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JVM, JRE, and JDK

Thinking

Understanding the JVM, JRE, and JDK is fundamental to grasping how Java works from writing code to running applications.

Each component plays a distinct role in Java development and execution, and knowing their differences helps in setting up and troubleshooting Java environments effectively.

Description

Java is a platform-independent language due to the Java Virtual Machine and its runtime architecture. The key components are:

  • JVM (Java Virtual Machine): An abstract machine that runs Java bytecode. It provides a runtime environment to execute compiled Java programs, translating bytecode into machine-specific instructions.
  • JRE (Java Runtime Environment): Includes the JVM along with core libraries and other components necessary to run Java applications. It does not include development tools.
  • JDK (Java Development Kit): A superset of the JRE that contains tools required for Java application development like the compiler (javac), debugger, and other utilities.

Video Resources

What is JVM?

A simple explanation of the Java Virtual Machine.

View PDF

Examples (code)

Example: Compile and Run a Java Program Using JDK


// Compile Java program
javac HelloWorld.java

// Run compiled bytecode on JVM
java HelloWorld

This shows how JDK is used to compile code into bytecode, which the JVM then executes.

Real-World Applications

JVM

Enables platform independence by allowing Java bytecode to run on any device with a compatible JVM.

JRE

Used by end-users to run Java applications without needing development tools.

JDK

Used by developers to write, compile, debug, and package Java applications.

Interview Questions

Q1: What is the role of the JVM in Java?

Show Answer

The JVM executes Java bytecode, enabling Java to be platform-independent by translating bytecode to machine-specific code at runtime.

Q2: How is JRE different from JVM?

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JRE includes the JVM along with core libraries and runtime components to run Java applications, whereas JVM is only the engine that executes bytecode.

Q3: Why do developers need the JDK?

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JDK provides tools such as the compiler and debugger that developers need to write, compile, and debug Java applications, which are not present in the JRE.

Q4: Can you run a Java program with just the JVM?

Show Answer

No, JVM alone cannot run Java programs. It requires the JRE, which includes the JVM plus necessary libraries and environment to run the application.