Server-Side Java Programming

1. Server-Side Java Programming

2. Java • Java started originally as OAK, for remote control of peripherals. • Applets, small portable web-page-embeddable programs made Java look appealing at first. • Today, applets are not very significant to Java's popularity or success. • Server-side Java has been most important driving force behind Java's direction.

3. Java Language • Java was designed by James Gosling from scratch to be a language that is: • Highly object-oriented • Garbage-collected • Multithreaded to the core • Secure/safe/managed • Dynamic and reflective • Interpreted (today, Just-In-Time compiled)‏ • The approach can be summarized as “first, do it right, then, improve efficiency.”

4. Packages and Classes • Directory structure parallels package hierarchy. • One .java file contains one public Java class by the same name. org/apache/logging/Main.java: package org.apache.logging; public class Main { ... } • Other non-public classes, nested classes and anonymous classes may also be defined in that same file. • Java compiles to intermediate “bytecode”; each class creates one .class file (may be multiple per .java file).

5. Bytecode • Unlike platform-specific executables, same bytecode can run on any platform that supports Java. • Bytecode is often smaller than source code (which is often much smaller than the corresponding compiled executable file). This speeds up Java code download on the network. • All external class references, methods, externally visible internal methods and fields will have names recorded in bytecode.

6. Symbols in Bytecode • Bytecode contains many symbols. • This allows classes with known interfaces to be dynamically loaded and used with ease. • Often, reverse engineering is quite possible. • Obfuscators can be used to make reverse engineering harder, but they require all classes to be gathered and obfuscated together (because public APIs will be jumbled up). • This eliminates the ability to dynamically load and use classes.

7. Standard Libraries • A very large standard library exists, is well-documented, and can be assumed available anywhere Java is available. • This helps reduce application code size. • Java motto is “Write once, run anywhere.” • You should quickly learn to read and navigate the API documentation. • We will use both Java SDK and Java EE libraries and APIs.

8. Java Syntax • There are no structs or properties in Java. • Objects are always passed-by-reference. • Strings and arrays are special types of objects; other objects can not do operator overloading (as in C++). • Generics and assertions were added later without changing the bytecode. • Annotations were added later and they changed Java significantly.

9. Server-Side Java • Lighter-weight three-tier Java “Web Applications” use Servlets and JSPs (JavaServer Pages). • Heavier-weight four-tier Java “Enterprise Applications” use EJBs. • In Java EE 5, annotation-based entities that contain database-persisted data can be used by both of these types of applications. • Web services and application client containers are also supported.

12. JSP • JSP is much like PHP. • It allows Java code to be embedded within HTML. • Java code runs on the server side and produces HTML. • Client only sees the HTML. • JSPs are compiled automatically to Servlets when a page is requested by a client.

13. JSTL • JSP pages used to mix presentation with business logic. • To separate code from the design, JSTL (JSP Tag Library) was created. • JSTL tags look like HTML tags, but are backed by Java code for each tag. • This allows JSP pages to stay smaller and use less Java code, making accidental change of code by designers less likely.

14. Servlets • Servlets and JavaBeans are often used to have persistent logic, and page flow control. • JSPs are best used for presentation (more HTML), Servlets can be used for business logic (more Java code). • Client sessions can be created and used in both Servlets and JSPs.

15. EJBs and Entities • In four-tier enterprise applications, EJBs usually handle the business logic. • In this case, JSPs and Servlets are used for presentation issues. • In Java EE 5, “entity beans” are now replaced with “entity objects”, which are part of the new JPA (Java Persistence API). • Entity objects are automatically saved and retrieved from database. • Enterprise Application Container handles EJB issues and persistence.

16. Types of EJBs • Stateless Session beans don't hold any client state. They are often pooled and shared amongst clients. • Stateful Session beans hold client state. They are created per-client. • Message-Driven beans allow asynchronous messaging; caller does not wait for a reply.

17. Enterprise Application Server • We will use Sun JSAS PE 9.0 (Sun Java System Application Server Platform Edition 9.0). • Sun JSAS is based on Glassfish open-source application server. • Java EE 5 greatly simplifies EJB development. • This is mostly due to object-relational mapping that automates object persistence by using annotations. • The standardized mapping was created by Oracle, in its open-source product “Toplink Essentials”.

18. JSP Code Examples

19. helloworld.jsp <%@page contentType="text/html“ %> <%@page pageEncoding="UTF-8“ %> <html> <head><title>JSP: Hello World!</title></head> <body> <h1>JSP Says:</h1> <% out.println("Hello World!"); %> </body> </html>

20. helloform.jsp 1/2: JSP Processing <%@page contentType="text/html“ %> <%@page pageEncoding="UTF-8“ %> <html> <head><title>JSP: Hello Xyz</title></head> <body> <% String user = request.getParameter("user"); if (user == null) user = "Stranger"; %> <h1>Hello <%= user %></h1>

21. helloform.jsp 2/2: HTML Form … (JSP processing goes here) <form method="POST" action="helloform.jsp"> Your Name: <input type="text" name="user" size="40"> <input type="submit" name="submit" value="OK"> </form> </body> </html>

22. colorbox.jsp Structure • Colorbox uses nested for loops to create a colorful table. • colorbox.jsp structure: • Page directives at the top • HTML on the outside • Initialize formatter to print hexadecimal later. • Outer for loop for red color • Nested HTML “<tr>” • Inner loop prints dynamic HTML

23. colorbox.jsp 1/2 <%@page contentType="text/html“ %> <%@page pageEncoding="UTF-8“ %> <%@page import="java.util.Formatter" %> <html> <head><title>JSP: Color Box</title></head> <body> <table border="0" cellspacing="0"> <% StringBuilder sb = new StringBuilder(); Formatter formatter = new Formatter(sb); int red, green;

24. colorbox.jsp 2/2 for (red = 0; red <= 255; red += 17) { // instead of <tr> below, we can use 'out.println("<tr>")' here %> <tr> <% for(green = 0; green <= 255; green += 17) { out.println(formatter.format( "<td bgcolor='%02x%02xff'>&nbsp;&nbsp;&nbsp;", red, green)); sb.setLength(0); // avoid appending next string } } %> </body></html>