-These requirements only apply to the Survival version of Minecraft, and not to the Classic version.
-Requirements may change depending on the server software, such as using Bukkit and/or mods.
-Java 8.1 is available for Minecraft Servers!
-Linux users should avoid using OpenJDK 6 for server hosting, as it has been known to cause issues
-Only the JRE version is required, however, Java JDK will still work.
-The more memory that your server has, the better. It is recommended to have at least 3 GB of physical RAM in the computer. That way, plenty of RAM is left over for the operating system, and you have the possibility of allocating more to the Minecraft server as needed.
-Minecraft servers, as of 1.1.0, can use multiple cores, so now they will not sit idle and will increase server performance.
Windows or Desktop based Linux distributions aren't recommended for hosting servers, use console based Linux/BSD for maximum performance.
Source:
https://minecraft.gamepedia.com/Server/Requirements