Exception Handling

Exceptions and language featuresto handle them

Exception unusual condition that occurs during

execution of a program may be error or not may be hardware or software based detected by

hardware (end-of-file) OS (file-not-found) PL (division-by-zero) program (test for zero denominator)

Exception handling

response to exception being detected terminate program ignore exception (temporarily) ‘handle’ exception within program –

special code to execute

Programmed exception handling

who handles the exception? procedure executing when exception is

raised calling procedure

provides instructions to called procedure by passing procedure parameter or reference

receives notification by return value from called procedure

special exception communication features in the language

design of exception handling

possible functions defining exceptions detecting and identifying exceptions propagating exceptions intercepting exceptions binding exceptions to handlers resolving exceptions

design of exception handling

features complete structure (eg Pascal procedure,

java class) or code segment? one general handler or many specific

handlers? program continuation options programmer control – defining &

detecting exceptions, defining handlers?

use of exception handling features

simplify code – remove error handling ‘clutter’

focus programmer attention on program correctness

give more program control in response to errors

use exceptions as legitimate design tool – not just for errors

open questions

should hardware errors be part of exception handling process?

should there be a default exception handling system?

should programmers have power to disable exceptions?

PL/I system pre-defined exceptions

e.g. ZERODIVIDE, ENDFILE(source) system default handlers – SYSTEM to use defaults:

ON SUBSCRIPTRANGE SYSTEM; /* redundant */...

(SUBSCRIPTRANGE): X(I) = Y(I+1) * 2; some exceptions always on, some

default on, some default off

PL/Iprogrammer can: create new exceptions

CONDITION BADINPUT; /* declaration */ raise (throw) the exception:

SIGNAL (X > 100) BADINPUT; trap and handle the exception like a

system exception

PL/I

to handle programmer exceptions ORto override SYSTEM handlers exception handlers are code segments

execute in environment where codedON <EXCEPTION_NAME>BEGIN; <statememts> END;

scope of control to end of block to next handler with same exception

PL/I

e.g. user defined exception declare, handle and throw

CONDITION BADINPUT

ON BADINPUT

BEGIN;

< code to handle exception>

END;

...

SIGNAL (X>100) BADINPUT;

PL/I exception handling problems

dynamic binding of handlers to exceptions is flexible but can be misleading: wrong handler can respond to raised exceptione.g. – several handlers for SUBSCRIPTRANGE associated with different arrays

complex continuation rules (what to do after handler executes)

Ada handlers are code segments in procedure

or block scope, at the end of the block static binding of exception to handler

after handler executes, procedure or block terminates, returns control to caller

unhandled exceptions propegated through dynamic links until handler found

if no handler found, task or program terminates

C++ try – throw – catch structure like java system exceptions and programmer

exceptions are distinct – programmer can’t handle system exceptions

anything can be thrown as exception (primitive or object) – no predefined exception classes

caught exceptions can be rethrown to caller

Java try – throw – catch structure with a

finally block option exceptions are legitimate objects in the

Throwable hierarchy some ‘handling’ can occur in the exception

object constructor as well as the catch continuation:

catch handlers can rethrow exceptions or throw different ones

hander can use return to terminate method otherwise execution continues after try/catch

block

Java Exception classes

Object Throwable

Error Exception

RuntimeException

class Throwable only objects that can be thrown by

jvm or by program contains

snapshot of execution stack for this thread

string message (maybe) cause (chain)- reference to another

throwable (trace history when one exception is caught and another is thrown) - recursive

class Error extends Throwable constructors

with / without message with / without cause

methods all inherited from Throwable

Errors should not be caught - serious abnormal conditions that should cause termination (“unchecked”)

class Exception extends Throwable

constructors with / without message with / without cause

methods all inherited from Throwable

Exceptions must be caught* - abnormal but manageable conditions that should be handled by program (“checked”)

* except RuntimeExceptions, next slide

class RuntimeException extends Exception

constructors with / without message with / without cause

methods all inherited from Throwable

RuntimeExceptions do not have to be caught - errors that should not occur (“unchecked”)

Java – exception classes unchecked exceptions (Error,

RuntimeException and subclasses) assumed throwable and propagatable from any class

checked exceptions (Exception) complier verifies that any exceptions a method throws are either caught or declared in header

which and why?? the official word

extending Error

AnnotationFormatError, AssertionError, AWTError, CoderMalfunctionError, FactoryConfigurationError, FactoryConfigurationError, IOError, LinkageError, ServiceConfigurationError, ThreadDeath, TransformerFactoryConfigurationError, VirtualMachineError

extending Exception - catch these AclNotFoundException, ActivationException, AlreadyBoundException, ApplicationException,

AWTException, BackingStoreException, BadAttributeValueExpException, BadBinaryOpValueExpException, BadLocationException, BadStringOperationException, BrokenBarrierException, CertificateException, ClassNotFoundException, CloneNotSupportedException, DataFormatException, DatatypeConfigurationException, DestroyFailedException, ExecutionException, ExpandVetoException, FontFormatException, GeneralSecurityException, GSSException, IllegalAccessException, IllegalClassFormatException, InstantiationException, InterruptedException, IntrospectionException, InvalidApplicationException, InvalidMidiDataException, InvalidPreferencesFormatException, InvalidTargetObjectTypeException, InvocationTargetException, IOException, JAXBException, JMException, KeySelectorException, LastOwnerException, LineUnavailableException, MarshalException, MidiUnavailableException, MimeTypeParseException, MimeTypeParseException, NamingException, NoninvertibleTransformException, NoSuchFieldException, NoSuchMethodException, NotBoundException, NotOwnerException, ParseException, ParserConfigurationException, PrinterException, PrintException, PrivilegedActionException, PropertyVetoException, RefreshFailedException, RemarshalException, RuntimeException, SAXException, ScriptException, ServerNotActiveException, SOAPException, SQLException, TimeoutException, TooManyListenersException, TransformerException, TransformException, UnmodifiableClassException, UnsupportedAudioFileException, UnsupportedCallbackException, UnsupportedFlavorException, UnsupportedLookAndFeelException, URIReferenceException, URISyntaxException, UserException, XAException, XMLParseException, XMLSignatureException, XMLStreamException, XPathException

extending RuntimeException AnnotationTypeMismatchException, ArithmeticException, ArrayStoreException,

BufferOverflowException, BufferUnderflowException, CannotRedoException, CannotUndoException, ClassCastException, CMMException, ConcurrentModificationException, DOMException, EmptyStackException, EnumConstantNotPresentException, EventException, IllegalArgumentException, IllegalMonitorStateException, IllegalPathStateException, IllegalStateException, ImagingOpException, IncompleteAnnotationException, IndexOutOfBoundsException, JMRuntimeException, LSException, MalformedParameterizedTypeException, MirroredTypeException, MirroredTypesException, MissingResourceException, NegativeArraySizeException, NoSuchElementException, NoSuchMechanismException, NullPointerException, ProfileDataException, ProviderException, RasterFormatException, RejectedExecutionException, SecurityException, SystemException, TypeConstraintException, TypeNotPresentException, UndeclaredThrowableException, UnknownAnnotationValueException, UnknownElementException, UnknownTypeException, UnmodifiableSetException, UnsupportedOperationException, WebServiceException

syntax of exceptions - Java… preceding codetry{

…code may throw Exceptions}catch (MyException mE){ \\ exception handler … exception handling code

}catch (MyNextException nE){

… handling by throwing new Exception}

… more exception handlersfinally{

… must be executed}

… following code

syntax of exceptions - Java… preceding codetry{

…code may throw Exceptions}catch (MyException mE){

… exception handling code}

catch (MyNextException nE){… handling by throwing new Exception}

… more exception handlersfinally{

… must be executed}

… following code

control flow with

1. no exception in try

2. exception caught by MyException

3. exception caught by MyNextException

4. exception not caught

control flow with

1. no exception in try

2. exception caught by MyException

3. exception caught by MyNextException

4. exception not caught