What order should programs look at JAVA_HOME, JRE_HOME, JDK_HOME for locating a Java version?

What order should programs look at JAVA_HOME, JRE_HOME, JDK_HOME for locating a version of Java to execute Java programs?

I want my program to find Java automatically using an environment variable, but there are at least three to choose from.

My guess is that I should go by semantics; I need any JRE to run java (both JDK or JRE would do) so I'm guessing the following order:

JRE_HOME (semantics: this is specifically intended as a JRE, not a JDK)
JAVA_HOME (semantics: ? Not sure if it is meant as any Java installation or if it is specifically meant as a JDK in contrast to JRE_HOME's significance as a JRE)

and since I don't specifically need/want a JDK, and JDK_HOME looks less common in the wild than JAVA_HOME, perhaps not try to use JDK_HOME, and rely on my users to set JRE_HOME or JAVA_HOME appropriately.

Solution

Your guess order looks good.

You could try all of them in any particular order since how those variables are set depends on the operating system and/or on the user. The script below is part of OpenSuse Linux distro that does the job for the user.

As it can be seen it tries both java and javac default commands to set variables.
It could be perfectly possible that both command belong to different versions depending on user's configuration.

cat /etc/profile.d/alljava.sh

#                                                                               
#    /etc/profile.d/alljava.sh                                                    
#                                                                               
# send feedback to http://bugs.opensuse.org

#
# This script sets some environment variables for default java.
# Affected variables: JAVA_BINDIR, JAVA_HOME, JRE_HOME, 
#                     JDK_HOME, SDK_HOME
#

if test -L /etc/alternatives/java -a -e /etc/alternatives/java; then
    ALTERNATIVES_JAVA_LINK=`realpath /etc/alternatives/java 2> /dev/null`
    export JRE_HOME=${ALTERNATIVES_JAVA_LINK%/bin/java}
    unset ALTERNATIVES_JAVA_LINK
fi

if test -L /etc/alternatives/javac -a -e /etc/alternatives/javac; then
    ALTERNATIVES_JAVAC_LINK=`realpath /etc/alternatives/javac 2> /dev/null`
    export JAVA_HOME=${ALTERNATIVES_JAVAC_LINK%/bin/javac}
    export JAVA_BINDIR=$JAVA_HOME/bin
    export JDK_HOME=$JAVA_HOME
    export SDK_HOME=$JAVA_HOME
    unset ALTERNATIVES_JAVAC_LINK
fi

The mentioned script is installed by this package

rpm -qf /etc/profile.d/alljava.sh
aaa_base-84.87+git20180409.04c9dae-150300.10.28.2.x86_64

values as shown by env here

env | grep -E 'JDK|JRE|JAVA' 
JAVA_HOME=/usr/lib64/jvm/java-17-openjdk-17
JDK_HOME=/usr/lib64/jvm/java-17-openjdk-17
JRE_HOME=/usr/lib64/jvm/java-17-openjdk-17
JAVA_BINDIR=/usr/lib64/jvm/java-17-openjdk-17/bin

Users may override those values

export JDK_HOME='/usr/lib64/jvm/java-1.8.0-openjdk-1.8.0'

env | grep -E 'JDK|JRE' 
JDK_HOME=/usr/lib64/jvm/java-1.8.0-openjdk-1.8.0
JRE_HOME=/usr/lib64/jvm/java-17-openjdk-17

Finally, those variables could not be set at all so version should be obtained from the java command itself

java -XshowSettings:properties -version 2>&1 | grep -E -e 'java\.(home|specification\.version)'

    java.home = /usr/lib64/jvm/java-17-openjdk-17
    java.specification.version = 17