Chapter 2 - Introduction to Java Applications

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Introduction
Read DD Sec 1.8 Sec 1.13THE Java tutorial -
http//java.sun.com/docs/books/tutorial/
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About Java

• Developed by Sun Microsystems
• Originally intended for intelligent
  consumer-electronic devices (smart refrigerators,
  microwave ovens, toasters..)

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About Java (cont.)

• Java Evolved into a general-purpose, high-level
  Java programming language which
• Provides a powerful software platform for the
  development of applications.
• Allows writing applets for the Web
• An applet is a program that runs within a
  Java-enabled browser.

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Java Applications Servers

• A Java server is a special kind of a Java
  application which serves and supports client
  programs on a network
• Examples of servers
• Mail servers
• Proxy serves
• Print servers

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A Servlet

• Another specialized java program
• Can almost be thought of as an applet that runs
  on the server side.
• Servlets are similar to applets in that they are
  runtime extensions of applications
• Instead of working in browsers, servlets run
  within Java Web servers, configuring or tailoring
  the server

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Basics of a Typical Java Environment (Sec 1.13)

• Java is unusual in that a program is both
  compiled and interpreted
• Java programs normally undergo five phases
• Edit
• Programmer writes program (and stores program on
  disk)
• Compile
• Compiler creates platform-independent codes
  called Java bytecodes

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Java Environment (cont.)

• Load
• Class loader stores bytecodes in memory
• Verify
• Verifier ensures bytecodes do not violate
  security requirements
• Execute
• Interpreter translates bytecodes into machine
  language

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Fig. 1.1 Typical Java environment.
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Java Virtual Machine (Java VM).

• Think of Java bytecodes as the machine code
  instructions for the Java Virtual Machine (Java
  VM).
• A Java interpreter is an implementation of the
  Java VM
• A Java interpreter can be
• A development tool that runs aplications
• A Web browser that can run applets

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Java Portability

• Java bytecodes help make "write once, run
  anywhere" possible.
• You can compile your program into bytecodes on
  any platform that has a Java compiler.
• The bytecodes can then be run on any
  implementation of the Java VM.
• Java portability
• As long as a computer has a Java VM, the same
  program written in Java can run on Windows 2000,
  a Solaris workstation, or on an iMac

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