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Adding an Annotation and a Filter to customize Tapestry's page rendering |
Meta-Programming Page Content
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By hooking into the component class transformation pipeline, we can change an annotation into meta-data that can be accessed by a filter.
Defining the Annotation
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package com.fnord.annotations; import java.lang.annotation.Documented; import java.lang.annotation.ElementType; import java.lang.annotation.Inherited; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; /** * Marker annotation for pages that should not allow framing. */ @Target({ ElementType.TYPE }) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited public @interface ForbidFraming { } |
This annotation presence is all that's needed; there aren't any additional attributes to configure it.
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Our key is just "forbid-framing", with values "true" and "false". The default is "false".
Defining the Constant
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package com.fnord; import org.apache.tapestry5.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"; } |
Setting the Meta-Data Default
Next, we'll create a module just for the logic directly related to framing. In the module, we'll define the default value for the meta-data.
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package com.fnord.services.forbidframing; import org.apache.tapestry5.ioc.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"); } } |
Mapping the Annotation
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 had attributes, we would have provided an implementation of MetaDataExtractor that examined those attributes to set the meta-data value. Since it has no attributes, 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()
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Everything comes together in the filter:
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package com.fnord.services.forbidframing; import org.apache.tapestry5.MarkupWriter; import org.apache.tapestry5.ioc.annotations.Inject; import org.apache.tapestry5.services.MarkupRenderer; import org.apache.tapestry5.services.MarkupRendererFilter; import org.apache.tapestry5.services.MetaDataLocator; import org.apache.tapestry5.services.RequestGlobals; import org.apache.tapestry5.services.Response; import org.apache.tapestry5.services.javascript.InitializationPriority; import org.apache.tapestry5.services.javascript.JavaScriptSupport; import com.fnord.FnordSymbols; public class ForbidFramingFilter implements MarkupRendererFilter { @Inject private RequestGlobals requestGlobals; @Inject private MetaDataLocator metaDataLocator; @Inject private Response response; @Inject private JavaScriptSupport jsSupport; public void renderMarkup(MarkupWriter writer, MarkupRenderer renderer) { String pageName = requestGlobals.getActivePageName(); boolean forbidFraming = metaDataLocator.findMeta( FnordSymbols.FORBID_FRAMING, pageName, boolean.class); if (forbidFraming) { response.setHeader("X-Frame-Options", "DENY"); jsSupport.addScript(InitializationPriority.IMMEDIATE, "Fnord.popOutOfFrame();"); } renderer.renderMarkup(writer); } } |
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); } } |
Conclusion
That's it: with the above code, simply adding the @ForbidFraming annotation to a page will add the response header and associated JavaScript; no inheritance hassles. This basic pattern can be applied to a wide range of cross-cutting concerns, such as security, transaction management, logging, or virtually any other kind of situation that would normally be solved with inheritance or ugly boilerplate code.
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The code in this example was designed for Tapestry version 5.2 and later. |