write once, run anywhere Nishad Abdulkareem, Lecturer MES College Marampally

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write once, run
anywhereNishad Abdulkareem, LecturerMES College
Marampally
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Index

• Java Programming language
• History
• Java Varieties
• Java world
• Buzz words
• Classpath
• Program

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Java Programming Language

• Java is a programming language originally
  developed by James Gosling at Sun Microsystems
  (which is now a subsidiary of Oracle Corporation)
  and released in 1995 as a core component of Sun
  Microsystems' Java platform.
• The language derives much of its syntax from C
  and C but has a simpler object model and fewer
  low-level facilities.
• Java applications are typically compiled to
  bytecode (class file) that can run on any Java
  Virtual Machine (JVM) regardless of computer
  architecture.
• It is intended to let application developers
  "write once, run anywhere".

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Once upon a time

• The original and reference implementation Java
  compilers, virtual machines, and class libraries
  were developed by Sun from 1995.
• As of May 2007, in compliance with the
  specifications of the Java Community Process, Sun
  relicensed most of its Java technologies under
  the GNU General Public License.
• James Gosling initiated the Java language project
  in June 1991 for use in one of his many set-top
  box projects.
• Java was originally designed for interactive
  television, but it was too advanced. The
  language, initially called Oak after an oak tree
  that stood outside Gosling's office, also went by
  the name Green and ended up later renamed as
  Java, from a list of random words.

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and grown up like

• Sun Microsystems released the first public
  implementation as Java 1.0 in 1995. It promised
  "Write Once, Run Anywhere" (WORA), providing
  no-cost run-times on popular platforms.
• Fairly secure and featuring configurable
  security, it allowed network- and file-access
  restrictions. Major web browsers soon
  incorporated the ability to run Java applets
  within web pages, and Java quickly became
  popular.
• With the advent of Java 2 (released initially as
  J2SE 1.2 in December 19981999), new versions had
  multiple configurations built for different types
  of platforms.
• The different platforms are J2EE,J2ME and J2SE

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Behind the names

• Java Platform, Enterprise Edition or Java EE is a
  widely used platform for server programming in
  the Java programming language.
• The Java platform (Enterprise Edition) differs
  from the Java Standard Edition Platform (Java SE)
  in that it adds libraries which provide
  functionality to deploy fault-tolerant,
  distributed, multi-tier Java software, based
  largely on modular components running on an
  application server.
• For marketing purposes, Sun renamed new J2
  versions as Java EE. The current version is
  called Java EE 6.
• Java EE includes several API specifications, such
  as JDBC, RMI, e-mail, JMS, web services, XML,
  etc, and defines how to coordinate them.

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JSE

• Java Standard Edition or Java SE is a widely used
  platform for programming in the Java language.
• It is the Java Platform used to deploy portable
  applications for general use.
• In practical terms, Java SE consists of a
  virtual machine, which must be used to run Java
  programs, together with a set of libraries (or
  "packages") needed to allow the use of file
  systems, networks, graphical interfaces, and so
  on, from within those programs.
• The "SE" is used to distinguish the base platform
  from Java EE and Java ME. The "2" was originally
  intended to emphasize the major changes
  introduced in version 1.2, but was removed in
  version 1.6.

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JME

• Java Platform, Micro Edition, or Java ME, is a
  Java platform designed for mobile devices and
  embedded systems.
• Target devices range from industrial controls to
  mobile phones (especially feature phones) and
  set-top boxes. Java ME was formerly known as Java
  2 Platform, Micro Edition (J2ME).

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JEE

• Java EE also features some specifications unique
  to Java EE for components. These include
  Enterprise JavaBeans, Connectors, servlets,
  portlets (following the Java Portlet
  specification), JavaServer Pages and several web
  service technologies.
• This allows developers to create enterprise
  applications that are portable and scalable, and
  that integrate with legacy technologies.
• A Java EE application server can handle
  transactions, security, scalability, concurrency
  and management of the components that are
  deployed to it, in order to enable developers to
  concentrate more on the business logic of the
  components rather than on infrastructure and
  integration tasks

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Java World
Provides a simplified, fast way to create dynamic
web content. JSP technology enables rapid
development of web-based applications that are
server- and platform-independent.

• Java Server Pages
• Java Servlets
• Java Messaging Services
• Java mail API
• Java API for XML Processing
• JDBC API

A servlet class extends the capabilities of
servers that host applications that are accessed
by way of a request-response programming model.
Although servlets can respond to any type of
request, they are commonly used to extend the
applications hosted by web servers
Messaging is a method of communication between
software components or applications. a messaging
client can send messages to and receive messages
from any other client. Each client connects to a
messaging agent that provides facilities for
creating, sending, receiving, and reading
messages.Eg Car inventory
Web applications can use the JavaMail API to send
email notifications. The API has two parts an
application-level interface that the application
components use to send email and a service
provider interface. Service providers implement
particular email protocols, such as SMTP
The Java API for XML Processing (JAXP), part of
the Java SE platform, supports the processing of
XML documents using the Document Object Model
(DOM), the Simple API for XML (SAX),
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JRE

• Java Runtime Environment
• The java programming language adds the
  portability by converting the source code to byte
  code version which can be interpreted by the JRE
  and gets converted to the platform specific
  executable ones.
• Thus for different platforms one has
  corresponding implementation of JRE. But JRE has
  to meet the specification JVM (Java Virtual
  Machine) Concept that serves as a link between
  the Java libraries and the platform specific
  implementation of JRE.
• Thus JVM helps in the abstraction of inner
  implementation from the programmers who make use
  of libraries for their programmes.    

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• Science Behind Java

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JDK

• Java Developer Kit contains tools needed to
  develop the Java programs, and JRE to run the
  programs. The tools include compiler (javac.exe),
  Java application launcher (java.exe),
  Appletviewer, etc
• The JDK(Java Developmental Tool kit) comes along
  with java libraries and JVM embedded in it.
• Apart from these it comes along with the utility
  tools for byte code compilation "javac",
  Executing the byte codes through java programmes
  through "java" and many more utilities found in
  the binary directory of java.
• Speaking practically JDK is essential for
  developers, which comes along with library
  packages to develop Software programmes.
• JRE is minimal set of programmes which executes
  the java class files developed by the software
  developers.
• Without jre one can only compile the application
  but cannot run the application Since jre contains
  the JVM which executes the byte code generated
  from the javac compiler.

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• JRE JVM Java Packages Classes(like util,
  math, lang, awt,swing etc)runtime libraries.
• Java Runtime Environment contains JVM, class
  libraries, and other supporting files. It does
  not contain any development tools such as
  compiler, debugger, etc. Actually JVM runs the
  program, and it uses the class libraries, and
  other supporting files provided in JRE. If you
  want to run any java program, you need to have
  JRE installed in the system
• The Java Virtual Machine provides a
  platform-independent way of executing code
  programmers can concentrate on writing software,
  without having to be concerned with how or where
  it will run

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• As we all aware when we compile a Java file,
  output is not an 'exe' but it's a '.class' file.
  '.class' file consists of Java byte codes which
  are understandable by JVM. Java Virtual Machine
  interprets the byte code into the machine code
  depending upon the underlying operating system
  and hardware combination. It is responsible for
  all the things like garbage collection, array
  bounds checking, etc JVM is platform dependent.
• The JVM is called "virtual" because it provides a
  machine interface that does not depend on the
  underlying operating system and machine hardware
  architecture. This independence from hardware and
  operating system is a cornerstone of the
  write-once run-anywhere value of Java programs.
• There are different JVM implementations are
  there. These may differ in things like
  performance, reliability, speed, etc. These
  implementations will differ in those areas where
  Java specification doesnt mention how to
  implement the features, like how the garbage
  collection process works is JVM dependent, Java
  spec doesnt define any specific way to do this.

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• JVM
• ByteCode
• JIT
• Applets

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Java Buzzwords

• Simple
• Java inherits the C/C syntax and many of the
  object oriented features of C ,most programmers
  have little trouble learning java.
• Object Oriented
• The object model in java is Simple and easy to
  extend, while primitive types ,such as integers
  ,are kept as high performance nonobjects.
• Robust
• Memory management
• Exception Handling
• Multithreaded
• Multiprocess synchronization
• Architectural Neutral
• Distributed
• RMI

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Classpath

• The Classpath is an argument - set either on the
  command-line, or through an environment variable
  - that tells the Java Virtual Machine where to
  look for user-defined classes and packages when
  running Java programs.
• When we invoke Java, we specify the name of the
  application to run org.mypackage.HelloWorld.
  However we must also tell Java where to look for
  the files and directories defining our package.
  So to launch the program, we use the following
  command
• java -classpath D\myprogram org.mypackage.HelloWo
  rld

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Say hello to Java

• First Program
• All program starts by executing by calling main()
• Public is the access specifier.main must be
  declared as public so that it can access from
  outside the class
• static allows main to be called without
  instantiating a particular class.
• Any information that are passed in to the method
  is received by variables specified within the
  parenthesis called parameters .
• String args declares a parameter named args,
  which is an array of instances of string.
• System is predefined class that provides access
  to system, out is the output stream that is
  connected to the console
• Any information need to pass to a method is
  specified within the set of parameters that
  follow the name of method_called parameters

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• Swap two numbers without using a third variable
• Reverse Number
• Expression

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javap - The Java Class File Disassembler

• The javap command disassembles a class file. Its
  output depends on the options used. If no options
  are used, javap prints out the package,
  protected, and public fields and methods of the
  classes passed to it. javap prints its output to
  stdout. For example, compile the following class
  declaration
• help Prints out help message for javap.
• -l Prints out line and local variable tables.
• -b Ensures backward compatibility with javap in
  JDK 1.1.
• -public Shows only public classes and members.
• -protected Shows only protected and public
  classes and members.
• -package Shows only package, protected, and
  public classes and members. This is the default.
• -private Shows all classes and members.

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• -Jflag Pass flag directly to the runtime system.
  Some examples javap -J-version javap
  -J-Djava.security.manager -J-Djava.security.policy
  MyPolicy MyClassName
• -s Prints internal type signatures.
• -c Prints out disassembled code, i.e., the
  instructions that comprise the Java bytecodes,
  for each of the methods in the class. These are
  documented in the Java Virtual Machine
  Specification.
• -verbose Prints stack size, number of locals and
  args for methods.
• -classpath path Specifies the path javap uses to
  look up classes. Overrides the default or the
  CLASSPATH environment variable if it is set.
  Directories are separated by semi-colons. Thus
  the general format for path is

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References

• www.wikipedia.com
• http//www.sun.com
• http//www.java-examples.com/
• The complete Reference Java