gem install rjb (1.6.4) error

An error occurred while installing rjb (1.6.4), and Bundler cannot continue.

Make sure that `gem install rjb -v '1.6.4' --source 'https://gems.ruby-china.com/'` succeeds before bundling.

Could not create Makefile due to some reason, probably lack of necessary

libraries and/or headers. Check the mkmf.log file for more details. You may

need configuration options.

Provided configuration options:

--with-opt-dir

--without-opt-dir

--with-opt-include

--without-opt-include=${opt-dir}/include

--with-opt-lib

--without-opt-lib=${opt-dir}/lib

--with-make-prog

--without-make-prog

--srcdir=.

--curdir

--ruby=/home/admin/.rvm/rubies/ruby-2.7.2/bin/$(RUBY_BASE_NAME)

extconf.rb:54:in `<main>': JAVA_HOME is not set. (RuntimeError)

extconf failed, exit code 1

1> java -version

Command 'java' not found, but can be installed with:

sudo apt install default-jre # version 2:1.11-72, or

sudo apt install openjdk-11-jre-headless # version 11.0.9.1+1-0ubuntu1~20.04

sudo apt install openjdk-8-jre-headless # version 8u275-b01-0ubuntu1~20.04

sudo apt install openjdk-13-jre-headless # version 13.0.4+8-1~20.04

sudo apt install openjdk-14-jre-headless # version 14.0.2+12-1~20.04

2> sudo apt install openjdk-8-jre-headless

3> export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64