Grasping Thumbnails in Alfresco 3

With Alfresco 3 (both Labs and Enterprise), Alfresco added a new thumbnail service. It isn’t documented too well yet so I thought I’d write up a quick example.

What is it

The Thumbnail Service is used to create alternate renditions of objects. Typically, those alternate renditions are small images called “thumbnails”. You can see a working application of the thumbnail service if you take a look at Alfresco Share’s document library. When you upload a document, the thumbnail service is invoked, and a small image is shown next to each item in the list.

Where thumbnails live

Like everything else in Alfresco, thumbnails are stored as nodes. Nodes are instances of cm:thumbnail and are stored as children of the object they represent. (You can see this for yourself by looking at the thumbnailed object in the node browser). Objects can have any number of thumbnails. This lets you have thumbnails of different sizes and mime types, for example.

Once Alfresco generates a thumbnail for an object, the object will have the cm:thumbnailed aspect applied to it so it is easy to find or filter objects based on whether or not they have at least one thumbnail.

Thumbnail definitions & thumbnail names

Every thumbnail has a thumbnail definition. The thumbnail definition keeps track of things like the mime type, transformation options, placeholder path, and thumbnail name. The thumbnail name uniquely identifies the thumbnail definition in the thumbnail registry. When you want to generate or display a thumbnail for an object, you must specify the name. For example, given a thumbnail definition named “scImageThumbnail”, you could use JavaScript to create a thumbnail for an object by calling the “createThumbnail” method on a ScriptNode like this:

document.createThumbnail("scImageThumbnail", true);

The first argument is the name of the thumbnail definition. The second argument says the thumbnail should be generated asynchronously.

Registering thumbnail definitions

The thumbnail registry needs to know about your thumbnail definitions. The out-of-the-box thumbnails are registered in the thumbnail-service-context.xml file. I don’t see a clean way to extend that without repeating the definitions, so in my example, I wrote a bean that calls the Thumbnail Registry and registers the custom thumbnail definitions provided in the Spring context file:


public class ThumbnailRegistryBootstrap {
  private ThumbnailService thumbnailService;
  private List<ThumbnailDefinition> thumbnailDefinitions;
  private Logger logger = Logger.getLogger(ThumbnailRegistryBootstrap.class);

public void init() {
  ThumbnailRegistry thumbnailRegistry = thumbnailService.getThumbnailRegistry();
    for (ThumbnailDefinition thumbDef : thumbnailDefinitions) {
      logger.info("Adding thumbnail definition:" + thumbDef.getName());
      thumbnailRegistry.addThumbnailDefinition(thumbDef);
    }
}

public void setThumbnailService(ThumbnailService thumbnailService) {
  this.thumbnailService = thumbnailService;
}

public void setThumbnailDefinitions(
  List<ThumbnailDefinition> thumbnailDefinitions) {
  this.thumbnailDefinitions = thumbnailDefinitions;
}

}

So this class will add all of my thumbnail definitions to the thumbnail registry. The class and the definitions are configured in a Spring context file. The config for a single thumbnail called “scImageThumbnail” which is a PNG 100 pixels high and retains the original aspect ratio of the image would be:


<bean id="someco.thumbnailRegistry"
 class="com.someco.thumbnails.ThumbnailRegistryBootstrap"
 depends-on="ThumbnailService"
 init-method="init">
  <property name="thumbnailService" ref="ThumbnailService" />
  <property name="thumbnailDefinitions">
    <list>
      <bean class="org.alfresco.repo.thumbnail.ThumbnailDefinition">
        <property name="name" value="scImageThumbnail" />
        <property name="mimetype" value="image/png"/>
        <property name="transformationOptions">
          <bean  class="org.alfresco.repo.content.transform.magick.ImageTransformationOptions">
            <property name="resizeOptions">
              <bean class="org.alfresco.repo.content.transform.magick.ImageResizeOptions">
              <property name="height" value="100"/>
              <property name="maintainAspectRatio" value="true"/>
              <property name="resizeToThumbnail" value="true" />
            </bean>
          </property>
        </bean>
     </property>
     <property name="placeHolderResourcePath" value="alfresco/extension/thumbnail/thumbnail_placeholder_scImageThumbnail.png" />
   </bean>
 </list>
 </property>
</bean>

The placeholder is a graphic that the thumbnail service can return as the thumbnail if the thumbnail for a given node has not been generated. In my example I just copied one of the out-of-the-box placeholders and renamed it but you could use anything you want there.

Example

I built a simple example to show how this works. Here is a screencast that shows it running or you can download the source and build it yourself.

In the example, a simple form is presented to allow a file to be uploaded. The form posts to a web script which creates a new node using the file provided. The form GET and POST web scripts are essentially the “helloworldform” web scripts from the Alfresco Developer Guide.

The “image list” is a simple GET web script that queries the folder where the images are uploaded to and writes out a list of image tags. The interesting thing to note here is the URL that’s used:

${url.serviceContext}/api/node/workspace/SpacesStore/${image.id}/content/thumbnails/scImageThumbnail?ph=true&c=queue

That URL is an out-of-the-box web script that returns the specified thumbnail for a given node reference. In my example I’m using the “ph” and “c” arguments. The “ph” argument says whether or not the placeholder image should be returned if the thumbnail does not exist. The “c” argument says that if a thumbnail doesn’t exist, queue a request for thumbnail creation. (Note that the descriptor says the queue create argument is “qc” but if you look at the controller source you’ll see it is actually just “c”. I’ll check to see if there’s a Jira on that).

When you add a new image and then go to the image list you’ll see the placeholder graphic. Behind the scenes, a thumbnail creation request has been queued. If you refresh the page, the thumbnail should show up because Alfresco has had a chance to generate it. If you wanted to queue the request when the node is created, you could either create a rule on the folder that holds the images, or you could add a call to “createThumbnail” in the upload POST web script controller, as shown earlier. (I’ve got an example of that commented out in the source).

That’s it

Hopefully this has given you some insight into the new thumbnail service in Alfresco. If you want to play with it yourself, you can download the source for the example and build it with Ant (make sure you set build.properties to match your environment first) by running “ant deploy”. Make sure you’ve got ImageMagick installed on your Alfresco server–the thumbnail service depends on it. You’ll also need the SDK to compile the registry bootstrap class. If you want to see what the thumbnail service is actually doing you’ll need the Alfresco source. None of the thumbnail source is included in the source code that currently accompanies the SDK.

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