Using Spring with Scala

Bernhard Wenzel
www.bernhardwenzel.com
Using Spring with Scala

Spring I/O
2016
Overview
1.Introduction & Motivation
2.Practical examples
3.Evaluation
4.Questions

Spring I/O
2016
Target audience
• Developers with Java/Spring background
• Scala curious or beginner

Spring I/O
2016
Why Spring + Scala?

Spring I/O
2016
Why Scala?
• For fun
• Learning a new language makes me a better
developer
• Functional Programming!
• For profit
• Scala is (becoming) popular: LinkedIn, Guardian,
Twitter, many more ...

Spring I/O
2016
What I like about Scala
(as a Java dev)
• Code tends to be more concise
• Scala was built for FP from ground up
• In Scala I can mix OO + FP
• Java interop allows to move gradually towards
Scala
• Java 8 is catching up (e.g. Lambdas) but Scala still
has many other features (e.g. Pattern matching)

Spring I/O
2016
Why Scala & Spring
• Learning a new language is enough, I’d like to keep
my favourite frameworks & tools (SpringBoot,
Gradle)
• New languages come with new frameworks that are
unproven
• I want to gradually introduce Scala into my existing
Java&Spring project

Spring I/O
2016
2. Examples
www.spring.io/guides

Spring I/O
2016
Translated guides from Java to Scala
• Copy & Paste of Java files into Scala dir
• Transformed each .java into .scala
• Kept Gradle as build tool
• Source code available at GitHub
(github.com/BernhardWenzel/spring-scala-
examples)

Spring I/O
2016
First example: serving web content
with Spring MVC

Spring I/O
2016
Make Gradle work with Scala
build.gradle
...
apply plugin: 'scala'
apply plugin: 'application'
mainClassName = "hello.Application"
...
dependencies {
...
compile 'org.scala-lang:scala-library:2.11.1'
...
}

Spring I/O
2016
Application class
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}
@SpringBootApplication
class Application {
}
object Application extends App {
SpringApplication.run(classOf[Application], args:_*)
}

Spring I/O
2016
Template
<!DOCTYPE HTML>
<html xmlns:th="http://www.thymeleaf.org">
<head>
<title>Getting Started: Serving Web Content</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body>
<p th:text="'Hello, ' + ${name} + '!'" />
</body>
</html>

Spring I/O
2016
Controller
@Controller
public class GreetingController {
@RequestMapping("/greeting")
public String greeting(
@RequestParam(value="name",required=false) String name,
Model model) {
model.addAttribute("name", name);
return "greeting";}}
@Controller
class GreetingController {
@RequestMapping(Array("/greeting"))
def greeting(
@RequestParam(value="name", required=false) name: String,
model: Model) : String = {
model.addAttribute("name", name)
"greeting"
}}

Spring I/O
2016
That’s it
$ gradle run
. . .
2016-05-06 16:05:52.644 INFO 1297 --- [ restartedMain] s.b.c.e.t.TomcatEmbeddedServletContainer : Tomcat
2016-05-06 16:05:52.650 INFO 1297 --- [ restartedMain] scala.App$class : Started App.class in
$ curl http://localhost:8080/greeting?name=SpringIO
<!DOCTYPE HTML>
<html>
<head>
<title>Getting Started: Serving Web Content</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body>
<p>Hello, SpringIO!</p>
</body>
</html>

Spring I/O
2016
Translation steps:
• Gradle: add scala plugin (optional application)
• Application class: split into companion object in
Scala and run with args:_*
• Controller: @RequestMapping annotation need to
convert String into Array(String)

Spring I/O
2016
Example: consuming REST
services

Spring I/O
2016
Beans in Java
Value.java
@JsonIgnoreProperties(ignoreUnknown = true)
public class Value {
private Long id;
private String quote;
public Long getId() {return this.id;}
public String getQuote() {return this.quote;}
public void setId(Long id) {this.id = id;}
public void setQuote(String quote) {this.quote = quote;}
@Override
public String toString() {
return "Value{" +
"id=" + id +
", quote='" + quote + ''' +
'}';
}}

Spring I/O
2016
Beans in Scala
Value.scala
@JsonIgnoreProperties(ignoreUnknown = true)
class Value {
@BeanProperty
var id: Long = _
@BeanProperty
var quote: String = _
override def toString : String =
"Value{id=" + id +", quote=" + quote +"}"
}

Spring I/O
2016
Application class
public void run(String... args) throws Exception {
RestTemplate restTemplate = new RestTemplate();
Quote quote = restTemplate.getForObject(
"http://gturnquist-quoters.cfapps.io/api/random",
Quote.class);
log.info(quote.toString());
}
override def run(args: String*): Unit = {
val restTemplate = new RestTemplate()
val quote : Quote = restTemplate.getForObject(
classOf[Quote])
log.info(quote.toString)
}

Spring I/O
2016
Asynchronous version using Lambdas
Application.java
AsyncRestTemplate asyncTemplate = new AsyncRestTemplate();
final ListenableFuture<ResponseEntity<Quote>> quoteFuture = asy
Quote.class);
quoteFuture.addCallback(
success -> log.info("async: " + quote),
failure -> log.error("Async error", failure));
void addCallback(
SuccessCallback<? super T> successCallback,
FailureCallback failureCallback);

Spring I/O
2016
Java 8 Lambdas vs.
Scala function literals
(a:Int, b:Int) => a + b
Scala function literal
(Integer a, Integer b) -> a + b
Java 8 lambdas

Spring I/O
2016
Asynchronous version
Application.scala
(success: SuccessCallback[ResponseEntity[Quote]])
=> log.info("async: " + quote),
(failure: FailureCallback)
=> log.error("Async error",failure))

Spring I/O
2016
Asynchronous version
Scala functions =! Java Lambdas
(success: SuccessCallback[ResponseEntity[Quote]])
=> log.info("async: " + quote),
(failure: FailureCallback)
=> log.error("Async error",failure))
Application.scala

Spring I/O
2016
We need ugly anonymous classes
Application.scala
quoteFuture.addCallback(new ListenableFutureCallback[ResponseEntit
override def onSuccess(entity : ResponseEntity[Quote]) : Unit =
log.info("async: " + entity.getBody.toString)
override def onFailure(t : Throwable) : Unit =
log.error("Async error", t)
})

Spring I/O
2016
Works in Scala
$ gradle run
. ____ _ __ _ _
/ / ___'_ __ _ _(_)_ __ __ _
( ( )___ | '_ | '_| | '_ / _` |
/ ___)| |_)| | | | | || (_| | ) ) ) )
' |____| .__|_| |_|_| |___, | / / / /
=========|_|==============|___/=/_/_/_/
:: Spring Boot :: (v1.3.3.RELEASE)
…
2016-05-02 11:44:49.052 INFO 3702 --- [ main] hello.Application : async: Quote{type='success'
over configuration, providing an experience on par with frameworks that excel
at early stage development, such as Ruby on Rails.}}
…
BUILD SUCCESSFUL

Spring I/O
2016
Translation steps
• Beans: use @BeanProperty annotation
• Java 8 Lambdas != Scala function literals, in Scala
have to use anonymous classes

Spring I/O
2016
Providing REST API using Spring Data
Rest

Spring I/O
2016
Person resource in Scala
@Entity
class Person {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@BeanProperty
var id: Long = _
@BeanProperty
var firstName: String = _
@BeanProperty
var lastName: String = _
}

Spring I/O
2016
Repository
@RepositoryRestResource(collectionResourceRel = "people",
path = "people")
public interface PersonRepository
extends PagingAndSortingRepository<Person, Long> {
List<Person> findByLastName(@Param("name") String name);
}
path = "people")
trait PersonRepository
extends PagingAndSortingRepository[Person, Long] {
def findByLastName(@Param("name") name: String):
List[Person]
}

Spring I/O
2016
Oops…
PersonRepository.scala:8: type arguments [hello.Person,Long]
do not conform to trait PagingAndSortingRepository's type
parameter bounds [T,ID <: java.io.Serializable]
trait PersonRepository extends
PagingAndSortingRepository[Person, Long] {
scala.Long != java.lang.Long

Spring I/O
2016
Repository
PersonRepository.scala
path = "people")
trait PersonRepository
extends PagingAndSortingRepository[Person, java.lang.Long] {
def findByLastName(@Param("name") name: String):
List[Person]
}
Explicitly refer to Java base type
import java.lang.Long

Spring I/O
2016
Spring Data Rest in Scala
$ curl http://localhost:8080
{
"_links" : {
"people" : {
"href" : "http://localhost:8080/people{?page,size,sort}",
"templated" : true
},
"profile" : {
"href" : "http://localhost:8080/profile"
}
}
}

Spring I/O
2016
Translation steps
• When using Java/Spring interface, beware of Java
base types

Spring I/O
2016
Final example: JDBC
public void run(String... strings) throws Exception {
jdbcTemplate.execute("DROP TABLE customers IF EXISTS");
jdbcTemplate.execute("CREATE TABLE customers(" +
"id SERIAL, first_name VARCHAR(255), last_name VARCHAR(255)
// Split up the array of whole names into an array of
// first/last names
List<Object[]> splitUpNames = Arrays
.asList("John Woo", "Jeff Dean", "Josh Bloch",
"Josh Long“)
.stream()
.map(name -> name.split(" "))
.collect(Collectors.toList());
. . .

Spring I/O
2016
JDBC: Java Batch Update
// Use a Java 8 stream to print out each tuple of the list
splitUpNames.forEach(
name -> log.info(String.format(
Inserting customer record for %s %s",
name[0], name[1])));
// Uses JdbcTemplate's batchUpdate operation to
// bulk load data
jdbcTemplate.batchUpdate(
"INSERT INTO customers(first_name, last_name) VALUES (?,?)",
splitUpNames);
log.info("Querying for customer records where first_name = 'Josh':");

Spring I/O
2016
jdbcTemplate.query(
"SELECT id, first_name, last_name FROM customers " +
"WHERE first_name = ?",
new Object[] { "Josh" },
(rs, rowNum) ->
new Customer(
rs.getLong("id"),
rs.getString("first_name"),
rs.getString("last_name"))
).forEach(customer -> log.info(customer.toString()));
public interface RowMapper<T> {
T mapRow(ResultSet rs, int rowNum) throws SQLException;
}
public <T> List<T> query(String sql, Object[] args, RowMapper<T> rowMapper)
JDBC: Java Query

Spring I/O
2016
override def run(args: String*): Unit = {
jdbcTemplate.execute("DROP TABLE customers IF EXISTS")
jdbcTemplate.execute("CREATE TABLE customers("id SERIAL,
first_name VARCHAR(255), last_name VARCHAR(255))“)
JDBC Scala create tables

Spring I/O
2016
JDBC Scala names
collection
import scala.collection.mutable.ListBuffer
val splitUpNames = ListBuffer(
"John Woo", "Jeff Dean", "Josh Bloch", "Josh Long“)
.map(_.split(" "))
splitUpNames.foreach(name => log.info(
"Inserting customer record for %s %s".format(name(0), name(1))))
Scala: collections are immutable
java.util.List <-> scala.collection.mutable.Buffer

Spring I/O
2016
"INSERT INTO customers(first_name, last_name) VALUES (?,?)“,
splitUpNames
.asInstanceOf[mutable.Buffer[Array[AnyRef]])
JDBC Scala batchUpdate
java.lang.Object <-> scala.AnyRef
List<Object[]> <-> mutable.Buffer[Array[AnyRef]]
public int[] batchUpdate(String sql, List<Object[]> batchArgs)

Spring I/O
2016
JDBC Scala batchUpdate
Still a Scala collection
splitUpNames
.asInstanceOf[mutable.Buffer[Array[AnyRef]])
java.lang.Object <-> scala.AnyRef
List<Object[]> <-> mutable.Buffer[Array[AnyRef]]
public int[] batchUpdate(String sql, List<Object[]> batchArgs)

Spring I/O
2016
Convert from Scala
JavaConverters.asJava <- Scala
import collection.JavaConverters._
splitUpNames
.asInstanceOf[mutable.Buffer[Array[AnyRef]].asJava)

Spring I/O
2016
jdbcTemplate.query(
"SELECT id, first_name, last_name FROM customers WHERE first_name = ?",
Array("Josh").asInstanceOf[Array[AnyRef]],
new RowMapper[Customer]{
override def mapRow(rs: ResultSet, rowNum: Int): Customer =
new Customer(
rs.getLong("id"),
rs.getString("last_name"))
})
JDBC Scala query
(again casting Object -> AnyRef)
(again implement anonymous class)

Spring I/O
2016
JDBC Scala print query
Returns Java collection
jdbcTemplate.query(
. . .
).foreach(…)

Spring I/O
2016
import collection.JavaConverters._
jdbcTemplate.query(
. . .
).asScala
.foreach((customer:Customer)
=> log.info(customer.toString))
Convert from Java
Java -> JavaConverters.asScala

Spring I/O
2016
Works in Scala
$ gradle run
. ____ _ __ _ _
/ / ___'_ __ _ _(_)_ __ __ _
( ( )___ | '_ | '_| | '_ / _` |
/ ___)| |_)| | | | | || (_| | ) ) ) )
' |____| .__|_| |_|_| |___, | / / / /
=========|_|==============|___/=/_/_/_/
:: Spring Boot :: (v1.3.3.RELEASE)
...[main] hello.Application: Creating tables
...[main] hello.Application: Inserting customer record for John Woo
...[main] hello.Application: Inserting customer record for Jeff Dean
...[main] hello.Application: Inserting customer record for Josh Bloch
...[main] hello.Application: Inserting customer record for Josh Long
...[main] hello.Application: Querying for customer records where first_name = 'Josh':
...[main] hello.Application: Customer[id=3, firstName=Josh, lastName=Bloch]
...[main] hello.Application: Customer[id=4, firstName=Josh, lastName=Long]

Spring I/O
2016
Translation steps
• Use Scala mutable collection when calling Java
• Type casting
• Java Object/Scala AnyRef,
• Java []/Scala Array
• Java List/Scala Buffer
• Collection transformations asJava and asScala

Spring I/O
2016
We „just“ need to translate from Scala to Java
and vice versa
3. Evaluation
Good news: it works!

Spring I/O
2016
Simple translation steps
• add Scala plugin to Gradle
• Application class requires object
• add Array to @Annotations(Array(…))
• Fields of beans require @BeanProperty annotation

Spring I/O
2016
Slightly annoying translation steps
• Avoiding Scala base types for Sprint interfaces
• Casting to/from Java (e.g. Java Object to Scala
AnyRef)
• Requiring scala.JavaTransformations toJava and
toScala with collections
• Using Scala mutable collections when interacting
with Java

Spring I/O
2016
Annoying translation step
• Can’t use Scala functions as Java 8 Lamdbas

Spring I/O
2016
Annoying translation step
Will work in Scala 2.12
jdbcTemplate.query(
"SELECT id, first_name, last_name FROM customers WHERE first_name = ?",
Array("Josh").asInstanceOf[Array[AnyRef]],
(rs: ResultSet, rowNum: Int) =>
new Customer(
rs.getLong("id"),
rs.getString("last_name")))
• Can’t use Scala functions as Java 8 Lamdbas

Spring I/O
2016
Scala SDK is getting better dealing with Java
• Additional frameworks?
• e.g. defunct (Pivotal) Spring-Scala project
• Eases some issues, e.g. no @BeanProperty
necessary
• Better to be consistent across all Java
dependencies

Spring I/O
2016
Conclusion
• It works. Spring is just another Java framework that
I can use from Scala

Spring I/O
2016
Conclusion
• Drawbacks using Spring instead of pure Scala
framework:
• not always able to write most idiomatic Scala
code
• translation from/to Java can be annoying
sometimes

Spring I/O
2016
Conclusion
• Advantages:
• I can keep my Spring productivity & use a well
proven framework while focusing on Scala
features
• Scala SDK constantly improving Java interop so
current issues can be expected to become
obsolete

Spring I/O
2016
Thank you
• Source code available at Github:
github.com/BernhardWenzel/spring-scala-examples
• Slides and talk online:
bernhardwenzel.com/blog/2016/04/22/using-spring-
with-scala/
Contact:
• http://bernhardwenzel.com
• twitter: @bernhardwenzel

Using Spring with Scala

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