实现 JSON 解析器的Java程序

JSON 通常用于在 Web 服务器之间来回交换数据。这里的关键是当我们从网络服务器获取数据时，它总是作为字符串返回，所以我们不需要在我们的Java程序中小心。

插图：

{"Geeks", "NIT", "Male", "30"}

此数据由 JSON 解析器作为 JavaScript 对象返回，因此它变为如下所示：

const object = JSON.parse({"Geeks", "NIT", "Male", "30"}) ;

所以在JSON解析数据的同时，对数组进行同样的处理。让我们编写一个Java程序调用方法，以便更简单地掌握JSON解析器是如何实现的。

例子

Java

// Java Program to Implement JSON Parser
  
// Importing required classes
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
  
// Defining constants for json parsers
enum CONSTANTS {
  
    CURLY_OPEN_BRACKETS('{'),
    CURLY_CLOSE_BRACKETS('}'),
    SQUARE_OPEN_BRACKETS('['),
    SQUARE_CLOSE_BRACKETS(']'),
    COLON(':'),
    COMMA(','),
    SPECIAL('|');
  
    private final char constant;
  
    // Constructor
    CONSTANTS(char constant) { this.constant = constant; }
  
    // Method
    // Overriding exiting toString() metohd
    @Override public String toString()
    {
        return String.valueOf(constant);
    }
}
  
// Class 1
// To parse json object
class JSONObject {
  
    private final static char specialChar;
    private final static char commaChar;
    private HashMap objects;
  
    static
    {
        specialChar = String.valueOf(CONSTANTS.SPECIAL)
                          .toCharArray()[0];
        commaChar = String.valueOf(CONSTANTS.COMMA)
                        .toCharArray()[0];
    }
  
    // Constructor if this class
    public JSONObject(String arg) { getJSONObjects(arg); }
  
    // Method 1
    // Storing json objects as key value pair in hash map
    public void getJSONObjects(String arg)
    {
  
        objects = new HashMap();
  
        if (arg.startsWith(String.valueOf(
                CONSTANTS.CURLY_OPEN_BRACKETS))
            && arg.endsWith(String.valueOf(
                CONSTANTS.CURLY_CLOSE_BRACKETS))) {
  
            StringBuilder builder = new StringBuilder(arg);
            builder.deleteCharAt(0);
            builder.deleteCharAt(builder.length() - 1);
            builder = replaceCOMMA(builder);
  
            for (String objects : builder.toString().split(
                     String.valueOf(CONSTANTS.COMMA))) {
  
                String[] objectValue = objects.split(
                    String.valueOf(CONSTANTS.COLON), 2);
  
                if (objectValue.length == 2)
                    this.objects.put(
                        objectValue[0]
                            .replace("'", "")
                            .replace("\"", ""),
                        objectValue[1]
                            .replace("'", "")
                            .replace("\"", ""));
            }
        }
    }
  
    // Method 2
    public StringBuilder replaceCOMMA(StringBuilder arg)
    {
  
        boolean isJsonArray = false;
  
        for (int i = 0; i < arg.length(); i++) {
            char a = arg.charAt(i);
  
            if (isJsonArray) {
  
                if (String.valueOf(a).compareTo(
                        String.valueOf(CONSTANTS.COMMA))
                    == 0) {
                    arg.setCharAt(i, specialChar);
                }
            }
  
            if (String.valueOf(a).compareTo(String.valueOf(
                    CONSTANTS.SQUARE_OPEN_BRACKETS))
                == 0)
                isJsonArray = true;
            if (String.valueOf(a).compareTo(String.valueOf(
                    CONSTANTS.SQUARE_CLOSE_BRACKETS))
                == 0)
                isJsonArray = false;
        }
  
        return arg;
    }
  
    // Method 3
    // Getting json object value by key from hash map
    public String getValue(String key)
    {
        if (objects != null) {
            return objects.get(key).replace(specialChar,
                                            commaChar);
        }
        return null;
    }
  
    // Method 4
    // Getting json array by key from hash map
    public JSONArray getJSONArray(String key)
    {
        if (objects != null)
            return new JSONArray(
                objects.get(key).replace('|', ','));
        return null;
    }
}
  
// Class 2
// To parse json array
class JSONArray {
  
    private final static char specialChar;
    private final static char commaChar;
  
    private ArrayList objects;
  
    static
    {
        specialChar = String.valueOf(CONSTANTS.SPECIAL)
                          .toCharArray()[0];
        commaChar = String.valueOf(CONSTANTS.COMMA)
                        .toCharArray()[0];
    }
  
    // Cosntructor of this class
    public JSONArray(String arg) { getJSONObjects(arg); }
  
    // Method 1
    // Storing json objects in array list
    public void getJSONObjects(String arg)
    {
  
        objects = new ArrayList();
  
        if (arg.startsWith(String.valueOf(
                CONSTANTS.SQUARE_OPEN_BRACKETS))
            && arg.endsWith(String.valueOf(
                CONSTANTS.SQUARE_CLOSE_BRACKETS))) {
  
            StringBuilder builder = new StringBuilder(arg);
  
            builder.deleteCharAt(0);
            builder.deleteCharAt(builder.length() - 1);
  
            builder = replaceCOMMA(builder);
  
            // Adding all elements
            // using addAll() method of Collections class
            Collections.addAll(
                objects,
                builder.toString().split(
                    String.valueOf(CONSTANTS.COMMA)));
        }
    }
  
    // Method 2
    public StringBuilder replaceCOMMA(StringBuilder arg)
    {
        boolean isArray = false;
  
        for (int i = 0; i < arg.length(); i++) {
            char a = arg.charAt(i);
            if (isArray) {
  
                if (String.valueOf(a).compareTo(
                        String.valueOf(CONSTANTS.COMMA))
                    == 0) {
                    arg.setCharAt(i, specialChar);
                }
            }
  
            if (String.valueOf(a).compareTo(String.valueOf(
                    CONSTANTS.CURLY_OPEN_BRACKETS))
                == 0)
                isArray = true;
  
            if (String.valueOf(a).compareTo(String.valueOf(
                    CONSTANTS.CURLY_CLOSE_BRACKETS))
                == 0)
                isArray = false;
        }
  
        return arg;
    }
  
    // Method  3
    // Getting json object by index from array list
    public String getObject(int index)
    {
        if (objects != null) {
            return objects.get(index).replace(specialChar,
                                              commaChar);
        }
  
        return null;
    }
  
    // Method 4
    // Getting json object from array list
    public JSONObject getJSONObject(int index)
    {
  
        if (objects != null) {
            return new JSONObject(
                objects.get(index).replace('|', ','));
        }
  
        return null;
    }
}
  
// Class 3
// To parse json string
public class Parser {
  
    // json string with user data
    // Custom data been passed as in arguments
    private final static String jsonString
        = "{'name':'user','id':1234,'marks':[{'english':85,'physics':80,'chemistry':75}]}";
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Parse json object for user data
        JSONObject user = new JSONObject(jsonString);
  
        // Get json array for user's marks
        JSONArray marks = user.getJSONArray("marks");
  
        // Get json object for subject's marks
        JSONObject subjects = marks.getJSONObject(0);
  
        // Print and display commands
        System.out.println(
            String.format("English marks - %s",
                          subjects.getValue("english")));
        System.out.println(
            String.format("Physics marks - %s",
                          subjects.getValue("physics")));
        System.out.println(
            String.format("Chemistry marks - %s",
                          subjects.getValue("chemistry")));
    }
}

输出

English marks - 85
Physics marks - 80
Chemistry marks - 75