01 java intro

www.corewebprogramming.com
Java is Web-Enabled?
• Myth: Java is only for the Web
– Java “applets” run in Web pages
– Java “applications” run stand-alone
– Current usage (roughly)
• Client (applet): 5%
• Desktop (application): 45%
• Server (servlets/JSP/EJB): 50%

Java is Safe?
• Truth: Restrictions on permissible
operations can be enforced
– No “raw” memory manipulation (directly or indirectly).
• Thus, it is easy to identify prohibited operations.
– Applets, by default, prohibited from:
• Reading from the local disk
• Writing to the local disk
• Executing local programs
• Opening network connections other than to HTTP
server
• Discovering private info about user (username,
directories, OS patch level, applications installed,
etc.).

Java
Source Code
Java Bytecode
Compiler
(javac)
Java Bytecode
Execution
JIT Compiler
or Interpreter
Compile Time Run Time
Java is Cross-Platform?
• Truth: Java programs can compile to
machine-independent bytecode
• Truth: All major operating systems have
Java runtime environments
– Most bundle it (Solaris, MacOS, Windows 2000, OS/2)

• Myth: Safety and machine independence
can be achieved with no performance
penalty
– Current systems are about 20% slower than C++
– Upcoming releases claim to lower or eliminate that gap
– Expect the gap to stay at 10% or more
• Myth: Java is interpreted
– Early releases were interpreted
– Many major “Just in Time” (JIT) compilers
– HotSpot and “native” compilers even faster (IBM,
Symantec, TowerJ, etc.)

•Truth: Java has a portable graphics library
“Native look & feel” -- Java 1.1 UI controls adapt to OS
•Myth: The graphics library has everything
most applications need.
AWT (Java 1.0 and 1.1) was weak. JFC/Swing (Java 2)
much more complete and powerful.

• Myth: Write Once Run Anywhere
– Cross-platform code can be achieved, but you must test
on all platforms you will deliver on.
• Java applications can execute local code
• The graphics library behaves slightly differently on
different platforms
• The behavior of the thread scheduler is only loosely
defined
• Myth: Java will kill Microsoft
– There is also no longer immediate danger of the reverse
(Microsoft killing Java)
– Microsoft wavered between trying to fight Java and
joining it and making money by dominating the market.
With .NET, they are back to fighting it again.

Java is Simple?
• Truth: Java greatly simplifies several
language features
– Java has automatic memory management
• Does Windows and takes out the garbage
• No dangling pointers. No memory leaks.
• A problem for real-time programs
– Java simplifies pointer handling
• No explicit reference/dereference operations
• Everything is a pointer (like Lisp)
– No makefiles
– No header files
– C++ syntax streamlined

Java is Powerful?
• Truth: Java has a rich set of standard
libraries
– Networking
– Threads (lightweight processes)
– Distributed objects
– Database access
– Graphics: GUI controls and drawing
– Data structure library
– Arbitrary precision integral and fixed-point arithmetic
– Digital signatures
– Serialization (transmitting/reassembling data structures)
– File and stream compression

Java is Powerful?
• Myth: Java will increase programmer
productivity for all applications by XXX%.
• Myth: Java will kill C++
• Myth: All software should be written in Java
– Unix utilities: C
– Small/medium Windows-only programs: Visual Basic
– String parsing: Perl
– High-performance, single-platform OO systems: C++
– Air traffic control, aircraft flight software: Ada
– Knowledge-based systems: Lisp/CLOS

Key Java Packages
and Protocols
• Core Technologies
– JDBC
– RMI
– Jini (Device Networking)
– JavaBeans
– Swing
– Java 2D
• Standard Extensions
– Servlets and JavaServer Pages (JSP)
– Enterprise Java Beans
– Java 3D

Java Packages and Protocols:
JDBC (Java DataBase Connectivity)
• Standardizes mechanism for making
connection to database server
– Requires server-specific driver on client. No change to
server.
• Standardizes mechanism for sending
queries
– Either regular or parameterized queries (stored
procedures)
• Standardizes data structure of query result
– Assumes relational data, so data structure is a table
• Does not standardize SQL syntax
– Queries are simply strings
– Server extensions and enhancements supported

Remote Method Invocation (RMI)
• Built-in Distributed Object Protocol
– RMI lets a developer access a Java object and manipulate
it in the normal manner. Behind the scenes, each function
call really goes over the network to a remote object.
– Arbitrary Java data structures can be sent over the
network with little or no special packaging, due to Java’s
“serialization” mechanism
– Similar to a simplified CORBA, but restricted to
Java-to-Java communication
• Jini
– RMI-based protocol for self-documenting services.
– Allows real “plug and play” devices -- no separate drivers
– Jury is out on eventual success. Security and industry
adoption are open questions.

JavaBeans
• JavaBeans is to Java as ActiveX is to
Visual C++.
– Lets you package a Java program
as a software “component”
– Visual tools can modify/manipulate
it without knowing anything about
it in advance
• For example, you can drop a Bean into
Visual Café, IBM VisualAge for Java,
Inprise (Borland) JBuilder, Sybase PowerJ,
Metrowerks CodeWarrior, Sun JavaWorkshop, etc.,
and it is automatically available from their tool palette
for drag-and-drop development
– Better security and portability than ActiveX
– More ActiveX components available

Swing
• Standard GUI-control (widget) library in Java
2
• Many more built-in controls
• Much more flexible and customizable
• Includes many small features aimed at
commercial applications
– Tooltips, tabbed panes, on-line
help, HTML support, dockable
toolbars, multi-document
interface, etc.
• Look and feel can be
changed at run time

Servlets and JavaServer Pages (JSP)
• Servlets: Java’s answer to CGI
– Efficient: thread, not process, per request
– Convenient: HTTP headers, cookies, etc.
– Powerful: persistence, session tracking, etc.
– Secure: no buffer overflows or shell escapes
• Supported by virtually all Web servers:
– Native support: Netscape/iPlanet, IBM WebSphere, Oracle
8i/9i and Oracle Application Server, BEA WebLogic,
Silverstream, Sapphire/Web, etc.
– Via add-on engine: Apache, Microsoft IIS and Personal
WebServer, Netscape FastTrack, O’Reilly WebSite, StarNine
WebSTAR for MacOS, etc.
• JavaServer Pages (JSP)
– Convenient and efficient way to combine servlets and
HTML. Portable alternative to ASP & ColdFusion.

Enterprise JavaBeans (EJB)
• EJBs are to server components
what regular JavaBeans are to
application components
• Standardizes access to
services like load balancing,
persistence, failover, etc.
• Builds on JavaBeans, CORBA, and RMI
“under the hood”
• Potentially accessible via non-Java
programs
• Application Servers Supporting EJB
– BEA WebLogic, IBM WebSphere, Netscape, Oracle,
Progress SW Apptivity, NetDynamics, Sybase, Bluestone
Saphire/Web etc.

Basic Hello World Application
• “Application” is Java lingo for a stand-alone
Java program
– Note that the class name and the filename must match
– A file can contain multiple classes, but only one can be
declared public, and that one’s name must match the
filename
• File HelloWorld.java:
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world.");
}
}

Basic Hello World Application
(Continued)
• Compiling:
javac HelloWorld.java
• Running:
java HelloWorld
• Output:
Hello, world.

Command Line Arguments
• File ShowArgs.java:
public class ShowArgs {
public static void main(String[] args) {
for(int i=0; i<args.length; i++) {
System.out.println("Arg " + i +
" is " + args[i]);
}
}
}

Command Line Arguments,
Results
• Compiling and Running:
> javac ShowArgs.java
> java ShowArgs fee fie foe fum
Arg 0 is fee
Arg 1 is fie
Arg 2 is foe
Arg 3 is fum

Basic Hello WWW Applet
• File HelloWWW.java:
import java.applet.Applet;
import java.awt.*;
public class HelloWWW extends Applet {
public void init() {
setBackground(Color.gray);
setForeground(Color.white);
setFont(new Font("SansSerif", Font.BOLD, 30));
}
public void paint(Graphics g) {
g.drawString("Hello, World Wide Web.", 5, 35);
//5=left, 35=bottom
}
}

(Continued)
• File HelloWWW.html:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
Transitional//EN">
<HTML>
<HEAD>
<TITLE>HelloWWW: Simple Applet Test.</TITLE>
</HEAD>
<BODY>
<H1>HelloWWW: Simple Applet Test.</H1>
<APPLET CODE="HelloWWW.class" WIDTH=400 HEIGHT=40>
<B>Error! You must use a Java enabled browser.</B>
</APPLET>
</BODY>
</HTML>

(Continued)
• Compiling:
javac HelloWWW.java
• Running:
Load HelloWWW.html in a Java-enabled browser

Customizing Applets with
PARAM
import java.applet.Applet;
import java.awt.*;
public class Message extends Applet {
private int fontSize;
private String message;
public void init() {
setBackground(Color.black);
setForeground(Color.white);
fontSize = getSize().height - 10;
setFont(new Font("SansSerif", Font.BOLD, fontSize));
// Read heading message from PARAM entry in HTML.
message = getParameter("MESSAGE");
}
public void paint(Graphics g) {
if (message != null)
g.drawString(message, 5, fontSize+5);
}
}

PARAM, cont.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>The Message Applet</TITLE>
</HEAD>
<BODY BGCOLOR="WHITE">
<H1>The <CODE>Message</CODE> Applet</H1>
<P>
<APPLET CODE="Message.class" WIDTH=325 HEIGHT=25>
<PARAM NAME="MESSAGE" VALUE="Tiny">
<B>Sorry, these examples require Java</B>
</APPLET>
<P>
<APPLET CODE="Message.class" WIDTH=325 HEIGHT=50>
<PARAM NAME="MESSAGE" VALUE="Small">
<B>Sorry, these examples require Java</B>
</APPLET>
...
</BODY>
</HTML>

PARAM, Result

Summary
• Java is a complete language, supporting
both standalone applications and Web
development
• Java is compiled to bytecode and can be
run on any platform that supports a Java
Virtual Machine
• Java 2 Platform is available in a Standard
Edition, Enterprise Edition, or Micro Edition
• Compiling: use “javac”
• Executing standalone programs: use “java”
• Executing applets: load HTML file in
browser

01 java intro

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01 java intro