import

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

• The import statment makes various pre-written classes available for use in this program.

class declaration

public class httpServer
{

• A document titled httpServer.java must have this declaration. Java enforces this relationship between document title and source code content.
• Enforced structure can make it easier to read other peoples' code.

global variables

String DocumentRoot = "/web/edu/topics/java/server/web";
int port = 81;

• These variables are needed in several methods within this class, so they are declared globally.

main method

public static void main(String args[])
{
   httpServer h = new httpServer();
   h.startIt();
}

• The main method is where program execution starts when the class file is run from the command line.
• In this example, the main method is used to instantiate an object of type httpServer.
• h.startIt(); is calling a method in the server that begins listening to the port specified above.

methods

public void startIt()
{
  try{
    ServerSocket sso=new ServerSocket(port);
    System.out.println("Java httpd awaiting socket connections on port "+port);
    while(true){
       Socket so=sso.accept();
       new httpHandler(so,DocumentRoot).start();
    }
  }
  catch(Exception e){
    System.out.println("ServerSocket exception: "+e);
  }
}

• The try/catch mechanism is a way to handle "exceptions" that occur in the running of the program.
• In this method, the ServerSocket class is used to listen to a port.
• The accept() method of the ServerSocket class is where the code stalls until a connection is made.
• The start() method causes the httpHandler (which extends the Thread class) to begin running.

threads

class httpHandler extends Thread {

• This class is used each time a new connection arrives. It handles the socket connection then stops.

 

variables inside a httpHandler object

Socket so;
String PrependingPath = null;

httpHandler(Socket so, String DocumentRoot){
    this.so=so;
    this.PrependingPath = DocumentRoot;
}

• These variables are given values when the httpHandler is constructed (above).
• Note that these variables are global within the httpHandler class.
• Arguments are passed to the httpHandler instance from the httpServer class through the constructor.

DataInputStream

• The readLine() method of this class is useful for grabbing lines from the input stream.
• The input stream in this case is coming from the browser.

StringTokenizer

• The nextToken() method of this class is used to grab individual words from the lines read from the stream.

FileInputStream

• This class helps to read the file from the hard drive so that its contents can be sent into the DataOutputStream back to the browser.

HTTP

if(s!=null && s.startsWith("POST ")){
   post=true;
}

• This server must first read the header to learn about the transaction.
• It has to handle the transaction differently if the method is POST vs. GET

Response

dos.writeBytes(header);
dos.write(buf);
dos.flush();

• A header is returned as well as the buffer containing the contents of the file.
• The flush() method forces the data to be sent over the Internet to the browser without delay.