Pengantar JSON-Java (org.json)

1. Pengantar JSON-Java

JSON (singkatan dari JavaScript Object Notation) adalah format pertukaran data ringan dan paling umum digunakan untuk komunikasi klien-server. Keduanya mudah dibaca / ditulis dan tidak bergantung pada bahasa. Nilai JSON dapat berupa objek JSON , larik, angka, string, boolean (true / false), atau null lainnya.

Dalam tutorial ini, kita akan melihat bagaimana kita dapat membuat, memanipulasi, dan mengurai JSON menggunakan salah satu pustaka pemrosesan JSON yang tersedia, yaitu pustaka JSON-Java yang juga dikenal sebagai org.json.

2. Prasyarat

Sebelum kita mulai, kita perlu menambahkan dependensi berikut di pom.xml kita :

 org.json json 20180130 

Versi terbaru dapat ditemukan di repositori Maven Central.

Perhatikan bahwa paket ini telah disertakan dalam Android SDK, jadi kami tidak boleh menyertakannya saat menggunakan yang sama.

3. JSON di Java [package org.json]

Pustaka JSON-Java juga dikenal sebagai org.json (jangan disamakan dengan org.json.simple Google) memberi kita kelas yang digunakan untuk mengurai dan memanipulasi JSON di Java.

Selain itu, pustaka ini juga dapat mengkonversi antara JSON, XML, HTTP Headers, Cookies, Comma-Delimited List atau Teks, dll.

Dalam tutorial ini, kita akan melihat:

  1. JSONObject - mirip denganobjek peta asli Javayang menyimpan pasangan nilai kunci yang tidak berurutan
  2. JSONArray - urutan nilai yang mirip dengan implementasi Vektor asli Java
  3. JSONTokener - alat yang memecah sepotong teks menjadi serangkaian token yang dapat digunakan oleh JSONObject atau JSONArray untuk mengurai string JSON
  4. CDL - alat yang menyediakan metode untuk mengubah teks yang dipisahkan koma menjadi JSONArray dan sebaliknya
  5. Cookie - mengubah dari String JSONmenjadi cookie dan sebaliknya
  6. HTTP - digunakan untuk mengonversi dari String JSONke header HTTP dan sebaliknya
  7. JSONException - ini adalah pengecualian standar yang diberikan oleh pustaka ini

4. JSONObject

Sebuah JSONObject adalah koleksi unordered kunci dan nilai pasangan, menyerupai asli Jawa Peta implementasi.

  • Kunci adalah String unik yang tidak boleh kosong
  • Nilai bisa apa saja dari Boolean , Number , String , JSONArray atau bahkan objek JSONObject.NULL
  • Sebuah JSONObject dapat diwakili oleh String tertutup dalam kurung kurawal dengan kunci dan nilai-nilai yang dipisahkan oleh titik dua, dan pasangan yang dipisahkan dengan koma
  • Ini memiliki beberapa konstruktor yang dapat digunakan untuk membuat JSONObject

Ini juga mendukung metode utama berikut:

  1. get (String key) - g ets objek yang terkait dengan kunci yang disediakan, melempar JSONException jika kunci tidak ditemukan
  2. opt (String key) - g ets objek yang terkait dengan kunci yang disediakan, null jika tidak
  3. put (String key, Object value) - menyisipkan atau mengganti pasangan kunci-nilai di JSONObject saat ini .

Metode put () adalah metode kelebihan beban yang menerima kunci bertipe String dan beberapa tipe untuk nilainya.

Untuk daftar lengkap metode yang didukung oleh JSONObject , kunjungi dokumentasi resmi.

Sekarang mari kita bahas beberapa operasi utama yang didukung oleh kelas ini.

4.1. Membuat JSON Langsung dari JSONObject

JSONObject memperlihatkan API yang mirip dengan antarmuka Peta Java . Kita bisa menggunakan metode put () dan memberikan kunci dan nilai sebagai argumen:

JSONObject jo = new JSONObject(); jo.put("name", "jon doe"); jo.put("age", "22"); jo.put("city", "chicago");

Sekarang JSONObject kita akan terlihat seperti:

{"city":"chicago","name":"jon doe","age":"22"}

Ada tujuh tanda tangan berlebih yang berbeda dari metode JSONObject.put () . Meskipun kuncinya hanya bisa unik, String bukan nol , nilainya bisa apa saja.

4.2. Membuat JSON dari Peta

Alih-alih langsung meletakkan kunci dan nilai dalam JSONObject , kita bisa membuat Peta khusus dan kemudian meneruskannya sebagai argumen ke konstruktor JSONObject .

Contoh ini akan memberikan hasil yang sama seperti di atas:

Map map = new HashMap(); map.put("name", "jon doe"); map.put("age", "22"); map.put("city", "chicago"); JSONObject jo = new JSONObject(map);

4.3. Creating JSONObject from JSON String

To parse a JSON String to a JSONObject, we can just pass the String to the constructor.

This example will produce same results as above:

JSONObject jo = new JSONObject( "{\"city\":\"chicago\",\"name\":\"jon doe\",\"age\":\"22\"}" );

The passed String argument must be a valid JSON otherwise this constructor may throw a JSONException.

4.4. Serialize Java Object to JSON

One of JSONObject's constructors takes a POJO as its argument. In the example below, the package uses the getters from the DemoBean class and creates an appropriate JSONObject for the same.

To get a JSONObject from a Java Object, we'll have to use a class that is a valid Java Bean:

DemoBean demo = new DemoBean(); demo.setId(1); demo.setName("lorem ipsum"); demo.setActive(true); JSONObject jo = new JSONObject(demo);

The JSONObject jo for this example is going to be:

{"name":"lorem ipsum","active":true,"id":1}

Although we have a way to serialize a Java object to JSON string, there is no way to convert it back using this library.

If we want that kind of flexibility, we can switch to other libraries such as Jackson.

5. JSONArray

A JSONArray is an ordered collection of values, resembling Java's native Vector implementation.

  • Values can be anything from a Number, String, Boolean, JSONArray, JSONObject or even a JSONObject.NULL object
  • It's represented by a String wrapped within Square Brackets and consists of a collection of values separated by commas
  • Like JSONObject, it has a constructor that accepts a source String and parses it to construct a JSONArray

The following are the primary methods of the JSONArray class:

  1. get(int index) – returns the value at the specified index(between 0 and total length – 1), otherwise throws a JSONException
  2. opt(int index) – returns the value associated with an index (between 0 and total length – 1). If there's no value at that index, then a null is returned
  3. put(Object value) – append an object value to this JSONArray. This method is overloaded and supports a wide range of data types

For a complete list of methods supported by JSONArray, visit the official documentation.

5.1. Creating JSONArray

Once we've initialized a JSONArray object, we can simply add and retrieve elements using the put() and get() methods:

JSONArray ja = new JSONArray(); ja.put(Boolean.TRUE); ja.put("lorem ipsum"); JSONObject jo = new JSONObject(); jo.put("name", "jon doe"); jo.put("age", "22"); jo.put("city", "chicago"); ja.put(jo);

Following would be contents of our JSONArray(code is formatted for clarity):

[ true, "lorem ipsum", { "city": "chicago", "name": "jon doe", "age": "22" } ]

5.2. Creating JSONArray Directly from JSON String

Like JSONObject the JSONArray also has a constructor that creates a Java object directly from a JSON String:

JSONArray ja = new JSONArray("[true, \"lorem ipsum\", 215]");

This constructor may throw a JSONException if the source String isn't a valid JSON String.

5.3. Creating JSONArray Directly from a Collection or an Array

The constructor of JSONArray also supports collection and array objects as arguments.

We simply pass them as an argument to the constructor and it will return a JSONArray object:

List list = new ArrayList(); list.add("California"); list.add("Texas"); list.add("Hawaii"); list.add("Alaska"); JSONArray ja = new JSONArray(list);

Now our JSONArray consists of:

["California","Texas","Hawaii","Alaska"]

6. JSONTokener

A JSONTokener takes a source String as input to its constructor and extracts characters and tokens from it. It's used internally by classes of this package (like JSONObject, JSONArray) to parse JSON Strings.

There may not be many situations where we'll directly use this class as the same functionality can be achieved using other simpler methods (like string.toCharArray()):

JSONTokener jt = new JSONTokener("lorem"); while(jt.more()) { Log.info(jt.next()); }

Now we can access a JSONTokener like an iterator, using the more() method to check if there are any remaining elements and next() to access the next element.

The tokens received from the previous example will be:

l o r e m

7. CDL

We're provided with a CDL (Comma Delimited List) class to convert comma delimited text into a JSONArray and vice versa.

7.1. Producing JSONArray Directly from Comma Delimited Text

In order to produce a JSONArray directly from the comma-delimited text, we can use the static method rowToJSONArray() which accepts a JSONTokener:

JSONArray ja = CDL.rowToJSONArray(new JSONTokener("England, USA, Canada"));

Our JSONArray now consists of:

["England","USA","Canada"]

7.2. Producing Comma Delimited Text from JSONArray

In order to reverse of the previous step and get back the comma-delimited text from JSONArray, we can use:

JSONArray ja = new JSONArray("[\"England\",\"USA\",\"Canada\"]"); String cdt = CDL.rowToString(ja);

The Stringcdt now contains:

England,USA,Canada

7.3. Producing JSONArray of JSONObjects Using Comma Delimited Text

To produce a JSONArray of JSONObjects, we'll use a text String containing both headers and data separated by commas.

The different lines are separated using a carriage return (\r) or line feed (\n).

The first line is interpreted as a list of headers and all the subsequent lines are treated as data:

String string = "name, city, age \n" + "john, chicago, 22 \n" + "gary, florida, 35 \n" + "sal, vegas, 18"; JSONArray result = CDL.toJSONArray(string);

The object JSONArray result now consists of (output formatted for the sake of clarity):

[ { "name": "john", "city": "chicago", "age": "22" }, { "name": "gary", "city": "florida", "age": "35" }, { "name": "sal", "city": "vegas", "age": "18" } ]

Notice that in this example, both data and header were supplied within the same String.There's an alternative way of doing this where we can achieve the same functionality by supplying a JSONArray that would be used to get the headers and a comma-delimited String working as the data.

Different lines are separated using a carriage return (\r) or line feed (\n):

JSONArray ja = new JSONArray(); ja.put("name"); ja.put("city"); ja.put("age"); String string = "john, chicago, 22 \n" + "gary, florida, 35 \n" + "sal, vegas, 18"; JSONArray result = CDL.toJSONArray(ja, string);

Here we'll get the contents of object result exactly as before.

8. Cookie

The Cookie class deals with web browser cookies and has methods to convert a browser cookie into a JSONObject and vice versa.

Here are the main methods of the Cookie class:

  1. toJsonObject(String sourceCookie) – converts a cookie string into a JSONObject

  2. toString(JSONObject jo) – this is reverse of the previous method, converts a JSONObject into a cookie String.

8.1. Converting a Cookie String into a JSONObject

To convert a cookie String to a JSONObject, well use the static method Cookie.toJSONObject():

String cookie = "username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC; path=/"; JSONObject cookieJO = Cookie.toJSONObject(cookie);

8.2. Converting a JSONObject into Cookie String

Now we'll convert a JSONObject into cookie String. This is reverse of the previous step:

String cookie = Cookie.toString(cookieJO);

9. HTTP

The HTTP class contains static methods that are used to convert HTTP headers to JSONObject and vice versa.

This class also has two main methods:

  1. toJsonObject(String sourceHttpHeader) – converts a HttpHeader String to JSONObject
  2. toString(JSONObject jo) – converts the supplied JSONObject to String

9.1. Converting JSONObject to HTTP Header

HTTP.toString() method is used to convert a JSONObject to HTTP header String:

JSONObject jo = new JSONObject(); jo.put("Method", "POST"); jo.put("Request-URI", "//www.example.com/"); jo.put("HTTP-Version", "HTTP/1.1"); String httpStr = HTTP.toString(jo);

Here, our String httpStr will consist of:

POST "//www.example.com/" HTTP/1.1

Note that while converting an HTTP request header, the JSONObject must contain “Method”,“Request-URI” and “HTTP-Version” keys, whereas, for response header, the object must contain “HTTP-Version”,“Status-Code” and “Reason-Phrase” parameters.

9.2. Converting HTTP Header String Back to JSONObject

Here we will convert the HTTP string that we got in the previous step back to the very JSONObject that we created in that step:

JSONObject obj = HTTP.toJSONObject("POST \"//www.example.com/\" HTTP/1.1");

10. JSONException

The JSONException is the standard exception thrown by this package whenever any error is encountered.

Ini digunakan di semua kelas dari paket ini. Pengecualian biasanya diikuti dengan pesan yang menyatakan apa yang sebenarnya salah.

11. Kesimpulan

Dalam tutorial ini, kami melihat JSON menggunakan Java - org.json - dan kami berfokus pada beberapa fungsionalitas inti yang tersedia di sini.

Potongan kode lengkap yang digunakan dalam artikel ini dapat ditemukan di GitHub.