Card 04/ 06
JDK vs JRE vs JVM
Three acronyms in every installation guide, and the one to install if you want to compile anything.
The JVM runs bytecode, the JRE (Java Runtime Environment) is the JVM plus the standard library, and the JDK (Java Development Kit) is the JRE plus the compiler and other tools. They nest, and only the JDK can compile code.
Installation guides mention all three in the same sentence, as though the difference were obvious. Picking the wrong one leaves you with a machine that can run Java and cannot compile it.
| Name | What it is | Contains | You need it to |
|---|---|---|---|
| JVM, the Java Virtual Machine | The engine that executes bytecode | Nothing else; it is the innermost piece | Run bytecode, and nothing more |
| JRE, the Java Runtime Environment | The JVM plus the standard library | The JVM, plus String, List, files, networking | Run a program somebody else compiled |
| JDK, the Java Development Kit | The JRE plus the tools that make bytecode | The JRE, plus javac, jar, jshell, a debugger | Compile anything at all |
They nest. Every JDK contains a JRE, and every JRE contains a JVM. Nothing contains a JDK, which is why installing the smallest one leaves you stuck.
The decision rule. On any machine where you write code, install the JDK. On a machine that only runs a jar somebody else built, such as a container image or a server, a JRE is enough and is smaller. If you are unsure which of the two a machine is, it is a development machine and it wants the JDK.
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