JVM, JDK, and JRE

JVM — Java Virtual Machine

The JVM is an abstract machine that provides a runtime environment to execute Java bytecode (.class files). It is not a physical machine; it is a specification implemented differently on each operating system. The JVM is responsible for loading class files, verifying bytecode for safety, executing instructions, managing memory through automatic garbage collection, and providing runtime data areas like the heap and stack. Because every platform has its own JVM implementation that understands the same bytecode format, a compiled Java program becomes portable.

JRE — Java Runtime Environment

The JRE is the JVM plus the core class libraries (java.lang, java.util, java.io, and so on) needed to run Java applications. If you only need to run a Java program — not develop one — the JRE is technically sufficient. Think of the JRE as "JVM + libraries needed at runtime."

JDK — Java Development Kit

The JDK is the full development kit: it includes the JRE plus development tools such as the compiler (javac), the debugger, javadoc for documentation generation, and other utilities like jar and jshell. To write and compile Java code, you need the JDK, not just the JRE.

How They Relate

JDK contains JRE, and JRE contains JVM. This is a strict containment relationship: JDK ⊇ JRE ⊇ JVM. When you install "Java" for development today, you install a JDK (for example, JDK 21), and that single installation gives you the compiler, the runtime, and the JVM together.

• JVM: Executes bytecode. Platform-specific implementation, platform-independent bytecode format.
• JRE: JVM + standard class libraries. Enough to run compiled Java programs.
• JDK: JRE + compiler and developer tools. Needed to write and build Java programs.

Example

// Step 1: Compile with the JDK's compiler (javac)
// Terminal: javac HelloJava.java
// This produces HelloJava.class (bytecode)

// Step 2: Run with the JVM (via the "java" launcher, included in JRE/JDK)
// Terminal: java HelloJava

public class HelloJava {
    public static void main(String[] args) {
        System.out.println("Compiled by JDK, executed by JVM");
    }
}

Compiled by JDK, executed by JVM

Exercise

Ask the running JVM to describe itself. Complete the program so it prints the Java version, the name of the JVM implementation, and the folder Java is installed in.

The output depends on your installation, for example a version such as 21.0.2 and a VM name such as OpenJDK 64-Bit Server VM.

public class RuntimeInfo {
    public static void main(String[] args) {
        // TODO: print these three system properties, one per line:
        // java.version, java.vm.name, java.home
    }
}

public class RuntimeInfo {
    public static void main(String[] args) {
        System.out.println("Version: " + System.getProperty("java.version"));
        System.out.println("VM: " + System.getProperty("java.vm.name"));
        System.out.println("Home: " + System.getProperty("java.home"));
    }
}

Quick quiz

Interview questions