How to dynamically load a Java class without using a class loader?

Question

I recently saw an online presentation of JRebel in which they explain what their product does, in one of the slides they mention they don't use class loaders (see screenshot below).

I have some projects in which I use class loaders for dynamically loading classes at runtime, for example for dependency injection (like in the Maker Factory framework that I developed). I thought the only way to load a class in Java was by using a class loader.

Answer 1

They don't use any new classloaders; they extend the default ones.

https://www.jrebel.com/learn/how-does-jrebel-work

Answer 2

Disclaimer: I'm involved with JRebel development

Technically, it is possible to bypass the classloader with some Unsafe magic, and JVM leverages that with lambdas when creating the runtime anonymous classes (in Java 8).

However, JRebel actually integrates with the existing classloaders and doesn't create new ones - that's what the slide mean. JRebel doesn't drop the existing classloader when it has to reload a class. Instead, it loads and versions the classes within the existing classloaders.

Answer 3

they mention they don't use class loaders.

Every class uses a ClassLoader (except primitives). However, a library doesn't have to create additional ClassLoader.

Even a "Hello World" program will have two class loaders (one for boostrapping)

I thought the only way to load a class in Java was by using a class loader.

It is, but you can force the existing classloader to load your class which is hack but might be simpler to use.

(Correction) If you use null as a ClassLoader, Unsafe.defineClass() will default to the ClassLoader of the caller.