Art of Software Reuse

I wrote earlier about using open source software for fulfilling functionality prior to building a custom implementation. Many projects have the need to transform object to text or XML – e.g. return XML response for REST-based services or return JSON string to support a web application etc.

Freemarker is a templating engine that can be leveraged to perform object to text transformations – with freemarker it is easy to generate RSS feeds or delimited text or plain old XML very easily. Below are some code fragments that illustrate freemarker usage:

Below is a code fragment for setting up the template configuration:

try {
	// Initialize configuration;
	cfg = new Configuration();
	cfg.setDirectoryForTemplateLoading(new File(templatesLocation));
	cfg.setTemplateUpdateDelay(0);
	cfg.setTemplateExceptionHandler(TemplateExceptionHandler.HTML_DEBUG_HANDLER);
	//Use beans wrapper (recommmended for most applications)
	cfg.setObjectWrapper(ObjectWrapper.BEANS_WRAPPER);
	cfg.setDefaultEncoding("ISO-8859-1");
	//charset of the output
	cfg.setOutputEncoding("UTF-8");
	//default locale
	cfg.setLocale(Locale.US);

} catch (Exception e) {
	//handle the exception
}

The templatesLocation variable is the path to a folder with freemarker templates (referred to as FTL templates). These are similar to templates used with frameworks such as Velocity. For example, below is a template fragment:

<Name>${myBean.firstName?xml} ${myBean.lastName?xml}</Name>

“myBean” is the java object that has first name and last name properties supported by JavaBeans style getXYZ() methods. The ?xml built-in function is used to encode any special XML characters.

Below is the code fragment when trying to invoke a template with a java object – the template file name is hard-coded for below example – it can be fetched from an input parameter or injected directly into this object as necessary:

// loading the template file
String templateFileName = "test-template.ftl";
Template template = cfg.getTemplate(templateFileName);
Writer out = new StringWriter();
template.process(dataObject, out);
String strResponse = out.toString();

The above code fragment essentially takes a java object (dataObject) as input and creates a string as output. Using the template above, the output for example could be:

<Name>John Smith</Name>

Freemarker also supports loops and conditions and offers a rich library of built-in functions as well.

This entry was posted on Sunday, August 22nd, 2010 at 10:00 pm and is filed under open source, Reuse. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.