Default GET and HEAD servlets

Sling provides a number of default GET and HEAD servlets, in the org.apache.sling.servlets.get bundle.

This provides useful functionality out of the box: JSON rendering of content for example, usually does not require custom code.

This page provides an overview of these default servlets.

Currently, only the DefaultGetServlet has configuration parameters. Those are found at /system/console/configMgr/org.apache.sling.servlets.get.DefaultGetServlet on a standard Sling setup, and should be self-explaining. One common use is to disable some of the default renderings listed below, as they might not be useful or desired on production systems.

If not otherwise mentioned for specific renderings the servlet does not support conditional requests as specified by RFC 7232 (i.e. the If-.... request headers are disregarded and the response will neither contain ETag nor Last-Modified headers).

Default renderings

Default JSON rendering

Adding a .json extension to a request triggers the default Sling GET servlet in JSON mode, unless a more specific servlet or script is provided for the current resource.

This servlet currently supports the following selectors:

• .tidy causes the JSON output to be formatted
• .harray causes child nodes to be output as arrays instead of objects, to preserve their order (requires org.apache.sling.servlets.get V2.1.10)
• A numeric value or .infinity as the last selector selects the desired recursion level

Note that the number of elements is limited by a configurable value, see the DefaultGetServlet configuration for more info.

Default HTML rendering

In a similar way, adding a .html extension to a request triggers the default Sling GET servlet in HTML mode. That rendering just dumps the current node values in a readable way, but it's only really useful for troubleshooting.

Default text rendering

A basic text rendering is also provided if the request has a .txt extension, unless more specific servlets or scripts are provided.

Default XML rendering

Adding a .xml extension triggers the default XML rendering, once again unless a more specific script or servlet is registered for the current resource.

That XML rendering currently uses the JCR "document view" export functionality directly, so it only supports rendering resources that are backed by JCR nodes.

StreamRendererServlet

Whenever the request carries the extension .res or no extension at all, the resource's input stream is spooled to the servlet response (leveraging Resource.adaptTo(InputStream.class)). This servlet supports conditional requests (RFC 7232) based on the last-modified response header by evaluating the resource's modification date from Resource.getResourceMetadata().getModificationTime() and range requests (RFC 7233).

In case the underlying resource's InputStream is an ExternalizableInputStream instead a redirect towards its URI is triggered (SLING-7140).

RedirectServlet

The RedirectServlet handles the sling:redirect resource type, using the sling:target property of the resource to define the redirect target, and the sling:status property to define the HTTP status to use (default is 302).

This is not to be confused with the sling:redirect property used under /etc/map, which is described in Mappings for Resource Resolution

SlingInfoServlet

The SlingInfoServlet provides info on the current JCR session, for requests that map to JCR nodes.

It is available at /system/sling/info.sessionInfo by default, and supports .json and .txt extensions.

JCR Versions Support

The extensions created for SLING-848 and SLING-4318 provide some access to JCR version management features, along with the Sling POST Servlet versioning-related features.

Here's an example that demonstrates this.

Use the org.apache.sling.servlets.get.impl.version.VersionInfoServlet OSGi configuration to activate the VersionInfoServlet which supports the .V.json selector shown below. That servlet is disabled by default to make sure the configurable V selector doesn't interfere with existing applications.

First, create a versionable node and check it in:

curl -u admin:admin -Fjcr:mixinTypes=mix:versionable -Fmarker=A http://localhost:8080/vtest
curl -u admin:admin -F :operation=checkin http://localhost:8080/vtest

The VersionInfoServlet, triggered by the V selector with our default configuration, shows the initial versions state:

curl -s -u admin:admin http://localhost:8080/vtest.V.json
{
  "versions": {
    "jcr:rootVersion": {
      "created": "Tue Jan 23 2018 14:08:09 GMT+0100",
      "successors": [
        "1.0"
      ],
      "predecessors": [],
      "labels": [],
      "baseVersion": "false"
    },
    "1.0": {
      "created": "Tue Jan 23 2018 14:08:35 GMT+0100",
      "successors": [],
      "predecessors": [
        "jcr:rootVersion"
      ],
      "labels": [],
      "baseVersion": "true"
    }
  }
}    

Now, create two additional versions with a different marker value:

curl -u admin:admin -F :autoCheckin=true -F :autoCheckout=true -Fmarker=B http://localhost:8080/vtest
curl -u admin:admin -F :autoCheckin=true -F :autoCheckout=true -Fmarker=C http://localhost:8080/vtest

The VersionInfoServlet now shows all versions (output abbreviated):

curl -s -u admin:admin http://localhost:8080/vtest.V.json
{
  "versions": {
    "jcr:rootVersion": {
      "successors": [
        "1.0"
      ],
      "predecessors": []
    },
    "1.0": {
      "successors": [
        "1.1"
      ],
      "predecessors": [
        "jcr:rootVersion"
      ]
    },
    "1.1": {
      "successors": [
        "1.2"
      ],
      "predecessors": [
        "1.0"
      ]
    },
    "1.2": {
      "successors": [],
      "predecessors": [
        "1.1"
      ]
    }
  }
}

And the ;v= URI path parameter gives access to each version (output abbreviated):

curl -s "http://localhost:8080/vtest.tidy.json;v=1.0"
{
  "marker": "A",
  "jcr:frozenUuid": "a6fd966d-917d-49e2-ba32-e7f942ff3a0f",
  "jcr:uuid": "74291bc8-e7cb-4a71-ab3a-224ba234be0a"
}

curl -s "http://localhost:8080/vtest.tidy.json;v=1.1"
{
  "marker": "B",
  "jcr:frozenUuid": "a6fd966d-917d-49e2-ba32-e7f942ff3a0f",
  "jcr:uuid": "18b38479-a3fc-4a21-9cd4-89c44daf917d"
}

curl -s "http://localhost:8080/vtest.tidy.json;v=1.2"
{
  "marker": "C",
  "jcr:frozenUuid": "a6fd966d-917d-49e2-ba32-e7f942ff3a0f",
  "jcr:uuid": "3d55430b-2fa6-4562-b415-638fb6608c0e"
}

Streaming binaries using the default GET servlet

There are scenarios where it is useful to stream a binary resource using the default GET servlet. However, there is no API to select a specific servlet. We can still stream using the default GET servlet by taking advantage of the fact that it is also registered for the res extension. The code to do what would be:

Resource toRender = /* code to obtain resource here */ null;
request
    .getRequestDispatcher(toRender.getPath() + ".res")
    .forward(request, response);  

See also SLING-8742 - Allow overriding the extension when using the RequestDispatcher for a discussion on providing an API for this use case.