JDK vs JRE vs JVM

“javac is not recognized”

What are the JDK, the JRE and the JVM?

Words you'll meet in this lesson

The three at a glance

Inside each box, and the different versions you'll meet

The three parts of the JVM

What the JRE adds, and what the JDK adds

Different JDK builds and JVM implementations

Look inside a real JDK on your computer

You don't need to take our word for it. Follow these steps on any computer that has a JDK (or skip to Part 7 to use the live editor).

1. Check what you have

   java -version
   javac -version

   openjdk version "21.0.12.1" 2026-08-18 OpenJDK Runtime Environment (build 21.0.12.1+1-1-24.04.4-Ubuntu) OpenJDK 64-Bit Server VM (build 21.0.12.1+1-1-24.04.4-Ubuntu, mixed mode, sharing) javac 21.0.12.1
   Look at the three lines: the version, the Runtime Environment (the JRE part) and the 64-Bit Server VM (the JVM, HotSpot). If javac -version fails, you have a runtime but not a JDK. This sample is from an Ubuntu build; your exact version numbers will differ.

2. Find the JDK folder

   On Windows it's usually under C:\Program Files\Eclipse Adoptium\ or C:\Program Files\Java\. On Linux, try /usr/lib/jvm/. On macOS, run /usr/libexec/java_home.

   This folder is what JAVA_HOME should point to: the JDK's top folder, not its bin subfolder.

3. Open the bin folder: these are the JDK tools

   You'll see programs such as java, javac, jar, javadoc, jdb, jshell, javap, jlink and jcmd.

   Only java is needed to run programs. Everything else is for developing. That's the difference between the JDK and the JRE. Interesting fact: javac is itself written in Java and runs on the JVM.

4. Look at lib and jmods: the class libraries

   The lib folder holds a file called modules, which packs all the standard library classes. The jmods folder holds the same modules in a form jlink can use.

   When your program uses String or LocalDate, the JVM's class loader reads those classes from this modules file. This is the “library” half of the JRE.

5. List the modules

   java --list-modules

   java.base@21.0.12.1 java.compiler@21.0.12.1 java.desktop@21.0.12.1 java.net.http@21.0.12.1 java.sql@21.0.12.1 …
   The JDK is split into modules. java.base (String, Object, collections…) is needed by every program. Modules starting with jdk., such as jdk.compiler (javac), are JDK-specific tools.

From “not recognized” to a running program: both views

Follow Kabir as he fixes his setup. Left: what he sees. Right: which box (JVM, JRE or JDK) is doing the work.

Who installs what in a real company

In a software team, different machines need different boxes. Here's how it usually looks.

Watch it with an example: Riya's chai-bill app goes to the shop

Riya builds the app on her laptop (JDK) and the chai shop only runs it (runtime only). See which part does which job.

Examples are simplified to show where each box fits. Each company's exact setup varies.

Ask the JVM about itself

Java can tell you which runtime and JVM it's running on, and which class loader loaded each class. The first three lines depend on the machine; the last two are the same everywhere.

public class Main {
    public static void main(String[] args) {
        // Which runtime and JVM is running this program?
        System.out.println("Java version: " + System.getProperty("java.version"));
        System.out.println("Vendor:       " + System.getProperty("java.vendor"));
        System.out.println("JVM:          " + System.getProperty("java.vm.name"));

        // Who loaded which class?
        System.out.println("Main loaded by:   " + Main.class.getClassLoader().getName());
        System.out.println("String loaded by: " + String.class.getClassLoader());
    }
}

Sample output from Eclipse Temurin 21. Your version and vendor will be whatever is installed.

• System.getProperty("java.version")Asks the runtime for a built-in setting. java.vendor is who built your JDK, java.vm.name is the JVM.
• Main.classAn object that describes the Main class itself. Every loaded class has one.
• getClassLoader().getName()Your own classes are loaded by the application class loader, named app.
• String.class.getClassLoader()Prints null! Core classes like String are loaded by the bootstrap loader, which is built into the JVM and isn't a Java object, so Java shows it as null.

Which box does what, at every line

Step through this program and watch which part of the JDK, JRE or JVM is working, along with the stack, heap and console.

Worked examples: real problems, solved and traced in memory

The JRE ships thousands of ready-made classes (the Java class library). These three programs use some of them for everyday business jobs. Watch the stack: when your code calls a library method, the JVM runs code that came with the JRE, not code you wrote.

today 2026-10-15, exam 2026-12-02

Exam on: 2026-12-02 (WEDNESDAY)
Days left: 48

principal 100000, rate 7%, 3 years

Maturity amount: Rs 122504
Interest earned: Rs 22504

"   asha VERMA  "

[   asha VERMA  ] length 15
[ASHA VERMA] length 10

Check yourself

Quick revision

Interview and viva questions

Practice problems

Use ready-made classes from the JRE (Math, String, LocalDate) for small tasks. Every output is fixed. The last three are business tasks.

144

12.0

public class Main {
    public static void main(String[] args) {
        System.out.println(Math.sqrt(144));
    }
}

75, 120

120

public class Main {
    public static void main(String[] args) {
        System.out.println(Math.max(75, 120));
    }
}

mumbai

MUMBAI

public class Main {
    public static void main(String[] args) {
        System.out.println("mumbai".toUpperCase());
    }
}

2026-10-15

2026-10-22

import java.time.LocalDate;

public class Main {
    public static void main(String[] args) {
        LocalDate d = LocalDate.of(2026, 10, 15);
        System.out.println(d.plusDays(7));
    }
}

249.67

250

public class Main {
    public static void main(String[] args) {
        System.out.println(Math.round(249.67));
    }
}

2026-10-28, 5 days

Delivery: 2026-11-02 (MONDAY)

import java.time.LocalDate;

public class Main {
    public static void main(String[] args) {
        LocalDate delivery = LocalDate.of(2026, 10, 28).plusDays(5);
        System.out.println("Delivery: " + delivery + " (" + delivery.getDayOfWeek() + ")");
    }
}

r = 3.5

Area: 38.48 sq m

public class Main {
    public static void main(String[] args) {
        double area = Math.PI * 3.5 * 3.5;
        System.out.println("Area: " + Math.round(area * 100) / 100.0 + " sq m");
    }
}

9876543210

XXXXXX3210

public class Main {
    public static void main(String[] args) {
        String phone = "9876543210";
        System.out.println("XXXXXX" + phone.substring(6));
    }
}

Real-world scenario questions (asked in company interviews)

These come straight from day-to-day work with JDKs and runtimes. Decide your answer first, then open the model answer.