Why is it possible to recover from a StackOverflowError?

When the stack overflows and StackOverflowError is thrown, the usual exception handling unwinds the stack. Unwinding the stack means:

• abort the execution of the currently active function
• delete its stack frame, proceed with the calling function
• abort the execution of the caller
• delete its stack frame, proceed with the calling function
• and so on...

... until the exception is caught. This is normal (in fact, necessary) and independent of which exception is thrown and why. Since you catch the exception outside of the first call to foo(), the thousands of foo stack frames that filled the stack have all been unwound and most of the stack is free to be used again.

When the StackOverflowError is thrown, the stack is full. However, when it's caught, all those foo calls have been popped from the stack. bar can run normally because the stack is no longer overflowing with foos. (Note that I don't think the JLS guarantees you can recover from a stack overflow like this.)

When the StackOverFlow occurs, the JVM will pop down to the catch, freeing the stack.

In you example, it get rids of all the stacked foo.