Write once, run anywhere Nishad Abdulkareem, Lecturer MES College Marampally Java

write once, run anywhere

Nishad Abdulkareem, LecturerMES College Marampally

Index• Java Programming language

• History

• Java Varieties

• Java world

• Buzz words

• Classpath

• Program

Department of MCA MES College Marampally

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".


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.


…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 1998–1999), new versions had multiple configurations built for different types of platforms.

• The different platforms are J2EE,J2ME and J2SE


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.


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.


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).


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


Java World

Java Server Pages

Java Servlets

Java Messaging Services

Java mail API

Java API for XML Processing

JDBC API

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.

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),


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.


• Science Behind Java


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.


• 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


• 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 doesn’t mention how to implement the features, like how the garbage collection process works is JVM dependent, Java spec doesn’t define any specific way to do this.


• JVM

• ByteCode

• JIT

• Applets


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


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.HelloWorld


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


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


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.

• Department of MCA MES College Marampally

-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:


References

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


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Write once, run anywhere Nishad Abdulkareem, Lecturer MES College Marampally Java