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Again, this could be done by having a specific base-class that included a beginRender()
method, but I think you'll see that the meta-programming approach is nearly as easy and much more flexible.
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package com.fnord;
import org.apache.tapestry5.http.services.BaseURLSource;
import com.fnord.annotations.ForbidFraming;
public class FnordSymbols {
/**
* Meta-data key; when true, MarkupRendererFilter will inject some extra
* content into the response to enforce that the content may not be framed
* (i.e., "stolen").
*
* @see ForbidFraming
*/
public static final String FORBID_FRAMING = "forbid-framing";
}
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package com.fnord.services.forbidframing; import org.apache.tapestry5.ioccommons.MappedConfiguration; import org.apache.tapestry5.ioc.annotations.Contribute; import org.apache.tapestry5.ioc.services.FactoryDefaults; import org.apache.tapestry5.ioc.services.SymbolProvider; import com.fnord.FnordSymbols; public class ForbidFramingModule { @Contribute(SymbolProvider.class) @FactoryDefaults public static void setupForbidFramingDefault( MappedConfiguration<String, String> configuration) { configuration.add(FnordSymbols.FORBID_FRAMING, "false"); } } |
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Most of the work has already been done for us: we just have to make a contribution to the MetaWorker service, which is already plugged into the component class transformation pipeline. MetaWorker spots the annotations we define and uses a second object, a MetaDataExtractor we provide, to convert the annotation into a meta-data value.
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@Contribute(MetaWorker.class) public static void mapAnnotationsToMetaDataValue( MappedConfiguration<Class, MetaDataExtractor> configuration) { configuration .add(ForbidFraming.class, new FixedExtractor<ForbidFraming>( FnordSymbols.FORBID_FRAMING)); } |
If the ForbidFraming annotation has had attributes, we would have provided an implementation of MetaDataExtractor that examined those attributes to set the meta-data value. Since it has no annotationsattributes, the FixedExtractor class can be used. The argument is the meta-data key, and the default value is "true".
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The work we ultimately want to do occurs when rendering a page. Tapestry defines a pipeline for that overall process. The point of a pipeline is that we can add filters to it. We'll add a filter that checks for the meta-data key and adds the response header and JavaScript.
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We contribute into the pipeline; the order is important: since the filter will need to write JavaScript, it must be added after the built-in filter that provides the JavaScriptSupport environmental object.
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@Contribute(MarkupRenderer.class) public static void addFilter( OrderedConfiguration<MarkupRendererFilter> configuration) { configuration.addInstance("ForbidFraming", ForbidFramingFilter.class, "after:JavascriptSupport"); } |
How do you know what filters are built-in and where to add your own? The right starting point is the JavaDoc for the method of TapestryModule that contributes the base set: contributeMarkupRenderer()
Implementing the Filter
Everything comes together in the filter:
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There's a bit going on in this short piece of code. The heart of the code is the MetaDataLocator service; given a meta-data key and a page name, it can not only extract the value, but then coerce it to a desired type, all in one go.
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This code makes one assumption: that the fnord application's Layout component added fnord.js to every page. That's necessary for the JavaScript that's added:
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Fnord = { popOutOfFrame : function() { if (top != self) top.location.replace(location); } } |
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Note |
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The code in this example was designed for Tapestry version 5.2 . Some names were changed to maintain the anonymity of the client (whose project is still secret at the time of writing)and later. |