exceptionhandlinginjava-140224181412-phpapp02.pptx

□ An exception is an event, which occurs during the
execution of a program, that disrupts the normal flow of
the program's instructions.

try {
//do something
} catch (ExceptionType name) {
} catch (ExceptionType name) {
} finally {
//clean up
}

□ Separating Error-Handling Code from "Regular" Code
□ Propagating Errors Up the Call Stack
□ Grouping and Differentiating Error Types

□ Part of the method signature
□ Compile type checking
□ Requires the programmer to handle the exception or
declare the method as throwing exception
□ Unique to java
□ e.g. FileNotFoundException

□ No need to declare the exception in the method’s
signature
□ No compile time checking
□ Usually indicate a programming error
□ e.g. NullPointerException

□ 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

□ 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.

□ 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

1. When you can handle the exception
2. When you need to throw a different type of exception
3. Refer to 1 & 2

□ 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();
}

□ 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!

□ 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

□ 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

□ Log the error and throw the same
exception again
□ Messy log file
□ Achieves nothing

□ The caller does not know the nature
of error – hinders error handling

□ We are masking programming errors

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;
}

catch (NoSuchMethodException e) {
throw new MyServiceException("Blah:
" +
e.getMessage());
}

LOG.error("Blah", e);
return null;
}

}

try {
blah();
} finally {
cleanUp();
}

ANTI PATTERNS - MULTI-LINE LOG
MESSAGES
LOG.debug("Using cache policy A");
LOG.debug("Using retry policy B");

public String foo() {
// Not supported in this
implementation.
return null;
}
□ Throw
UnsupportedOperationException

□
□
□
□
□
□
□
□
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

try{
..some code that throws
XXXException
}catch(XXXException ex){
throw new RuntimeException(ex);
}

□ 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*

□ 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

Exceptions and Transactions
□ @ApplicationException(rollback=true)
public class FooException extends
Exception ...

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-1003-
generics.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

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

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