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Java powers enterprise applications, build tools like Maven and Gradle, and platforms like Jenkins, Elasticsearch, and Minecraft servers. This guide covers installing OpenJDK, the standard open-source Java implementation.

Prerequisites

• Ubuntu 22.04/24.04 or Debian 11/12 VPS
• Root or sudo access

Step 1 — Install OpenJDK

sudo apt update
sudo apt install default-jdk -y

Or a specific version:

sudo apt install openjdk-21-jdk -y

Step 2 — Verify Installation

java -version
javac -version

Step 3 — Set JAVA_HOME

sudo nano /etc/environment

Add:

JAVA_HOME="/usr/lib/jvm/default-java"

source /etc/environment
echo $JAVA_HOME

Installing Multiple Java Versions

sudo apt install openjdk-17-jdk openjdk-21-jdk -y

Switching Between Java Versions

sudo update-alternatives --config java

Step 4 — Test with a Simple Program

nano HelloWorld.java

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello from Java!");
    }
}

javac HelloWorld.java
java HelloWorld

Installing Maven (Java Build Tool)

sudo apt install maven -y
mvn -version

Installing Gradle (Alternative Build Tool)

sudo apt install gradle -y
gradle -version

Running a Java Application (JAR File)

java -jar myapp.jar

Running a Java Application as a systemd Service

sudo nano /etc/systemd/system/myapp.service

[Unit]
Description=My Java Application
After=network.target

[Service]
ExecStart=/usr/bin/java -jar /opt/myapp/myapp.jar
Restart=always
User=deploy

[Install]
WantedBy=multi-user.target

sudo systemctl daemon-reload
sudo systemctl enable --now myapp

Common Errors

"java: command not found" — installation failed or PATH isn't updated; verify with which java and reinstall if needed.

Wrong Java version being used — use update-alternatives --config java to select the correct default among installed versions.

Best Practices

• Use OpenJDK for most applications unless a specific product requires Oracle's commercial JDK
• Set JAVA_HOME for build tools and applications that require it
• Run production Java applications as a managed systemd service, not a manual foreground process

FAQ

Should I install the JRE or JDK?
Install the JDK — it includes both the runtime (JRE) and development tools, and is the recommended choice for nearly all server use cases, including simply running pre-built applications.

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