JDK vs JRE vs JAR vs JVM

JDK

Java Development Kit — build + run toolkit
The JDK is the full Java toolkit for developing Java apps.

• Compile: .java → .class using javac
• Run: run .class / .jar using java
• Includes extra tools: jar, debuggers, diagnostic tools, etc.

JRE

Java Runtime Environment — run only
The JRE is the Java runtime needed to run Java applications.

• Includes the JVM + core libraries
• Does not include developer tools like javac
• You use it when you only need to run an app, not build it

JAR

Java Archive — packaged application output
A JAR is a package file (like a zip) that bundles compiled Java classes and resources.

• Spring Boot often builds an executable “fat/uber” JAR that includes dependencies.
• Run it with:

java -jar app.jar

JVM

JVM stands for Java Virtual Machine. It’s the runtime engine that runs Java programs.
Here’s the typical pipeline:

• You write Hello.java
• The Java compiler (javac) compiles it into bytecode: Hello.class
• The JVM loads and executes that bytecode on your machine