Pengenalan kepada JsonPath

1. Gambaran keseluruhan

Salah satu kelebihan XML adalah ketersediaan pemprosesan - termasuk XPath - yang ditakrifkan sebagai standard W3C. Untuk JSON, alat serupa yang disebut JSONPath telah muncul.

Artikel ini akan memberikan pengenalan kepada Jayway JsonPath , implementasi Java dari spesifikasi JSONPath. Ini menerangkan penyediaan, sintaks, API umum, dan demonstrasi kes penggunaan.

2. Persediaan

Untuk menggunakan JsonPath, kita hanya perlu memasukkan kebergantungan dalam Maven pom:

 com.jayway.jsonpath json-path 2.4.0 

3. Sintaksis

Struktur JSON berikut akan digunakan dalam bahagian ini untuk menunjukkan sintaks dan API 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. Notasi

JsonPath menggunakan notasi khas untuk mewakili nod dan sambungannya ke nod bersebelahan dalam jalan JsonPath. Terdapat dua gaya notasi, iaitu titik dan tanda kurung.

Kedua-dua jalur berikut merujuk pada simpul yang sama dari dokumen JSON di atas, yang merupakan elemen ketiga dalam medan lokasi node pencipta , iaitu anak dari objek jsonpath milik alat di bawah simpul akar.

Dengan notasi titik:

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

Dengan notasi kurungan:

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

Tanda dolar ($) mewakili objek anggota akar.

3.2. Pengendali

Kami mempunyai beberapa pengendali yang berguna di JsonPath:

Node akar ($) : Simbol ini menunjukkan anggota akar struktur JSON tidak kira ia adalah objek atau array. Contoh penggunaannya dimasukkan dalam sub-bahagian sebelumnya.

Node semasa (@) : Merupakan nod yang sedang diproses, kebanyakannya digunakan sebagai bahagian ungkapan input untuk predikat. Katakan kita berurusan dengan susunan buku dalam dokumen JSON di atas, buku ungkapan [? (@. Harga == 49.99)] merujuk kepada buku pertama dalam susunan itu.

Wildcard (*) : Menyatakan semua elemen dalam skop yang ditentukan. Contohnya, buku [*] menunjukkan semua nod di dalam susunan buku .

3.3. Fungsi dan Penapis

JsonPath juga mempunyai fungsi yang dapat digunakan hingga akhir jalan untuk mensintesis ekspresi output jalur itu: min () , max () , avg () , stddev () , length () .

Akhirnya - kami mempunyai penapis; ini adalah ungkapan boolean untuk mengehadkan senarai nod yang dikembalikan kepada hanya yang diperlukan oleh kaedah panggilan.

Beberapa contoh adalah persamaan ( == ), padanan ungkapan biasa ( = ~ ), penyertaan ( dalam ), periksa kekosongan ( kosong ). Penapis digunakan terutamanya untuk predikat.

Untuk senarai lengkap dan penjelasan terperinci mengenai operator, fungsi, dan penapis yang berbeza, sila rujuk projek JsonPath GitHub.

4. Operasi

Sebelum kita mula beroperasi, nota sampingan cepat - bahagian ini menggunakan struktur contoh JSON yang kita tentukan sebelumnya.

4.1. Akses ke Dokumen

JsonPath mempunyai cara mudah untuk mengakses dokumen JSON, iaitu melalui API baca statik :

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

Yang dibaca API boleh bekerja dengan API fasih statik untuk menyediakan lebih banyak fleksibiliti:

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

Varian pembacaan berlebihan yang lain dapat digunakan untuk berbagai jenis sumber JSON, termasuk Objek , InputStream , URL , dan File .

Untuk mempermudah , ujian untuk bahagian ini tidak memasukkan predikat dalam senarai parameter ( varargs kosong ); predikat akan dibincangkan dalam sub-bahagian kemudian.

Mari mulakan dengan menentukan dua jalan contoh untuk dikendalikan:

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

Seterusnya, kami akan membuat objek DocumentContext dengan menguraikan sumber JSON yang diberikan jsonDataSourceString . Objek yang baru dibuat kemudian akan digunakan untuk membaca kandungan menggunakan jalan yang ditentukan di atas:

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

API yang dibaca pertama mengembalikan String yang mengandungi nama pencipta JsonPath, sementara yang kedua mengembalikan senarai alamatnya. Dan kami akan menggunakan JUnit Assert API untuk mengesahkan kaedah berfungsi seperti yang diharapkan:

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

4.2. Predikat

Sekarang setelah kita selesai dengan asas-asasnya, mari kita tentukan contoh JSON baru untuk dikerjakan dan menggambarkan penciptaan dan penggunaan predikat:

{ "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.

Walaupun JsonPath mempunyai beberapa kekurangan, seperti kekurangan operator untuk mencapai node ibu bapa atau saudara, ia sangat berguna dalam banyak senario.

Pelaksanaan semua contoh dan coretan kod ini terdapat dalam projek GitHub .