Protocol

The Hypertext Transfer Protocol (HTTP) is an application protocol for distributed, collaborative, hypermedia information systems. It is a stateless request-reponse protocol used to access data on another computer.

Versions

Request

HTTP requests are messages sent by the client to initiate an action on the server. A request consists of:

  • A request line
    • HTTP method
    • Request target (absolute path)
    • HTTP version
  • Headers (optional key-value pairs)
  • An empty line
  • A message body (optional)
  GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/html

Methods

The following HTTP methods are defined:

MethodRequest BodySafeIdempotentCacheable
GETOptionalYesYesYes
HEADOptionalYesYesYes
POSTYesNoNoYes
PUTYesNoYesNo
DELETEOptionalNoYesNo
PATCHYesNoNoNo
OPTIONSOptionalYesYesNo
CONNECTOptionalNoNoNo
TRACENoYesYesNo

Headers

HTTP headers are key-value pairs that provide additional information about the request or response.

Common headers include:

  • Accept: Media types accepted by the client
  • Accept-Encoding: Content encodings accepted by the client
  • Accept-Language: Human languages accepted by the client
  • Authorization: Credentials for authentication
  • Cache-Control: Cache directives
  • Content-Type: Media type of the body
  • Cookie: Cookies sent by the client
  • Host: Domain name of the server
  • Location: Redirect location
  • Referer: Referrer URL (spelling mistake in the spec)
  • User-Agent: Information about the client

Response

HTTP responses are messages sent by the server in response to a client’s request. A response consists of:

  • A status line
    • HTTP version
    • Status code
    • Message
  • Headers (optional key-value pairs)
  • An empty line
  • A message body (optional)
  HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 35

<!DOCTYPE html><html>Hello, World!</html>

Java Webserver

An example web server implementation in Java that returns the current date.

  public class HttpDemoServer {
    public static void main(String[] args) throws IOException {
        HttpServer server = HttpServer.create(new InetSocketAddress(80), 0);
        server.createContext("/date", new DateHandler());
        server.start();
    }
}

public class DateHandler implements HttpHandler {
    @Override
    public void handle(HttpExchange exchange) throws IOException {
        exchange.getResponseHeaders().add("Content-Type", "text/html");
        String response = "<b>" + LocalDateTime.now() + "</b> for " + exchange.getRequestURI();
        exchange.sendResponseHeaders(200, response.length());
        try (OutputStream os = exchange.getResponseBody()) {
            os.write(response.getBytes());
        }
    }
}

Java Servlet

Java Servlets are server-side Java components that generate dynamic content. They are part of the Java EE (Enterprise Edition) platform.

Lifecycle Methods

  • void init(ServletConfig config): Called by the servlet container to indicate that the servlet is being placed into service.
  • void destroy(): Called by the servlet container to indicate that the servlet is being taken out of service.
  • void service(ServletRequest req, ServletResponse res): Called by the servlet container to allow the servlet to respond to a request.

HTTP Servlet

The HttpServlet class extends the GenericServlet class and provides methods to handle HTTP-specific requests. The service method dispatches requests to the appropriate doXXX method based on the HTTP method.

  • void doGet(HttpServletRequest req, HttpServletResponse res)
  • void doPost(HttpServletRequest req, HttpServletResponse res)
  • void doPut(HttpServletRequest req, HttpServletResponse res)
  • void doDelete(HttpServletRequest req, HttpServletResponse res)
  • void doHead(HttpServletRequest req, HttpServletResponse res)
  • void doOptions(HttpServletRequest req, HttpServletResponse res)
  • void doTrace(HttpServletRequest req, HttpServletResponse res)

Example

Currency converter web application using a servlet.

  <form>
    <input name="amt" size="5" value="1">
    <select name="from">
        <option value="USD">USD</option>
        <option value="EUR">EUR</option>
        <option value="GBP">GBP</option>
    </select>
    <select name="to">
        <option value="USD">USD</option>
        <option value="EUR">EUR</option>
        <option value="GBP">GBP</option>
    </select>
    <input type="submit" value="Convert">
</form>
  
  public class Converter extends HttpServlet {
    public void doPost(HttpServletRequest request, HttpServletReponse response)
            throws ServletException, IOException {
        double amount = Double.parseDouble(request.getParameter("amt"));
        String from = request.getParameter("from");
        String to = request.getParameter("to");
        double result = computeResult(amount, from, to);
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<html>\n<body bgcolor=\"white\">");
        out.println("<h1>Currency Converter</h1>")
        out.printf("%s %.2f = %s %.2f\n", from, amount, to, result);
        out.println("</body>\n</html>");
    }

    private double computeResult(double amount, String from, String to) {
        // Conversion logic
    }
}
  
  <?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="https://jakarta.ee/xml/ns/jakartaee"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="https://jakarta.ee/xml/ns/jakartaee
 https://jakarta.ee/xml/ns/jakartaee/web-app_6_0.xsd"
 version="6.0">
 <servlet>
 <servlet-name>CurrencyConverter</servlet-name>
 <servlet-class>ch.fhnw.ds.Converter</servlet-class>
 </servlet>
 <servlet-mapping>
 <servlet-name>CurrencyConverter</servlet-name>
 <url-pattern>/convert</url-pattern>
 </servlet-mapping>
</web-app>