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From David Pollak <>
Subject Scala, bytecode and such
Date Tue, 01 Dec 2009 00:06:08 GMT

There were some recent rumblings about what kind of byte-code Scala
generates.  I'd like to take a few minutes discussing the issue.

The Scala compiler and language were designed primarily by Martin
Everybody on this list has used Martin's most famous work: javac.  Martin
wrote and maintained the Java compiler versions 1.1-1.4.  Martin also
co-designed Java generics.  Martin knows his way around JVM byte-code, has a
direct line to John Rose <>.  Seeing Martin and
John when they get together is quite a treat for the hardcore JVM geek.

Because Martin co-designed Java Generics and implemented them in the
compiler, it's pretty safe to say that Scala's claims of full compatibility
with Java libraries is a true one and with > 250K LoC of Scala code that
I've written or managed, I can say that there are no cases where Scala is
not compatible with Java's object model.

Scala takes a different approach to libraries than does Java.  Scala's
language libraries are skewed towards immutable data structures.  A
prominent member of the Java ecosystem took a sample of data used by
enterprise apps and ran that data through Java's collections and Scala's
collections.  It turns out that Scala's collections were marginally more
efficient (about 4%).  This is because Java's mutable collections
preallocate buffers that are typically not used where Scala only allocates
what it needs.  This puts more pressure on the memory system (more temporary
objects), but it turns out that it is more efficient.  (Sorry I don't have a
lot more data as to JVM versions, etc. as this data was relayed to me by
said Java ecosystem member over > 1 pint of beer.)

It has been my experience that caching and immutability go hand in hand.  In
Lift, we cache a lot of information (e.g., page templates) and do not need
to make defensive copies of cached information because, like Java Strings,
immutable objects can be passed to any thread or returned from any method
without worrying about synchronization or mutation.

While I'm not an expert on the byte-code generated by the Scala compiler
(I'm a consumer of Scala rather than a producer of the language), I did this
presentation at the JVM Language Summit.  It might be helpful for folks to
understand how Scala translates various constructs into byte-code:

Also, it's been my experience (and this in on the 1.6 JVM) that even though
Scala produces far more in terms of temporary objects than does Java, that
performance, even in highly threaded, high CPU load on multi-core Xeon
boxes, this does not negatively impact performance.  While I'm all for
tuning apps and I love benchmarks 'cause that's how we can improve code, I'm
a lot less worried about lots of short-lived objects and a lot more
concerned about things like lots of array copies.



Lift, the simply functional web framework
Beginning Scala
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