JsonPath and JsonPathMatcher

A tool and syntax for selecting elements within JSON-like structures

Despite the name, JsonPathMatcher and JsonPath expressions may be used in other markup languages besides JSON that support it, such as YAML. When we refer to "JSON" keep in mind that JsonPathMatcher and JsonPath expressions apply to markup languages that have a JsonPathMatcher implementation, such as YAML.

JsonPathMatcher provides methods for matching the given cursor location using a provided JsonPath expression. "JsonPath" (sometimes stylized as JSONPath) is a query language for JSON, similar to how XPath is a query language for XML. Despite the name, JsonPathMatcher can be used in other languages besides JSON, such as YAML.

JsonPathMatcher and JsonPath expressions go hand-in-hand. A JsonPath expression specifies a path to an element in the JSON or YAML structure. A JsonPathMatcher provides methods that visitors can use to make decisions about whether an LST element is a desired target.

JsonPath expression notation

A JsonPath expression specifies a path to an element, or set of elements, in a JSON-like structure.

The JsonPath expression syntax OpenRewrite provides is a subset implementation of the JsonPath syntax.

Syntax components

Paths can use dot notation or bracket notation. For example, these two JsonPath expressions are equivalent:

$.spec.containers[0].image
$['spec']['containers'][0]['image']

Additional syntax subcomponents of a JsonPath expression are shown below:

Expression	Description
$	The root object of the document.
.property	Selects a specific property name.
['property']	Selects a specific property name. Use this notation if the property name contains special characters such as spaces, or begins with a character other than A..Za..z_.
[n]	Selects the n-th element from an array. Indexes are 0-based.
[:n]	Selects the first n elements of the array. Returns a list.
..property	Recursive descent. This searches for the specified property name recursively and returns an array of all values with this property name. Always returns a list, even if just one property is found.
*	Wildcard selects all elements in an object or an array, regardless of their names or indexes. For example, address.* means all properties of the address object, and book[*] means all items of the book array.
[(expression)]	Script expressions can be used instead of explicit property names or indexes.
@	Used in filter expressions to refer to the current node being processed.

Filter expressions

Filters are logical expressions used to filter arrays. An example of a JsonPath expression with a filter is:

$.subjects[?(@.kind == 'ServiceAccount')].kind

where [?(@.kind == 'ServiceAccount')] is the actual filter expression, and @ represents the current item being processed.

The supported operators of a filter expression are shown below:

Operator	Description
==	Equals to. String values should be enclosed in single quotes. For example, [?(@.kind == 'Deployment')].
=~	Matches a JavaScript regular expression. For example, [?(@.uses =~ 'actions/setup-java@v2.*')] matches items which uses starts with actions/setup-java@v2, and so would include actions/setup-java@v2.1.1.

Configuring recipes using JsonPath expressions

Recipes that take JsonPath expressions as arguments take them as strings. In YAML, that looks like this:

---
type: specs.openrewrite.org/v1beta/recipe
name: com.yourorg.ChangeValueExample
displayName: ChangeValueExample
recipeList:
  - org.openrewrite.yaml.ChangeValue:
      oldKeyPath: $.subjects[?(@.kind == 'ServiceAccount')].kind
      value: Deployment

Constructing a similarly configured instance of the same recipe in Java:

ChangeValue v = new ChangeValue("$.subjects[?(@.kind == 'ServiceAccount')].kind", "Deployment", null);

Which, for example, would change the following YAML document:

Before

example.yml

---
subjects:
  - kind: User
    name: some-user
  - kind: ServiceAccount
    name: monitoring-tools

After

example.yml

---
subjects:
  - kind: User
    name: some-user
  - kind: Deployment
    name: monitoring-tools

Authoring recipes with JsonPathMatcher

org.openrewrite.json.JsonPathMatcher (or org.openrewrite.yaml.JsonPathMatcher) are the classes that most recipes will use JsonPath expression patterns with. Instances are created by providing the JsonPath expression pattern to the JsonPathMatcher constructor:

JsonPathMatcher matcher = new JsonPathMatcher("$.spec.containers[?(@.name == 'app')].image");

Once you have constructed your JsonPathMatcher with a JsonPath expression as the "evaluation target", you can use the JsonPathMatcher to determine whether a given Cursor at a point within the tree matches what you're looking for as demonstrated by the following example from rewrite-json:

@Override
public TreeVisitor<?, ExecutionContext> getVisitor() {
    JsonPathMatcher matcher = new JsonPathMatcher(oldKeyPath);
    return new JsonIsoVisitor<ExecutionContext>() {
        @Override
        public Json.Member visitMember(Json.Member member, ExecutionContext ctx) {
            Json.Member m = super.visitMember(member, ctx);
            if (matcher.matches(getCursor()) && (!(m.getValue() instanceof Json.Literal) || !((Json.Literal) m.getValue()).getValue().equals(value))) {
                String source = ChangeValue.this.value;
                if (source.startsWith("'") || source.startsWith("\"")) {
                    source = source.substring(1, source.length() - 1);
                }
                if (!(m.getValue() instanceof Json.Literal) || !((Json.Literal) m.getValue()).getSource().equals(ChangeValue.this.value)) {
                    m = m.withValue(new Json.Literal(randomId(), m.getValue().getPrefix(), Markers.EMPTY, ChangeValue.this.value, source));
                }
            }
            return m;
        }
    };
}