Page History

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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.

  @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.

  @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:

Fnord = {
  popOutOfFrame : function() {
    if (top != self)
      top.location.replace(location);
  }
}

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