Velocity Template Engine – Part 2 (Getting templates from a JAR)

In my previous post I described the basics of Velocity template engine. In that post, we were making use of a template (the .vm file) which we keep outside the application archive. But there are some potential issues in doing that – sometimes those file may get corrupted or deleted. So a better idea will be to keep it with in the application archive itself. In this post I will explain how we can make use of a template file with in an archive.

Sample Template File – testvelocity.vm

Hello $name! Welcome to Velocity!

Java Program – TestVelocity.java

A java program that refers to the the testvelocity.vm template.

public static void handleTemplate()
{
	VelocityEngine engine = null;
	String myTemplateBody = null;
	VelocityContext context = null;
	StringResourceRepository repository = null;
	Template template = null;
	StringWriter writer = null;

	try
	{
		// Getting Velocity Engine
		engine = getVelocityEngine(engine);

		// Reading Template Body from the template file(.vm file) in the jar
		myTemplateBody = getTemplateFromJar();

		// Setting the template body in string repository with a template
		// name. Here the template name is used as a key for future mapping.
		repository = StringResourceLoader.getRepository();
		repository.putStringResource("myTemplateName", myTemplateBody);

		// Getting the context with placeholder values
		context = getVelocityContext();

		// Fetch Template to a StringWriter
		template = engine.getTemplate("myTemplateName");
		writer = new StringWriter();
		template.merge(context, writer);

		System.out.println("VM Template:\n" + myTemplateBody);
		System.out.println("Output:\n" + writer.toString());
	}
	catch (Exception e)
	{
		System.out.println("Oops! We have an exception");
		e.printStackTrace();
	}

}

Getting the template engine
Velocity supports different kind of resource loaders.  In this context we need to use a String resource loader. Know more about it here.

private static VelocityEngine getVelocityEngine(VelocityEngine engine)
throws Exception
{
	// Initializes the velocity engine with properties. We should specify
	// the resource loader as string and the class for
	// string.resource.loader in properties
	Properties p = new Properties();

	p.setProperty("resource.loader", "string");
	p.setProperty("string.resource.loader.class",
		"org.apache.velocity.runtime.resource.loader.StringResourceLoader");
	engine = new VelocityEngine();
	engine.init(p);

	return (engine);
}

Getting Template file from JAR
Know more about JAR and deployment here.

// Reading the file contents from the JAR
inStream = TestVelocity.class
		.getResourceAsStream("/path/to/velocity/template/testvelocity.vm");
stringBuilder = new StringBuilder();
streamReader = new InputStreamReader(inStream);
bufferedReader = new BufferedReader(streamReader);

while ((line = bufferedReader.readLine()) != null)
{
	stringBuilder.append(line);
}

Getting Velocity context

context = new VelocityContext();
context.put("name", "mathew");

Output

Please check this post get additional information on building a JAR file & executing it: https://javabeanz.wordpress.com/2009/01/29/running-an-executable-jar-from-command-line/

Velocity Template Engine

Velocity is an open source template engine developed by an international volunteer community and hosted by the Apache Software Foundation.

A template engine is a software that is designed to process web templates and content information to produce output web documents. Any velocity project contains two things – Velocity Template & Java Program.

Velocity Template
A velocity template is a text or text file (with .vm extension – that’s the general practice), that contains normal text and velocity specific placeholders (e.g. $name).

A sample template file – testvelocity.vm

My name is $name

Java Program – TestVelocity.java
A java program that refers to the the testvelocity.vm template.

import java.io.StringWriter;
import org.apache.velocity.app.VelocityEngine;
import org.apache.velocity.Template;
import org.apache.velocity.VelocityContext;
public class TestVelocity
{
	public static void main( String[] args )
	throws Exception
	{
		// Initializes engine
		VelocityEngine ve = new VelocityEngine();
		ve.init();

		// Getting the Template
		Template temp = ve.getTemplate("testvelocity.vm");

		// Create a context and add data to the template placeholder
		VelocityContext context = new VelocityContext();
		context.put("name", "Velocity");

		// Fetch template into a StringWriter
		StringWriter wr = new StringWriter();
		temp.merge( context, wr );
		System.out.println( wr.toString() );
	}
}

Output
When you compile and run,the output will be;

My name is Velocity

Note:
The template file is expected to be in the class-path. If you want to load a template file, add the following code block to the code.

Properties velocityProperties = new Properties();
velocityProperties.setProperty("file.resource.loader.path",
    "/path/to/my/velocity/template/dir");

// Use this properties to initialize the engine
VelocityEngine ve = new VelocityEngine();
ve.init(velocityProperties);

Directives – What is it?
There are different directives for velocity.

#set(), #if(), #else, #elseif(), #end, #foreach(), #include(), #parse(), #macro()

Example:

#if ( $value > 10 )
display me. I am greater than 10.
#elseif ( $value < 10 ) display me. I am lesser than 10. #else display me. #end [/sourcecode] I will be adding more about it in my future posts.