Why is creating a Thread said to be expensive?

Question

The Java tutorials say that creating a Thread is expensive. But why exactly is it expensive? What exactly is happening when a Java Thread is created that makes its creation

Answer 1

Obviously the crux of the question is what does 'expensive' mean.

A thread needs to create a stack and initialize the stack based on the run method.

It needs to set up control status structures, ie, what state it's in runnable, waiting etc.

There's probably a good deal of synchronization around setting these things up.