Sometimes, when working with Java payloads, it might be necessary to switch to a different Java version or use specific version in order to run a program. Recently I needed to switch from OpenJDK version 17 to 11 in order to get a ysoserial payload to generate. One of the suggestions available when researching this was to use alternatives to switch versions, but this seems like a relatively permanent solution to a temporary problem.

Java versions installed on Debian operating systems are stored in /usr/lib/jvm. In there are nested bin directories containing java. By setting the PATH environment variable and including java’s directory first, the correct java command will be used. Java also requires a home directory set in the JAVA_HOME directory for java JDK reasons.

• switching to Java 11 on Kali

export PATH=/usr/lib/jvm/java-11-openjdk-arm64/bin:$PATH
export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-arm64

Developing a Version Selector Script

Since I like automating everything, I decided to come up with a Java version selector script in order to do this on the spot when I need to. Since the script is setting environment variables, I need to actually source it. The easiest way to do this is to define a bash alias in my bashrc, which is available on github in my environment scripts.

alias java-selector='source $ENVIRON_BASEPATH/zet/20240107171624/java-selector'

I am using the fzf program to create an interactive menu to and return the path to the selected java binary. A simple awk script to cut the directory path on slashes and remove the last one helps find the relevant paths.

#!/bin/bash

javapath=$(find /usr/lib/jvm -name java | fzf | awk 'BEGIN {OFS=FS="/"} {NF--; print}')
javadir=$(echo "$javapath" | awk 'BEGIN {OFS=FS="/"} {NF--; print}')
export PATH=$javapath:$PATH
export JAVA_HOME=$javadir

• example of switching java versions

┌──(parallels㉿kali-gnu-linux-2023)-[~/environment/zet/20240107171624]
└─$ java-selector

┌──(parallels㉿kali-gnu-linux-2023)-[~/environment/zet/20240107171624]
└─$ java --version
Picked up _JAVA_OPTIONS: -Dawt.useSystemAAFontSettings=on -Dswing.aatext=true
openjdk 11.0.20-ea 2023-07-18
OpenJDK Runtime Environment (build 11.0.20-ea+7-post-Debian-1)
OpenJDK 64-Bit Server VM (build 11.0.20-ea+7-post-Debian-1, mixed mode)

┌──(parallels㉿kali-gnu-linux-2023)-[~/environment/zet/20240107171624]
└─$ java-selector

┌──(parallels㉿kali-gnu-linux-2023)-[~/environment/zet/20240107171624]
└─$ java --version
Picked up _JAVA_OPTIONS: -Dawt.useSystemAAFontSettings=on -Dswing.aatext=true
openjdk 17.0.9 2023-10-17
OpenJDK Runtime Environment (build 17.0.9+9-Debian-2)
OpenJDK 64-Bit Server VM (build 17.0.9+9-Debian-2, mixed mode, sharing)