Introducción a JsonPath

1. Información general

Una de las ventajas de XML es la disponibilidad de procesamiento, incluido XPath, que se define como un estándar W3C. Para JSON, ha surgido una herramienta similar llamada JSONPath.

Este artículo dará una introducción a Jayway JsonPath , una implementación de Java de la especificación JSONPath. Describe la configuración, la sintaxis, las API comunes y una demostración de casos de uso.

2. Configuración

Para usar JsonPath, simplemente necesitamos incluir una dependencia en el pom de Maven:

 com.jayway.jsonpath json-path 2.4.0 

3. Sintaxis

La siguiente estructura JSON se utilizará en esta sección para demostrar la sintaxis y las API de JsonPath:

{ "tool": { "jsonpath": { "creator": { "name": "Jayway Inc.", "location": [ "Malmo", "San Francisco", "Helsingborg" ] } } }, "book": [ { "title": "Beginning JSON", "price": 49.99 }, { "title": "JSON at Work", "price": 29.99 } ] }

3.1. Notación

JsonPath usa una notación especial para representar nodos y sus conexiones a nodos adyacentes en una ruta de JsonPath. Hay dos estilos de notación, a saber, punto y corchete.

Las dos rutas siguientes se refieren al mismo nodo del documento JSON anterior, que es el tercer elemento dentro del campo de ubicación del nodo creador , que es un elemento secundario del objeto jsonpath que pertenece a la herramienta debajo del nodo raíz.

Con notación de puntos:

$.tool.jsonpath.creator.location[2]

Con notación de corchetes:

$['tool']['jsonpath']['creator']['location'][2]

El signo de dólar ($) representa el objeto miembro raíz.

3.2. Operadores

Tenemos varios operadores útiles en JsonPath:

Nodo raíz ($) : este símbolo denota el miembro raíz de una estructura JSON sin importar si es un objeto o matriz. Sus ejemplos de uso se incluyeron en la subsección anterior.

Nodo actual (@) : representa el nodo que se está procesando, que se utiliza principalmente como parte de expresiones de entrada para predicados. Supongamos que estamos tratando con una matriz de libros en el documento JSON anterior, la expresión libro [? (@. Price == 49,99)] se refiere al primer libro de esa matriz.

Comodín (*) : expresa todos los elementos dentro del alcance especificado. Por ejemplo, libro [*] indica todos los nodos dentro de una matriz de libros .

3.3. Funciones y filtros

JsonPath también tiene funciones que se pueden usar hasta el final de una ruta para sintetizar las expresiones de salida de esa ruta: min () , max () , avg () , stddev () , length () .

Finalmente, tenemos filtros; estas son expresiones booleanas para restringir las listas de nodos devueltos a solo aquellos que necesitan los métodos de llamada.

Algunos ejemplos son igualdad ( == ), coincidencia de expresiones regulares ( = ~ ), inclusión ( en ), verificación de vacío ( vacío ). Los filtros se utilizan principalmente para predicados.

Para obtener una lista completa y explicaciones detalladas de los diferentes operadores, funciones y filtros, consulte el proyecto JsonPath GitHub.

4. Operaciones

Antes de entrar en operaciones, una nota al margen rápida: esta sección hace uso de la estructura de ejemplo JSON que definimos anteriormente.

4.1. Acceso a documentos

JsonPath tiene una forma conveniente de acceder a documentos JSON, que es a través de API de lectura estática :

 T JsonPath.read(String jsonString, String jsonPath, Predicate... filters);

Las API de lectura pueden funcionar con API fluidas estáticas para proporcionar más flexibilidad:

 T JsonPath.parse(String jsonString).read(String jsonPath, Predicate... filters);

Se pueden usar otras variantes de lectura sobrecargadas para diferentes tipos de fuentes JSON, incluidos Object , InputStream , URL y File .

Para simplificar las cosas, la prueba de esta parte no incluye predicados en la lista de parámetros ( varargs vacíos ); Los predicados se discutirán en subsecciones posteriores.

Comencemos por definir dos rutas de muestra en las que trabajar:

String jsonpathCreatorNamePath = "$['tool']['jsonpath']['creator']['name']"; String jsonpathCreatorLocationPath = "$['tool']['jsonpath']['creator']['location'][*]";

A continuación, crearemos un objeto DocumentContext analizando la fuente JSON jsonDataSourceString dada . El objeto recién creado se utilizará para leer contenido utilizando las rutas definidas anteriormente:

DocumentContext jsonContext = JsonPath.parse(jsonDataSourceString); String jsonpathCreatorName = jsonContext.read(jsonpathCreatorNamePath); List jsonpathCreatorLocation = jsonContext.read(jsonpathCreatorLocationPath);

La primera API de lectura devuelve una cadena que contiene el nombre del creador de JsonPath, mientras que la segunda devuelve una lista de sus direcciones. Y usaremos la API JUnit Assert para confirmar que los métodos funcionan como se esperaba:

assertEquals("Jayway Inc.", jsonpathCreatorName); assertThat(jsonpathCreatorLocation.toString(), containsString("Malmo")); assertThat(jsonpathCreatorLocation.toString(), containsString("San Francisco")); assertThat(jsonpathCreatorLocation.toString(), containsString("Helsingborg"));

4.2. Predicados

Ahora que hemos terminado con lo básico, definamos un nuevo ejemplo JSON para trabajar e ilustrar la creación y el uso de predicados:

{ "book": [ { "title": "Beginning JSON", "author": "Ben Smith", "price": 49.99 }, { "title": "JSON at Work", "author": "Tom Marrs", "price": 29.99 }, { "title": "Learn JSON in a DAY", "author": "Acodemy", "price": 8.99 }, { "title": "JSON: Questions and Answers", "author": "George Duckett", "price": 6.00 } ], "price range": { "cheap": 10.00, "medium": 20.00 } }

Predicates determine true or false input values for filters to narrow down returned lists to only matched objects or arrays. A Predicate may easily be integrated into a Filter by using as an argument for its static factory method. The requested content can then be read out of a JSON string using that Filter:

Filter expensiveFilter = Filter.filter(Criteria.where("price").gt(20.00)); List

We may also define our customized Predicate and use it as an argument for the read API:

Predicate expensivePredicate = new Predicate() { public boolean apply(PredicateContext context) { String value = context.item(Map.class).get("price").toString(); return Float.valueOf(value) > 20.00; } }; List

Finally, a predicate may be directly applied to read API without the creation of any objects, which is called inline predicate:

List

All the three of the Predicate examples above are verified with the help of the following assertion helper method:

private void predicateUsageAssertionHelper(List predicate) { assertThat(predicate.toString(), containsString("Beginning JSON")); assertThat(predicate.toString(), containsString("JSON at Work")); assertThat(predicate.toString(), not(containsString("Learn JSON in a DAY"))); assertThat(predicate.toString(), not(containsString("JSON: Questions and Answers"))); }

5. Configuration

5.1. Options

Jayway JsonPath provides several options to tweak the default configuration:

• Option.AS_PATH_LIST: Returns paths of the evaluation hits instead of their values.
• Option.DEFAULT_PATH_LEAF_TO_NULL: Returns null for missing leaves.
• Option.ALWAYS_RETURN_LIST: Returns a list even when the path is definite.
• Option.SUPPRESS_EXCEPTIONS: Makes sure no exceptions are propagated from path evaluation.
• Option.REQUIRE_PROPERTIES: Requires properties defined in the path when an indefinite path is evaluated.

Here is how Option is applied from scratch:

Configuration configuration = Configuration.builder().options(Option.).build();

and how to add it to an existing configuration:

Configuration newConfiguration = configuration.addOptions(Option.);

5.2. SPIs

JsonPath's default configuration with the help of Option should be enough for the majority of tasks. However, users with more complex use cases can modify the behavior of JsonPath according to their specific requirements – using three different SPIs:

• JsonProvider SPI: Lets us change the ways JsonPath parses and handles JSON documents
• MappingProvider SPI: Allows for customization of bindings between node values and returned object types
• CacheProvider SPI: Adjusts the manners that paths are cached, which can help to increase performance

6. An Example Use Cases

Now that we have a good understanding of the functionality that JsonPath can be used for – let's look at an example.

This section illustrates dealing with JSON data returned from a web service – assume we have a movie information service, which returns the following structure:

[ { "id": 1, "title": "Casino Royale", "director": "Martin Campbell", "starring": [ "Daniel Craig", "Eva Green" ], "desc": "Twenty-first James Bond movie", "release date": 1163466000000, "box office": 594275385 }, { "id": 2, "title": "Quantum of Solace", "director": "Marc Forster", "starring": [ "Daniel Craig", "Olga Kurylenko" ], "desc": "Twenty-second James Bond movie", "release date": 1225242000000, "box office": 591692078 }, { "id": 3, "title": "Skyfall", "director": "Sam Mendes", "starring": [ "Daniel Craig", "Naomie Harris" ], "desc": "Twenty-third James Bond movie", "release date": 1350954000000, "box office": 1110526981 }, { "id": 4, "title": "Spectre", "director": "Sam Mendes", "starring": [ "Daniel Craig", "Lea Seydoux" ], "desc": "Twenty-fourth James Bond movie", "release date": 1445821200000, "box office": 879376275 } ]

Where the value of release date field is duration since the Epoch in milliseconds and box office is revenue of a movie in the cinema in US dollars.

We are going to handle five different working scenarios related to GET requests, supposing that the above JSON hierarchy has been extracted and stored in a String variable named jsonString.

6.1. Getting Object Data Given IDs

In this use case, a client requests detailed information on a specific movie by providing the server with the exact id of that one. This example demonstrates how the server looks for requested data before returning to the client.

Say we need to find a record with id equaling to 2. Below is how the process is implemented and tested.

The first step is to pick up the correct data object:

Object dataObject = JsonPath.parse(jsonString).read("$[?(@.id == 2)]"); String dataString = dataObject.toString();

The JUnit Assert API confirms the existence of several fields:

assertThat(dataString, containsString("2")); assertThat(dataString, containsString("Quantum of Solace")); assertThat(dataString, containsString("Twenty-second James Bond movie"));

6.2. Getting the Movie Title Given Starring

Let's say we want to look for a movie starring an actress called Eva Green. The server needs to return title of the movie that Eva Green is included in the starring array.

The succeeding test will illustrate how to do that and validate the returned result:

@Test public void givenStarring_whenRequestingMovieTitle_thenSucceed() { List

6.3. Calculation of the Total Revenue

This scenario makes use of a JsonPath function called length() to figure out the number of movie records, to calculate the total revenue of all the movies. The implementation and testing are demonstrated as follows:

@Test public void givenCompleteStructure_whenCalculatingTotalRevenue_thenSucceed() { DocumentContext context = JsonPath.parse(jsonString); int length = context.read("$.length()"); long revenue = 0; for (int i = 0; i < length; i++) { revenue += context.read("$[" + i + "]['box office']", Long.class); } assertEquals(594275385L + 591692078L + 1110526981L + 879376275L, revenue); }

6.4. Highest Revenue Movie

This use case exemplifies the usage of a non-default JsonPath configuration option, namely Option.AS_PATH_LIST, to find out the movie with the highest revenue. The particular steps are described underneath.

At first, we need to extract a list of all the movies' box office revenue, then convert it to an array for sorting:

DocumentContext context = JsonPath.parse(jsonString); List revenueList = context.read("$[*]['box office']"); Integer[] revenueArray = revenueList.toArray(new Integer[0]); Arrays.sort(revenueArray);

The highestRevenue variable may easily be picked up from the revenueArray sorted array, then used for working out the path to the movie record with the highest revenue:

int highestRevenue = revenueArray[revenueArray.length - 1]; Configuration pathConfiguration = Configuration.builder().options(Option.AS_PATH_LIST).build(); List pathList = JsonPath.using(pathConfiguration).parse(jsonString) .read("$[?(@['box office'] == " + highestRevenue + ")]");

Based on that calculated path, title of the corresponding movie can be determined and returned:

Map dataRecord = context.read(pathList.get(0)); String title = dataRecord.get("title");

The whole process is verified by the Assert API:

assertEquals("Skyfall", title);

6.5. Latest Movie of a Director

This example will illustrate the way to figure out the lasted movie directed by a director named Sam Mendes.

To begin with, a list of all the movies directed by Sam Mendes is created:

DocumentContext context = JsonPath.parse(jsonString); List

That list is used for extraction of release dates. Those dates will be stored in an array and then sorted:

List dateList = new ArrayList(); for (Map item : dataList) { Object date = item.get("release date"); dateList.add(date); } Long[] dateArray = dateList.toArray(new Long[0]); Arrays.sort(dateArray);

The lastestTime variable, which is the last element of the sorted array, is used in combination with the director field's value to determine the title of the requested movie:

long latestTime = dateArray[dateArray.length - 1]; List

The following assertion proved that everything works as expected:

assertEquals("Spectre", title);

7. Conclusion

This tutorial has covered fundamental features of Jayway JsonPath – a powerful tool to traverse and parse JSON documents.

Aunque JsonPath tiene algunos inconvenientes, como la falta de operadores para llegar a los nodos principales o hermanos, puede ser muy útil en muchos escenarios.

La implementación de todos estos ejemplos y fragmentos de código se puede encontrar en un proyecto de GitHub .