Linux Java Runtime Environment | TOP-RATED ✧ |

-Xms512m -Xmx2g -XX:+UseG1GC | Variable | Purpose | |----------|---------| | JAVA_HOME | Root directory of JRE/JDK | | JRE_HOME | Sometimes used for JRE-specific path | | _JAVA_OPTIONS | Default JVM options (overrides command line) | | JAVA_TOOL_OPTIONS | Options for tools like java , javac |

Example:

sudo useradd -r -s /bin/false javauser sudo chown -R javauser:javauser /opt/myapp sudo -u javauser java -jar /opt/myapp/app.jar 10.1 "java: command not found" Cause : JRE not installed or PATH not set. linux java runtime environment

FROM alpine:3.19 RUN apk add --no-cache openjdk17-jre Create app user RUN addgroup -g 1000 -S appuser && adduser -u 1000 -S appuser -G appuser -Xms512m -Xmx2g -XX:+UseG1GC | Variable | Purpose |

sudo apt update sudo apt install temurin-17-jre # Download from https://adoptium.net/ wget https://github.com/adoptium/temurin17-binaries/releases/download/jdk-17.0.9%2B9/OpenJDK17U-jre_x64_linux_hotspot_17.0.9_9.tar.gz Extract to /opt sudo tar -xzf OpenJDK17U-jre_x64_linux_hotspot_17.0.9_9.tar.gz -C /opt Set environment variables echo "export JAVA_HOME=/opt/jdk-17.0.9+9-jre" | sudo tee /etc/profile.d/java.sh echo "export PATH=$JAVA_HOME/bin:$PATH" | sudo tee -a /etc/profile.d/java.sh source /etc/profile.d/java.sh 5. Managing Multiple JRE Versions Linux allows multiple JREs side-by-side. Use update-alternatives (Debian/Ubuntu) or manual symlinks. 5.1 Using update-alternatives (Debian family) # Register a new JRE sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/java-17-openjdk-amd64/bin/java 1700 List available JREs sudo update-alternatives --config java Switch version interactively (enter selection number) 5.2 Manual management via JAVA_HOME # Set per user (in ~/.bashrc) export JAVA_HOME=/usr/lib/jvm/java-17-openjdk-amd64 export PATH=$JAVA_HOME/bin:$PATH Or system-wide (in /etc/environment) JAVA_HOME="/usr/lib/jvm/java-17-openjdk-amd64" 6. Verifying Your JRE Installation | Command | Purpose | |---------|---------| | java -version | Shows JRE vendor, version, and VM mode | | which java | Shows path to java binary | | update-alternatives --list java | Lists all installed JREs (Debian) | | readlink -f $(which java) | Resolves symlinks to actual binary | | ldd $(which java) | Shows linked system libraries | Example validation script: #!/bin/bash echo "Java version:" java -version 2>&1 | head -1 echo "JAVA_HOME: $JAVA_HOME:-not set" echo "Java binary: $(readlink -f $(which java))" 7. JRE in Headless Environments (Servers & Containers) For server applications (no GUI), use headless JRE : Use update-alternatives (Debian/Ubuntu) or manual symlinks

# Check JAR's required version javap -verbose MyClass.class | grep "major version" # 52 = Java 8, 55 = Java 11, 61 = Java 17, 65 = Java 21 Minimal Dockerfile using Alpine + OpenJDK JRE :