Collection Selection

Selection is a powerful expression language feature that lets you transform a source collection into another collection by selecting from its entries.

Selection uses a syntax of .?[selectionExpression]. It filters the collection and returns a new collection that contains a subset of the original elements. For example, selection lets us easily get a list of Serbian inventors, as the following example shows:

  • Java

  • Kotlin

List<Inventor> list = (List<Inventor>) parser.parseExpression(
		"members.?[nationality == 'Serbian']").getValue(societyContext);
val list = parser.parseExpression(
		"members.?[nationality == 'Serbian']").getValue(societyContext) as List<Inventor>

Selection is supported for arrays and anything that implements java.lang.Iterable or java.util.Map. For an array or Iterable, the selection expression is evaluated against each individual element. Against a map, the selection expression is evaluated against each map entry (objects of the Java type Map.Entry). Each map entry has its key and value accessible as properties for use in the selection.

Given a Map stored in a variable named #map, the following expression returns a new map that consists of those elements of the original map where the entry’s value is less than 27:

  • Java

  • Kotlin

Map newMap = parser.parseExpression("#map.?[value < 27]").getValue(Map.class);
val newMap = parser.parseExpression("#map.?[value < 27]").getValue() as Map

In addition to returning all the selected elements, you can retrieve only the first or the last element. To obtain the first element matching the selection expression, the syntax is .^[selectionExpression]. To obtain the last element matching the selection expression, the syntax is .$[selectionExpression].

The Spring Expression Language also supports safe navigation for collection selection.

See Safe Collection Selection and Projection for details.