Exception & LoggingAzeem MumtazSoftware Engineer

Introduction

•Why Exceptions?▫To describe exceptional circumstances▫To interrupt the current flow

•In Java▫Exception are thrown and caught

Exception Class Hierarchy

Throwable

Error Exception

RuntimeException

ArithmeticException

IOException

FileNotFoundException

Types of Exception

•Within JVM there are 2 types of exceptions▫Checked

Enforced by methods▫Unchecked

May lead to thread unraveling and poor visibility

Catching Exceptions

•Try ▫Tells that the enclosed code segment could

raise a potential exception•Catch

▫What exceptions to handle▫If no catch for a checked exception, the

compiler will generate an error.

Guaranteed Execution of Cleanup Code•To write code that executes when control

leaves a code segment▫Finally

Throwing Exceptions

•Making account for an exceptional problem by throwing an exception

•The execution will not continue on current code path

Catching Multiple Exceptions•Using different catch clauses

▫Most-Specific to Most-General•A new Java construct

▫“|” operator▫Java 7 Feature

•Pokemon® Exception▫catch (Exception e)▫Catches every exception type and treat them

same▫May catch unwanted exceptions (i.e.

OutOfMemoryException)

Catching the Uncaught Exceptions•How to know when a thread is being

terminated because of an uncaught exception?▫Register an ExceptionHandler

Per thread Thread.currentThread().setUncaughtExceptionH

andler Globally

Thread.setDefaultUncaughtExceptionHandler

Managing Resource with try/catch•Why?

▫Managing resources problematic ▫Should properly dispose once used▫Limited and cause performance

degradation•ARM

▫Automatic Resource Management▫Java 7▫Try-with-resource block▫java.lang.AutoClosable

Creating an Exception Class• extend RuntimeException

▫Unchecked exceptions▫But,

Leads to poor exception handling• extend Exceptions• Rule of Thumb:

▫ If it’s possible to recover from an exception, extend the Exception class.

▫Else, if it is not reasonable to expect to be recovered from the exception (i.e. NullPointerException), extend RuntimeException.

Re-throwing the Caught Exception•With Java 7, JVM will bubble the

exception to the appropriate type•Checked exceptions have to be defined in

the method declaration

Logging Events

•SLF4J▫A Simple Logging Facade for Java▫An abstraction for various logging

frameworks Java Logging API Log4j Jakarta Commons Logging

log4j

•Enabling logging at runtime•Fully Configurable at runtime via external

configuration file•Gentle learning curve•3 Main Parts

▫Loggers▫Appenders▫Layouts

log4j

•Logger▫Named entities ▫Has a naming hierarchy▫Provides run-time control on which

statements are printed or not▫Root logger

It always exists It cannot be retrieved by name

Logger.getRootLogger ();

log4j• Log Levels - http://goo.gl/jOWg8

▫ TRACE▫ DEBUG

Messages in this level contain extensive contextual information. They are mostly used for problem diagnosis.

▫ INFO contain some contextual information

▫ WARN a potential problem in the system – i.e. a security valnarability

▫ ERROR▫ FATAL

• Level inheritance• A logging request is said to be enabled,

▫ If the level of the logging request >= logger’s log level

log4j

•Basic Selection Rule▫A log request of level p in a logger with

(either assigned or inherited, whichever is appropriate) level q, is enabled if p >= q.

•TRACE < DEBUG < INFO < WARN < ERROR < FATAL

log4j

• .getLogger(…)▫Will always return a reference to the exact

same object•Log4j makes it easy to name loggers by

software component▫statically instantiating a logger in each class▫Logger name as the class name▫A way to define loggers

The origin of the log message is easily identified

log4j• Loggers • Appenders

▫ log4j allows logging requests to print to multiple destinations

▫An output destination is called appender. Currently appenders exists for; Console Files GUI Components Remote Sockets, Servers, JMS, NT Event Loggers,

Syslog Daemons Logging Asynchronously

log4j

•Appenders▫Each enabled logging request for a given

logger will be forwarded to all the appenders in that logger as well as the appenders higher in the hierarchy

•Appender Additivly▫The output of a log statement of logger C

will go to all the appenders in C and its ancestors.

▫Additivity flag

log4jroo

t

S X

X.Y

X.Y.Z

Console

File

log4j

Aditivity = false

root

S X

X.Y

X.Y.Z

Console

File

log4j•Loggers•Appenders•Layout

▫Customizes log output format▫Conversion pattern like C languages printf▫http://logging.apache.org/log4j/1.2/apidocs/org

/apache/log4j/PatternLayout.html▫Ex:

conversion pattern “%r [%t] %-5p %c - %m%n [%x]”

176 [main] INFO org.foo.Bar - Hello World [1581616 4464 116416 1161]

log4j

•Nested Diagnostic Contexts (NDC)•Performance and Best Practices

▫Log message parameter construction▫logger.isDebugEnabled▫Choose appropriate logging level▫Log message format▫Logging in a production environment

Logging sensitive data Log file management Correlating specific execution order

Thank You