Package org.neo4j.procedure

Annotation Interface UserAggregationFunction

@Target(METHOD) @Retention(RUNTIME) public @interface UserAggregationFunction
Declares a method as an aggregation function, meaning the method can be called from the cypher query language.

An aggregation function must returned an instance of a class with two annotated methods, one annotated with UserAggregationUpdate and one annotated with UserAggregationResult

Resource declarations

The aggregation function method itself can contain arbitrary Java code - but in order to work with the underlying graph, it must have access to the graph API. This is done by declaring fields in the function class, and annotating them with the Context annotation. Fields declared this way are automatically injected with the requested resource. This is how functions gain access to APIs to do work with.

All fields in the class containing the function declaration must either be static; or it must be public, non-final and annotated with Context.

Resources supported by default are as follows:

Lifecycle and state

The class that declares your function method may be re-instantiated before each call. Because of this, no regular state can be stored in the fields of the function.

If you want to maintain state between invocations to your function, simply use a static field. Note that functions may be called concurrently, meaning you need to take care to ensure the state you store in static fields can be safely accessed by multiple callers simultaneously.

  • Optional Element Summary

    Optional Elements
    Modifier and Type
    Optional Element
    Description
    String
    deprecatedBy
    When deprecating a function it is useful to indicate a possible replacement procedure that clients might show in warnings.
    String
    name
    Synonym for value()
    String
    value
    The namespace and name for the aggregation function, as a period-separated string.

  • Element Details

    • value

      String value
      The namespace and name for the aggregation function, as a period-separated string. For instance myfunctions.myfunction. If this is left empty, the name defaults to the package name of the class the procedure is declared in, combined with the method name. Notably, the class name is omitted.
      Returns:
      the namespace and procedure name
      Default:
      ""

    • name

      String name
      Synonym for value()
      Returns:
      the namespace and procedure name.
      Default:
      ""

    • deprecatedBy

      String deprecatedBy
      When deprecating a function it is useful to indicate a possible replacement procedure that clients might show in warnings.
      Returns:
      a string representation of the replacement procedure.
      Default:
      ""