Custom Function Libraries

For a more in-depth look at the process of creating a custom function library, please refer to the demo project. It provides a walkthrough of the entire process and contains extensive examples: https://github.com/heutelbeck/sapl-demos/tree/master/sapl-demo-extension .

The standard functions can be extended by custom functions. Function libraries available in SAPL documents are collected in the PDP’s function context. The embedded PDP provides an AnnotationFunctionContext where Java classes with annotations can be provided as function libraries:

SAPL functions must not perform any IO operations. Functions are to be used as “immediate” data transformation functions.

• To be recognized as a function library, a class must be annotated with @FunctionLibrary. The optional annotation attribute name contains the library’s name as it will be available in SAPL policies. The attribute value must be a string consisting of one or more identifiers separated by periods. If the attribute is missing, the name of the Java class is used. The optional annotation attribute description contains a string describing the library for documentation purposes.

  @FunctionLibrary(name = "sample.functions", description = "a sample library")
  public class SampleFunctionLibrary {
      ...
  }
  

• The annotation @Function identifies a function in the library. An optional annotation attribute name can contain a function name. The attribute is a string containing an identifier. By default, the name of the Java function will be used. The annotation attribute docs can contain a string describing the function.

  @Function(docs = "returns the length")
  public static Val length(@Text Val parameter) {
      ...
  }
  

  Each parameter can be annotated with any number of @Array, @Bool, @Int, @JsonObject, @Long, @Number, and @Text. The annotations describe which types are allowed for the parameter (in the case of multiple annotations, each of these types is allowed).