Java 8 optimized, memory efficient, speedy template engine producing statically typed, plain java objects
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Fix hot reload system property name in readme
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Rocker Templates by Fizzed

Build Status Maven Central

Fizzed, Inc. (Follow on Twitter: @fizzed_inc)

Sponsored by

Rocker is proudly sponsored by Greenback. We love the service and think you would too.


More engineering. Less paperwork. Expenses made simple.


Rocker is a Java 8 optimized (runtime compat with 6+), near zero-copy rendering, speedy template engine that produces statically typed, plain java object templates that are compiled along with the rest of your project. No more "warm-up" time in production, slow reflection-based logic, or nasty surprises that should have been caught during development.

Write your templates using an intuitive, tagless syntax with standard Java expressions for logic, iteration, and values. Use Rocker's special ? presence operator for null-safe evaluation. All the heavy lifting is done by the Rocker parser during development -- which keeps the runtime dependencies down to just a handful of classes. Rocker will parse your templates and generate well-documented Java source files (so you can easily inspect and understand how it works).


Based on the following template benchmark, Rocker is the clear winner. ~250% faster than Freemarker while also requiring orders-of-magnitude less memory.

Template Comparison

Two-minute drill

Most templates are used for websites, so here is a quick sample showing how Rocker templates work and can call each other during the rendering process. Create a template containing a common header and footer as well as a placeholder for body content. Create template src/main/java/views/main.rocker.html

@args (String title, RockerBody content)


The template we actually plan on showing to a user will render its content within the context of the common/header footer. In Java terms, it's passing a block of rendering code to be executed within another template. Create template src/main/java/views/index.rocker.html

@args (String message)

@views.main.template("Home") -> {
    <h1>Hello @message!</h1>

Hey, what about the RockerBody content argument? We cover it in more detail in the syntax readme, but for now just understand that its the only special type of argument and instructs Rocker that a template expects a "body" to be passed to it.

The Rocker parser will generate a Java source file for each template. They will be target/generated-sources/rocker/views/ and target/generated-sources/rocker/views/ In your application, you can render the index template like so.

static public void main(String[] args) {

    String output = views.index.template("World")


The output will equal:

        <h1>Hello World!</h1>

Once you generate the Java sources and peek inside the code, it's simple to see how this works. The views.index class creates a views.main template instance and hands off rendering to it -- while also passing a block of itself that it will render when views.main calls the @content variable. The syntax is identical to how a lambda is defined in Java 8 (implemented with lambdas for Java 8 and anonymous inner classes for Java 6/7). Rocker does a number of things behind the scenes to make sure templates that create other templates share the same rendering context (output buffer, application-specific context/implicit state).


  • Templates are runtime compatible with Java 6+

  • Optimizations enabled when targeting Java 8+ -- using Lambdas and type inference under-the-hood

  • Near zero-copy rendering

  • Hot reload support in two flavors

  • Elegant, intuitive, tagless syntax that infers when your logic ends for control / dynamic content. All dynamic / control code uses standard Java syntax.

  • A special ? presence operator extends syntax for simplified handling of null values.

  • Parsed templates become normal POJOs with defined arguments -- allowing you to tap into your IDEs code completion, syntax highlighting, etc.

  • Support for injecting intermediate application-specific super classes during parsing & generating phase -- thereby creating your own app-specific template engine where you can make implicit variables/methods available to all templates.

  • Since templates are just Java classes -- your logic / dynamic content can call out to any other Java code. Your templates can be as advanced or as simple as you need. No reflection used.

  • No runtime configuration/engine required -- there isn't any sort of RockerEngine class required to execute templates. Each compiled template is ready-to-go and knows how to render itself.

  • Templates retain enough information about the original template to throw exceptions at runtime (during render()) that let you track down the problematic line in the original template source file.


Checkout the file for a comprehensive deep dive on the rocker syntax.

Framework integrations

Rocker has a growing list of frameworks that it has been seamlessly integrated with. If you want to link to a new framework added, please file an issue or submit a PR:

Near zero-copy rendering

Static (plain text) for each Rocker template is (by default) stored internally as static byte arrays already converted into your target charset (e.g. UTF-8). When a template is rendered -- the static byte arrays are reused across all requests. Rocker renders to an optimized output stream that stores a composite (linked list) view of the reused byte arrays plus your dynamic content. Since templates consist mostly of static content rendered into the same charset over and over again, rather than allocating new memory, copying that content, and then converting it into your target charset for each request -- Rocker simply uses a pointer to it over and over again. This technique produces fast and memory efficient renders.

Let's say you have a template consisting of 9000 bytes of plain static text and 1000 bytes of dynamic content. Without this optimization, it would require ~100MB of memory to service 10000 requests (10000 bytes x 10000 requests). With this optimization, it would require ~10MB of memory to service 10000 requests (1000 bytes x 10000 requests). Besides lower memory, you also cut out 90MB of memory copies and 90MB of UTF-8 String->byte conversions. A pretty useful optimization.

No reflection

Everything is compiled by your project's compiler along with your other Java source code. Any dynamic code in your template is ultimately converted into standard Java and compiled. No reflection used.

Hot reloading

Version 0.10.0 introduced support for hot reloading templates during development. Hot reloading allows you to modify the template source code, save it, and have the changes active on the next request -- without having to restart your JVM. Rocker offers two different flavors of hot reloading for flexibility.

Flavor 1: static interface, dynamic rendering

The major feature of Rocker templates is that your templates are compile-time checked for usage, arguments, logic, etc. by the Java compiler.

In version 0.10.0 the underlying structure of a template was modified where a template generates two underlying classes. Each template generates a model class (its interface) and an implementation class (its renderer). Your application will only interact directly with the model, therefore allowing Rocker to dynamically recompile and reload the implementation class.

The major benefit of flavor one is that your application code remains the same and is compile-time checked by the Java compiler, while the template content can be modified and automatically reloaded at runtime. Only in the case where you actually change the template arguments, will you need to restart your application.

Flavor 2: dynamic interface, dynamic rendering

If you prefer the convenience of fully dynamic templates, flavor two supports hot reloading of both the template model class (its interface) as well as the implementation class (its renderer). Your application will lose some of the compile-time checking and a small performance hit, but gain the convenience of everything being reloadable. The way your application will use templates is different as well.

import com.fizzed.rocker.Rocker


// dynamic interfaces, dynamic implementation
String rendered = Rocker.template("views/index.rocker.html")
    .bind("val", "ValueA")

The template path and arguments will be runtime-checked. Please note that each bindable value must match the name and type declared in your template.

Activate hot reloading

Support for hot reloading is added to your generated templates by default in version 0.10.0. If you'd like to disable support, set the configuration/system property rocker.optimize to true during your build. Since the code is present in your templates by default, you merely need to turn it on at runtime.

Add dependency

The rocker-compiler dependency needs to be added to your build. This dependency only needs to be present during development and can be removed in production. In Maven, this means you'll want to add the dependency in the provided scope.


Enable at runtime

Activate hot reloading at runtime. You can activate hot reloading either with a system property or programmatically. For activating hot reloading with a system property in maven.

mvn -Drocker.reloading=true of args...

Alternatively, you can activate hot reloading programmatically.

import com.fizzed.rocker.runtime.RockerRuntime



Try out hot reloading

There is a simple example demonstrating hot reload in action. This project uses Blaze to help script tasks. Run the following

java -jar blaze.jar hot_reload

Point your browser to http://localhost:8080

Then modify & save rocker-test-reload/src/test/java/views/index.rocker.html and refresh your browser.

Getting started

Rocker consists of two components - the parser/generator and the runtime. To use Rocker in your project, add the runtime dependency to your application, then enable the parser in your build tool followed by creating your first template.

Add dependency

Rocker is published to Maven central. To add as a dependency in Maven:


<!-- for hot-reloading support only during development -->

To add as a dependency in Gradle:

repositories {

dependencies {
    compile group: 'com.fizzed', name: 'rocker-runtime', version: '0.24.0'
    // add rocker-compiler dependency as needed

Integrate parser/generator in build tool

Rocker supports Maven and Gradle out-of-the box.


Add the following to your pom


By default, Rocker will recursively process any template files ending with .rocker.html in src/main/java. The directory the template is saved will become the standard Java package the generated Java classes will be placed into. The generated Java source files will be saved to target/generated-sources/rocker. The plugin will take care of adding this generated directory to your sources root.

The following properties are supported:

  • templateDirectory is the base directory to recursively start from when locating and parsing template files. The Java package a template will be generated to will use this directory as its base. So if you have ${templateDirectory}/views/mytemplate.rocker.html then Rocker will generate ${outputDirectory}/views/ Defaults to ${}.

  • outputDirectory is the directory the parser will generate sources for templates. Defaults to ${}/generated-sources/rocker

  • classDirectory is the directory the hot reload feature will (re)compile classes to at runtime. Defaults to ${}

  • failOnError determines whether any parsing/generating errors cause Maven to fail. Defaults to true.

  • skip determines whether execution of the plugin should be skipped. Defaults to false.

  • touchFile is the file to "touch" after successfully generating Java sources. Useful for triggering other workflow. Many IDEs will not automatically reload generated sources for code completion unless either explicitly told to reload OR if the maven pom.xml file is changed. Thus, this value is by default set to ${basedir}/pom.xml. It's usually harmless to keep this enabled.

  • skipTouch disables touchFile. Defaults to false.

  • addAsSources will add the outputDirectory to maven as sources to be compiled. Defaults to true.

  • addAsTestSources will adds the outputDirectory to maven as test sources to be compiled. Defaults to false. If true, this is evaluated before addAsSources and effectively tells maven to compile your templates as test code.

The following properties are also supported, but it's important to understand these are essentially passthrough overrides to the parser and they all default to Rocker's default value.

  • javaVersion is the Java version you'd like your templates compile & runtime compatible with. Defaults to the Java version of the JVM executing maven (e.g. "1.8").

  • optimize determines if hot reloading support will be removed from the generated templates. False by default.

  • extendsClass is the class that all template implementations should extend. Useful for application-specific intermediate classes that you'd like all templates to extend. Defaults to Rocker's default.

  • extendsModelClass is the class that all template models should extend. Useful for application-specific intermediate classes that you'd like all template models to extend. Defaults to Rocker's default.

  • discardLogicWhitespace determines whether whitespace in templates that is determined to be only a part of logic/control blocks should be discarded. Helps make rendered content look more professional, while still keeping much of your formatting intact. Defaults to Rocker's default.

  • targetCharset is the target charset for template output. Defaults to Rocker's default.

  • suffixRegex is the regular expression to use to find templates to parse. Defaults to Rocker's default.


Thanks to @victory and @mnlipp for contributing the gradle plugin. @etiennestuder also had an alternative Gradle plugin you may want to consider as well. Rocker's gradle plugin is published to Just add the following to your build script:

plugins {
  id "com.fizzed.rocker" version "0.24.0"

sourceSets {
    main {
        rocker {

rocker {
    // (All settings are shown with their defaults)
    // Skips building templates all together
    skip false
    // Base directory for generated java sources, actual target is sub directory 
    // with the name of the source set. The value is passed through project.file(). 
    outputBaseDirectory = "$buildDir/generated-src/rocker"
    // Base directory for the directory where the hot reload feature 
    // will (re)compile classes to at runtime (and where `rocker-compiler.conf`
    // is generated, which is used by RockerRuntime.getInstance().setReloading(true)).
    // The actual target is a sub directory with the name of the source set. 
    // The value is passed through project.file().
    classBaseDirectory = "$buildDir/classes"
    failOnError true
    skipTouch true
    // must not be empty when skipTouch is equal to false
    touchFile ""
    javaVersion '1.8'
    extendsClass null
    extendsModelClass null
    optimize null
    discardLogicWhitespace null
    targetCharset null
    suffixRegex null
    postProcessing null

Create first template

The template syntax is described in detail below, but for now create a new file in ${templateDirectory}/views/HelloWorld.rocker.html

 Example of hello world
@args (String message)

Hello @message!

Use compiled template

Time to compile your project and starting using the template. You can call it from java like so:

static public void main(String[] args) {

    String output = views.HelloWorld


Use optimized output

Rocker is heavily optimized (by default) to output templates as byte arrays. The default RockerOutput a template will render to is of the type com.fizzed.rocker.runtime.ArrayOfByteArraysOutput. This is an excellent choice for byte arrays or asynchronous IO. However, the framework has the capability for optimized rendering to Strings (or other custom outputs).

To efficiently render to a String:

import com.fizzed.rocker.runtime.StringBuilderOutput;

static public void main(String[] args) {

    StringBuilderOutput output = views.HelloWorld

    String text = output.toString();


To efficiently render to an OutputStream:

import com.fizzed.rocker.runtime.OutputStreamOutput;

static public void main(String[] args) throws Exception {

    final OutputStream os = new FileOutputStream(new File("test"));

    OutputStreamOutput output = views.HelloWorld
        .render((contentType, charsetName) -> new OutputStreamOutput(contentType, os, charsetName));


Please note that if there is an exception during the render the OutputStream would have a partial template rendered (up to the point of the exception). In most cases it would be better to render to the default com.fizzed.rocker.runtime.ArrayOfByteArraysOutput and write its buffer of byte arrays out directly to your OutputStream.

Other demos?

There are numerous demos of Rocker in action. From parsing templates into a model to asynchronously sending results in an HTTP server. This project uses Blaze to help script tasks. Run the following for a complete list:

java -jar blaze.jar -l


Copyright (C) 2015 Fizzed, Inc.

This work is licensed under the Apache License, Version 2.0. See LICENSE for details.