Java Development Kit (JDK) vs Java Runtime Environment (JRE)

Java has three key components: JDK (Java Development Kit), JRE (Java Runtime Environment), and JVM (Java Virtual Machine). Let’s break them down in detail.

1. JDK (Java Development Kit)

The JDK is a software development kit that provides all the necessary tools to develop, compile, debug, and run Java applications. It contains both JRE and development tools.

JDK Contains:

1. JRE (Java Runtime Environment) – Includes JVM and libraries to run Java programs.
2. Compiler (javac) – Converts Java source code into bytecode (.class files).
3. Debugger (jdb) – Used for debugging Java applications.
4. Interpreter (java) – Runs Java bytecode inside the JVM.
5. Jar tool (jar) – Used to create and manage .jar files.
6. JavaDoc (javadoc) – Generates documentation from Java comments.
7. Other Development Tools – Includes utilities like jconsole, javap, jstack, jstat, and jvisualvm.

JDK Versions:

• JDK 8 – Popular version with long-term support (LTS).
• JDK 11 – Another LTS version, widely used in modern projects.
• JDK 17 – Latest LTS version (recommended for new applications).

JDK Uses:

• Writing Java programs.
• Compiling Java source code.
• Debugging Java applications.
• Running Java programs (since it includes JRE).

2. JRE (Java Runtime Environment)

The JRE is a subset of JDK that allows you to run Java applications but does not include development tools like javac.

JRE Contains:

1. JVM (Java Virtual Machine) – Converts Java bytecode into machine code and executes it.
2. Core Libraries – Essential Java class libraries (like java.lang, java.util, etc.).
3. Runtime Libraries – Libraries required to run Java programs.
4. Supporting Files – Configuration and property files.

JRE Uses:

• Running Java applications.
• Required for Java-based software and web applications.
• Used in production environments.

3. JVM (Java Virtual Machine)

The JVM is the core of Java’s “Write Once, Run Anywhere” (WORA) principle. It translates Java bytecode into machine-specific code.

JVM Components:

1. Class Loader – Loads Java classes into memory.
2. Runtime Memory (Heap & Stack) – Manages memory allocation.
3. Execution Engine – Interprets and executes bytecode.
4. Garbage Collector (GC) – Automatically manages memory.

JVM Types:

• HotSpot JVM – Used by Oracle and OpenJDK.
• OpenJ9 JVM – Lightweight alternative to HotSpot.

Key Differences Between JDK and JRE

Summary

• JDK = JRE + Development tools (Used for coding & compiling).
• JRE = JVM + Libraries (Used for running Java apps).
• JVM = Converts bytecode to machine code (Provides platform independence).