Riktig feilhåndtering

...og ja, checked exceptions er skadelig

Trond Arve Wasskog – JavaZone 2008

0Feilhåndtering

� Exception handling done right (and yes, checked exceptions are not necessary!)

- Trond Arve - BEKK - Confluence

� [JavaSpecialists 162] - Exceptions in Java

� Unchecked Exceptions — The Controversy (The Java™ Tutorials > Essential

Classes > Exceptions)

� java.net: Exception-Handling Antipatterns

� java.net: Three Rules for Effective Exception Handling

� Java Exceptions Handling

� The Art and Craft of Great Software Architecture and Development: Java

Exception Handling Anti-Patterns

� Neal Gafter's blog: Closures for Java

� Neal Gafter's blog: Removing Language Features?

� Exception handling problems in Java, Part I

� Exception handling problems in Java, Part II

� Bruce Eckel's MindView, Inc: Does Java need Checked Exceptions?

� Exceptions in Java: Nothing exceptional about them - Java World

� News & Ideas Forum (Closed for new topic posts) - Failure and Exceptions

� The Trouble with Checked Exceptions

� BEKK forum :: View topic - Håndtering av exceptions - erfaringer/diskusjoner

� Thinking Inside a Bigger Box » A Hopeful Idea: The End of Checked

Exceptions?

� Should checked exceptions be removed from Java?

� The RedirectException - The Daily WTF

� Dont Use Exceptions For Flow Control

� Exception Patterns

� Exception Tunneling

� Checked Exceptions Are Of Dubious Value

� Octopull/Java: More Exceptional Java

� Java theory and practice: The exceptions debate

� James Gosling Chimes In on Checked Exceptions

� Exception handling - Wikipedia, the free encyclopedia

� cat /dev/random » Remove checked exceptions?

� Java's checked exceptions were a mistake (and here's what I would like to do

about it)

� DOTNET Archives -- November 2000, week 1 (#349)

� Java Practices -> Checked versus unchecked exceptions

� Java Exception Handling - BEKK - Confluence

� Java Exception Handling - BEKK - Confluence

� Feilhåndtering - Prosjektkvalitet - Confluence

� XO.NET-metoden - Unntakshåndtering - BEKK - Confluence

� The Coad Letter: Modeling and Design Edition, Issue 90, Exceptional Strategies

� Effective Java Exceptions

� cache:DxQf_u_Vue8J:dev2dev.bea.com/lpt/a/541 java exception design

strategies - Google-søk

� David Walend's Blog: Design For Exceptions

� SwTech.com - Java Design Patterns

� Single Sign On - Login

� Exception Patterns

� Page 2 - Framework Patterns: Exception Handling, Logging, and Tracing

� Alexander Klimetschek's Blog : Exception Handling Best Practices Part 1

� towards_xcptn_hndling.pdf (application/pdf Object)

� Andre's Java Links

� JForum - Få slutt på galskapen: Stem "ja" til å fjerne checked exceptions fra

Java

� Beware the dangers of generic Exceptions - Java World

� Designing with exceptions - Java World

� Forward - Tips and Guidelines for Programming Java GUI Applications

� warning unchecked exception - Google-søk

� Exception (Java 2 Platform SE v1.4.2)

� LISTSERV 15.0 - RMI-USERS Archives

� Google Answers: The origin of checked exceptions

� Java (programming language) - Wikipedia, the free encyclopedia

� Hacking Architect : Weblog

� Old News: Java Checked vs. Unchecked Exceptions « Bees Are Social By Nature

� Opinion: Exception Handling Debate is Back

� Manageability - The Exception Debate: Why Gosling and Hejlsberg are Both In

Error

� Mistaeks I Hav Made: Generic Throws: Another Little Java Idiom

� DataAccessException.gif (GIF Image, 916x397 pixels)

� Antonio Goncalves' Weblog

� A Test-Driven Exploration of the Advanced Features of EJB 3.0

Dårlig feilhåndtering er vanlig

1972

ReturnCodes

1964

On ErrorExceptions

1964 1972 1983 1992 1995 2001

ReturnCodes

On ErrorExceptions

1964 1972 1983 1992 1995 2001

ReturnCodes

On ErrorExceptions

Exceptions

1964 1972 1983 1992 1995 2001

ReturnCodes

On ErrorExceptions

Exceptions

CheckedExceptions

1964 1972 1983 1992 1995 2001

ReturnCodes

On ErrorExceptions

Exceptions

CheckedExceptions

Exceptions

Java Exceptions

Throwable

Java Exceptions

Throwable

Exception

Checked

Exceptions

Java Exceptions

Throwable

Exception

Runtime

ExceptionChecked

Exceptions

Unchecked

Exceptions

Java Exceptions

Throwable

Error Exception

Runtime

ExceptionVM Errors

(Unchecked)Checked

Exceptions

Unchecked

Exceptions

Et alternativt design

Throwable

Error Exception

VM Errors

<interface>

Unchecked

Exception

Unchecked

Exceptions

Checked

Exceptions

1Hvem bryr seg om feilhåndtering?

Brukeren er opptatt av at en feil har oppstått og at situasjonen er håndtert,

ikke hva som har skjedd

Forretningen forstår funksjonelle feil og vil unngå kritiske feil

Drift vil vite hva de skal gjøre med feilen

Utviklere vil vite hva som har skjedd, hvor, når, hvorfor, hvem, tilstand og kontekst

Feilhåndtering har ulike interessenter med sine spesifikke behov

Typisk er få representert som kravstillere

2Feilhåndtering

Hva kan man gjøre når en feil oppstår?

Ignorere?

Håndtere?

Prøv igjen?

Prøv et alternativ?

Avslutte forespørselen

Hvor skal feil håndteres?

Lokal feilhåndtering

Sentral feilhåndtering

De aller fleste applikasjoner er tjent med sentral feilhåndtering

Feilhåndtering vs feilretting

Hva skjer om en annen Exception

eller Error kastes?

Er det viktig at operasjonen eller

applikasjonen fungerer?

Feilhåndtering sørger for konsistens

Feilhåndtering er kritisk for applikasjonen

Automatisert feilretting er ofte komplekst, kostbart og risikabelt

Feilhåndtering er viktigere enn feilretting

3Tre feiltyper

Application Exception

System Exception

Programming Exception

a.k.aRuntimeException

Type Håndter Feilfiks og

utrulling

Prøv igjen?

Application

Exception

Mulig Nei Nei

System

Exception

Nei Nei Mulig

Programming

Exception

Nei Ja Nei

4Overordnet Design

Lettvekts exception-hierarki - Distribuert

Felles exception-klasser med feilkode - Sentralisert

AbstractException- Long id

ApplicationException SystemException

java.lang

RuntimeException

Hybrid – Sentrale exception-klasser, distribuert hierarki

AbstractException- Long id

ApplicationException SystemException

java.lang

RuntimeException

dd<System Exceptions>

dd<Application Exceptions>

AbstractException-Long id

- Map context

-----------------------------

addContext(key, value)

Rammeverk = Distribuert

Applikasjon = Sentralisert

5Checked Exceptions

Ondskapens akse

JimWaldo

JamesGosling

Ann Wollrath

There was a time when Oak and the earliest version of Java did not have checked exceptions. Exception handling was advisory, and it was an unsafe world out there. It was our group (Jim Waldo and I in particular :-) that recommended that there be exceptions checked by the compiler. Jim was quite persuasive in his arguments, telling of a world where robust code would reign. After some consideration, Java was retooled to have checked exceptions. Only those exceptions for which there was norecovery or reflect application errors would be unchecked (e.g.,OutOfMemoryError, NullPointerException respectively). And the world was safe again

-- Ann Wollrathhttp://archives.java.sun.com/cgi-bin/wa?A2=ind9901&L=RMI-USERS&P=R25037

Ringbærerne

Item 58: Use chekced exceptions for recoverable conditions

and runtime exceptions for programming errors

Item 59: Avoid unnecessary use of checked exceptions

-- Joshua Bloch

I believe the language could be simplified by treating all exception types as unchecked without breaking existing programs. This could also result in a simplification of future

language extensions and APIs. But would the language and

platform be better off without checked exceptions?

-- Neil Gafter

The overhead of checked exceptions was having the opposite effect of what was intended, something that can happen when

you experiment (and I now believe that checked exceptions were

an experiment based on what someone thought was a good idea,

and which I believed was a good idea until recently).

-- Bruce Eckel

Source: http://java.net/pub/pq/163

“Checked exceptions are gold. Developers that don't like them are those that are too lazy to actually

think about error cases.”

http://www.javaworld.com/javaforums/showflat.php?Cat=0&Number=35228&Main=2629

“Were all exceptions unchecked, no exceptions would be caught until the corresponding bugs

began to present themselves.”

http://www.javaworld.com/javaforums/showflat.php?Cat=0&Number=35228&Main=2629

“Checked exceptions seem like a really good idea at first…

However, the kind of code you must write around these things and

the common phenomenon of "swallowed" exceptions begins to

suggest there's a problem. ”

http://www.mindview.net/Etc/Discussions/CheckedExceptions

“I am so tired of all the comments singing the praise of checked

exceptions on a theoretical basis. I don’t care about what’s good in

theory!”

http://www.javaworld.com/community/?q=comment/reply/1128

“The HibernateException, which wraps most of the errors that can occur in a Hibernate

persistence layer, is an unchecked exception (it wasn't in older versions of Hibernate).

In our opinion, we shouldn't force the application developer to catch an

unrecoverable exception at a low layer. In most systems, unchecked and fatal exceptions

are handled in one of the first frames of the method call stack”

http://www.hibernate.org/hib_docs/reference/en/html/transactions.html

Checked exceptions forurenser APIet

Checked exceptions fører til unnamanøvre som forpester

koden

Checked exceptions påtvinger lokal ”feilhåndtering”

java.text.ParseException

java.net.MalformedURLException

java.lang.CloneNotSupportedException

Checked exceptions kan brukes når klienten alltid både kan og må

håndtere feilen.

God feilhåndtering er viktig

Exceptions er bra

Dokumenter feilsituasjoner

Skal kompilatoren påtvinge feilhåndtering?

Checked exceptions påfører applikasjoner stor skade

Checked exceptions bør fjernes

6Slik, ikke slik!

Ikke bruk exceptions for normal forretningslogikk

Bruk eksisterende exceptions

IllegalStateExceptionUnsupportedOperationException

IllegalArgumentExceptionNoSuchElementException

NullPointerException

Ikke sluk exceptions

Fallgruver med try-catch-finally

Dokumenter exceptions

Russisk kone

ERROR [btpool0-3] JDBCExceptionReporter.logExceptions(78) | Violation

of unique constraint $$: duplicate value(s) for column(s) $$: SYS_CT_88 in statement [insert into

app_user (id, account_expired, account_locked, address, city, country, postal_code, province,

credentials_expired, email, account_enabled, first_name, last_name, password, password_hint, phone_number,

username, version, website) values (null, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?

org.springframework.dao.DataIntegrityViolationException: could not insert: [edu.ksu.model.User]; nested

exception is org.hibernate.exception.ConstraintViolationException: could not insert: [edu.ksu.model.User]

at

org.springframework.orm.hibernate3.SessionFactoryUtils.convertHibernateAccessException(SessionFactoryUtils.java:

624)

at

org.springframework.orm.hibernate3.HibernateAccessor.convertHibernateAccessException(HibernateAccessor.java:412)

at org.springframework.orm.hibernate3.HibernateTemplate.doExecute(HibernateTemplate.java:424)

at

org.springframework.orm.hibernate3.HibernateTemplate.executeWithNativeSession(HibernateTemplate.java:374)

at org.springframework.orm.hibernate3.HibernateTemplate.saveOrUpdate(HibernateTemplate.java:744)

<snip/>

Caused by: org.hibernate.exception.ConstraintViolationException: could not insert: [edu.ksu.model.User]

at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:71)

at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43)

<snip/>

Caused by: java.sql.SQLException: Violation of unique constraint $$:

duplicate value(s) for column(s) $$: SYS_CT_88 in statement [insert into app_user (id, account_expired,

account_locked, address, city, country, postal_code, province, credentials_expired, email, account_enabled,

first_name, last_name, password, password_hint, phone_number, username, version, website) values (null, ?, ?, ?,

?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)]

at org.hsqldb.jdbc.Util.throwError(Unknown Source)

at org.hsqldb.jdbc.jdbcPreparedStatement.executeUpdate(Unknown Source)

at

org.apache.commons.dbcp.DelegatingPreparedStatement.executeUpdate(DelegatingPreparedStatement.java:101)

at

org.apache.commons.dbcp.DelegatingPreparedStatement.executeUpdate(DelegatingPreparedStatement.java:101)

at org.hibernate.id.insert.AbstractSelectingDelegate.performInsert(AbstractSelectingDelegate.java:33)

... 143 more

Unngå både logging og kasting

Drift trenger unik feilkode for åidentifisere feilen

Utviklere trenger unik instans-ID for å identifisere feilen

Exceptions skal også testes

Bruk stubber for testing av eksterne systemer

<Stub>

Fagsystem A

<interface>

Fagsystem A

Fagsystem A

<interface>

Fagsystem A

Gjør logging enkelt

Gjør logging enkelt

Legg til kontekst der feilen oppstår

BEKK CONSULTING ASSKUR 39, VIPPETANGEN. P.O. BOX 134 SENTRUM, 0102 OSLO, NORWAY. WWW.BEKK.NO

Takk for oppmerksomheten!

trond.arve.wasskog[at]bekk.no