11.4

An Example of Exceptions

EXCEPTIONS

Asynchronous exceptions are rare in Java. They occur only as a result of:

An invocation of the

stop

 methods of class

Thread

 ( 20.20.15,  20.20.16) or

ThreadGroup

 ( 20.21.8,  20.21.9)

An

InternalError

 ( 11.5.2.2) in the Java Virtual Machine

The

stop

 methods may be invoked by one thread to affect another thread or all the

threads in a specified thread group. They are asynchronous because they may

occur at any point in the execution of the other thread or threads. An

InternalError

is considered asynchronous so that it may be handled using the

same mechanism that handles the

stop

 method, as will now be described.

Java permits a small but bounded amount of execution to occur before an

asynchronous exception is thrown. This delay is permitted to allow optimized

code to detect and throw these exceptions at points where it is practical to handle

them while obeying the semantics of the Java language.

A simple implementation might poll for asynchronous exceptions at the point

of each control transfer instruction. Since a Java program has a finite size, this

provides a bound on the total delay in detecting an asynchronous exception. Since

no asynchronous exception will occur between control transfers, the code genera 

tor has some flexibility to reorder computation between control transfers for

greater performance.

The paper

Polling Efficiently on Stock Hardware

by Mark Feeley,

Proc. 1993

Conference on Functional Programming and Computer Architecture

, Copen 

hagen, Denmark, pp. 179 187, is recommended as further reading.

Like all exceptions, asynchronous exceptions are precise ( 11.3.1).

11.4   An Example of Exceptions

Consider the following example:

class TestException extends Exception {

TestException() { super(); }

TestException(String s) { super(s); }

}

class Test {

public static void main(String[] args) {

for (int i = 0; i < args.length; i++) {

206