Einführung in JsonPath

1. Übersicht

Einer der Vorteile von XML ist die Verfügbarkeit der Verarbeitung - einschließlich XPath -, die als W3C-Standard definiert ist. Für JSON wurde ein ähnliches Tool namens JSONPath entwickelt.

Dieser Artikel enthält eine Einführung in Jayway JsonPath , eine Java-Implementierung der JSONPath-Spezifikation. Es beschreibt Setup, Syntax, allgemeine APIs und eine Demonstration von Anwendungsfällen.

2. Setup

Um JsonPath verwenden zu können, müssen wir lediglich eine Abhängigkeit in den Maven-Pom aufnehmen:

 com.jayway.jsonpath json-path 2.4.0 

3. Syntax

Die folgende JSON-Struktur wird in diesem Abschnitt verwendet, um die Syntax und APIs von JsonPath zu demonstrieren:

{ "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. Notation

JsonPath verwendet eine spezielle Notation, um Knoten und ihre Verbindungen zu benachbarten Knoten in einem JsonPath-Pfad darzustellen. Es gibt zwei Arten der Notation, nämlich Punkt und Klammer.

Beide der folgenden Wege beziehen sich auf demselben Knoten aus dem obigen JSON Dokument, das ist das dritte Element innerhalb des Standortfeld creator Knoten, dass ein Kind ist der jsonpath Objekt gehörenden Werkzeug unter dem Wurzelknoten.

Mit Punktnotation:

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

Mit Klammernotation:

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

Das Dollarzeichen ($) steht für das Stammelementobjekt.

3.2. Betreiber

Wir haben mehrere hilfreiche Operatoren in JsonPath:

Stammknoten ($) : Dieses Symbol kennzeichnet das Stammelement einer JSON-Struktur, unabhängig davon, ob es sich um ein Objekt oder ein Array handelt. Die Verwendungsbeispiele wurden im vorherigen Unterabschnitt aufgenommen.

Aktueller Knoten (@) : Stellt den Knoten dar, der verarbeitet wird und hauptsächlich als Teil von Eingabeausdrücken für Prädikate verwendet wird. Angenommen , wir beschäftigen uns mit Buch - Array in der oben JSON Dokument der Ausdruck Buch [? (@. Preis == 49.99)] bezieht sich auf das erste Buch in dieser Reihe.

Platzhalter (*) : Drückt alle Elemente innerhalb des angegebenen Bereichs aus. Zum Beispiel Buch [*] zeigt alle Knoten innerhalb eines Buches Array.

3.3. Funktionen und Filter

JsonPath verfügt auch über Funktionen, die bis zum Ende eines Pfads verwendet werden können, um die Ausgabeausdrücke dieses Pfads zu synthetisieren: min () , max () , avg () , stddev () , length () .

Endlich - wir haben Filter; Dies sind boolesche Ausdrücke, um zurückgegebene Listen von Knoten auf diejenigen zu beschränken, die aufrufende Methoden benötigen.

Einige Beispiele sind Gleichheit ( == ), Übereinstimmung mit regulären Ausdrücken ( = ~ ), Einbeziehung ( in ), Überprüfung auf Leere ( leer ). Filter werden hauptsächlich für Prädikate verwendet.

Eine vollständige Liste und detaillierte Erklärungen zu verschiedenen Operatoren, Funktionen und Filtern finden Sie im JsonPath GitHub-Projekt.

4. Operationen

Bevor wir mit den Operationen beginnen, eine kurze Randnotiz: In diesem Abschnitt wird die zuvor definierte JSON-Beispielstruktur verwendet.

4.1. Zugriff auf Dokumente

JsonPath bietet eine bequeme Möglichkeit, über statische Lese- APIs auf JSON-Dokumente zuzugreifen :

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

The read APIs can work with static fluent APIs to provide more flexibility:

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

Other overloaded variants of read can be used for different types of JSON sources, including Object, InputStream, URL, and File.

To make things simple, the test for this part does not include predicates in the parameter list (empty varargs); predicates will be discussed in later sub-sections.

Let's start by defining two sample paths to work on:

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

Next, we will create a DocumentContext object by parsing the given JSON source jsonDataSourceString. The newly created object will then be used to read content using the paths defined above:

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

The first read API returns a String containing the name of JsonPath creator, while the second returns a list of its addresses. And we'll use the JUnit Assert API to confirm the methods work as expected:

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

4.2. Predicates

Now that we're done with the basics, let's define a new JSON example to work on and illustrate the creation and usage of predicates:

{ "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 expensive = JsonPath.parse(jsonDataSourceString) .read("$['book'][?]", expensiveFilter); predicateUsageAssertionHelper(expensive);

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 expensive = JsonPath.parse(jsonDataSourceString) .read("$['book'][?]", expensivePredicate); predicateUsageAssertionHelper(expensive);

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

List expensive = JsonPath.parse(jsonDataSourceString) .read("$['book'][?(@['price'] > $['price range']['medium'])]"); predicateUsageAssertionHelper(expensive);

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 dataList = JsonPath.parse(jsonString) .read("$[?('Eva Green' in @['starring'])]"); String title = (String) dataList.get(0).get("title"); assertEquals("Casino Royale", title); }

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 dataList = context.read("$[?(@.director == 'Sam Mendes')]");

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 finalDataList = context.read("$[?(@['director'] == 'Sam Mendes' && @['release date'] == " + latestTime + ")]"); String title = (String) finalDataList.get(0).get("title");

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.

Obwohl JsonPath einige Nachteile aufweist, z. B. das Fehlen von Operatoren zum Erreichen von Eltern- oder Geschwisterknoten, kann es in vielen Szenarien sehr nützlich sein.

Die Implementierung all dieser Beispiele und Codefragmente finden Sie in einem GitHub-Projekt .