Mastering JsonPath Filters: From Existence Checks to Array Length Queries

JSON Data

A sample JSON document is defined, containing a store with an array of books and a bicycle, plus additional fields such as "expensive" and an integer array "ss".

JSONObject json = JSON.parseObject("{\"store\":{\"book\":[{\"category\":\"reference\",\"author\":\"Nigel Rees\",\"title\":\"Sayings of the Century\",\"price\":8.95},{\"category\":\"fiction\",\"author\":\"Evelyn Waugh\",\"title\":\"Sword of Honour\",\"price\":12.99},{\"category\":\"fiction\",\"author\":\"Herman Melville\",\"title\":\"Moby Dick\",\"isbn\":\"0-553-21311-3\",\"price\":8.99},{\"category\":\"fiction\",\"author\":\"J. R. R. Tolkien\",\"title\":\"The Lord of the Rings\",\"isbn\":\"0-395-19395-8\",\"price\":22.99}],\"bicycle\":{\"color\":\"red\",\"price\":19.95}},\"expensive\":10,\"ss\":[32,32,4,23]}");

Checking Property Existence

The JsonPath expression $..book[?(@.isbn)] selects all book objects that contain an isbn field.

Object read = JsonPath.read(json, "$..book[?(@.isbn)]");
output(JSONArray.parseArray(read.toString()));

The console output shows the two books that have an isbn attribute.

Value Comparison

String comparison example: $..book[?(@.isbn == '0-395-19395-8')] returns the book with that exact ISBN.

Numeric comparison example: $..book[?(@.price > 20)] returns books whose price exceeds 20.

Object read = JsonPath.read(json, "$..book[?(@.price > 20)]");
output(JSONArray.parseArray(read.toString()));

Comparing Property to Property

A nested JsonPath expression can compare a property against another property in the same document, e.g., $..book[?(@.price > $['expensive'])] selects books whose price is greater than the top‑level expensive value.

Object read = JsonPath.read(json, "$..book[?(@.price > $['expensive'])]");
output(JSONArray.parseArray(read.toString()));

Array Length Evaluation

The length() function returns the size of an array: $..book.length() yields the number of books (4). A noted pitfall is that applying length() after a filter changes its semantics, returning the length of the filtered result as a numeric array rather than a single integer.

Object read = JsonPath.read(json, "$..book.length()");
output(read);