1 Exception Handling Introduction to Exception Handling Exception Handling in PLs –Ada –C++ –Java Sebesta Chapter 14

2 Why Exception Handling? No exception handling –Unchecked errors abort the program –Clutter program with if-clauses checking for errors –Use the return value to indicate errors –Pass an error-label parameter in all subprograms –Pass an errors handling subprogram to all subprograms With exception handling –Programs can catch exceptions, handle the problems and continue –Programmers are encouraged to consider all possible errors –Exception propagation allows for reuse of exception handling code

3 Basic Concepts An exception –An exceptional state in program execution, typically an error Exception handling –Built-in mechanism for special flow control when exception occurs Exception handler –Code unit that handles an exception An exception is raised (or thrown) –When the associated exceptional event occurs

4 Design Issues How and where are exception handlers specified? What is the scope of an exception handler? How is an exception event bound to an exception handler? Is the exception info available in the handler? Where does control go at the end of an exception handler? Is finalization supported? Can user define her own exceptions and how? Should there be default exception handlers? Can built-in exceptions be explicitly raised? Are hardware errors exceptions that can be handled? Are there any built-in exceptions? Can exceptions be disabled and how?

5 Exception Handling Control Flow

6 Exception Handling in Ada An exception handler is –a subprogram body, or –a package body, or –a task, or –a block Exception handlers do not have parameters –They are usually with the code where an exception can be raised Handlers are at the end of the block in which they occur

7 Exception Handlers Syntax Handler form when exception { | exception } => statements... [when others => statements] exception form exception_name | others

8 Propagation of Exceptions If exception is raised in a unit without a handler for that exception, the unit is terminated Then, the exception is propagated –From a procedure to the caller –From a block to parent scope –From a package body to the declaration part of the declaring unit –In a task No propagation, just mark it "Completed" If the unit where an exception is propagated to doesn't handle it, that unit is terminated, too

9 Other Syntax User-defined exceptions form exception_name_list : exception; Raising exceptions form raise [exception_name] –If the exception name is omitted, the same exception is propagated Exception conditions can be disabled with pragma SUPPRESS(exception_list)

10 Predefined Exceptions CONSTRAINT_ERROR –index constraints, range constraints, etc. NUMERIC_ERROR –illegal numeric operation (overflow, division by zero, etc.) PROGRAM_ERROR –call to a subprogram whose body has not been elaborated STORAGE_ERROR –system heap overflow TASKING_ERROR –Some tasks' error

11 Evaluation of Ada's Exception Handling Reflects the state-of-the-art in PL design in 1980 A significant advance over PL/1 Ada was the only widely used language with exception handling before it was added to C++

12 Exception Handling in C++ Exception handling was added in 1990 Design is based on CLU, Ada, and ML Syntax try { -- code that may raise an exception } catch (formal parameter) { -- handler code } catch (formal parameter) { -- handler code … }

13 catch Function catch is an overloaded name of all handlers –The formal parameter of each catch must be unique The formal parameter doesn't need to be a variable –It can be a type name to distinguish it from others The formal parameter can supply the handler with information about the exception If the formal parameter is an ellipsis, it handles all exceptions not yet handled After a handler is executed, control flows to the first statement after the sequence of catch es An unhandled exception is propagated to the caller The propagation continues to the main function If no handler is found, the default handler is called

14 Throwing Exceptions Exceptions are raised explicitly by throw [expression]; A throw with no operand re-raises the exception –It can only appear within a handler The type of the expression disambiguates the handlers

15 Other Design Choices All exceptions are user-defined Exceptions are neither specified nor declared The default handler unexpected terminates the program – unexpected can be redefined by the user A function can list the exceptions it may raise A function can raise any exception (the throw clause) –No specification is required

16 Evaluation of Exception Handling in C++ Advanced compared to Ada However, some weird design choices Reliability –Hardware- and system software-exceptions can't be handled Readability –Exceptions are not named –Exceptions are bound to handlers via the type of the parameter

17 Exception Handling in Java Based on that of C++ More OOP-compliant All exceptions are objects of subclasses of the Throwable class

18 Classes of Exceptions Superclasse Throwable has two subclasses Error –System events such as heap overflow –User programs aren't required to handle these –Usually they don't handle them Exception –User-defined exceptions are usually subclasses –Has rich subtree of predefined subclasses NullPointerException, IOException, ArrayIndexOutOfBoundsException, … Typically, user programs must handle these

19 Java Exception Handlers try clause is exactly like in C++ catch clause is like in C++ –But every catch must have a declared named parameter whose type is a subclass of Throwable Exceptions are thrown as in C++ with throw, but an object must be thrown –It must bean instance of a subclass of Throwable An exception is bound to the 1 st handler whose parameter type matches the thrown object –It has the same class or is a its superclass A handler that catch es a Throwable parameter will catch all unhandled exceptions –This insures that all exceptions are caught –Of course, it must be the last in the try construct

20 The finally Clause Optional, at the end of a try-catch-catch construct Form finally {... } Specifies code that will be always executed, regardless of what happens in the try construct

21 Example with finally A try construct with a finally clause can be used outside exception handling context try { for (i = 0; i < 100; i++) { … if (…) {return i;} // index found, return it } } finally { … // clean up }

22 Continuation If no handler is found in the try construct, the search is continued in the nearest enclosing try construct, etc. If no handler is found in the method, the exception is propagated to the method’s caller If no handler is found (all the way to main ), the program is terminated

23 Checked and Unchecked Exceptions The Java throws clause is different from C++ Unchecked exceptions –Exceptions of class Error and RunTimeException and their subclasses Checked exceptions –All other exceptions A method that may throw a checked exception must –Either handle it, or –List it in the throws clause A method cannot declare more exceptions in its throws clause than the method it overrides

24 Handling Choices An exception can be –Caught and ignored Usually a bad, bad choice –Caught and handled completely –Propagated I.e. not caught –Caught, handled and re-thrown By throw ing it in the handler –Caught, handled, then a different exception is thrown Often the best choice as system exceptions carry insufficient information

25 Assertions Statements in the program specifying whether the current state of the computation is as expected Must form a boolean expression that –When evaluated to true nothing happens The program state is ok –When evaluated to false throws AssertionError Can be disabled during runtime without program modification or recompilation Two forms – assert condition; – assert condition: expression;

26 Evaluation The types of exceptions makes more sense than in the case of C++ The throws clause is better than that of C++ –The throw clause in C++ says little to the programmer The finally clause is often useful The Java interpreter throws a variety of exceptions that can be handled by user programs