Exception handling in java
- 1. Exception handling in java
- 2. What is Exception An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions.
- 3. Try catch finally try { //do something } catch (ExceptionType name) { } catch (ExceptionType name) { } finally { //clean up }
- 4. Advantages Separating Error-Handling Code from "Regular" Code Propagating Errors Up the Call Stack Grouping and Differentiating Error Types
- 6. Checked exceptions Part of the method signature Compile type checking Requires programmer to handle the exception or declare the method as throws exception Unique to java e.g. FileNotFoundException
- 7. Unchecked exceptions No need to declare the exception in method’s signature No compile time checking Usually indicate programming error e.g. NullPointerException
- 8. Error Indicate error in the underlying JVM Error are external to the application Application does not usually have to deal with these class of Exceptions e.g. OutOfMemoryError
- 9. When to throw exceptions Exceptions indicate a broken contract Precondition (e.g. file is open for read) Postcondition (e.g. read a character from file) Your method encounters an abnormal condition that it can't handle If your method is unable to fulfill its contract, throw either a checked or unchecked exception.
- 10. What to throw? Exceptions v/s Errors Errors are for JVM Exceptions for rest of us Checked v/s Unchecked exceptions Can caller recover from this error? Yes: checked No: unchecked
- 11. When to catch exception 1. When you can handle the exception 2. When you need to throw a different type of exception 3. Refer to 1 & 2
- 12. When not to throw an exception To achieve Flow control using exception try { } while (true) { increaseCount(); } } catch (MaximumCountReachedException ex) { } //Continue execution public void increaseCount() throws MaximumCountReachedException { if (count >= 5000) throw new MaximumCountReachedException(); }
- 13. 3 rules What went wrong? Where did it go wrong? Why did it go wrong? If your exception does not provide answers to all these questions, you are doing something wrong!
- 14. Performance implications of exceptions Exceptions are expensive for the JVM Creating stack traces requires resources and CPU the Java VM requires more efforts to handle a thrown exception than a normal method
- 15. Anti Patterns Log and Throw Throwing Generic Exception Catching Generic Exception Destructive Wrapping Log and Return Null Catch and Ignore (a.k.a. Head in the Sand) Throw from Within Finally Multi-Line Log Messages Unsupported Operation Returning Null
- 16. Anti Patterns - Log and Throw Log the error and throw the same exception again Messy log file Achieves nothing
- 17. Anti Patterns - Throwing Generic Exception The caller does not know the nature of error – hinders error handling
- 18. Anti Patterns - Catching Generic Exception We are masking programming errors
- 19. public SomeInterface buildInstance(String className) { SomeInterface impl = null; try { Class clazz = Class.forName(className); impl = (SomeInterface)clazz.newInstance(); } catch (Exception e) { log.error("Error creating class: " + className); } return impl; }
- 20. Anti Patterns - Destructive Wrapping catch (NoSuchMethodException e) { throw new MyServiceException("Blah: "+ e.getMessage()); }
- 21. Anti Patterns - Log and Return Null catch (NoSuchMethodException e) { LOG.error("Blah", e); return null; }
- 22. Anti Patterns - Catch and Ignore (a.k.a. Head in the Sand) catch (NoSuchMethodException e) { }
- 23. Anti Patterns - Throw from Within Finally try { blah(); } finally { cleanUp(); }
- 24. Anti Patterns - Multi-Line Log Messages LOG.debug("Using cache policy A"); LOG.debug("Using retry policy B");
- 25. Anti Patterns - Unsupported Operation Returning Null public String foo() { // Not supported in this implementation. return null; } Throw UnsupportedOperationException
- 26. Best practices Throw checked exception when caller can recover from error Throw runtime exception when the caller cannot recover Throw runtime exception for programming error Throw early, catch late Use NestedException Don’t catch an exception if you cant do any thing about it. Log exception only once, and at the latest possible time Default Error Page in presentation layer for all Runtime Exceptions
- 27. Exception chaining try{ ..some code that throws XXXException }catch(XXXException ex){ throw new RuntimeException(ex); }
- 28. Exception logging Log all internal states Log all parameters to the method that failed Log all data required to trace the error Ensure log statements don’t cause NPE*
- 29. Exceptions in a typical enterprise applications Define a hierarchy of exceptions. Lower level module throws lower level exceptions, higher level module encapsulate lower level exceptions Define which exceptions will cause transaction to rollback
- 30. Exceptions and Transactions @ApplicationException(rollback=true) public class FooException extends Exception ...
- 31. Questions
- 32. references Best practices in EJB exception handling http://www.ibm.com/developerworks/library/j-ejbexcept.html Beware the dangers of generic Exceptions http://www.javaworld.com/javaworld/jw-10-2003/jw-1003generics.html Exception Handling in Web Applications http://weblogs.java.net/blog/crazybob/archive/2004/02/exception_han dl.html Designing with Exceptions http://www.artima.com/designtechniques/desexceptP.html Build a better exception-handling framework http://www.ibm.com/developerworks/java/library/j-ejb01283.html
- 33. References (cont…) JAVA EXCEPTIONS http://www.javaolympus.com/J2SE/Exceptions/JavaExceptions.jsp Exception-Handling Antipatterns http://today.java.net/pub/a/today/2006/04/06/exception-handling-antipatterns.html Three Rules for Effective Exception Handling http://today.java.net/pub/a/today/2003/12/04/exceptions.html 13 Exceptional Exception Handling Techniques http://www.manageability.org/blog/stuff/exceptional-exception-handling-techniques Best Practices for Exception Handling http://www.onjava.com/pub/a/onjava/2003/11/19/exceptions.html Lesson: Exceptions http://java.sun.com/docs/books/tutorial/essential/exceptions/index.html
Editor's Notes
- #21: Class not found Class cast to the interface SomeInterface Both types of exceptions are handled in the catch block, but that’s not evident by casual inspection of the code.