A JVM (Java Virtual Machine) running alongside PostgreSQL.
This is not production ready, just a proof of concept.
It only works on Linux, but it should be possible to run it on other OSes (feel free to create a PR).
You need to compile first the Java part, as the generated Jar file will be included as part of the extension. This is a standard Maven program:
cd java
mvn package
A jar file should have been generated under java/target/pgj-<VERSION>.jar.
You need a suitable JDK (version 6 or above) and export its path via the JAVA_HOME environment variable.
export JAVA_HOME=/path/to/jdk
cd pgxs
make
sudo make install
You need to make the JVM library (typically called libjvm.so) available in the LIBRARY_PATH, like:
sudo ln -s /usr/lib/jvm/java-8-oracle/jre/lib/amd64/server/libjvm.so /usr/lib
Add the library to postgresql.conf's shared_preload_libraries:
shared_preload_libraries = '... other libraries if any... , pgj' # (change requires restart)
And restart PostgreSQL.