FreeMarker is a template Java-based template language engine that is a great alternative to JSP. FreeMarker is ideal for situations where your action results can possibly be loaded from outside a Servlet container. For example, if you wished to support plugins in your application, you might wish to use FreeMarker so that the plugins could provide the entire action class and view in a single jar that is loaded from the classloader.
For more information on FreeMarker itself, please visit the FreeMarker website.
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The framework utilizes FreeMarker because
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the engine includes strong error reporting, built-in internationalization and powerful macro libraries.
Support is also included for Velocity templates. For a comparison of Velocity vs FreeMarker see here.
Getting Started
Getting started with FreeMarker is as simple as ensuring all the dependencies are included in your project's classpath. Typically, the only dependency is the {{freemarker.jar}. Other than that, struts-default.xml already configures the FreeMarker Result needed to process your application's templates.
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Then in test-success.ftl:
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Where name is a property on your action. That's it! Read the rest of this document for details on how templates are loaded, variables are resolved, and tags can be used.
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Assuming there's an attribute with name myApplicationAttribute in the Application scope.
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or
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Session Scope Attribute
Assuming there's an attribute with name mySessionAttribute in the Session scope.
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or
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Request Scope Attribute
Assuming there's an attribute with name 'myRequestAttribute' in the Request scope.
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or
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Request Parameter
Assuming there's a request parameter myParameter (eg. http://host/myApp/myAction.action?myParameter=one).
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or
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Context parameter
Assuming there's a parameter with the name myContextParam in framework context.
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or
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Template Loading
The framework looks for FreeMarker templates in two locations (in this order):
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In addition, you can specify a location (directory on your file system) through the templatePath or TemplatePath context variable (in the {{ web.xml)}. If a variable is specified, the content of the directory it points to will be searched first.
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This variable is currently NOT relative to the root of your application.
Variable Resolution
When using FreeMarker with the framework, variables are looked up in several different places, in this order:
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Note that the action context is looked up after the value stack. This means that you can reference the variable without the typical preceding has marker (#) like you would have to when using the JSP s:property tag. This is a nice convenience, though be careful because there is a small chance it could trip you up.
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The built-in variables that Struts-FreeMarker integration provides are:
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FreeMarker includes complete tag support. See the FreeMarker Tags documentation for information on how to use the generic Struts Tags provided by Struts. In addition to this, you can use any JSP tag, like so:
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Where mytag.tld is the JSP Tag Library Definition file for your tag library. Note: in order to use this support in FreeMarker, you must enable the JSPSupportServlet in web.xml documented as follows:-
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Tips and Tricks
There are some advanced features that may be useful when building Struts applications with FreeMarker.
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FreeMarker has built in support for formatting dates and numbers. The formatting rules are based on the locale associated with the action request, which is by default set in struts.properties but can be over-ridden using the I18n Interceptor. This is normally perfect for your needs, but it is important to remember that these formatting rules are handled by FreeMarker and not by the framework's Type Conversion support.
If you want the framework to handle the formatting according to the Type Conversion you have specified, you shouldn't use the normal ${...} syntax. Instead, you should use the property tag. The difference is that the property tag is specifically designed to take an OGNL expression, evaluate it, and then convert it to a String using any Type Conversion rules you have specified. The normal ${...} syntax will use a FreeMarker expression language, evaluate it, and then convert it to a String using the built in formatting rules.
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To extend the Freemarker support, develop a class that extends org.apache.struts2.views.freemarker.FreemarkerManager, overriding methods as needed, and plugin the class through the struts.properties:
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ObjectWrapper Settings
Once you get familiar with FreeMarker, you will find certain subtletieswith it that may become frustrating. The most common thing you'll likely run in to is the BeansWrapper provided by FreeMarker. If you don't know what this is, don't worry. However, if you do, know this:
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INLINE
Syntax Notes
As of FreeMarker 2.3.4, an alternative syntax is supported. This alternative syntax is great if you find that your IDE (especially IntelliJ IDEA) makes it difficult to work with the default syntax. You can read more about this syntax here.
Cache
You can enable FreeMarker cache mechanism by specifying below options in struts.xml:
<constant name="struts.freemarker.mru.max.strong.size" value="250" />- this option will be used by freemarker.cache.MruCacheStorage
<constant name="struts.freemarker.templatesCache.updateDelay" value="1800" />- default update cache interval (5 seconds)
<constant name="struts.freemarker.templatesCache" value="true" />- *DEPRECATED* this option will use a internal ConcurrentHashMap in FreemarkerTemplateEngine but not freemarker native cache
Setting devMode to true will disable cache and updateDelay immediately, but you can explicit specify these constants to enable cache even in devMode, see devMode
Incompatible Improvements
By default Struts is using FreeMarker in way to be backward compatible as much as possible but if you need to enable new features you can do it via freemarker.properties by defining incompatible improvements settings, ie.:
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You can also pass this setting via ServletContext <init-param/> (since Struts 2.5.13):
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This can impact your freemarker powered pages and Struts tags as well, so please careful test this change.