servlet api - first day

servlet-api1.md

Servlet Api Training

webapp

The servlet specification defines the directory structure for a web application:

├── WEB-INF
│   ├── classes
│   │   └── com
│   │       └── acme
│   │           └── servlet
│   │               └── HelloServlet.class
│   ├── lib
│   │   ├── logback-classic.jar
│   │   ├── logback-core.jar
│   │   └── slf4j.jar
│   └── web.xml
└── index.html

As a minimum you need a WEB-INF/web.xml with a content of a few lines:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xmlns="http://java.sun.com/xml/ns/javaee" 
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" 
  id="WebApp_ID" version="2.5">
  
  <display-name>sampleWebApp</display-name>
</web-app>

javax.servlet.Servlet

• init(ServletConfig)
• service(ServletRequest, ServletResponse)
• destroy()

ServletConfig.getInitParameter()

HttpServlet

service() methos delegates to:

• doGet
• doPost
• doPut
• doDelete

url pattern

• path : /something/*
• extension: *.extension
• exact: anything else
• default: / - if nothing matches default servlet should serve. (static resources)

generating html:

resp.getWriter().println("<h1>Hello</h1>");
resp.getWriter().println("<div class=\"msg\">");
resp.getWriter().println("some message");
resp.getWriter().println("</div>");

http

• url: http://host:port/context/path?query

Request response based message protocol where both have the same structure:

• start-line (request-line / status-line)
• headers: Host, Content-type, Accept-Encoding, Accept-Language, Cookie, ...
• body: in reguest optional (only for POSTed forms), in response: html

status codes:

• 1xx: Informational
• 2xx: Successful:
  • 200: Ok
  • 201: Created
• 3xx: Redirection
  • 301: Moved Permanently
  • 302: Found
  • 304: Not Modified
• 4xx:Client Error
  • 401: Unauthorized
  • 403: Forbidden
  • 404: Not Found
• 5xx: Server Error
  • 500: Internal Server Error
  • 501: Not Implemented
  • 503: Service Unavailable

Forms

sample html form:

<form action="breaktime" method="post">
  <input type="text" name="name" />
  <input type="text" name="age" />
  <input type="submit" value="add">
</form>

java code that handles form submission:

String name = request.getParameter("name");
String age = request.getParameter("age");

Scopes

Scopes help to store information with get/setAttribute() methods.

• request: request.getAttribute()
• session: request.getSession().getAttribute()
• application: getServletContext().getAttribute()

Note: be carefull with session scope! Think about 1 million user ...

include/forward/redirect

There are several ways, how 2 servlet can communicate with eachother:

• include: get the content reproduced by an other servlet an include it in the output of the current one. (footer,header)
• forward: let an other servlet create the complete output html. (error handling)
• redirect: send an 304 response, browser sends a new request. (bookmark)

forward/include

RequestDispatcher dispatcher = getServletContext().getRequestDispatcher("/path");

dispatcher.forward(request, response);
...
dispatcher.include(request, response);