JDK, JRE and JVM | Core Java

What are JDK, JRE and JVM?

JVM (Java Virtual Machine), JRE (Java Runtime Environment), and JDK (Java Development Kit) are the fundamental parts of the Java platform. They work together to develop, run, and execute Java applications efficiently.

JVM (Java Virtual Machine)

The JVM is an abstract machine responsible for running Java bytecode. It translates compiled Java code into machine-specific instructions. It provides memory management, security, and garbage collection, ensuring Java’s platform independence – “Write Once, Run Anywhere.”

Example:

class Demo { public static void main(String[] args) { System.out.println("Java Program Running..."); } }

When you run this program, the JVM executes the compiled Demo.class file and produces output:

Java Program Running...

JRE (Java Runtime Environment)

The JRE contains the JVM along with the essential libraries and class files required to execute Java applications. It does not include development tools such as compilers or debuggers. JRE is used primarily to run existing Java applications.

Example:

When a user installs Java to play a game or run software, they are installing the JRE, which provides the runtime environment to execute the compiled program.

JDK (Java Development Kit)

The JDK includes the JRE, compiler (javac), debugger, and development tools needed to create Java applications. Developers use the JDK to write, compile, and debug Java programs.

Example:

When you write a Java program in an IDE like IntelliJ or Eclipse, you are using the JDK. The command:

javac HelloWorld.java java HelloWorld

Here, javac compiles the code, and java runs it using the JRE and JVM.

Differences Between JDK, JRE, and JVM

Component	Includes	Usage
JVM	Execution engine only	Runs Java bytecode
JRE	JVM + Libraries	Runs Java applications
JDK	JRE + Development Tools	Develops and runs Java programs

Best Practices

✔ Install the JDK if you are developing Java applications.

✔ Use the JRE when you only need to run Java applications.

✔ Always match your Java version (e.g., 8, 11, 17) with your project requirements.

✔ Keep your JDK updated for the latest compiler improvements and security updates.

Summary Example

Suppose you write a simple program HelloWorld.java:

class HelloWorld { public static void main(String[] args) { System.out.println("Hello Java"); } }

- The JDK compiles it into bytecode (HelloWorld.class).
- The JRE provides libraries and environment to run it.
- The JVM executes the bytecode on your system.

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