How can I quickly switch between JDKs in cygwin?

Question

When working on the command line (using CYGWIN), I frequently need to switch between different versions of java. Are there any utilities that can take care of setting up JAVA_HOME, PATH etc. for me each time I need to switch?

Answer 1

I use a shell (bash, ksh, ...) function for this; Functions execute in the context of the current shell process and therefore can affect its environment:

# Switch current JDK (JAVA_HOME) on Cygwin
function jdkswitch {
    local version=$1
    local -a JDKS
    JDKS[8]='/cygdrive/c/apps/JDK/x64/jdk1.8.0_231'
    JDKS[11]='/cygdrive/c/apps/JDK/x64/jdk-11.0.5'

    if [[ -z ${version} ]] ; then
        echo "Current JDK: ${JAVA_HOME}"
        echo "Available JDKs: ${JDKS[*]}"
    else
        local jdkhome=${JDKS[${version}]}
        if [[ -d ${jdkhome} ]] ; then
            # Cygwin paths do not work for JAVA_HOME, must use Windows-style
            export JAVA_HOME=$(cygpath -w ${jdkhome})
            PATH=${jdkhome}/bin:${PATH}
            echo "Switched JDK to:"
            java -version
        else
            echo "Usage: jdkswitch version"
            echo "Available versions: ${!JDKS[*]}"
        fi
    fi
}

I typically save my functions one-per-file in ~/bin/functions and load them in my .profile (bash lacks the ksh "autoload" feature):

FPATH=~/bin/functions
for FUNC in ${FPATH}/* ; do
    . ${FUNC}
done

Then to switch JDK it's simply:

$ jdkswitch 11

There's a minor bug there where successive switches will prepend to the PATH each time, it might be fixable with some trickery but I've not experienced a problem to-date.

As an aside (since the question specifies Cygwin), here is a MacOS version:

# Switch current JDK (JAVA_HOME) based on available Mac installs
function jdkswitch {
    local version=$1
    local jdkhome
    if [[ -z ${version} ]] ; then
        /usr/libexec/java_home --verbose
    else
        jdkhome=$(/usr/libexec/java_home -v${version})
        if [[ -d ${jdkhome} ]] ; then
            export JAVA_HOME=${jdkhome}
            echo "Switched to ${JAVA_HOME}"
            java -version
        fi
    fi
}