Pengenalan JSON-Java (org.json)

Pengaturcaraan

1. Pengenalan JSON-Java

JSON (akronim untuk Notasi Objek JavaScript) adalah format pertukaran data ringan dan paling sering digunakan untuk komunikasi pelayan pelanggan. Ia senang dibaca / ditulis dan bebas bahasa. Nilai JSON boleh menjadi objek, susunan, nombor, rentetan, boolean (benar / salah) atau nol JSON yang lain .

Dalam tutorial ini, kita akan melihat bagaimana kita dapat membuat, memanipulasi dan menguraikan JSON menggunakan salah satu perpustakaan pemprosesan JSON yang ada, iaitu, perpustakaan JSON-Java juga dikenali sebagai org.json.

2. Pra-Syarat

Sebelum memulakan, kita perlu menambahkan kebergantungan berikut dalam pom.xml kami :

 org.json json 20180130

Versi terbaru boleh didapati di repositori Maven Central.

Perhatikan bahawa pakej ini sudah disertakan dalam Android SDK, jadi kami tidak boleh memasukkannya saat menggunakan yang sama.

3. JSON di Java [pakej org.json]

Perpustakaan JSON-Java juga dikenali sebagai org.json (tidak boleh dikelirukan dengan org.json.simple Google) menyediakan kami kelas yang digunakan untuk mengurai dan memanipulasi JSON di Java.

Selain itu, perpustakaan ini juga boleh menukar antara JSON, XML, HTTP Header, Cookies, Comma-Delimited List atau Text, dll.

Dalam tutorial ini, kita akan melihat:

  1. JSONObject - mirip dengan Peta asli Javaseperti objek yang menyimpan pasangan nilai-kunci yang tidak tersusun
  2. JSONArray - urutan nilai yang disusun serupa dengan pelaksanaan Vektor asli Java
  3. JSONTokener - alat yang memecahkan sekeping teks menjadi rangkaian token yang boleh digunakan oleh JSONObject atau JSONArray untuk menguraikan rentetan JSON
  4. CDL - alat yang menyediakan kaedah untuk menukar teks yang dipisahkan koma menjadi JSONArray dan sebaliknya
  5. Cookie - menukar dari JSON String ke cookies dan sebaliknya
  6. HTTP - digunakan untuk menukar dari JSON String ke tajuk HTTP dan sebaliknya
  7. JSONException - ini adalah pengecualian standard yang dilemparkan oleh perpustakaan ini

4. JSONObjek

A JSONObject adalah koleksi tidak tertib kunci dan nilai pasangan, menyerupai asli Jawa Peta pelaksanaan.

  • Kekunci adalah rentetan unik yang tidak boleh kosong
  • Nilai boleh berupa apa sahaja dari objek Boolean , Number , String , JSONArray atau bahkan objek JSONObject.NULL
  • A JSONObject boleh diwakili oleh String disertakan dalam pendakap kerinting dengan kunci dan nilai-nilai dipisahkan dengan noktah bertindih, dan pasangan yang dipisahkan oleh koma
  • Ia mempunyai beberapa pembina untuk membina JSONObject

Ia juga menyokong kaedah utama berikut:

  1. get (kekunci String) - g letakkan objek yang berkaitan dengan kunci yang disediakan, lemparkan JSONException jika kunci tidak dijumpai
  2. opt (kunci String) - g Ets objek yang berkaitan dengan kunci yang dibekalkan, null sebaliknya
  3. put (Kekunci rentetan, Nilai objek) - memasukkan atau menggantikan pasangan nilai-kunci dalam JSONObject semasa .

Kaedah put () adalah kaedah yang terlalu banyak yang menerima kunci jenis String dan beberapa jenis untuk nilai.

Untuk senarai lengkap kaedah yang disokong oleh JSONObject , lawati dokumentasi rasmi.

Sekarang mari kita bincangkan beberapa operasi utama yang disokong oleh kelas ini.

4.1. Membuat JSON Secara Langsung dari JSONObject

JSONObject memaparkan API yang serupa dengan antara muka Peta Java . Kita boleh menggunakan kaedah 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 kami akan kelihatan seperti:

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

Terdapat tujuh tandatangan yang berbeza dari kaedah JSONObject.put () . Walaupun kuncinya hanya String yang unik dan tidak kosong , nilainya boleh menjadi apa sahaja.

4.2. Membuat JSON dari Peta

Daripada meletakkan kunci dan nilai secara langsung dalam JSONObject , kita dapat membina Peta tersuai dan kemudian menyampaikannya sebagai hujah kepada pembangun JSONObject .

Contoh ini akan menghasilkan 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 pakej ini. Pengecualian biasanya diikuti oleh mesej yang menyatakan apa yang salah.

11. Kesimpulannya

Dalam tutorial ini, kami melihat JSON menggunakan Java - org.json - dan kami memberi tumpuan kepada beberapa fungsi teras yang terdapat di sini.

Coretan kod lengkap yang digunakan dalam artikel ini boleh didapati di GitHub.