Wanted: the ability to create an instance of an Apex class from the class name

Suppose you have a managed package that has an Apex class called PackageController and a Visualforce page called PackagePage. But for a particular customer the behavior of the class needs to be modified. Now this could be done using public custom setting switches (that can be set in the customer’s org that the managed package is deployed in) but that would mean that the new behavior has to be added to the managed package. Where there are many or arbitrary behavior variations this isn’t a viable approach.

One solution is to copy the source code of PackageController and PackagePage into the customer’s org and modify it for each customer. But then you no longer have an upgradeable package or any intellectual property protection or indeed a product any more.

The approach I am looking to use instead is the strategy pattern. This allows parts of the PackageController’s behavior to be plugged in. The PackageController defines one or more global interfaces and uses a factory to create instances of concrete classes that implement those interfaces and then uses those instances. That factory takes the names of the concrete classes from public custom settings and these are defaulted to the names of the standard implementation concrete classes contained in the managed package. The code would look something like this:

// Example of an interface that abstracts a piece of behavior
global interface InterestStrategy {
    Decimal calculate(Decimal price, Decimal rate, Integer days);
}
// Example of a factory method
public class Factory {
    public InterestStrategy createInterestStrategy() {
        // This creates an instance of the class named by its argument by invoking that class's no-arg constructor
        return (InterestStrategy) System.newInstance(Strategies__c.InterestStrategyClassName__c);
    }
}
// Example of a controller using a strategy
public with sharing class PackageController {
    public void calculate() {
        ...
        Decimal result = new Factory().createInterestStrategy().calculate(price, rate, days);
        ...
    }
}

Given this approach, alternate concrete class implementations of the strategy interfaces can be written in the customer’s org and configured to be used by the PackageController by setting their names in the public custom settings. This way, the new behavior goes where it should go, in the org that is using the managed package not in the managed package itself. The managed package follows the open/closed principle – it is open for extension without needing to be modified to achieve the extension.

Apex already has all the necessary mechanisms e.g. interfaces except one: the ability to create an instance of a class starting from the class name. In the above code this is represented by the non-existent method System.newInstance. The equivalent mechanism in Java is Class.forName(“ClassName”).newInstance().

I’m posting this because the lack of this feature is really hurting our ability to deliver what our customers need. I (and others) have posted the need on the IdeaExchange e.g. Allow Apex code customization of a managed package but there is no commitment from salesforce to deliver this. If you have a managed package or are thinking of creating one, I suggest that you too should push for this feature to be delivered.

PS Type.newInstance was delivered in Summer ’12; see the current Force.com documentation for more detail.