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WebObjects with Scala

Version 333.1 by Ravi Mendis on 2010/03/07 17:40
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What is Scala?

Scala is a language for concurrent computing.
In this day and age of multi-core processors, concurrent computing can't be ignored.

Many of Scala's features have been designed with concurrency in mind.
Some of these may not be unfamiliar to Objective-C or WebObjects developers.

Here's a quick summary:

                                            Objective-C                                                                                Java           Scala                                          
 Separation of Mutable & Immuable Datatypes  Collections e.g: NSArray/NSMutableArray                                                    No             Yes                                             
 Closures                                        Blocks (Extension)                                                                     No             Anonymous Functions                             
 Static variables                                 Yes                                                                                     Yes             No                                             
 Static methods or 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                       

Why Use Scala?

Scala is inherently thread-safe.
It has concurrency that is effectively built-in to the language.

So for WebObjects developers, Scala offers itself as a powerful, safe and easy-to-use solution for concurrent applications. (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.
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 Talent 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 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

An example of accessing variables in WebObjects with the following languages:

          Objective-C                    Java                       Scala                
  getter   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

One of the benefits of Scala is its very powerful, concurrency-ready collection classes - primarily List, Map 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[Studio] = {
    storedValueForKey(_Studio.Keys.MOVIES).asInstanceOf[NSArray[Studio]]
}
 
def moviesList: List[Studio] = {
    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

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