When you install Java via yum install java on CentOS, the system typically creates symlinks in /usr/bin, but the actual Java installation resides elsewhere. The JAVA_HOME environment variable should point to the root directory of your JDK/JRE installation, not to the binary symlink.

First, let's find where Java is actually installed:

# For OpenJDK installations:
sudo alternatives --config java

# Or use this command to follow symlinks:
readlink -f $(which java)

Example output might show: /usr/lib/jvm/java-11-openjdk-11.0.19.0.7-1.el7_9.x86_64/bin/java

The JAVA_HOME should be the parent directory of the bin folder containing the java executable. From the above example:

export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-11.0.19.0.7-1.el7_9.x86_64

To make this persistent across reboots:

Option 1: System-wide configuration (requires root)

echo "export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-11.0.19.0.7-1.el7_9.x86_64" >> /etc/profile.d/java.sh
chmod +x /etc/profile.d/java.sh

Option 2: User-specific configuration

echo "export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-11.0.19.0.7-1.el7_9.x86_64" >> ~/.bashrc
source ~/.bashrc

After setting the variable, verify it works correctly:

echo $JAVA_HOME
$JAVA_HOME/bin/java -version

For more complex environments, you might want an automated way to set JAVA_HOME:

#!/bin/bash

# Find Java home automatically
if type -p java &> /dev/null; then
    JAVA_EXEC=$(readlink -f $(which java))
    if [[ "$JAVA_EXEC" == */bin/java ]]; then
        export JAVA_HOME=${JAVA_EXEC%/bin/java}
        echo "Discovered JAVA_HOME: $JAVA_HOME"
    else
        echo "Could not determine JAVA_HOME from java executable path"
    fi
else
    echo "Java not found in PATH"
fi

• Never set JAVA_HOME to /usr/bin (this is just symlinks)
• For multiple Java versions, use alternatives to manage them
• Some applications expect trailing slash in JAVA_HOME, others don't

When you install Java via yum on CentOS, the system places the Java binaries in standard paths like /usr/bin/java, but this isn't where JAVA_HOME should point. JAVA_HOME must reference the JDK/JRE installation directory containing bin, lib, and other critical folders.

First, identify where Java is truly installed:

readlink -f $(which java)
# Typical output for OpenJDK: /usr/lib/jvm/java-11-openjdk-11.0.19.0.7-1.el7_9.x86_64/bin/java

Strip the trailing /bin/java to get your JAVA_HOME:

dirname $(dirname $(readlink -f $(which java)))

For permanent configuration, add this to ~/.bashrc or /etc/profile:

export JAVA_HOME=$(dirname $(dirname $(readlink -f $(which java))))
export PATH=$PATH:$JAVA_HOME/bin

Check your configuration with:

echo $JAVA_HOME
java -version
javac -version  # If JDK is installed

For multi-Java environments, use alternatives:

sudo alternatives --config java

Here's a robust bash function for Java environment setup:

setup_java_env() {
    local java_path=$(readlink -f $(which java))
    if [[ -z "$java_path" ]]; then
        echo "Java not found!" >&2
        return 1
    fi
    
    export JAVA_HOME=$(dirname $(dirname "$java_path"))
    export PATH=$PATH:$JAVA_HOME/bin
    
    cat <> ~/.bashrc
# Java Environment
export JAVA_HOME=$JAVA_HOME
export PATH=\$PATH:\$JAVA_HOME/bin
EOF
    
    echo "Java environment configured for $JAVA_HOME"
}

When managing several Java installations, consider this pattern:

# In /etc/profile.d/java.sh
export JAVA_8_HOME=$(dirname $(dirname $(readlink -f $(which java))))
export JAVA_11_HOME=/usr/lib/jvm/java-11-openjdk
# Default to Java 11
export JAVA_HOME=$JAVA_11_HOME