How Java Works (JVM, JRE, JDK)

Understand the journey from Java source code to running program: the compiler, bytecode, and the Java Virtual Machine.

Overview

When you write Java code and run it, several things happen behind the scenes. First your source code (.java file) is compiled by javac into bytecode (.class file). Bytecode is not machine code - it is a platform-neutral set of instructions that any Java Virtual Machine (JVM) can execute, regardless of the underlying operating system or hardware. This two-step process (compile to bytecode → execute in JVM) is what enables "Write Once, Run Anywhere". The JVM is the layer that translates bytecode into instructions the actual CPU understands, handling differences between Windows, macOS, and Linux transparently. Understanding JDK vs JRE vs JVM helps you know what to install and why. The JDK (Java Development Kit) is the full toolkit for developers. The JRE (Java Runtime Environment) is just enough to run Java programs. The JVM is the engine inside both.

Key Points

•Source code (.java) → compiler (javac) → bytecode (.class) → JVM → output
•Bytecode is platform-independent; the JVM converts it to native machine code at runtime
•JDK = JRE + development tools (javac compiler, javadoc, debugger)
•JRE = JVM + standard library (enough to run Java, but cannot compile)
•JVM = the engine that interprets or JIT-compiles bytecode to native instructions
•JIT (Just-In-Time) compilation inside the JVM makes Java run fast by compiling hot code paths to native code

Examples

The Java execution pipeline

Step 1: Write source code
  → HelloWorld.java

Step 2: Compile with javac
  → javac HelloWorld.java
  → produces HelloWorld.class (bytecode)

Step 3: Run with java
  → java HelloWorld
  → JVM reads HelloWorld.class
  → JVM executes bytecode
  → Output appears in terminal

The .class file is the portable artifact. You can copy it to any machine with a JVM and run it there.

JDK, JRE, and JVM relationship

┌─────────────────────────────────┐
│              JDK                │
│  ┌───────────────────────────┐  │
│  │           JRE             │  │
│  │  ┌─────────────────────┐  │  │
│  │  │        JVM          │  │  │
│  │  └─────────────────────┘  │  │
│  │  + Standard Library       │  │
│  └───────────────────────────┘  │
│  + javac, javadoc, tools        │
└─────────────────────────────────┘

Every component is nested. The JVM is the core runtime; the JRE adds the class library; the JDK adds developer tools.

Reading bytecode with javap (disassembler)

// After compiling HelloWorld.java, run:
javap -c HelloWorld

// Output (abbreviated):
//   0: getstatic  #2   // Field System.out
//   3: ldc        #3   // String "Hello, World!"
//   5: invokevirtual #4 // Method println
//   8: return

javap lets you inspect the bytecode instructions. This is what the JVM actually executes.

Common Mistakes

Installing only the JRE thinking you can compile Java - you need the full JDK to run javac
Confusing "bytecode" with "machine code" - bytecode runs on the JVM, not directly on the CPU
Forgetting that the .class file name must match the class name exactly, not the .java file name

Note

The JVM also manages memory automatically through garbage collection. You never manually free memory in Java - the JVM detects objects that are no longer needed and reclaims their memory.

How Java Works (JVM, JRE, JDK)

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

What file is produced when you compile a .java source file?