Practical Exception Specifications ECOOP Workshop Exception Handling in

Practical Exception Specifications ECOOP Workshop Exception Handling in Object Oriented Systems Donna Malayeri and Jonathan Aldrich 25 Jul 2005 1

Exceptions: a double-edged sword? o Useful for writing error-handling code n n o o allows localizing handler code harder to ignore error conditions But can produce confusing control flow Have been compared to GOTO 25 Jul 2005 2

Are tools the answer? o o Most require whole-program analysis, and are not scalable Don’t allow specifying and enforcing a policy n bad for team development 25 Jul 2005 3

Are exception specifications the answer? o Not current specification systems: n n too heavyweight too fine-grain difficult to maintain little or no help for understanding exception flow 25 Jul 2005 4

Example: Java throws declarations User Preferences Module class User. Prefs public set. Key(String key, Object val) { Pref. Keys. get. Key(key). set. Value(val); } public save() { Serializer. save(); } class Pref. Keys set. Value(Object val) { Serializer. set. Value(this, val); } Error: unhandled exception SQLException class Serializer static set. Value(key, val) { the. DB. save(key, cache. store(key, value); val); } 25 Jul 2005 5

Example: Java throws declarations User Preferences Module Error: unhandled exception SQLException class User. Prefs public set. Key(String key, Object val) { Pref. Keys. get. Key(key). set. Value(val); } public save() { Serializer. save(); } class Pref. Keys Error: unhandled exception SQLException set. Value(Object val) throws SQLException { Serializer. set. Value(this, val); } class Serializer static set. Value(key, val) throws SQLException { the. DB. save(key, cache. store(key, value); val); } 25 Jul 2005 6

Example: Java throws declarations User Preferences Module class User. Prefs public key, Object val) throws publicset. Key(String Object val) SQLException { Pref. Keys. get. Key(key). set. Value(val); } public save() Problem! { Serializer. save(); } Programmer didn’t realize that User. Prefs is. Pref. Keys supposed to hide storage details class set. Value(Object val) throws SQLException User Preferences abtraction { Serializer. set. Value(this, val); } is violated! class Serializer static set. Value(key, val) throws SQLException { the. DB. save(key, cache. store(key, value); val); } 25 Jul 2005 7

So, Java “throws” is not a solution o o o Not expressive Not lightweight Difficult to maintain Easily become imprecise No aid in comprehension of exception flow 25 Jul 2005 8

In contrast, practical exception specifications should be: o o o Expressive Lightweight Easy to maintain An aid for understanding exception flow and exception policies Exn. Java has all of these properties 25 Jul 2005 9

Exn. Java is more expressive Specify exception policies for Java packages Programmer adds specification: package util. user. Prefs may only throw Preference. Exception 25 Jul 2005 10

Exn. Java combines inference and annotations o o o Programmer writes throws declarations for public and protected methods only Other declarations are inferred Performs a per-package analysis n n scalable future work: modules instead of packages 25 Jul 2005 11

Exn. Java User Preferences Module Error: unhandled exception SQLException class User. Prefs public set. Key(String key, Object val) { Pref. Keys. get. Key(key). set. Value(val); } public save() Error: { Serializer. save(); } Public methods in util. user. Prefs may not class Pref. Keys inferred throw SQLException set. Value(Object val) throws SQLException { Serializer. set. Value(this, val); } class Serializer inferred static set. Value(key, val) throws SQLException { the. DB. save(key, cache. store(key, value); val); } 25 Jul 2005 12

Exn. Java User Preferences Module class User. Prefs public set. Key(String key, Object val) { try { Pref. Keys. get. Key(key). set. Value(val); } catch (SQLException e) { throw new Preference. Exception(e); } } public save() { Serializer. save(); } 25 Jul 2005 13

Exn. Java specifications are maintainable o Fewer declarations means n n o fewer declarations to maintain fewer declarations that can become imprecise Exn. Java includes tools n n fix imprecise declarations propagate throws declarations 25 Jul 2005 14

Demo 25 Jul 2005 15

Related Work o Robillard & Murphy n o Jex Sinha et al n n exception analysis views for exception flow 25 Jul 2005 16

Summary of Exn. Java Properties o Lightweight annotations n o Expressive n o supports package exception policies Easy to maintain n n o combination of inference and user annotations fewer annotations tools: Propagate Throws, Fix Imprecise Aids understanding exception flow n detailed views 25 Jul 2005 17

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