What is JVM, JDK and JRE ?

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Short Answer

Java Virtual Machine (JVM)

The JVM ensures that Java applications can run on any device or operating system that has the JVM installed, making Java programs platform-independent.

Java Development Kit (JDK)

The JDK is a full-featured software development kit for Java developers. It includes everything needed to develop Java applications and applets, including the JVM, the Java Runtime Environment (JRE), an interpreter/loader (java), a compiler (javac), an archiver (jar), a documentation generator (Javadoc) and other tools needed in Java development.

Java Runtime Environment (JRE)

The JRE is a subset of the JDK. The JRE consists of the JVM, core libraries, and other components to run applications and applets written in Java. The JRE does not contain tools and utilities such as compilers or debuggers for developing applets and applications.

Detailed Answer

Java Virtual Machine (JVM)

The Java Virtual Machine (JVM) is a crucial component of the Java Runtime Environment (JRE) that allows Java applications to run on any device or operating system whether it’s a Windows PC, a Mac, or a Linux system. Think of the JVM as the heart of Java’s “write once, run anywhere” magic. When you run a Java program, the JVM reads the bytecode and translates it into machine code to understand it by computer.

In another words you can also define it as The JVM is an abstract computing machine that enables a computer to run a Java program. It is platform-dependent and responsible for converting bytecode into native machine code. The JVM ensures that Java applications can run on any device or operating system that has the JVM installed, making Java programs platform-independent. The JVM performs several tasks, including loading code, verifying code, executing code, and providing runtime environment.

Java Development Kit (JDK)

The Java Development Kit, known as JDK, is like a big toolbox for Java programmers. It comes with the JVM, so you can run Java programs. But it also has tools to write and test your own Java programs. For example, it includes a compiler called javac that turns your Java code into bytecode that the JVM can understand. It also has libraries, which are collections of pre-written code you can use in your programs. These libraries make it easier to do things like connect to the internet or create a user interface.

In another word you can also define it as The JDK is a full-featured software development kit for Java developers. It includes everything needed to develop Java applications and applets, including the JVM, the Java Runtime Environment (JRE), an interpreter/loader (java), a compiler (javac), an archiver (jar), a documentation generator (Javadoc) and other tools needed in Java development. Essentially, the JDK is the package that allows developers to create Java-based applications.

Java Runtime Environment (JRE)

The Java Runtime Environment, or JRE, is like a smaller version of the JDK. It includes the JVM and the libraries you need to run Java programs, but it doesn’t include the tools to create Java programs. This means if you just want to run a Java app someone else made, you only need the JRE. It’s lighter and easier for people who aren’t developing Java apps.

In another word you can also define it as The JRE is a subset of the JDK. It is the runtime portion of Java software, which is all you need to run it in your Web browser. The JRE is what you get when you download Java software. The JRE consists of the JVM, core libraries, and other components to run applications and applets written in Java. The JRE does not contain tools and utilities such as compilers or debuggers for developing applets and applications.

Important Points with Examples

  1. Cross-Platform Compatibility: Because of the JVM, a Java program written on a Mac can run on Windows or Linux without changes. For instance, a simple Java calculator app will work the same on all these platforms.
  2. Development Tools in JDK: The JDK’s tools, like the javac compiler, help turn your Java code into runnable form. For example, when you write a Java program to say “Hello, World!”, javac compiles it into bytecode that the JVM understands.
  3. Libraries for Functionality: Both the JDK and JRE include libraries, which are like building blocks for Java programs. Say you’re making a program that needs to draw graphics. Java libraries have tools for that, so you don’t have to start from scratch.
  4. Running Java Applications: To run a Java web application, you need the JRE installed on your server. This application could be a simple web page counter that tracks visitors.

In conclusion, the JVM, JDK, and JRE work together to make Java versatile, easy to develop with, and able to run anywhere. The JVM ensures Java’s compatibility across different systems. The JDK provides the tools needed for development, and the JRE allows Java programs to run on any device equipped with it. This trio makes Java one of the most accessible and widely used programming languages in the world.