mcrepl

This plugin enables operators on your server to open a personal Java REPL they can interact with through a simple textual interface. With this, they can execute arbitrary Java code from inside the game. They will even have access to the entire Bukkit API and the APIs of all the plugins running on the server.

The plugin implements a custom REPL on top of JShell, the official Java shell added in Java 9's JDK. You can find the User Guide for JShell here. Because this plugin requires JShell, that means your server must be started using the JRE provided with the Java 9 JDK. If you don't know what that means, this plugin is probably not for you. This plugin is intended for developers and is not recommended for production servers unless you really know what you're doing.

This plugin gives authorized users unrestricted access to your server via the Java code they enter. That means that anyone allowed to use this REPL can potentially delete all the files on your server. YOU HAVE BEEN WARNED!

Permissions
mcrepl.use - Gives access to the /mcrepl command which starts a REPL for the user. OPs have this permission by default.
mcrepl.modal - Silences all messages sent to the user except those sent from the REPL while the REPL is in use. This is enabled by default for all users.