Java Template Engines, fight!

One of the greatest features Java provides for enterprise applications, are the many
frameworks, libraries and tools provided by many third-party developers. In the web development
front, we find many useful resources and today I'm gonna compare the most popular template
systems a Java developer will find.


Basically, instead of using several HTML pages, you produce some templates which allows you
to easily maintain most of the static content of a site without having to touch 40 different files. This
way, if we want to modify the header of any site, simply by editing our header template, all the headers
on our site will change.


A template system is comprimised of three main components:

1. Template Engine
2. Content Resource
3. Template Resource

You can find more information about the theory behind template systems in the Wikipedia Entry.

J2EE Templates

Our candidates are:

• Freemarker
• Velocity
• StringTemplate
• MVEL

Freemarker

License:

BSD-style

Tested Version:

2.3.16

Dependencies:

None

Advantages:

• XML/HTML Templates (no need to learn a new language).
• Very simple and straight to the point.
• Plugins for lots of IDEs.

Disadvantages:

• Latest version is almost 8 months old.
• Simplicity comes at a price: not as powerful as the others.

As stated on the projects' site "It simply generates text" and it does it greatly. It was very easy to get to work with it and appart from the example below, I was able to generate
SQL queries, HTML pages and Configuration files without a problem, from a file or from in-code templates. Also, having the ability to provide a OutputWriter was great.

public class FreemarkerExample extends TemplateExample{

    Configuration config = new Configuration();
    Template freemarkerTemplate;
    OutputStreamWriter output = new OutputStreamWriter(System.out);
    
    Map<String,String> dataModel = new HashMap<String,String>();
    
    public FreemarkerExample(String user){
        this.user = user;
        dataModel.put("user", this.user);
    }
    
    @Override
    public void print() {
        try {
            freemarkerTemplate = new Template("Template", new StringReader("Hello Freemarker, I'm ${user}\n"), config);
            freemarkerTemplate.process(this.dataModel, this.output);
        } catch (TemplateException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        
    }

    @Override
    public void printFromFile() {
        try {
            freemarkerTemplate = config.getTemplate("templates/freemarker.xml");
            freemarkerTemplate.process(this.dataModel, this.output);
        } catch (TemplateException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Velocity Engine

License:

Apache

Tested Version:

1.6.4

Dependencies:

None

Advantages:

• Very alive. This version is from a few weeks back.
• The most powerful.
• Most popular.

Disadvantages:

• Ugly syntax.
• Complex.

Without a doubt, the most powerful and complex project. If you take into account all the parts that conform Velocity (Engine, Tools, DVSL, Texen, Anakia, etc.) you have almost
all of your needs covered. If you take only the Velocity Engine, you'll want to rip your eyes off because of the syntax. Also, the variable names are very easy to
get lost inside complex pieces of code, and when we're talking about velocity, they will.

I really wish they followed Python's way with the syntax. Instead of using all those counter intuitive pound signs. I mean, for any Unix/Linux user, those are comments
and believe me, your mind won't get used to them, ever!

It was the hardest to get running and creating a new context can be a mess in big systems when developers start putting data in there.

public class VelocityExample extends TemplateExample {

    VelocityEngine engine = new VelocityEngine();
    VelocityContext context = new VelocityContext();
    
    public VelocityExample(String user) {
        this.user = user;
        context.put("user", this.user);
    }
    
    @Override
    public void print() {
        StringWriter writer = new StringWriter();
        StringReader reader = new StringReader("\nHello Velocity, I'm $user\n");
        try {
            Velocity.evaluate(context, writer, this.user, reader);
            System.out.println( writer.toString() ); 
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public void printFromFile() {
        StringWriter writer = new StringWriter();
        try {
            Template template = engine.getTemplate("templates/velocity.vm");
            template.merge( context, writer );
            System.out.println( writer.toString() ); 
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

StringTemplate

License:

BSD

Tested Version:

3.2.1

Dependencies:

ANTLR

Advantages:

• Powerful and simple
• Not limited to Web.
• Not a Java only project.

Disadvantages:

• Counter-intuitive usage of the dollar sign.
• Single person project. Not much activity.
• Not much documentation.
• No IDE plugins

Watching an article about StringTemplate is what drove me into writing this post. I thought: "finally someone is implementing a StringBuilder that behaves like it should". But no, it was another template engine, but I wanted to see how it works. And it was fast and easy to learn, but the way the the dollar sign is used kept me head banging on the table. You only have to take a look at this to cry: $if(title)$<title>$title$</title>$endif$. Now imagine a thousand lines of it.

If you get used to this syntax, having a template engine that works in several programming languages is a very good thing.

public class StringTemplateExample extends TemplateExample{
    
    StringTemplate template = new StringTemplate("Hello StringTemplate, I'm $user$");
    
    public StringTemplateExample(String user) {
        this.user = user;
        template.setAttribute("user", this.user);
    }
    
    @Override
    public void print(){
        System.out.println(template);
    }
    
    @Override
    public void printFromFile(){
        StringTemplateGroup templateGroup = new StringTemplateGroup("xml group", "templates");
        StringTemplate xmlTemplate = templateGroup.getInstanceOf("xml");
        xmlTemplate.setAttribute("user", this.user);
        
        System.out.println(xmlTemplate.toString());
    }
}

MVEL2

License:

Apache 2.0

Tested Version:

2.0.17

Dependencies:

None

Advantages:

• Easiest to use of all
• Nice syntax
• Very alive. Latest version is from March 2010

Disadvantages:

• The eval method should accept a File object directly.
• No IDE plugins

I really liked MVEL2 for its simplicity and power. You get a very nice template engine and a very nice expression language which will look very familiar to Java programmers. No weird syntax and the usage of @{var} was a good decision since it's not something you see in any other place, allowing the programmer to easily adjust to the engine.

public class MVELExample extends TemplateExample{
    
    Map<String,String> map = new HashMap<String,String>();

    public MVELExample(String user) {
        this.user = user;
        map.put("user", user);
    }
    
    @Override
    public void print() {
        System.out.println((String)TemplateRuntime.eval("Hello MVEL, I'm @{user}\n", map));
    }

    @Override
    public void printFromFile() {
        try {
            System.out.println(TemplateRuntime.eval(new FileInputStream(new File("templates/mvel.xml")), map));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}

Conclusion

For an enterprise system the decision is clear: Velocity. It's the most known and powerful tool of them all. It might also have the ugliest syntax but if you get your mind around it and install your IDE plugin, things start looking better. If Velocity wasn't the most known of them all, I would definitely go with MVEL2 for everything, from complex systems to simple ones. From Web apps, to Swing apps.