How to use String as Velocity Template?

Question

What is the best way to create Velocity Template from a String?

I'm aware of Velocity.evaluate method where I can pass String or StringReader, but I'm curios is there a better way to do it (e.g. any advantage of creating an instance of Template).

Solution 1

There is some overhead parsing template. You might see some performance gain by pre-parsing the template if your template is large and you use it repeatedly. You can do something like this,

RuntimeServices runtimeServices = RuntimeSingleton.getRuntimeServices();
StringReader reader = new StringReader(bufferForYourTemplate);
Template template = new Template();
template.setRuntimeServices(runtimeServices);

/*
 * The following line works for Velocity version up to 1.7
 * For version 2, replace "Template name" with the variable, template
 */
template.setData(runtimeServices.parse(reader, "Template name")));

template.initDocument();

Then you can call template.merge() over and over again without parsing it everytime.

BTW, you can pass String directly to Velocity.evaluate().

Solution 2

The above sample code is working for me. It uses Velocity version 1.7 and log4j.

private static void velocityWithStringTemplateExample() {
    // Initialize the engine.
    VelocityEngine engine = new VelocityEngine();
    engine.setProperty(RuntimeConstants.RUNTIME_LOG_LOGSYSTEM_CLASS, "org.apache.velocity.runtime.log.Log4JLogChute");
    engine.setProperty("runtime.log.logsystem.log4j.logger", LOGGER.getName());
    engine.setProperty(Velocity.RESOURCE_LOADER, "string");
    engine.addProperty("string.resource.loader.class", StringResourceLoader.class.getName());
    engine.addProperty("string.resource.loader.repository.static", "false");
    //  engine.addProperty("string.resource.loader.modificationCheckInterval", "1");
    engine.init();

    // Initialize my template repository. You can replace the "Hello $w" with your String.
    StringResourceRepository repo = (StringResourceRepository) engine.getApplicationAttribute(StringResourceLoader.REPOSITORY_NAME_DEFAULT);
    repo.putStringResource("woogie2", "Hello $w");

    // Set parameters for my template.
    VelocityContext context = new VelocityContext();
    context.put("w", "world!");

    // Get and merge the template with my parameters.
    Template template = engine.getTemplate("woogie2");
    StringWriter writer = new StringWriter();
    template.merge(context, writer);

    // Show the result.
    System.out.println(writer.toString());
}

A similar so question.

Solution 3

This works in Velocity 2.1

// Initialize the engine
VelocityEngine velocityEngine = new VelocityEngine();
velocityEngine.setProperty(Velocity.RESOURCE_LOADERS, "string");
velocityEngine.setProperty("resource.loader.string.class", StringResourceLoader.class.getName());
velocityEngine.setProperty("resource.loader.string.cache", true);
velocityEngine.setProperty("resource.loader.string.modification_check_interval", 60);
velocityEngine.init();

// Add template to repository
StringResourceRepository repository = StringResourceLoader.getRepository()
repository.putStringResource("hello_world", "Hello $w");

// Set parameters
VelocityContext context = new VelocityContext();
context.put("w", "world!");

// Process the template
StringWriter writer = new StringWriter();
velocityEngine.getTemplate('hello_world').merge( context, writer );

System.out.println(writer.toString());

Solution 4

RuntimeServices rs = RuntimeSingleton.getRuntimeServices();            
StringReader sr = new StringReader("Username is $username");
SimpleNode sn = rs.parse(sr, "User Information");

Template t = new Template();
    t.setRuntimeServices(rs);
    t.setData(sn);
    t.initDocument();

VelocityContext vc = new VelocityContext();
vc.put("username", "John");

StringWriter sw = new StringWriter();
t.merge(vc, sw);

System.out.println(sw.toString());

Solution 5

Velocity 2 can be integrated into the JSR223 Java Scripting Language Framework which make another option to transform string as a template:

ScriptEngineManager manager = new ScriptEngineManager();
manager.registerEngineName("velocity", new VelocityScriptEngineFactory());
ScriptEngine engine = manager.getEngineByName("velocity");


System.setProperty(VelocityScriptEngine.VELOCITY_PROPERTIES, "path/to/velocity.properties");
String script = "Hello $world";
Writer writer = new StringWriter();
engine.getContext().setWriter(writer);
Object result = engine.eval(script);
System.out.println(writer);

View more solutions

Comments

[tomsame]

Exactly what I was looking for. Thanks. For other people's reference, runtimeServices is an instance of org.apache.velocity.runtime.RuntimeInstance

[ZZ Coder]

Missed one-line. For completeness, I added it.

[Bernhard Hofmann]

+1 for the mention of Velocity.Evaluate since that's exactly what I was looking for.

[Ben.Yan]

Wow, this is a unique approach. how velocity engine load resource seems is internal implementation, could property "string.resource.loader.class" subject to change?

[vektor]

You seem to have a typo in Velocity.RESOURCE_LOADERS - should probably be Velocity.RESOURCE_LOADER.

[devatherock]

The RuntimeConstants interface which is implemented by Velocity class has both constants - RESOURCE_LOADERS and RESOURCE_LOADER. RESOURCE_LOADERS was the one I tested with

[ltlBeBoy]

This worked for me for some time. But then I found internal Velocity exceptions in the log file. I changed to the following solution, and everything works fine now: Velocity template as a String in Java. It uses Velocity#evaluate(context, out, logTag, instring), as mentioned in another answer here.

[joseph]

compilation error due to too many parens. should be template.setData(runtimeServices.parse(reader, "Template name")); stackoverflow complains Edits must be at least 6 characters; is there something else to improve in this post?.