Introduction to Java

Java Background: History

Java

� was created in 1991

� by James Gosling et al. of Sun Microsystems.

� Initially called Oak, in honor of the tree outside Gosling's window, its name

was changed to Java because there was already a language called Oak.

Java Background: History

Java

� The original motivation for Java

The need for platform independent language that could be embedded in various

consumer electronic products like toasters and refrigerators.

� One of the first projects developed using Java

a personal hand-held remote control named Star.

� At about the same time, the World Wide Web and the Internet were gaining

popularity. Gosling et. al. realized that Java could be used for Internet

programming.

Java Background:

What is Java Technology?

The Java technology is:

� A programming language

� A development environment

� An application environment

� A deployment environment

Java Technology: Programming Language

As a programming language, Java can create all kinds of

applications that you could create using any conventional

programming language.

Java Technology: A Development Environment

As a development environment, Java technology provides

you with a large suite of tools:

� A compiler (javac)

� An interpreter (java)

� A documentation generator (javadoc)

� A class file packaging tool

and so on...

Java Technology: An Application and Runtime Environment

Java technology applications are typically general-purpose

programs that run on any machine where the Java runtime

environment (JRE) is installed.

There are two main deployment environments:

1. The JRE supplied by the Java 2 Software Development Kit (SDK)

contains the complete set of class files for all the Java technology

packages, which includes basic language classes, GUI component

classes, and so on.

2. The other main deployment environment is on your web browser.

Most commercial browsers supply a Java technology interpreter and

runtime environment

Java Features

 Some features of Java:

� The Java Virtual Machine

� Garbage Collection

� Code Security

Java Features:

The Java Virtual Machine

 Java Virtual Machine (JVM)

� an imaginary machine that is implemented by emulating software on a real

machine

� provides the hardware platform specifications to which you compile all Java

technology code

 Bytecode

� a special machine language that can be understood by the Java Virtual

Machine (JVM)

� independent of any particular computer hardware, so any computer with a

Java interpreter can execute the compiled Java program, no matter what

type of computer the program was compiled on1

Java Features:

Garbage Collection

Garbage collection thread

� responsible for freeing any memory that can be freed. This happens

automatically during the lifetime of the Java program.

� programmer is freed from the burden of having to deallocate that

memory themselves

Java Features:

Code Security

 Code security is attained in Java through the implementation

of its Java Runtime Environment (JRE).

 JRE

� runs code compiled for a JVM and performs class loading (through

the class loader), code verification (through the bytecode verifier)

and finally code execution1 13

Java Features:

Code Security

 Class Loader

� responsible for loading all classes needed for the Java program

� adds security by separating the namespaces for the classes of the

local file system from those that are imported from network sources

� After loading all the classes, the memory layout of the executable is

then determined. This adds protection against unauthorized access

to restricted areas of the code since the memory layout is

determined during runtime

Java Features:

Code Security

 Bytecode verifier

� tests the format of the code fragments and checks the code

fragments for illegal code that can violate access rights to objects