Using FreeMarker templates (FTL)- Tutorial

FreeMarker Tutorial. This tutorial explains how can you define FreeMarker templates and how can you generate output based on these templates. It also demonstrates the usage of macros.

1. Introduction to FreeMarker

FreeMarker is a Java-based template engine which can be used in stand-alone or servlet-based Java programs.java

In FreeMarker you define templates, which are text files that contain the desired output, except that they contain placeholders like ${name}, and even some logic like conditionals, loops, etc. In your Java program you supply the actual values for these placeholders and the final output is generated based on this input.web

The input of templates is a bunch of named variables that you usually provide as a Map<String, Object> (the Map entries will be the variables) or as a JavaBean (the JavaBean properties will be the variables). The variable values can be simple strings, numbers and such primitive values, but also lists, maps, or arbitrary Java objects whose methods you can call from the template. Note that when accessing JavaBean properties, myObject.myProperty syntax should be used instead ofmyObject.getMyProperty().api

The output of templates is written into a Writer that you provide, so it can go into a HTTP response (for dynamic web pages), into a local file, into a String, etc.app

It is configurable from where FreeMarker reads the templates; commonly used options are loading from a file-system directory, from the class-path, from the servlet context ( WEB-INF/templates or such), or even from a database table. It’s also possible to "load" templates directly from String objects.less
2. Installation of FreeMarker
To use FreeMarker download the latest version of it from the following webpage and add it to the classpath of your Java project.eclipse
http://freemarker.org/freemarkerdownload.html
3. Eclipse Integration
FreeMarker code completion and syntax highlighting is part of the JBoss Tools. Add the following update site to your Eclipse installation via Help ▸ Install New Software…ide
http://download.jboss.org/jbosstools/updates/stable/kepler/
4. Basic example
Create a new Java project called com.vogella.freemarker.first. Create a new folder called lib and add the Freemarker library to it. Add this library to the classpath for your project.oop

If you don’t know how to achieve that, please see the Eclipse IDE Tutorial for instructions on the required steps.flex
Create a new folder called templates inside the folder of the com.vogella.freemarker.first package. Inside that, create the following file with name helloworld.ftl.
<html>
<head>
  <title>${title}
</head>
<body>
  <h1>${title}</h1>

  <p>${exampleObject.name} by ${exampleObject.developer}</p>

  <ul>
    <#list systems as system>
      <li>${system_index + 1}. ${system.name} from ${system.developer}</li>
    </#list>
  </ul>

</body>
</html>
Create the following class which demonstrates the usage of Java objects in templates.
package com.vogella.freemarker.first;

public class ValueExampleObject {

        private String name;
        private String developer;

        public ValueExampleObject(String name, String developer) {
                this.name = name;
                this.developer = developer;
        }

        public String getName() {
                return name;
        }

        public String getDeveloper() {
                return developer;
        }

}
Create the following class which creates the input for this template and creates the output.
package com.vogella.freemarker.first;

import java.io.File;
import java.io.FileWriter;
import java.io.OutputStreamWriter;
import java.io.Writer;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Locale;
import java.util.Map;

import freemarker.template.Configuration;
import freemarker.template.Template;
import freemarker.template.TemplateExceptionHandler;
import freemarker.template.Version;

public class MainTest {

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

                // 1. Configure FreeMarker
                //
                // You should do this ONLY ONCE, when your application starts,
                // then reuse the same Configuration object elsewhere.

                Configuration cfg = new Configuration();

                // Where do we load the templates from:
                cfg.setClassForTemplateLoading(MainTest.class, "templates");

                // Some other recommended settings:
                cfg.setIncompatibleImprovements(new Version(2, 3, 20));
                cfg.setDefaultEncoding("UTF-8");
                cfg.setLocale(Locale.US);
                cfg.setTemplateExceptionHandler(TemplateExceptionHandler.RETHROW_HANDLER);

                // 2. Proccess template(s)
                //
                // You will do this for several times in typical applications.

                // 2.1. Prepare the template input:

                Map<String, Object> input = new HashMap<String, Object>();

                input.put("title", "Vogella example");

                input.put("exampleObject", new ValueExampleObject("Java object", "me"));

                List<ValueExampleObject> systems = new ArrayList<ValueExampleObject>();
                systems.add(new ValueExampleObject("Android", "Google"));
                systems.add(new ValueExampleObject("iOS States", "Apple"));
                systems.add(new ValueExampleObject("Ubuntu", "Canonical"));
                systems.add(new ValueExampleObject("Windows7", "Microsoft"));
                input.put("systems", systems);

                // 2.2. Get the template

                Template template = cfg.getTemplate("helloworld.ftl");

                // 2.3. Generate the output

                // Write output to the console
                Writer consoleWriter = new OutputStreamWriter(System.out);
                template.process(input, consoleWriter);

                // For the sake of example, also write output into a file:
                Writer fileWriter = new FileWriter(new File("output.html"));
                try {
                        template.process(input, fileWriter);
                } finally {
                        fileWriter.close();
                }

        }
}
5. Useful FTL tricks
5.1. Reuse common template fragments

When you find yourself copy-pasting common parts between templates a lot, you should probably use macros.
Continuing our last example, create a new folder called lib inside the templates directory, and there create a file called utils.ftl, with this content:
<#macro page>
  <html>
  <head>
    <title>${title}
  </head>
  <body>
    <h1>${title}</h1>

    <#-- This processes the enclosed content:  -->
    <#nested>
  </body>
  </html>
</#macro>

<#macro otherExample p1 p2>
  <p>The parameters were: ${p1}, ${p2}</p>
</#macro>
Now you can simplify helloworld.ftl like this:
<#import "lib/utils.ftl" as u>

<@u.page>
  <p>${exampleObject.name} by ${exampleObject.developer}</p>

  <ul>
    <#list systems as system>
      <li>${system_index + 1}. ${system.name} from ${system.developer}</li>
    </#list>
  </ul>

  <#-- Just another example of using a macro: -->
  <@u.otherExample p1=11 p2=22 />
</@u.page>
Another way of reusing template fragments is moving the common fragment into its own ftl file. Then just insert it with <#include "lib/myfragment.ftl">. This is less flexible than macros, but simpler in concept: it mimics copy-pasting.
5.2. Variables
You can define and assign content to variables inside the FTL files for easy reuse.
<#assign var_link = "http://www.vogella.com/people/larsvogel.html">

<a href="${var_link}">About Lars Vogel</a>
5.3. Handling null/undefined values
FreeMarker requires you to provide an explicit default for variables, so avoid values that are null or undefined:

<p>Color: ${color!'N/A'}</p>


<#if color??>
  <p>Color: ${color}</p>
</#if>
5.4. Escape
When generating HTML, it’s important to escape <, &, etc. in values that were not meant to store HTML and can contain these problematic characters. You can apply such escaping like ${message?html}. You can also ask FreeMarker to add ?html to all ${} -s in a section like this:
<#escape x as x?html>
  <p>Sender: ${from}
  <p>Title: ${title}
  <p>Message: ${body}
</#escape>
It’s important to understand that #escape only affects the ${} bits that are inside the enclosed section in the template file when you look at it in a text editor. That means, ${} embracements which are in other templates or macros called from there, won’t be affected.

轉載自：http://www.vogella.com/tutorials/FreeMarker/article.html