Java 21 LTS is the current Long-Term Support release of Java, featuring virtual threads (Project Loom), pattern matching, and record patterns. This guide installs OpenJDK 21 on Ubuntu 24.04 LTS.

Tested and valid on:

  • Ubuntu 24.04 LTS

Prerequisites

  • Ubuntu 24.04 LTS server
  • A user with sudo privileges

Step 1 – Install OpenJDK 21

OpenJDK 21 is available in Ubuntu 24.04 repositories:

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

Step 2 – Verify the Installation

Check the Java version:

java -version
javac -version

Step 3 – Set JAVA_HOME

Add JAVA_HOME to your environment:

echo 'export JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64' >> ~/.bashrc
echo 'export PATH=$PATH:$JAVA_HOME/bin' >> ~/.bashrc
source ~/.bashrc

Step 4 – Manage Multiple Java Versions

If multiple JDKs are installed, switch with update-alternatives:

sudo update-alternatives --config java

Step 5 – Create and Run a Hello World Program

Write a simple Java program:

mkdir ~/hello && cat > ~/hello/Hello.java << 'EOF'
public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello from Java 21 on Ubuntu 24.04!");
    }
}
EOF
javac ~/hello/Hello.java
java -cp ~/hello Hello

Step 6 – Install Maven or Gradle (optional)

Install the Maven build tool:

sudo apt install maven -y
mvn -version

Or install Gradle via SDKMAN:

curl -s 'https://get.sdkman.io' | bash
source ~/.sdkman/bin/sdkman-init.sh
sdk install gradle

Step 7 – Run a JAR File

Execute a compiled JAR application:

java -jar myapp.jar

Conclusion

OpenJDK 21 LTS is now installed on Ubuntu 24.04 LTS. It supports all major Java frameworks including Spring Boot, Quarkus, and Micronaut. Use virtual threads for high-throughput server applications.