Installing a JDK, and JAVA_HOME

Installing a JDK is easy. Knowing which one you just installed is the part that matters, because a machine can have four and the one that runs is decided by something you have not looked at.

#Installing

macOS — Homebrew is the least trouble: brew install --cask temurin. Installers from the distribution's site work too.

Linux — your package manager (apt install temurin-21-jdk with Adoptium's repository, or your distribution's openjdk-21-jdk), or unpack a tarball anywhere and point at it.

Windows — the .msi from the distribution, which offers to set JAVA_HOME and PATH for you. Let it.

Any of them — a version manager (SDKMAN! on Unix, jabba, or your IDE's downloader) if you expect to switch between versions. Which you will, the first time you work on two projects.

The install is not the lesson. This is:

#Prove what you got

Two commands, and they answer different questions:

which java        # WHERE the java that runs comes from
java -version     # WHAT version that one is

Run both, every time you are surprised. java -version alone is the command people run and it cannot tell you that there are three other JDKs on the machine.

$ which java
/usr/local/bin/java
 
$ java -version
openjdk version "21.0.12" 2026-07-21 LTS
OpenJDK Runtime Environment Temurin-21.0.12+8 (build 21.0.12+8-LTS)
OpenJDK 64-Bit Server VM Temurin-21.0.12+8 (build 21.0.12+8-LTS, mixed mode, sharing)

And check the compiler separately, because it is a different binary and can be a different version:

javac -version

#The two variables

PATH is a list of directories. When you type java, the shell walks it in order and runs the first match. That is what which is reporting.

JAVA_HOME points at the root of a JDK installation — not at the binary:

JAVA_HOME=/usr/lib/jvm/temurin-21     # yes: the directory containing bin/, lib/
JAVA_HOME=/usr/lib/jvm/temurin-21/bin/java   # no

PATH is for the shell. JAVA_HOME is for other programs — Maven, Gradle, application servers, scripts — which need more than the launcher: they need javac, the libraries, the whole installation.

They can disagree, and that is the confusing afternoon above. Set both, to the same JDK:

export JAVA_HOME=/usr/lib/jvm/temurin-21
export PATH="$JAVA_HOME/bin:$PATH"

Putting $JAVA_HOME/bin first is what makes that JDK win over anything installed elsewhere.

Those two lines belong in your shell's startup file (~/.zshrc, ~/.bashrc) — export in a terminal lasts only as long as that terminal.

#What you actually installed

A JDK is a directory. Look inside bin/ and you find about thirty programs:

jar jarsigner java javac javadoc javap jcmd jconsole jdb jdeprscan jdeps
jfr jhsdb jimage jinfo jlink jmap jmod jpackage jps jrunscript jshell
jstack jstat jstatd jwebserver keytool rmiregistry serialver

Most you will never type. Six are worth knowing the names of now, so that when a lesson later says "run jcmd", it is a tool you have rather than something to install:

The last two are diagnostics, and the fact that they ship with the platform is unusual and genuinely useful. You do not install anything to debug a production Java process.

#Try it yourself