Java to scala

Oct 7, 20103 likes382 views

The document describes the Guardian's migration from a Java-based Content API to one written in Scala. It started by writing integration tests in Scala and enjoyed it so much that after a month it decided to convert the whole app to Scala. The migration was relatively smooth, with an incremental embrace of Scala features. It took some time for the team to realize that many of their Java loops could be written as map or flatMap in Scala. They also used implicit conversions to hide issues with Java libraries returning nulls.

(partly)

How we moved from
^

Java to Scala
Graham Tackley
guardian.co.uk

Thursday, 7 October 2010

History

• Java shop since 2006
• guardian.co.uk: java + spring + velocity +
hibernate + oracle
• ~100k lines production java code (and
~35k xml...)

Thursday, 7 October 2010

Content API
• Provide API to access all of our website
content
• Limited beta released early 2009
• Started implementation of ﬁnal version late
2009
• Live April 2010:
http://content.guardianapis.com

Thursday, 7 October 2010

December 2009

• Started new Content API implementation
• java + guice + guice servlets + apache solr

Thursday, 7 October 2010

December 2009

• A few people had played with scala
• No production scala code

Thursday, 7 October 2010

January 2010
• Comprehensive integration tests were very
important to us
• Started writing our integration tests in
Scala with the awesome ScalaTest
• Already a maven project, so mixed scala-
java support easy with maven-scala-plugin

Thursday, 7 October 2010

January 2010

• A fair bit of our test code was java-
without-semicolons as we learnt about
Scala...

Thursday, 7 October 2010

February 2010

• ... despite that we loved it so much that
after a month we decided to convert the
whole app to Scala
• scala + guice + guice servlets + apache solr

Thursday, 7 October 2010

February 2010

Still some very java-like code...
Thursday, 7 October 2010

February 2010

... immediate win with non-IDE generated constructors ...
Thursday, 7 October 2010

February 2010

... and some things much more readable than Java
Thursday, 7 October 2010

July 2010

• Switched from maven to simple-build-tool
• Mainly for incremental compilation
• ~ test-quick and ~ prepare-webapp also
invaluable

Thursday, 7 October 2010

Feb-Oct 2010

• Much learning within the team as to how to
write concise, readable Scala code

Thursday, 7 October 2010

Option
or How we learned to stop worrying and love the Option

We’d read “don’t use nulls, use Option instead”

DAY ONE
s/x == null/x.isDefined

DAY THREE
x match {
case None => ...
case Some(value) => ...
}
Thursday, 7 October 2010

Option
or How we learned to stop worrying and love the Option

WEEK THREE

TODAY

We now think of Options as lists with zero or one entries

Thursday, 7 October 2010

map & ﬂatMap

it took us a while to realise that most of the loops we
wrote in Java were actually “map” or “ﬂatMap”

Thursday, 7 October 2010

Java Libraries
Content API extensively uses Solr/J,
which is good but returns loads of nulls

And don’t get me started on HttpServletRequest...

We used implicit conversions to hide the Java yuk

Thursday, 7 October 2010

Summary
• Migration path from Java relatively smooth
• Take it easy, and don’t fear java-without-
semicolons in the early days
• An incremental embrace of Scala features
worked really well for us
• Hide the Java yuk with implicit conversions

Thursday, 7 October 2010

• http://content.guardianapis.com
• http://www.guardian.co.uk/open-platform
• http://github.com/guardian/open-platform-
content-api-scala-client
• http://blog.tackley.net
graham.tackley@guardian.co.uk
Thursday, 7 October 2010

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The document discusses The Guardian's migration from a Java-based architecture to one using Scala for their Content API project. They started by writing integration tests in Scala with ScalaTest in January 2010 and enjoyed it so much that after a month, in February 2010, they decided to convert the whole application to Scala. It took some time for the team to learn how to write good Scala code, like embracing the Option type to avoid nulls and using map and flatMap instead of loops. The migration was relatively smooth by embracing Scala features incrementally. [END SUMMARY]

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