Introduction to JSON-Java

最后修改: 2019-10-09T09:46:06Z

    1. Introduction to JSON-Java

    JSON (an acronym for JavaScript Object Notation) is a lightweight data-interchange format and is most commonly used for client-server communication. It's both easy to read/write and language-independent. A JSON value can be another JSON object, array, number, string, boolean (true/false) or null.

    In this tutorial, we'll see how we can create, manipulate and parse JSON using one of the available JSON processing libraries, i.e., JSON-Java library is also known as org.json.

    2. Pre-Requisite

    Before we get started, we'll need to add the following dependency in our pom.xml:

    <dependency>
    <groupId>org.json</groupId>
    <artifactId>json</artifactId>
    <version>20180130</version>
</dependency>

    The latest version can be found in the Maven Central repository.

    Note that this package has already been included in Android SDK, so we shouldn't include it while using the same.

    3. JSON in Java [package org.json]

    The JSON-Java library is also known as org.json (not to be confused with Google's org.json.simple) provides us with classes that are used to parse and manipulate JSON in Java.

    Furthermore, this library can also convert between JSON, XML, HTTP Headers, Cookies, Comma-Delimited List or Text, etc.

    In this tutorial, we'll have a look at:

    1. JSONObject – similar to Java's native Map like object which stores unordered key-value pairs
    2. JSONArray – an ordered sequence of values similar to Java's native Vector implementation
    3. JSONTokener – a tool that breaks a piece of text into a series of tokens which can be used by JSONObject or JSONArray to parse JSON strings
    4. CDL – a tool that provides methods to convert comma-delimited text into a JSONArray and vice versa
    5. Cookie – converts from JSON String to cookies and vice versa
    6. HTTP – used to convert from JSON String to HTTP headers and vice versa
    7. JSONException – this is a standard exception thrown by this library

    4. JSONObject

    A JSONObject is an unordered collection of key and value pairs, resembling Java's native Map implementations.

    • Keys are unique Strings that cannot be null
    • Values can be anything from a Boolean, Number, String, JSONArray or even a JSONObject.NULL object
    • A JSONObject can be represented by a String enclosed within curly braces with keys and values separated by a colon, and pairs separated by a comma
    • It has several constructors with which to construct a JSONObject

    It also supports the following main methods:

    1. get(String key) – gets the object associated with the supplied key, throws JSONException if the key is not found
    2. opt(String key)- gets the object associated with the supplied key, null otherwise
    3. put(String key, Object value) – inserts or replaces a key-value pair in current JSONObject.

    The put() method is an overloaded method which accepts a key of type String and multiple types for the value.

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

    Let's now discuss some of the main operations supported by this class.

    4.1. Creating JSON Directly from JSONObject

    JSONObject exposes an API similar to Java's Map interface*.* We can use the put() method and supply the key and value as an argument:

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

    Now our JSONObject would look like:

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

    There are seven different overloaded signatures of JSONObject.put() method. While the key can only be unique, non-null String, the value can be anything.

    4.2. Creating JSON from Map

    Instead of directly putting key and values in a JSONObject, we can construct a custom Map and then pass it as an argument to JSONObject‘s constructor.

    This example will produce same results as above:

    Map<String, String> 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<String> 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 String cdt 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.

    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.

    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);

    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", "http://www.example.com/");
jo.put("HTTP-Version", "HTTP/1.1");
String httpStr = HTTP.toString(jo);

    Here, our String httpStr will consist of:

    POST "http://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 \"http://www.example.com/\" HTTP/1.1");

    10. JSONException

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

    This is used across all classes from this package. The exception is usually followed by a message that states what exactly went wrong.

    11. Conclusion

    In this tutorial, we looked at a JSON using Java – org.json – and we focused on some of the core functionality available here.

    The complete code snippets used in this article can be found over on GitHub.