All code running in IJava flows through the kernel. This makes it the place to register magics, add things to the classpath, and perform many jupyter related operations.
IJava injects a function for getting the active kernel instance and additional helpers for making use of the kernel at runtime. These are defined in the runtime Kernel class.
Get a reference to the current kernel. It may return null if called outside of a kernel context but should be considered @NonNull when inside a notebook or similar. The kernel api has lots of goodies, look at the JavaKernel class for more information. Specifically there is access to adding to the classpath, getting the magics registry and maven resolver, and access to eval.
The eval function provides full access to the code evaluation mechanism of the kernel. It evaluates the code in the same scope as the kernel and returns an object. This object is an object that lives in the kernel!
The given expression can be anything you would write in a cell, including magics.
(int) eval("1 + 2") + 3