Basics of Servlets: Structure and Utilities for Web Development

1. Chapter 3 Servlet Basics

2. Servlet Basics • Recall the Servlet Role • Basic Servlet Structure • A simple servlet that generates plain text • A servlet that generates HTML • Servlets and packages • Some utilities that help build HTML • The servlet life cycle • Servlet debugging strategies

3. 1. Recall the Servlet Role • Read the explicit data sent by the client • Read the implicit HTTP request data sent by the browser

4. 1. Recall the Servlet Role • Generate the results • Send the explicit data (i.e., the document) to the client • Send the implicit HTTP response data

5. 2. Basic Servlet Structure • Listing 3.1 outlines a basic servlet that handles GET requests

6. 2. Basic Servlet Structure • GET requests are the usual type of browser requests for Web pages • A browser generates this request when the user enters a URL on the address line, follows a link from a Web page • Or submits an HTML form that either does not specify a METHOD or specifies METHOD="GET" • Post method: Submit button

7. 2. Basic Servlet Structure • Servlets extend • HttpServlet • override doGet or doPost, depending on whether the data is being sent by GET or by POST • Servlet can take the same action for both GET and POST requests, simply have doGet call doPost.

8. 2. Basic Servlet Structure • Both doGet and doPost take two arguments : • HttpServletRequest • HttpServletResponse • The HttpServletRequest gets at all of the incoming data. • The HttpServletResponse specifies outgoing information such as HTTP status codes (200, 404, etc.) and response headers (Content-Type, Set-Cookie, etc.). • HttpServletResponse obtains a PrintWriter, use to send document content back to the client.

9. 2. Basic Servlet Structure • doGet and doPost throw two exceptions: • ServletException • IOException •  Require to include them in the method declaration. • Import classes: • java.io (for PrintWriter, etc.) • javax.servlet (for HttpServlet, etc.) • javax.servlet.http (for HttpServletRequest and HttpServletResponse).

10. 3. A Servlet That Generates Plain Text 3.1. Hello World Problem 3.2. Hello World Solution 3.3. A Servlet That Generates Plain Text

11. 3.1. Hello World Problem • Write a program that shows “Hello World” as below:

12. 3.2. Hello World Solution

13. 3.3. A Servlet That Generates Plain Text • First, reviewing the process of installing, compiling, and running this simple servlet • server is set up properly • CLASSPATH refers to the necessary three entries • execute successfully

14. 3.3. A Servlet That Generates Plain Text • Second • type "javac HelloWorld.java" • ->create HelloWorld.class • Third • move HelloWorld.class to the directory that server uses to store servlets that are in the default Web application • install_dir/.../WEB-INF/classes • Last • Invoke servlet. • Using either the default URL of http://host/servlet/ServletName

15. 4. A Servlet That Generates HTML 4.1. Hello Problem 4.2 Hello Solution 4.3. A servlet that generates HTML

16. 4.1. Hello Problem • Write a program that shows an HTML with content “Hello” as below:

17. 4.2. Hello Solution

18. 4.3. A Servlet That Generates HTML • To generate HTML, add three steps to the process just shown: • Tell the browser that you're sending it HTML. • Modify the println statements to build a legal Web page. • Check your HTML with a formal syntax validator.

19. 4.3. A Servlet That Generates HTML • response.setContentType("text/html"); • Servlets to generate Excel spreadsheets (content type application/vnd.ms-excel—see Section 7.3), • JPEG images (content type image/jpeg—see Section 7.5), • and XML documents (content type text/xml) • println statements output HTML

20. 5. Servlet Packaging 5.1. Hello Problem with Packaging 5.2. Hello Packaging Solution 5.3. Servlet Packaging

21. 5.1. Hello Problem with Packaging • Shows the servlet accessed by means of the default URL

22. 5.2. Hello Packaging Solution

23. 5.3. Servlet Packaging • Since even a single Web application can be large, need the standard Java solution for avoiding name conflicts: packages • Step • Place the files in a subdirectory that matches the intended package name • Insert a package statement in the class file • package somePackage • install_dir/webapps/ROOT/WEB-INF/classes/coreservlets • Listing 3.4 coreservlets/HelloServlet2.java

24. 6. Simple HTML-Building Utilities • An HTML document is structured as follows: • Listing 3.5 coreservlets/ServletUtilities.java • Listing 3.6 coreservlets/HelloServlet3.java

25. 6. Simple HTML-Building Utilities

26. 6. Simple HTML-Building Utilities

27. 6. Simple HTML-Building Utilities • Result of http://localhost/servlet/coreservlets.HelloServlet3

28. 7. The Servlet Life Cycle • When the servlet is first created, its init method is invoked, so init is where you put one-time setup code. • After this, each user request results in a thread that calls the service method of the previously created instance. • The service method then calls doGet, doPost, or another doXxx method, depending on the type of HTTP request it received. • Finally, if the server decides to unload a servlet, it first calls the servlet's destroy method.

29. 7. The Servlet Life Cycle • The service Method • Each time the server receives a request for a servlet, the server spawns a new thread and calls service • The service method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc., as appropriate

30. 7. The Servlet Life Cycle • The init method definition looks like this: • public void init() throws ServletException { // Initialization code... } • The init method performs two varieties of initializations: • general initializations • and initializations controlled by initialization parameters.

31. General Initializations • Init: creates or loads some data that will be used throughout the life of the servlet, or it performs some one-time computation. • Listing 3.7 shows a servlet that uses init to do two things. • First, it builds an array of 10 integers, complex calculations . • So, doGet looks up the values that init computed, instead of generating them each time. The results of this technique are shown in Figure 3-6.

32. General Initializations • Book • The destroy Method : • asked by the server administrator • Or the servlet is idle for a long time • It closes database connections, • halt background threads, • write cookie lists or hit counts to disk, and perform other such cleanup activities

33. 8. Servlet Debugging Strategies • Use print statements • Use an integrated debugger in your IDE. • Use the log file. • Write separate classes • Look at the HTML source. • Look at the request data separately • Look at the response data separately • Stop and restart the server.

34. Practice Exercise • Problem 1: Create a servlet page that view all the month and day of month of 2011. • Problem 2: Create a servlet page that view all of lucky numbers from 1 to 1000. A lucky number is an integer number that have sum of digits mod 10 equal 7 For example: 7 is lucky number 16 is lucky number ( 1 + 6 = 7 and 7 % 10 = 7) 250 is lucky number (1 + 5 + 0 = 7 and 7 % 10 = 7) 962 is lucky number ( 9 + 6 + 2 = 17 and 17 % 10 = 7)