Lecture 1

Basic Web Technologies

The client is a computer running a browser that can display webpages. The client interacts via a network with the server which can serve webpages and other resources to the client.

The client and server rely on HTTP (Hypertext Transfer Protocol) to send requests and responses.

A HTTP request has a method, such as:

• GET
• POST
• PUT
• DELETE

HTTP is built on top of TCP/IP.

Server Solutions

Common Gateway Interface

The Common Gateway Interface (CGI) is a interface specification. It allows programs to be called through the web. It relates a program (with it's parameters) to a url

Example:

http://example.com/cgi-bin/printenv.pl/somepath?and=a&query=string

here the parameters are and=a and query=string.

A typical use for CGI is an interface for a database: the HTML page displayed is the result of a query, the query can be changed with the parameters of the url.

The main limitation of CGI is poor scalability, a new process is started for each new HTTP request. Meanwhile servlets replaces this by creating a new thread for each request (which has much lower overhead)

Java Servlets

Servlet

A java Servlet is a java class that allows you to build functionality on top of a web server. Servlets execute in a network server like Apache Tomcat.

HTTP Servlets allow you to take an incoming http request to the network server and provide a response (request-response communication model).

Servlets allow you to generate dynamic content, i.e you can return a custom response to the client based on the request.

Unlike CGIs, Servlets allow peristent context for applications.

Servlet Typical Execution Flow

1. Server receives HTTP request sent by user that points to a Servlet.
2. Invokes Servlet by passing the HTTP request to it.
3. Processes request and generates result
   • Http response (HTML page, json document, xml document, etc...)
4. Servlet returns an HTTP response.
5. HTTP Response is sent to the client side.

Servlet Invocation

Example url: http://localhost:8080/bookQuote/bookQuote?isbn=1

url format

[protocol]://[host:port]/webApp/Servlet?Parameters

• A single instance of the Servlet is used to handle all requests over the lifetime of the Servlet.

Servlet Invocation With Web Page

<form action="./bookQuote">
    <p>Enter ISBN number: <input type="text" name="isbn" /></p>
    <input type="submit">
</form>

Servlet Interaction

Basic Servlet

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ServletTemplate extends HttpServlet {
    public void doGet(
        HttpServletRequest request
        HttpServletResponse response
        ) throws ServletException, IOException {
        // Use ‘request’ to read incoming HTTP headers
        // (e.g., cookies) and parameters (query data)
        // Use ‘response’ to specify the HTTP response status
        // code and headers (e.g., the content type, cookies)PrintWriterout = response.getWriter();
        // Use ‘out’ to send content to browser
    }
}

Servlet Example

public class BookQuote extends HttpServlet {
    private Quoter quoter;

    public void init() throws ServletException {
        quoter = new Quoter();
    }

    public void doGet(
        HttpServletRequest request
        HttpServletResponse response
    ) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        String html = // html
        out.println(html);
    }
}

Development Process Comparison

.WAR File Structure (TOMCAT)

Web applications are packaged in .WAR (Web ARchive) files in order to be deployed

WAR files contain the code necessary to run the application:

• Java bytecode
• libraries
• HTML files
• CSS files
• pictures etc

Example WAR file structure:

/
│   *.html (all html files)
│
└───WEB-INF
    │   web.xml (deployment descriptor)
    │
    └───classes
    │   │   *.class (Java bytecode)
    │
    └───lib
    │   *.jar (libraries)

Deployment Descriptor Example

The deployment descriptor allows you to map a Servlet to a URL. The deployment descriptor is the WEB-INF/web.xml file an example of a deployment descriptor is:

<?xmlversion="1.0"encoding="ISO-8859-1"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xmlns="http://xmlns.jcp.org/xml/ns/javaee"xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaeehttp://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"version="3.1">
<description>BookQuoteServlet</description>
    <display-name>BookQuoteServlet</display-name>
    <servlet>
    <!-- Binds Servlet name to code -->
        <servlet-name>BookQuote</servlet-name>
        <servlet-class>nl.utwente.di.BookQuote</servlet-class>
    </servlet>
    <servlet-mapping>
    <!-- Binds Servlet name to URL -->
        <servlet-name>BookQuote</servlet-name>
        <url-pattern>/bookQuote</url-pattern>
    </servlet-mapping>
</web-app>

Java Annotations

Java annotations are an alternative to the deployment descriptor. They map a servlet class to a url but instead of being in a separate web.xml file it is defined in the Servlet that we want to map to a URL.

Example:

@WebServlet(description="BookQuote Servlet", urlPatterns={"/bookQuote"})
public class BookQuote extends HttpServlet {
    ...
}

Javascript

Javascript is a lightweight interpreted programming language. It was created to be a scripting language for web pages.

• javascript is object-based.
• javascript has dynamic typing: variable data types are not declared.
• properties and methods can be added to any object dynamically.

Javascript allows you to change the client page or parts of the page dynamically. Therefore pages can become more responsive.

Web Services

Web Service

A web service is a software system designed to support interoperable machine-to-machine interaction over a network.

A web service is different form a web page. A web service provides data with no formatting while a webpage is a document that can have a defined formatting.

Web Service Flavours

• SOAP: Web services implemented on top of the SOAP protocol.

  • considered as 'heavyweight'

• RESTful: Based on the REST principles.

  • considered as 'lightweight'

REST

This is an important topic so it has it's dedicated page ✨

Overview