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Install JDK 21

1. What is the JDK

The JDK (Java Development Kit) is the toolset required to write and run Java programs. It includes:

  • javac — the compiler, which translates your Java source code into bytecode
  • java — the virtual machine (JVM), which runs that bytecode
  • The standard library and debugging tools

Without the JDK, your computer cannot understand or execute any Java code.

Why JDK 21?

JDK 21 is the latest LTS (Long-Term Support) release — Oracle guarantees security patches until at least 2031. It is the standard choice for new projects and most current enterprise environments.

What is Temurin?

Eclipse Temurin (provided by Adoptium) is a free, open-source JDK distribution trusted by the community and major companies (Microsoft, Red Hat, IBM). It is equivalent to Oracle JDK in performance and features, with no commercial licensing restrictions.

2. Installation

Step 1 — Download the installer

  1. Open a browser and go to the Temurin download page:
    https://adoptium.net/temurin/releases/
  2. Set the filters at the top:
    • Version: 21 - LTS
    • OS: Windows
    • Architecture: x64 (standard 64-bit PC; select aarch64 for ARM devices like Surface Pro X)
    • Package Type: JDK
    • File type: .msi
  3. Click Download to save the .msi file (about 170 MB).

Step 2 — Run the installer

  1. Locate the downloaded .msi file (usually in Downloads), double-click to open it.
  2. The User Account Control prompt asks "Do you want to allow this app to make changes?" — click Yes.
  3. The installer welcome screen appears — click Next.

  4. The Custom Setup screen appears — leave all options at their defaults, no changes needed.

    Important

    Make sure "Add to PATH" and "Set JAVA_HOME variable" are checked (shown in green). The Temurin installer handles this automatically — just don't uncheck them.

  5. Click Next → Install → wait 30–60 seconds → click Finish.

Step 3 — Verify the installation

  1. Press the Windows key, type cmd, press Enter to open Command Prompt.

  2. Type the following and press Enter:

    java -version

  3. The correct output looks like this:

    openjdk version "21.0.x" 2024-xx-xx LTS
OpenJDK Runtime Environment Temurin-21.0.x+xx (build 21.0.x+xx)
OpenJDK 64-Bit Server VM Temurin-21.0.x+xx (build 21.0.x+xx, mixed mode, sharing)

  4. Also verify the compiler:

    javac -version

    Expected output: javac 21.0.x

Installation successful if

Both java -version and javac -version show 21.x.x. You can skip the troubleshooting section below.

Step 1 — Install Homebrew (if not already installed)

Homebrew is the most popular package manager for macOS — similar to an app store but for developer tools.

  1. Open Terminal: press Cmd + Space, type Terminal, press Enter.

  2. Run the following command (copy and paste the entire line):

    /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

  3. Enter your macOS password when prompted (the cursor will not move — this is normal). Press Enter.

  4. Installation takes 2–5 minutes. When complete, the terminal shows Installation successful!.

Already have Homebrew?

Run brew --version to check. If a version number appears, skip this step.

Step 2 — Install JDK 21 via Homebrew

brew install --cask temurin@21

This downloads and installs Temurin JDK 21. It takes 2–5 minutes depending on your connection speed.

Step 3 — Verify the installation

java -version
javac -version

Expected output:

openjdk version "21.0.x" 2024-xx-xx LTS
OpenJDK Runtime Environment Temurin-21.0.x+xx (build 21.0.x+xx)
OpenJDK 64-Bit Server VM Temurin-21.0.x+xx (build 21.0.x+xx, mixed mode)

Manual installation (without Homebrew)

If you prefer not to use Homebrew:

  1. Go to https://adoptium.net/temurin/releases/
  2. Select Version: 21 - LTS, OS: macOS, Architecture: aarch64 (Apple Silicon M1/M2/M3) or x64 (Intel), Package Type: JDK, File type: .pkg
  3. Download and run the .pkg file → follow the on-screen instructions.

Ubuntu / Debian (APT)

sudo apt update
sudo apt install -y wget apt-transport-https gpg

# Add the Adoptium repository
wget -qO - https://packages.adoptium.net/artifactory/api/gpg/key/public \
  | gpg --dearmor \
  | sudo tee /etc/apt/trusted.gpg.d/adoptium.gpg > /dev/null

echo "deb https://packages.adoptium.net/artifactory/deb \
  $(awk -F= '/^VERSION_CODENAME/{print$2}' /etc/os-release) main" \
  | sudo tee /etc/apt/sources.list.d/adoptium.list

sudo apt update
sudo apt install -y temurin-21-jdk

Fedora / RHEL / CentOS (DNF)

sudo dnf install -y java-21-openjdk-devel

Fedora uses OpenJDK from its default repository

This version is equivalent to Temurin in performance.

Arch Linux (Pacman)

sudo pacman -S jdk21-openjdk

Verify

java -version
javac -version

SDKMAN — Install and manage multiple JDK versions

SDKMAN lets you install and switch between multiple JDK versions with a single command — useful when working on projects that require different Java versions.

# Install SDKMAN
curl -s "https://get.sdkman.io" | bash
source "$HOME/.sdkman/bin/sdkman-init.sh"

# Install Temurin 21
sdk install java 21.0.3-tem

# Verify
java -version

3. What is JAVA_HOME and why it matters

JAVA_HOME is an environment variable — a system label pointing to the JDK installation directory. Many tools (Maven, Gradle, IDEs) read this variable to locate the JDK.

Check if JAVA_HOME is already set

Open Command Prompt:

echo %JAVA_HOME%

Correct output (example):

C:\Program Files\Eclipse Adoptium\jdk-21.0.3.9-hotspot

If the output is empty (just %JAVA_HOME% or a blank line) — see the troubleshooting section below.

echo $JAVA_HOME

Correct output (example):

/Library/Java/JavaVirtualMachines/temurin-21.jdk/Contents/Home

Set JAVA_HOME manually (if missing)

  1. Press Windows + R, type sysdm.cpl, press Enter → Advanced tab → Environment Variables.
  2. Under System variables, click New:
    • Variable name: JAVA_HOME
    • Variable value: path to the JDK folder (e.g. C:\Program Files\Eclipse Adoptium\jdk-21.0.3.9-hotspot)
  3. Find the Path variable in System variables, click Edit → New, add: %JAVA_HOME%\bin
  4. Click OK on all windows. Open a new Command Prompt to apply the changes.

Open your shell config file:

  • Zsh (macOS default): ~/.zshrc
  • Bash: ~/.bashrc or ~/.bash_profile

Add these two lines at the end of the file:

export JAVA_HOME=$(/usr/libexec/java_home -v 21)  # macOS
# or on Linux:
# export JAVA_HOME=/usr/lib/jvm/temurin-21-amd64

export PATH=$JAVA_HOME/bin:$PATH

Apply immediately:

source ~/.zshrc   # or source ~/.bashrc

4. Troubleshooting

java is not recognized as an internal or external command (Windows)

Cause: The JDK has not been added to PATH.

Fix: 1. Check that JAVA_HOME is set (see the section above). 2. Make sure %JAVA_HOME%\bin is included in the Path variable. 3. Important: Open a completely new Command Prompt window — the old one won't pick up the new environment variables.

java -version shows a different version (not 21)

Cause: Multiple JDK versions are installed and an older one takes priority.

Check which java executable is being used:

where java

The first path in the list is the active version. Make sure the path to JDK 21 appears first.

which java

brew: command not found (macOS)

Homebrew is not installed. Re-run the Homebrew installation command in Step 1.

Permission denied error (Linux)

Prepend sudo to the command. Example: sudo apt install -y temurin-21-jdk.

5. Summary

Command Purpose
java -version Verify the JVM is installed (runs programs)
javac -version Verify the compiler is installed
echo %JAVA_HOME% Check the environment variable (Windows)
echo $JAVA_HOME Check the environment variable (macOS/Linux)

Once both java -version and javac -version show 21.x.x, you are ready for the next lesson: Install IDE.

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