You can use native API's using JNI.
You can also use SIGAR which is Apache licensed library which does much more then what you are asking for.It has wrapped the native calls in Java API's so that you dont have to know the inner workings of JNI.
You can also spawn processes from inside the code to run OS specific commands so that you can collect the system stats e.g. systeminfo |find "Available Physical Memory" or wmic command on windows or Linux cat /proc/meminfo . This can be achieved via java.lang.ProcessBuilder.
