Author: Rory Winston

Criteria queries are enormously useful feature in Hibernate, as they allow you to dynamically build up a complex query. They are very useful, for instance, when you have a search form UI (or a set of search forms) and you want to build up a search query from any combination of the search parameters contained in the form. However, there is one feature regarding Criteria queries which is non-obvious – you need to create a sub-Criteria for each composite property that you wish to map. For instance, the code below will not work:

Criteria hibCriteria = getDao().createCriteria(Event.class);
hibCriteria.add(Expression.like("title", name));
hibCriteria.add(Expression.ge("calendar.startDate", startDate);
hibCriteria.add(Expression.le("calendar.endDate", endDate);


If you want to reference a nested property in a Criteria query, you need to create a separate Criteria instance for each property:

Criteria hibCriteria = getDao().createCriteria(Event.class);
hibCriteria.add(Expression.like("title", name));

// Add a sub-criteria for the nested property set
Criteria calCriteria = hibCriteria.createCriteria("calendar");
calCriteria.add(Expression.ge("startDate", startDate);
calCriteria.add(Expression.le("endDate", endDate);

The current project that I am working on makes very heavy use of Ajax. In fact, the application UI has developed to the point where the logical flow of the application has begun to resemble a thick-client (e.g. Swing) application, with the various JSP UI elements firing off asynchronous notifications from within event handlers and updating the UI when data becomes available. This has made a huge difference to the usability of the UI from the end user’s point of view. The application makes heavy use of Prototype, the Javascript framework, and we have had great success with its XmlHttpRequest handling capabilities. The controller tier is built on Struts, which delegates to a Spring-managed service layer. For instance, a sample request on the client might be invoked via Javascript as follows:

/**
* Update the list of unit names, given a handle to a <select> element with a currently
* selected unit type value
*/
fireUnitNameUpdate: function(sel) {
var unit = sel.options[sel.selectedIndex].value;
var url = Utils.contextRoot() + actionPath;
var pars = "action=showApprovalTree&type=" + unit;

var myAjax = new Ajax.Request (
url,
{
method: 'post',
postBody: pars,
onComplete: updateApprovalTree
});
}

The contextRoot() and actionPath variables construct a path to a Struts handler, which picks up the request and unpacks the parameters (Note that extra validation could be done on the client at this point). It then calls the service layer to create an XML document (constructed using Dom4J) and transforms it via an XSLT stylesheet:

public ActionForward showApprovalTree(ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws Exception {
String xml = approvalChainService.buildXmlApprovalTree(new Event());

// Transform the xml
TransformerFactory factory = TransformerFactory.newInstance();
Transformer transformer = factory.newTransformer(new StreamSource(new File(Constants.APPROVAL_TREE_XSL)));
response.setContentType(Constants.HTML);
transformer.transform(new StreamSource(new StringReader(xml)), new StreamResult(response.getWriter()));
response.getWriter().flush();
response.getWriter().close();
return null;
}

The Struts action returns an XML tree which is deserialized in the Ajax handler callback method on the client:

updateApprovalTree: function(originalRequest) {
var dom = XMLUtil.createXMLDom(originalRequest.responseText);
var nodes = XMLUtil.parseXPath(dom, "//unit");
UIUtils.updateDropDownWithXML('approvalTree', nodes);
}

This approach has proven to be very flexible and allows us to easily construct simple event handlers for UI event triggers. Building reusable libraries of cross-browser JavaScript code is now (with the help of Prototype and Behaviors) very easy. The only downside is the reduced debgging capabilities on the client side.

A lot of Java libraries that need regex functionality, but also need to retain compatability across 1.3 and earlier VMs use the famous Jakarta ORO library. This is a great (and fully-featured) regex implementation. An example of a project that uses this is one of the projects that I am a committer on, Jakarta Commons-Net. This is a very popular library which is mainly used for its FTP functionality, e.g. Ant’s FTP task uses it as its FTP engine.
However, it is designed to be compatible with 1.3 and earlier VMs. One of the implications of this is that its regex parsing is done by ORO, which necessitates extra “baggage” in the form of a separate jar download. As an exercise, I decided to see how easy it would be to convert the existing ORO code to a JDK-based approach. I first created a branch in SVN (here), and started to work on the ORO code.
It turned out to be trivial to make the changes, in fact the APIs were so similar I have a strong feeling that the Sun engineers who implemented the java.util.regex package may have been heavily influenced by the design of the ORO interface.
First, look at the ORO version:

And now look at the equivalent Java 1.4+ version: