pair to the internal array
     */
    public void addNameValue(String name, String value)
    {
        try
        {
            NameValue temp[] = new NameValue[ headerpairs.length + 1 ];
            System.arraycopy(headerpairs, 0, temp, 0, headerpairs.length);
            temp[ headerpairs.length ] = new NameValue(name, value);
            headerpairs = temp;
        }
        catch (NullPointerException npe)
        {
            System.out.println("NullPointerException while adding name-value: 
" + npe);
        }
    }

    /**
     * Renders the contents of the class in String format
     */
    public String toString()
    {
        String s = method + " " + file + " " + version + "\n";
        for (int x = 0; x < headerpairs.length; x++ )
             s += headerpairs[x] + "\n";
        return s;
    }
}