Extension Parameters

spmallette edited this page Aug 16, 2012 · 11 revisions

Extension methods marked with an @ExtensionDefinition annotation contain the logic to be executed by the Rexster. To do anything useful an extension method must have access to the resources that Rexster has to from the context of the request, the general graph configuration and other such parameters. Extension methods access these resources through method parameters that are marked with the @RexsterContext or @ExtensionRequestParameter annotations. Rexster will read these marked parameters and try to inject an object based on its type.

Parameters marked with one of these annotation that is not one of an expected type (described more below) will be set to null (unless a default value is specified). Furthermore, parameters that are not marked by annotation at all will be set to null.

Rexster Context

The @RexsterContext annotation tells Rexster to inject resources internal to Rexster to the extension method and can be applied to parameters of the following types:

type description
Graph The requested blueprints graph.
RexsterApplicationGraph The requested blueprints graph.
ExtensionMethod Provides access to the reflected Method as well as the ExtensionDefinition and ExtensionDescriptor. It also provides the getExtensionApiAsJson helper method for getting API information to return as JSON.
UriInfo The requested URI.
HttpServletRequest The actual servlet request made.
SecurityContext Allows access to the name of the user who logged in (if authentication services are turned on).
RexsterResourceContext A container for the UriInfo, HttpServletRequest, ExtensionMethod, RexsterApplicationGraph, SecurityContext and two request objects: one that is mapped to JSON and one that just contains the raw values passed to Rexster
Edge The requested edge given a GRAPH or EDGE @ExtensionPoint.
Vertex The requested edge given a GRAPH or EDGE @ExtensionPoint.

Consider the following example taken from the SimpleRootExtension extension in the Sample Kibbles project:

public ExtensionResponse doWorkOnGraph(@RexsterContext Graph graph) {

In this example, Rexster will inject the Graph object from the request into the doWorkOnGraph method.

Extension Request Parameter

The @ExtensionRequestParameter annotation tells Rexster to inject parameters from the request object into a parameter. This annotation takes several parameters: name, an optional description, defaultValue and parseToJson. The name refers to the key in the root of the request object. Rexster will try to coerce values from the request object into the specified type of the parameter. The following types are supported:

  • String
  • Integer
  • Float
  • Double
  • Long
  • Boolean
  • JSONObject
  • JSONArray

The defaultValue allows specification of a default for that parameter if the parameter is not supplied on the request. This parameter takes a string, but the value will be coerced to the data type of the value to which the annotation is attached. If this value cannot be coerced, then it will be defaulted to null.

It is important to note that defaultValue parameter takes an array of strings as its value. This is a workaround to a shortcoming of Java that prevents a null assignment to annotation parameters. If no value, is specified, the defaultValue is actually an empty array, which Rexster will evaluate to null. If there is more than one value, only the first value in the array is taken into account.

The parseToJson parameter is defaulted to true and when set as such, tells Rexster to inject the value of the parameter as mapped from JSON. When set to false, Rexster will inject the raw value of the parameter without the mapping. This is especially useful when dealing with parameters that contain characters that incorrectly parse to JSON when a string value is really what is needed.

Consider this snippet from the PingExtension in the Sample Kibbles project:

@ExtensionDefinition(extensionPoint = ExtensionPoint.GRAPH)
@ExtensionDescriptor(description = "Ping me.")
public ExtensionResponse evaluatePing(@RexsterContext RexsterResourceContext context,
                                      @RexsterContext Graph graph,
                                      @ExtensionRequestParameter(name="reply", description="a value to reply with") String reply) {
    Map<String, String> map = new HashMap<String, String>();
    map.put("ping", reply);
    return ExtensionResponse.ok(map);

When accessing this URI:


The value of the reply URI query string parameter, “ping-a-ling” is injected into the reply method parameter on the extension.

The next example demonstrates usage of the defaultValue parameter on the ExtensionRequestParameter. This example is taken from the ParametersExtension in the Sample Kibbles project.

@ExtensionDefinition(extensionPoint = ExtensionPoint.GRAPH, path = "object-default")
@ExtensionDescriptor(description = "pass an object parameter to be used in the response.")
public ExtensionResponse evaluateSomeObjectWithDefaults(@RexsterContext RexsterResourceContext context,
                                                        @RexsterContext Graph graph,
                                                        @ExtensionRequestParameter(name = "a", defaultValue = "1", description = "an integer to reply with") Integer reply,
                                                        @ExtensionRequestParameter(name = "b", defaultValue = "{ \"a\":\"marko\",\"b\":true, \"c\": { \"a\" : \"peter\" } }", description = "an object to reply with") JSONObject replyObject,
                                                        @ExtensionRequestParameter(name = "c", defaultValue = "[ \"marko\",\"povel\" ]", description = "a list to reply with") JSONArray replyList) {
    Map<String, Object> map = new HashMap<String, Object>();
    map.put("a", reply);
    map.put("b", replyObject);
    map.put("c", replyList);
    return ExtensionResponse.ok(map);

Note that the defaultValue is specified by a string for all three parameters. Rexster will coerce those values to their appropriate data types. So therefore resolving:


will return:

  "b": {
    "c": { "a":"peter" }
  "c": [ "marko", "povel" ]

or alternatively, resolving this URI:


will return:

  "b": {
    "c": { "a":"peter" }
  "c": [ "marko", "povel" ]