Killing SSJS - Passing a parameter using Expression Language - Part 2

In part 1, I showed how you can expand the DataObject implementation to allow you to pass a parameter to a method and return the result. It's not a perfectly elegant solution, and it has some limitations, but it's a very solid technique and not at all difficult to implement or maintain. But I kept running into roadblocks. What if I have to compute part of the expression? What if I have to get a value based on another parameterized getter? You can't compute part of an EL expression in your EL.

So after some research and experimentation, I created the GetMap. This isn't a new concept - it was developed for JSF 1.1 (which is what XPages is built on) but the rest of the world has moved on to JSF 2.0 and so some of these older techniques can be a little tricky to unearth since this hasn't been an issue in the JSF world for years.
A GetMap works on the concept that JSF knows how to talk to Map objects. CompositeData is a Map, and EL can do #{compositeData.dataSource[compositeData.fieldName]}. With that in mind, let me show you the GetMap:

public abstract class GetMap<V> implements Map<String, V> {

public abstract V get(Object key);

}

Pretty simple, isn't it? The Map will require you to implement a number of other methods, but just have them return null or throw an UnsupportedOperationException. We aren't using any of them.

And here is how we implement it in a bean:

public class MyBean implements Serializable {

private static final long serialVersionUID = 1L;

transient private GetMap<Boolean> readOnlyGetter = new GetMap<Boolean>() {

@Override
public Boolean get(final Object key) {

if(null==unid || !(unid instanceof String)){

throw new IllegalArgumentException();

}
return isReadOnly(key);

};

public GetMap<Boolean> getReadOnly() {
return readOnlyGetter;

}

}

And here is the expression language:
isRendered="#{myBean.readOnly['firstName']"

And one thing you'll notice is that you can compute the value passed to the map. Like so:
isRendered="#{myBean.readOnly[compositeData.fieldName]}"

The way this works:

1. EL translates myBean.readOnly into myBean.getReadOnly()
2. getReadOnly() returns a Map, which EL recognises and understands.
3. ['firstName'] is seen as a property of the map, so EL calls ((Map) myBean.getReadOnly()).get('firstName').
4. Instead of being an actual map implementation, our GetMap simply runs some code when get is called, returning an object based on the parameter it receives.

This concept can be expanded further by returning a GetMap, enabling EL such as:

value="#{myBean.supportDoc['lineItems'].quantity['capitalItems']}"

Let me know what you think!