Java architecture, JRE, Java Platform, JVM, JDK
Java is much more than just a popular programming language instead it's a complete architecture. The entire Java archirecture can be broadly categorized into three technologies which work together to make Java a powerful software development architecture. The components of the Java architecture are:-
- The Java Programming Language - JLS (Java Langauge Specifications) provide the syntax, constructs, grammar, etc. of the Java language.
- The Java APIs - Java Application Programming Interfaces (APIs) make available plethora of functionalities to the application developers so that they can focus on their core business functionality without bothering about the common tasks including those from the areas - I/O, Network, Language Constructs, Multithreading, Serialization, etc. Many vendors have implemented these APIs including the standard implementation given by Sun Microsystems which is normally used as a benchmark. The Java APIs are simply a collection of Java .class files. A .class file is the bytecode representation of a valid Java program. Bytecodes are a collection operator-operand tuples which are converted into the native machine level instructions by the underlying JVM.
- The Java Virtual Machine - The JVM is the abstract machine which runs all the bytecodes on a particular machine. JVM converts the bytecodes into native instructions which are executed by the underlying Operating System.
JVM - Java Virtual Machine
This software component is the key to achieve Platform Independence feature of Java. Most of the popular platforms have their own JVM implementations and hence a Java program written and compiled on a Java-compliant platform can be run on any other Java-compliant platform.
JVM is mainly composed of two components - Classloader and Execution Engine. The first component loads the .class files after converting them into the implementation dependent internal data structures. The Execution Engine uses these data structures are converts the bytecode instructions into machine-level instructions for the underlying Operating System which are subsequently executed by the OS.
Java Runtime Environment (JRE) / Java Platform
Both the terms JRE and Java Platform represent the same thing. Java Platform started with the verion 1.2, and it was called The Java 2 Platform. JRE is nothing but a combination of JVM and Java APIs - Java SE APIs as well as Java Web (Deployment) which consists of Java Web App Development/Deployment, Java Web Start, and Applet (plug-in).
Java Development Kit (JDK)
As the name suggest it's a Kit and it contains all the software components used for compiling, documenting, and executing Java programs. It's basically a logical collection of the The Java Programming Language, JRE (JVM + Java APIs), and Tools & Tool APIs, which include java, javac, javadoc, apt, jar, javap, Deploy, Monitoring, Java VisualVM, Scripting, etc.