Skip to Content

WebObjects with Scala

Version 442.1 by Ravi Mendis on 2010/12/23 06:48
Warning
For security reasons, the document is displayed in restricted mode as it is not the current version. There may be differences and errors due to this.

What is Scala?

Scala is a JVM language that is a hybrid of Object-Oriented and Functional styles.  It is useful as complete general purpose replacement for Java.  But its built-in Actors library makes it especially attractive 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, primarily a preference for immutability and the use of other functional language paradigms.
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 can help you to write thread-safe code.
It has concurrency that is built-in to the standard library, primarily via Actors.

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.

However, tool support is a weak point. You should use Eclipse 3.6 with bundle-less WO builds.  If your Application class is in Scala then you will have to create an Eclipse launch configuration manually. One weird thing is that once you add the Scala nature to a WO project WOD completion will stop working - this is unfortunate. Also the Scala plugin is rather slow and still very 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. Is this true?  I don't think Scala "object" instances (with the object keyword) are guaranteed to be thread-safe; they are just singletons

So you don't have to worry about synchronizing access to shared mutable 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"

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


Talent.ENTITY_NAME

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

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.  Alternatively, you could generate an actual new scala.List instance by calling myNSArray.toList.

How to Add Scala to a WO 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.

Warning

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 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).

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