Explore the Java Platform part2 - Resources

Explore the Java Platform
Part 2
Resources

In this part we are going to define the resources used to develop and run a java application.

2. Java Runtime Environment (JRE)
The Java Runtime Environment, or JRE, is a software bundle from Sun Microsystems that allows a computer system to run a Java application. Java applications are in widespread use and necessary to view many Internet pages.
The software bundle consists of the Java Virtual Machine and the application and programming interface (API). The API provides a set of standard class libraries. The virtual machine and API have to be consistent with each other and are therefore bundled together as the JRE. This can be considered a virtual computer in which the virtual machine is the processor and the API is the user interface. Sun also distributes a superset of the JRE called the Java 2 SDK (more commonly known as the JDK), which includes development tools such as the Java compiler, Javadoc, and debugger.
2.1 Components of the JRE

2.1.1 Java Virtual machine (JVM)
A Java Virtual Machine (JVM) is a virtual machine that interprets and executes Java bytecode. This code is most often generated by Java language compilers, although the JVM can also be targeted by compilers of other languages. JVM's may be developed by other companies as long as they adhere to the JVM standard published by Sun.
The JVM is a crucial component of the Java Platform. The availability of JVMs on many types of hardware and software platforms enables Java to function both as middleware and a platform in its own right. Hence the expression "Write once, run anywhere." The use of the same bytecode for all platforms allows Java to be described as "Compile once, run anywhere", as opposed to "Write once, compile anywhere", which describes cross-platform compiled languages.

2.1.2 Libraries
Java libraries are the compiled byte codes of source code developed by the JRE implementer to support application development in Java. Examples of these libraries are:

The core libraries, which include:
-Collection libraries which implement data structures such as lists, dictionaries, trees and sets
-XML Parsing libraries
-Security
-Internationalization and localization libraries

The integration libraries, which allow the application writer to communicate with external systems. These libraries include:
-The Java Database Connectivity (JDBC) API for database access
-Java Naming and Directory Interface (JNDI) for lookup and discovery
-RMI and CORBA for distributed application development

User Interface libraries, which include:
-The (heavyweight, or native) Abstract Windowing Toolkit (AWT), which provides GUI components, the means for laying out those components and the means for handling events from those components
-The (lightweight) Swing libraries, which are built on AWT but provide (non-native) implementations of the AWT.
-APIs for audio capture, processing, and playback

There are more libraries than mentioned above, but the three categories mentioned above are the main categories.

2.1.3 Plugins
Plugins enable a special kind of java applications called “Applets” to be run in Web Browsers. We will talk about applets in coming articles.

2.1.4 Java Web Start
Java Web Start is a framework developed by Sun Microsystems which allows application software for the Java Platform to be started directly from the Internet using a web browser. Unlike Java applets, Web Start applications do not run inside the browser, and the sandbox in which they run does not have to be as restricted, although this can be configured. One chief advantage of Web Start over applets is that they overcome many compatibility problems with browsers' Java Plugins and different JVM versions. On the other hand, Web Start programs cannot communicate with the browser as easily as applets. To assist migration, a Java Applet can also be used as a Java Web Start application.

2.1.5 Licensing and documentation

2.2 APIs
The classes in the Java APIs are organized into separate groups called packages. Each package contains a set of related interfaces, classes and exceptions. Refer to the separate platforms for a description of the packages available.
Sun has defined three platforms targeting different application environments and segmented many of its APIs so that they belong to one of the platforms.
The platforms are:

Java Platform, Micro Edition (Java ME) — targeting environments with limited resources like cellular phones.
Java Platform, Standard Edition (Java SE) — targeting workstation environments.
Java Platform, Enterprise Edition (Java EE) — targeting large distributed enterprise or Internet environments.

As mentioned before, java platform is very wide in terms of functionality; in addition, java is an open source platform which makes it possible for any one to edit and add more functionality, so, in order to utilize this feature, there is a standard process defined to allow interested parties to be involved in the definition of future versions and features of the Java platform. This process is called the Java Community Process (JCP).

The JCP involves the use of Java Specification Requests (JSR), which are formal documents that describe proposed specifications and technologies to be added to the Java platform. Formal public reviews of JSRs are conducted before the JSR becomes final and is voted on by the JCP Executive Committee. A final JSR provides a reference implementation which provides a free implementation of the technology in source code form and a Technology Compatibility Kit to verify the API specification.

Extensions and related architectures
The extensions to standard Java are typically in javax.* packages. They are not included in the JDK or JRE. Extensions and architectures closely tied to the Java programming language include:
Java EE (previously J2EE) (Java Platform, Enterprise Edition—for distributed enterprise applications)
Java ME (previously J2ME) (Java Platform, Micro Edition—for PDAs and cellular phones)
JMF (Java Media Framework)
JNDI (Java Naming and Directory Interface)
JSML (Java Speech API Markup Language)
JDBC (Java DataBase Connectivity)
JDO (Java Data Objects)
JAI (Java Advanced Imaging)
JAIN (Java API for Integrated Networks)
JDMK (Java Dynamic Management Kit)
Jini (a network architecture for the construction of federated distributed systems)
Jiro
Java Card (Java for smart cards)
JavaSpaces
JML (Java Modeling Language)
JMI (Java Metadata Interface)
JMX (Java Management Extensions)
JSP (JavaServer Pages)
JSF (JavaServer Faces)
JNI (Java Native Interface)
JXTA (Open Protocols for Peer-to-Peer (P2P) Virtual Network)
Java 3D (A high level API for 3D graphics programming)
JOGL (Java OpenGL—A low level API for 3D graphics programming, using OpenGL)
LWJGL (Light Weight Java Game Library—A low level API providing access to OpenGL, OpenAL and various input devices)
OSGi (Dynamic Service Management and Remote Maintenance)

2.3 Summary and coming articles
This article can be considered the entry point for discussing the java platform in a more detailed view. We described the components of the JRE including the JVM and APIs.
Starting from the next article, we are going to discuss the main three platforms of java one by one showing the functionality of each platform.