JDK, JRE & JVM

When you start learning Java, three terms come up immediately — JDK, JRE, and JVM.

Most beginners either skip over them or get confused. But understanding what each one does — and how they relate to each other — makes everything else in Java easier to understand.

The Simple Analogy First

Before any definitions, here's an analogy that makes the relationship click.

Think of building and delivering a package:

• ›The JDK is the factory — where you build the package (write and compile Java code)
• ›The JRE is the delivery truck — it carries everything needed to deliver the package to any destination
• ›The JVM is the engine inside the truck — the actual mechanism that makes it move and run

You need all three to build and run a Java program — but they serve different purposes at different stages.

The Relationship — One Inside the Other

This is the most important thing to understand:

The key insight: JDK contains JRE, and JRE contains JVM.

• ›If you install the JDK, you get all three automatically
• ›If you only install the JRE, you get the JRE and JVM — but not the compiler
• ›The JVM alone doesn't exist as a standalone install — it always comes with the JRE

What is the JVM?

JVM = Java Virtual Machine

The JVM is the engine that actually runs your Java program. It takes the Bytecode (the .class file your compiler produces) and translates it into instructions your specific operating system understands.

What the JVM does internally:

• ›Class Loader — loads the .class file into memory
• ›Bytecode Verifier — checks the code is safe to run before executing it
• ›Interpreter / JIT Compiler — converts Bytecode into native machine instructions and runs them
• ›Garbage Collector — automatically frees memory that is no longer in use

What is the JRE?

JRE = Java Runtime Environment

The JRE is everything needed to run a Java program on a machine. It includes:

• ›The JVM (to execute the Bytecode)
• ›The Java Standard Library — thousands of pre-built classes like ArrayList, String, System, Math
• ›Supporting files and configuration that the JVM needs to work

Who needs the JRE? Anyone who wants to run Java programs — but not necessarily write or compile them. For example, if someone downloads your Java application, they need the JRE installed to run it. They don't need the full JDK.

What is the JDK?

JDK = Java Development Kit

The JDK is the complete package for writing, compiling, and running Java programs. It includes everything in the JRE plus the tools developers need:

• ›javac — the Java compiler (converts .java source files into .class Bytecode)
• ›java — the launcher that starts the JVM and runs your program
• ›javadoc — generates documentation from your code comments
• ›jdb — the Java debugger
• ›jar — packages compiled classes into .jar archive files
• ›jshell — an interactive REPL for trying Java code without writing a full program

Who needs the JDK? Every Java developer. If you are writing Java code, you must install the JDK. It is the starting point for everything.

Side-by-Side Comparison

Which One Should You Install?

Always install the JDK.

If you are learning Java or writing Java code, install the JDK. It contains the JRE and JVM inside it — so you get all three in one installation.

Which version?

Install Java 21 LTS (the latest Long-Term Support release). It is free, stable, and widely supported by all major tools and frameworks.

Which distribution?

There are several free JDK distributions. The most commonly used are:

• ›Eclipse Temurin (OpenJDK) — recommended for most beginners and professionals
• ›Oracle JDK — official Oracle release, free for development use
• ›Amazon Corretto — Amazon's OpenJDK build, widely used in cloud environments

How They Work Together — The Full Picture

Here is what happens from the moment you write code to the moment it runs:

In plain English:

1. ›You write code in a .java file — the JDK's compiler converts it to Bytecode (.class)
2. ›The JRE provides the standard libraries your code depends on and loads the class into memory
3. ›The JVM reads the Bytecode and executes it on your machine — producing the output you see

Common Beginner Confusion

"Do I need to install all three separately?"

No. Install the JDK and you get all three. The JDK contains the JRE, which contains the JVM — one installation covers everything.

"What is the difference between JVM and the compiler?"

The compiler (javac) is part of the JDK — it converts your .java source code into Bytecode before the program runs. The JVM runs that Bytecode. They are separate tools doing separate jobs at different stages.

"Is the JVM the same on every computer?"

The JVM is different for each operating system — there's a Windows JVM, a Mac JVM, a Linux JVM. But all of them understand the same Bytecode. That's exactly why your compiled Java program runs on any machine without recompiling.

The 3 Things to Remember From This Page

Summary

• ›JVM executes Bytecode — it's the engine that makes Write Once, Run Anywhere possible
• ›JRE = JVM + Standard Libraries — everything needed to run a Java program
• ›JDK = JRE + Compiler + Dev Tools — everything needed to write and run Java code
• ›JDK contains JRE, which contains JVM — they are nested, not separate
• ›As a developer, always install the JDK — it gives you all three
• ›Use Java 21 LTS with Eclipse Temurin (OpenJDK) for a free, stable, widely-supported setup

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