Package openjdk-7-jdk is not available 발생시

KoNLPy를 사용하기 위해 우분투(여기서는 Synology Docker 상의 우분투)에서 자바를 설치하려고 하는데 다음과 같은 오류메시지가 출력된다.

Package openjdk-7-jdk is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source

sudo apt-get install g++ openjdk-7-jdk python-dev python3-dev

Reading package lists... Done

Building dependency tree       

Reading state information... Done

Package openjdk-7-jdk is not available, but is referred to by another package.

This may mean that the package is missing, has been obsoleted, or

is only available from another source

E: Package 'openjdk-7-jdk' has no installation candidate

이 때에는 ppa를 추가해주면 된다.

sudo add-apt-repository ppa:openjdk-r/ppa 

sudo apt-get update  

sudo apt-get install openjdk-7-jre

sudo apt-get install g++ openjdk-7-jdk python-dev python3-dev

그런데 add-apt-repository가 없다고 나올 경우에는,

sudo apt-get install software-properties-common

그럼에도 openjdk-7-jdk를 못찾는다고 나올 경우에는 openjdk-8-jdk를 설치하면 된다. (호환상 문제 없음)

참고: https://launchpad.net/~openjdk-r/+archive/ubuntu/ppa

       https://itsfoss.com/add-apt-repository-command-not-found/