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ONOS is a Java based platform. In order to be able to compile the code, you need to install Oracle Java 1.8. The OpenJDK equivalent might work, but it's not suggested, since it has never been tested.

Since Oracle had an license update on April 16 2019, anyone who want to download java8 has to register account, the apt methods of installing will not work.

Go to Oracle JDK download website, login and download the tar ball : jdk-8uxxx-linux-x64.tar.gz

sudofor apt-getJavaCommand install software-properties-common -y && \
sudo add-apt-repository ppa:webupd8team/java -y && \
sudo apt-get update && \
echo "oracle-java8-installer shared/accepted-oracle-license-v1-1 select true" | sudo debconf-set-selections && \
sudo apt-get install oracle-java8-installer oracle-java8-set-default -yin java java2groovy javac javadoc javafxpackager javah javap javapackager javaws
do
    sudo update-alternatives --install /usr/bin/$JavaCommand $JavaCommand /your_path_extracted_jdk/bin/$JavaCommand 1
done

Install bazel

currently  bazel.0.21.0 is the lastest one that support building ONOS, please refer to Install bazel for how to install bazel.