Skip to content

JVM.md

Latest commit

 

History

History
53 lines (32 loc) · 2.43 KB

JVM.md

File metadata and controls

53 lines (32 loc) · 2.43 KB
Raw

The Java Execution Process: From Source Code to Machine Code

Java is unique because it is both compiled and interpreted. This process allows Java to follow the "Write Once, Run Anywhere" (WORA) philosophy.

1. The Java Development Hierarchy (JDK vs. JRE vs. JVM)

To understand how Java works, you must understand the nested relationship between these three core components:

Visual Representation

  • JVM (Java Virtual Machine): The "engine" that actually runs the Java bytecode. It is platform-dependent (different for Windows, Mac, and Linux).
  • JRE (Java Runtime Environment): Contains the JVM + Library files (jars) and other files that Java needs to run. It’s what you need to run Java apps.
  • JDK (Java Development Kit): Contains the JRE + Development tools (like javac and jdb). It’s what you need to write and compile Java apps.

2. The Execution Workflow

The journey of a Java program happens in four distinct stages:

Step 1: Writing the Code (.java)

The programmer writes source code in a text editor or IDE (like IntelliJ or VS Code).

  • File Extension: Main.java

Step 2: Compilation (.class)

The developer runs the Java Compiler (javac). Instead of turning the code into machine-specific instructions (like C++ does), it turns it into Bytecode.

  • Command: javac Main.java
  • Result: Main.class (Platform-independent bytecode)

Step 3: Loading and Verification

When you run the program, the JVM’s ClassLoader puts the .class file into memory. The Bytecode Verifier checks the code for security violations and illegal code to ensure it won't crash the host system.

Step 4: Execution (JIT Compilation)

The JVM interprets the bytecode. However, to make it faster, it uses a JIT (Just-In-Time) Compiler.

  • The JIT compiler analyzes the code as it runs and compiles "hot spots" (frequently used code) directly into native Machine Code for the specific processor you are using.

3. Summary Flowchart

  1. Source Code (.java)
  2. ➜ Compiled by JDK (javac) 3. ➜ Bytecode (.class)
  3. ➜ Executed by JRE (JVM) 5. ➜ Interpreted/JIT Compiled to Native Machine Code (0101...)

4. Why this matters?

Because of the JVM, the same .class file you compile on a Windows machine can be moved to a Linux server or a Mac laptop and it will run exactly the same way, as long as that machine has a JRE installed.