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Added by Felix Meschberger, last edited by Felix Meschberger on Nov 07, 2009  (view change)

Frequently Asked Questions

This page lists a series of common questions and answers. It is of course work in progress ...

RESTful API

How do I create a node by posting a json document to a URL?

At the moment, you cannot do this. (Soon to change as per SLING-1172!) Instead, each value must be a field in the request POST. For example, suppose you have the json document:

  {
    "greetings":"Hello, World!",
    "multi" : ["first","second"],
    "translations" : { "en": "Hello", "zh", "你好" }
  }

You would do a post such as:

curl -F"greetings=Hello, World!" -F"mult=first" -F"multi=second" -F"translations/en=Hello" -F"translations/zh=你好" http://admin:admin@localhost:8080/content/../../..

What so special about the 'content','apps' and '*' urls?

'apps' is reserved for matching scripts evaluated by sling.

The "*" url is used for POSTing to a child node.

By default, if a resource cannot be found from the root url, sling will try appending "content". For example, if you request the following non-existent resource:

http://localhost:8080/blog/first_post

Sling will look in:

http://localhost:8080/content/blog/first_post

Before returning a 404.

I posted a resource, where did it go?

Let's start by creating a resource:

curl -F"greetings=Hello, World" -F"translations/en=Hello" -F"translations/es=hola" http://admin:admin@localhost:8080/content/greet

We can now view the resource with:

curl http://admin:admin@localhost:8080/content/greet.json
{"greetings":"Hello, World","jcr:created":"Fri Nov 06 2009 16:26:23 GMT-0800","jcr:primaryType":"sling:Folder"}

Notice that the "greet" resource is a sling:Folder. Also notice that it's a little hard to read the result. Let's tidy it up:

curl http://admin:admin@localhost:8080/content/greet.tidy.json
{
  "greetings": "Hello, World",
  "jcr:created": "Fri Nov 06 2009 16:26:23 GMT-0800",
  "jcr:primaryType": "sling:Folder"
}

But where did our translations go? To get them, we have to request 2 nodes down into the tree:

curl http://admin:admin@localhost:8080/content/greet.tidy.2.json
{
  "greetings": "Hello, World",
  "jcr:created": "Fri Nov 06 2009 16:26:23 GMT-0800",
  "jcr:primaryType": "sling:Folder",
  "translations": {
    "en": "Hello",
    "jcr:created": "Fri Nov 06 2009 16:26:23 GMT-0800",
    "es": "hola",
    "jcr:primaryType": "sling:Folder"
  }
}

However, if we try and get the resource without an extension, we get nothing found. This is because we're requesting a folder, so sling tries to find either an index.html or return a directory list, just like a normal directory on a webserver.

To fix this, we can use a script and a sling resource type. Let's update our greeting document:

curl F"sling:resourceType=greeting" -F"greetings=Hello, World" -F"translations/en=Hello" -F"translations/es=hola" http://admin:admin@localhost:8080/content/greet

Then we'll post a simple esp script to apps:

GET.esp
<html>
  <head><title></title></head>
  <body>
    <h1><%= currentNode.greetings %></h1>
  </body>
</html>

curl -X MKCOL http://admin:admin@gandalf.local:8080/apps/greeting
curl -T GET.esp http://admin:admin@localhost:8080/apps/greeting/GET.esp

Now you should be able to see an HTML version of the resource at http://localhost.local:8080/content/greet. This script matches the sling:resourceType we set and the HTTP method we used. Note that resourceType matches must be exact.

Scripts and Servlets

How do I find out why a given script or servlet is preferred to another when processing a request?

See SLING-580, the SlingServletResolver class logs detailed information (at the DEBUG level) to indicate in which order the candidate scripts and servlets are considered for processing a request.

How to replace the default json renderer (for example) with my own?

The JSON rendering is done by the DefaultGetServlet, which is hardwired to use the JsonRendererServlet for .json extensions.

If a servlet or script is registered for the sling/servlet/default resource type, but with a specific sling.servlet.extensions property (set using the @scr.property annotation), it will take over and process GET requests which have a .json extension and no specific servlet or script.

As scripts and servlets are equivalent in Sling, the simplest way to do this to create a script at apps/sling/servlet/default/json.esp, for example.

The same logic applies to other extensions (html, txt, ...) handled by the DefaultGetServlet.

How to execute scripts directly?

The following servlet (inspired from the Sakai ScriptRunner) executes scripts directly when called with the script URL and a .runscript selector (for example /foo/myscript.esp.runscript.html).

Note that this can be insecure: if users are allowed to upload scripts, they can execute any code supported by Sling, so use that only if you know what you're doing.

ScriptRunnerServlet
/* @scr.component
 *    immediate="true" label="ScriptRunner"
 *    description="Runs scripts using the .runscript selector"
 *
 * @scr.service
 *    interface="javax.servlet.Servlet"
 * @scr.property
 *    name="sling.servlet.resourceTypes"
 *    value="sling/servlet/default"
 * @scr.property
 *    name="sling.servlet.selectors"
 *    value="runscript"
 * @scr.property
 *    name="sling.servlet.methods"
 *    value="GET"
 */
public class ScriptRunnerServlet extends SlingAllMethodsServlet {

protected void doGet(
  SlingHttpServletRequest req,
  SlingHttpServletResponse resp)
  throws ServletException, IOException {
    Servlet s = req.getResource().adaptTo(Servlet.class);
    if(s == null) {
      throw new
        ServletException("Resource "
        + req.getResource()
        + " does not adapt to a Servlet");
    }
    s.service(req, resp);
  }
}

How do I create a new script engine?

As I write this, we don't have documentation on how to create more script engines, but that's not too hard to do if you take one of the simple existing engines as an example.

The JRuby engine for example, implemented in the scripting/ruby module, is built out of two simple classes, one that inherits from AbstractSlingScriptEngine, and one that inherits from AbstractScriptEngineFactory. The code is very simple, it's basically only a wrapper around the JRuby engine, that adapts it for Sling.

If creating a script engine, don't forget the META-INF/services/javax.script.ScriptEngineFactory file, which lets scripting subsystem know about the factory class, so that the engine is activated when the bundle that contains it is loaded.

Once the script engine is created, loading its bundle into Sling should be enough to activate scripts having the extension defined by the engine. If several scripts are found with the same name but different script extensions, the priority in selecting them is currently unspecified.

The javascript and freemarker engines source code also shows how to add automated tests to a script engine, including making a JCR repository available to the tests.

To go further, the javascript and jsp script engines are the most interesting ones to study.

The javascript engine provides wrappers to make it easier to access JCR and Sling objects from server-side javascript, and also uses a clever EspReader (sorry it's not that ESP) to convert .esp scripts to plain javascript code.

The jsp engine is actually a compiler, so it can be an interesting example if your language needs or can benefit from compiling.

Working with bundles

Is there an easy way to update bundles in a running installation during development?

The Sling Maven Plugin provides an install goal which is able to install or update a bundle in a running Sling application (if the Sling web console is deployed). If the plugin properties are configured accordingly you can just mvn clean package org.apache.sling:maven-sling-plugin:install and the bundle is uploaded.

You can use the settings.xml to set the url to your Sling application. See the Sling Maven Plugin for more information.

Classloading issues

Accessing Classes from the Environment

Mostly when using the Sling Web Application, that is running Sling inside a web application deployed into some servlet container, you might want to share classes between the servlet container and Sling. Some examples of such sharing are:

  1. Accessing EJB from the Application Server
  2. Sharing classe with another web application such as a Jackrabbit instance
  3. Using other container features

For such cases the OSGi Core Specification provides a functionality to declare such class sharing. The functionality is defined in terms of two Framework properties org.osgi.framework.system.packages and org.osgi.framework.bootdelegation:

  1. org.osgi.framework.bootdelegation - All classes matching any entry in this list are always loaded from the parent class loader and not through the OSGi framework infrastructure. This property is a comma separated list of package names. A package name may be terminated by a wildcard character such that any package starting with the list entry matches the entry and thus will be used from the parent class loader.
  2. org.osgi.framework.system.packages - Additional package declarations for packages to be exported from the system bundle. This property is a simple package declaration list just like any Export-Package manifest header. In a sense the org.osgi.framework.system.packages property may be seen as the Export-Package manifest header of the system bundle. Namely these entries may not contain wildcards (as is allowed for the bootdelegation property) and may contain directives and attributes such as the uses directive and the version attribute. It is recommended to provide this additional information to help in resolving the bundles. The OSGi Core Specification even prescribes the use of the uses directive.

The problem with the org.osgi.framework.bootdelegation property is, that it completely bypasses any bundle import wirings and just asks the parent classloader. Such situations are not easily recognizable. Therefore the Sling Console will be enhanced to mark any package import which matchs an entry in the org.osgi.framework.bootdelegation appropriately (SLING-148).

Also note, that any package listed as an import in a bundle must be resolveable for the bundle resolve. The import resolution process does not take the org.osgi.framework.bootdelegation configuration into account. This means, that regardless of whether a package is listed in the org.osgi.framework.bootdelegation property or not, if the package is listed as a required import in the Import-Package header, it must be exported by some other bundle.

How are the sling.bootdelegation properties used ?

Sling uses the sling.bootdelegation.class property name prefix to define lists of classes that must be added to the org.osgi.framework.bootdelegation property. In case you want to have a closer look, this is implemented in the org.apache.sling.launcher.app.Sling.resolve() method.

If a Sling property name starts with the sling.bootdelegation.class. prefix, the list of packages defined as the property value is appended to the org.osgi.framework.bootdelegation property, but only if the fully qualified class taken from the rest of the property name exists in the parent class loader.

Here's an example, from the jcr-client.properties file:

sling.bootdelegation.class.javax.jcr.Repository = \
 javax.jcr, \
 javax.jcr.lock, \
 javax.jcr.nodetype, \
 javax.jcr.observation, \
 javax.jcr.query, \
 javax.jcr.util, \
 javax.jcr.version

This means that, if the javax.jcr.Repository class is available in the parent class loader, all packages listed will be added to the org.osgi.framework.bootdelegation, making the corresponding classes available to OSGi bundles.

If the property name does not start with this sling.bootdelegation.class. property, the list of packages is just appended to the org.osgi.framework.bootdelegation property.

How does Sling support the org.osgi.framework.system.packages Property ?

Currently extending the org.osgi.framework.system.packages property in a Sling configuration file is only possibly by setting the org.apache.sling.launcher.system.packages property. The value of this property, which must start with a comma, is just appended to the org.osgi.framewrok.system.packages property.

A more elaborate support as is supported for the org.osgi.framework.bootdelegation Property has been prepared (SLING-147).

Should the org.osgi.framework.bootdelegation or the org.osgi.framework.system.packages Property be used ?

So, what mechanism should be used ? The answer is, that it depends.

Most of the time, you will want to use the org.osgi.framework.system.packages property. Because this property ensures that you will allways benefit from the normal class resolution mechanism through package imports and exports.

This allows creating the bundles normally by having the package import lists being built according to the packages used by the bundle classes. For example you may use the Apache Felix Maven Bundle Plugin to build your OSGi bundles and the imports are automatically calculated (by default).

The drawback of this method is, that there may be bundles in your system, which export packages also listed in the org.osgi.framework.system.packages property. Depending on the export version, the wrong package may be bound. So to prevent such collisions you should not install such bundles.

An example of such a declaration is the Servlet API packages (javax.servlet, javax.servlet.http and javax.servlet.resources). These packages are imported into the OSGi framework by the SlingServlet of the launcher/webapp project as part of the org.osgi.framework.system.packages property. To have this work correctly, no bundle should export the respective packages. In the case of Sling, this means, the org.apache.felix.commons.sling-api bundle must not be installed.

If on the other hand you cannot prevent the installation of such bundles and hence the export of the respective packages, you might want to set the org.osgi.framework.bootdelegation property conditionally as described above in the answer to how this property is supported in Sling. This ensures the property is only set, if the classes are actually available. This should be used as a fall back only, if the org.osgi.framework.system.packages method does not work.

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