Kotlin language specification

An exception type declaration is any type declaration that meets the following criteria:

Any object of an exception type may be thrown or caught.

A try-expression becomes active once the execution of the program enters it and stops being active once the execution of the program leaves it. If there are several active try-expressions, the one that became active last is currently active.

If an exception is thrown while a try-expression is currently active and this try-expression has any catch-blocks, those catch-blocks are checked for applicability for this exception. A catch-block is applicable for an exception object if the runtime type of this expression object is a subtype of the bound exception parameter of the catch-block.

If a catch-block is applicable for the exception thrown, the code inside the block is evaluated and the value of the block is returned as the value of a try-expression. If the try-expression contains a finally-block, the body of this block is evaluated after the body of the selected catch block. If these evaluations results in throwing other exceptions (including the one caught by the catch-block), they are propagated as if none of the catch-blocks were applicable.

If none of the catch-blocks of the currently active try-expression are applicable for the exception, the finally block (if any) is still evaluated, and the exception is propagated, meaning the next active try-expression becomes currently active and is checked for applicability.

If there are no active try-blocks, the execution of the program finishes, signaling that the exception has reached top level.

Throwing an exception object is done using throw-expression. A valid throw expression throw e requires that:

Throwing an exception results in checking active try-blocks.