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Spring with ImmutabilityMatt Stine

Community Engineer, Cloud Foundrymstine@gopivotal.com

http://mattstine.comTwitter: @mstine

RELOADED

Item #15Minimize mutability

Recipe

• Don’t provide mutators

• Ensure the class can’t be extended

• Make all fields final

• Make all fields private

• Ensure exclusive access to any mutable components

Simple!

Inherently thread-safe!

Can be shared freely.

Can share internals.

BigInteger

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int[] = 100101

BigInteger

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BigInteger

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BigInteger

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BigInteger

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Calling negate()

Make great building blocks for other objects.

Require separate object for each distinct value.

Assume immutability during design.

If mutability necessary, limit as much as possible.

Make every field final unless there is a compelling reason to

do otherwise.

Thinking in Immutability

Forces New Style

Procedure

Arguments

Possible Return Val

Side Effects

Procedural Out

Function

Arguments

Return Value

Functional In

Immutability is key to functional programming because it matches the goals of minimizing the parts that change, making it easier to

reason about those parts.

Neal Fordhttp://www.ibm.com/developerworks/library/j-ft4/

Ease into Programming with Immutability

Ease into Functional Programming

?Big Questions

Big Questions

• Performance Concerns

• Construction of Large Objects

• Framework Concerns

Performance Concerns

BigInteger

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int[] = 100101

BigInteger

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BigInteger

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BigInteger

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BigInteger

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Revisiting negate()

What about large collections of objects?

Persistent Data Structures

Persistent List

From Programming Concurrency on the JVM, Venkat Subramaniam

Persistent Hash Trie

From Programming Concurrency on the JVM, Venkat Subramaniam

Persistent Hash Trie

From Programming Concurrency on the JVM, Venkat Subramaniam

Library Support

• TotallyLazyhttp://code.google.com/p/totallylazy/

• Functional Javahttp://functionaljava.org/

• CLJ-DS (Clojure’s data structures modified for use outside of Clojure)https://github.com/krukow/clj-ds

Code Examples = Spockhttp://spockframework.org

Construction of Large Objects

Telescoping Constructor Pattern

Doesn’t scale!

JavaBean PatternFAIL

http://en.wikipedia.org/wiki/File:Builder_UML_class_diagram.svg

The Builder Pattern

Builder Pattern

• Static member class of class it builds

• Simulates named optional parameters

• Can provide validation

Bob the Builderhttp://code.google.com/a/eclipselabs.org/p/bob-the-builder/

1. Download Jar

2. Add to dropins folder

Works with Helios, Indigo,and MyEclipse 10

Framework Concerns

Anything that...

• Requires a no-arg constructor

• Requires setter methods

Bridge Pattern

Morrison Bridge, Portland, OR

Marick’s Restructuring Layer

Nugget

Nugget

ImmutableNugget

Mutable World

Mutable World

Restructuring

Restructuring

Dozerhttp://dozer.sourceforge.net/https://github.com/DozerMapper/dozer

JavaBean PatternFAIL

Use the Builder!http://codeslut.blogspot.com/2010/05/mapping-immutable-value-objects-with.html

Frameworks Supporting Immutability

JIRM

Java Immutable object Relational Mapperhttps://github.com/agentgt/jirm

Leverages Spring JDBC

Code Samples

https://github.com/mstine/immutability-examples

Matt StineCommunity Engineer, Cloud Foundry

mstine@gopivotal.comhttp://mattstine.com

Twitter: @mstinePlease fill out an evaluation!

Spring with

Immutability

Image Credits

• “Think,” sculpture by Nathan Sawaya, photo by Matt Stine

• “Blue Sky,” sculpture by Nathan Sawaya, photo by Matt Stine

• “Title Unknown,” sculpture by Nathan Sawaya, photo by Matt Stine

• “Yellow,” sculpture by Nathan Sawaya, photo by Matt Stine

• “Morrison Bridge, Portland, OR,” photo by Matt Stine

• “CAT Dozer,” http://www.flickr.com/photos/mkw87/4877121037