Skip to Content

WebObjects with Scala

Last modified by Ravi Mendis on 2011/05/10 02:10

What is Scala?

Scala is a modern, multi-paradigm JVM language that is most often compared to Groovy, Clojure and Erlang. Its functional language foundations and built-in Actors library make it especially attractive for concurrent computing. (Scala is an abbreviation for "scalable" hinting at its design goals).

In this day and age of multi-core processors concurrent computing can not be ignored. Many of the design features of Scala have been chosen with concurrency in mind, some of which may not be unfamiliar to Objective-C or WebObjects developers. Here's a summary:

 

Objective-C

Java

Scala

Immutability

Partial - via collections e.g: NSArray/NSMutableArray

No

Yes

Closures

Yes - via Blocks (Extension)

No

Yes - via Anonymous Functions

Static variables

Yes

Yes

No

Static methods

Yes

Yes

No

Concurrency

Yes - via Grand Central Dispatch (Extension)

Yes - via Threads

Yes - via Actors

 

Weakly Typed

Strongly Typed

Strongly Typed

Other notable features include:

 

Objective-C

Java

Scala

Parametered methods

Yes e.g: addObject: to:

No

Yes e.g: add(object= ,to=)

Class composition

Yes - via Categories

Yes - via Interfaces

Yes - via Traits

Why Use Scala?

For WebObjects developers, Scala offers itself as a powerful, safe and easy-to-use solution for concurrent computing. (In other words, Scala Actors can be used for problems that would have normally required threads).

Can WebObjects be Programmed In Scala?

Yes. It is very simple.
 By virtue of being a JVM-language, Scala compiles to java bytecode.

Furthermore, being a multi-paradigm language grants Scala easy WebObjects-interoperability.

Caveats

Legacy tool support is often cited as a weak point. The Eclipse Scala plugin has been found to be slow at times and sometimes buggy.

WebObjects In Scala

The following highlights some of the differences between Java and Scala in WebObjects:

EOs in Scala

Thread-Safe Shared Vars

Scala doesn't have static variables or methods. Instead Scala employs the Singleton Pattern which is built into the language and is thread-safe: a class can have a Companion Object that will allow you to achieve something equivalent to static variables - but better.

You don't have to worry about synchronizing access to shared mutable fields in a concurrent application.
 (This is not however true when for example you have a val declared as a NSMutableArray. You will still have to synchronize when adding to or removing from this mutable field).

The following is an example of the use of a Companion Object for Talent in Scala instead of Talent static fields in Java.

Java:


public class Talent extends EOGenericRecord {
public static final String ENTITY_NAME = "Talent";
}

Scala:


object Talent {
val ENTITY_NAME = "Talent"
}

This value will be accessed exactly the same way in both languages:


Talent.ENTITY_NAME

Compacted imports

In Java:


import com.webobjects.eocontrol.EOGenericRecord;
import com.webobjects.eocontrol.EORelationshipManipulation;

In Scala:


import com.webobjects.eocontrol.{EOGenericRecord, EORelationshipManipulation}

WOComponents in Scala

Compact Constructors

Scala allows for simpler use of multi-valued constructors than Java.

In Java:


public class MenuHeader extends WOComponent {

   public MenuHeader(WOContext aContext) {
        super(aContext);
   }
}

In Scala:


class MenuHeader(context: WOContext) extends WOComponent(context: WOContext)

Simplified Exception Handling

Scala doesn't force you to catch exceptions unlike in Java.
 In addition, the syntax employs Scala's very powerful pattern matching to handle exceptions.

In Java:


try {
    EditPageInterface epi = D2W.factory().editPageForNewObjectWithEntityNamed(_manipulatedEntityName, session());
    epi.setNextPage(context().page());
    nextPage = (WOComponent) epi;
} catch (IllegalArgumentException e) {
    ErrorPageInterface epf = D2W.factory().errorPage(session());
    epf.setMessage(e.toString());
    epf.setNextPage(context().page());
    nextPage = (WOComponent) epf;
}

In Scala:


try {
     var epi: EditPageInterface = D2W.factory.editPageForNewObjectWithEntityNamed(_manipulatedEntityName, session)
     epi.setNextPage(context.page)
     nextPage = epi.asInstanceOf[WOComponent]
} catch {
     case e: IllegalArgumentException => {
            var epf: ErrorPageInterface = D2W.factory.errorPage(session)
            epf.setMessage(e.toString)
            epf.setNextPage(context.page)
            nextPage = epf.asInstanceOf[WOComponent]
     }
}

Scala Annotations vs. Generated Accessors

Here's an example of accessing variables in the following languages:

 

Objective-C

Java

Scala

getter

[WO:object name]
object.name()
object.name

setter

[object setName:aName]
object.setName(aName)
object.name = aName

Of course in Java, we may generate WebObjects classes with "get" methods as well in order to stick to convention.
 In scala there is an additional convenience we may use to produce "get" and "set" methods in addition to the default Scala accessors - Scala Annotations.

E.g, in Main.scala we annotate our component keys with @BeanProperty to automatically create public "set" and "get" methods.
 These variables can then be accessed via KVC.


import scala.reflect.BeanProperty

@BeanProperty var username = new String()
@BeanProperty var password = new String()
@BeanProperty var isAssistantCheckboxVisible = false

How to Use Scala Collections with EOF

To use the Scala Collections API with an NSArray or NSDictionary you simply need to add an import:


import scala.collection.JavaConversions._

After that, you may access the typical Scala collection methods directly on NSArray.  This employs a feature of Scala known as implicit conversions to automagically cast a NSArray (a Java Iterable) into a Scala Iterable while leaving the actual object unchanged.

How to Add Scala to a WO Project (in Eclipse)

Adding Scala Support to a WOLips Project

Failed to execute the [include] macro. Cause: [You must specify a 'reference' parameter pointing to the entity to include.]. Click on this message for details.

WO Scala Example

The following example is an almost 100% Scala WO app. In reality it is a mixed Java/Scala app:
 All the EO logic and WO components are in Scala.
 Only the Application class remains Java.

It is based on the D2W Movies example.

Unknown macro: attachments. Click on this message for details.

Setup

  1. Install the Scala eclipse IDE
  2. Right-click on Application.java and run as a WOApplication (as usual).
Warning

Application can be made into a Scala class as well, but then you will have to create a launcher in Eclipse manually.

EO Templates

When you create your .eogen file, be sure to make the following changes in the EOGenerator Editor:

  1. Point to the local Scala versions of the .eotemplate files for Entity and _Entity
  2. Change the File Names Extension to "scala"
  3. In Destination Paths set the Superclass Package (e.g: base)
  4. Uncheck Java under Options

How to Build & Deploy a WebObjects Scala Project with Ant

  1. Download and install Scala
  2. Set scala.home (the location Scala has been installed onto) in the project build.properties file
  3. Add the scalac task and properties to the ant build.xml file
  4. Run from the project directory: sudo ant clean install