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WebObjects with Scala

Version 355.1 by Ravi Mendis on 2009/10/13 23:34
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What is Scala?

Scala is a modern language not unlike Groovy.
It is said to be more powerful (and faster) than Groovy or Ruby which has been the reason for its adoption at sites like Twitter.

Many of its features and paradigms favor multi-threading and concurrency. Some of these may not be unfamiliar to Objective-C and WebObjects developers. Here's a summary:

                             Objective-C                                                                                Java           Scala                                          
 Mutable/Immuable Datatypes  Collections e.g: NSArray/NSMutableArray                                                    No             Yes                                             
 Closures                        Blocks (Extension)                                                                     No             Anonymous Functions                             
 Static variables                 Yes                                                                                     Yes             No                                             
 Static methods/functions         Yes                                                                                     Yes             No                                             
 Concurrency                 Grand Central Dispatch (Extension)  Threads      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           Categories                                  Interfaces  Traits                       

A fuller description of Scala can be found here.

Why Use Scala?

With Web 2.0, building concurrent WebObjects applications is a must.
Developing and maintaining a concurrent or multi-threaded WebObjects application can be challenging.

Scala offers concurrency that is (effectively) built-in to the language and is inherently thread-safe.
In other words, developing Ajax (i.e asynchronous communication) with WO will require concurrent request handling and thread-safe code, for which Scala is a better choice than Java.

In addition it may offer new solutions for concurrency in WebObjects and EOF.

Can WebObjects be Programmed In Scala?

Yes. It is very simple.
Scala compiles to java bytecode. Hence using it with WebObjects is fairly straightforward.

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. However, a class can have a Companion Object that will allow you to achieve something equivalent to static variables.
One of the advantages of this approach is that it is thread-safe, so you don't have to worry about synchronizing access to these fields in a concurrent application.

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

Java:


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

Scala:


object _Talent extends  EOGenericRecord {
val ENTITY_NAME = "Talent"

Compacted imports

Two lines in Java are compacted into one in Scala.

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 different 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]
     }
}

How to Use Scala Collections with EOF

One of the benefits of Scala is its very powerful, concurrency-ready collection classes - primarily List, Map, Seq and Set.
Employing these instead of NSArray and NSDictionary in WebObjects/EOF may be challenging.

But one may modify the EO templates to produce API such as:


def movies: NSArray[EOGenericRecord] = {
    storedValueForKey(_Studio.Keys.MOVIES).asInstanceOf[NSArray[EOGenericRecord]]
}
 
def moviesList: List[EOGenericRecord] = {
    movies.objects.toList
}

How to Add Scala to a WO Project

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Warning

Note

This is for Eclipse/WOLips IDE

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 is Java.

It is based on the D2W Movies example.

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Setup

  1. Install the Scala eclipse IDE
  2. Install and start the OpenBase OBMovies database.
  3. Right-click on Application.java and run as a WOApplication (as usual).

EO Templates

See: Scala templates

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

  1. Point to the local .eotemplate files for Entity and Entity
  2. Change the File Names Extension to "scala"
  3. Uncheck Java under Options