java verifies whether the string is in json format

 

Taken from the  original link

I think it’s a very interesting question. How to verify whether the string is in the standard json format. The first answer is to convert jsonObject or jsonArray. If it can be converted, it is a json string. If it can’t, throw an exception and catch the exception.

But this method is not allowed. So check the information from the string and character.

package com.xiaozu.gateway.util;

import java.text.CharacterIterator;
import java.text.StringCharacterIterator;

/**
 * @author bumblebee
 * @version 1.0
 * @date 2021/1/15 14:18
 * @desc
 */
public class JsonValidator {
    private CharacterIterator it;
    private char c;
    private int col;

    public JsonValidator() {
    }

    /**
     * 验证一个字符串是否是合法的JSON串
     *
     * @param input 要验证的字符串
     * @return true-合法 ，false-非法
     */
    public boolean validate(String input) {
        input = input.trim();
        boolean ret = valid(input);
        return ret;
    }

    private boolean valid(String input) {
        if ("".equals(input)) return true;

        boolean ret = true;
        it = new StringCharacterIterator(input);
        c = it.first();
        col = 1;
        if (!value()) {
            ret = error("value", 1);
        } else {
            skipWhiteSpace();
            if (c != CharacterIterator.DONE) {
                ret = error("end", col);
            }
        }

        return ret;
    }

    private boolean value() {
        return literal("true") || literal("false") || literal("null") || string() || number() || object() || array();
    }

    private boolean literal(String text) {
        CharacterIterator ci = new StringCharacterIterator(text);
        char t = ci.first();
        if (c != t) return false;

        int start = col;
        boolean ret = true;
        for (t = ci.next(); t != CharacterIterator.DONE; t = ci.next()) {
            if (t != nextCharacter()) {
                ret = false;
                break;
            }
        }
        nextCharacter();
        if (!ret) error("literal " + text, start);
        return ret;
    }

    private boolean array() {
        return aggregate('[', ']', false);
    }

    private boolean object() {
        return aggregate('{', '}', true);
    }

    private boolean aggregate(char entryCharacter, char exitCharacter, boolean prefix) {
        if (c != entryCharacter) return false;
        nextCharacter();
        skipWhiteSpace();
        if (c == exitCharacter) {
            nextCharacter();
            return true;
        }

        for (; ; ) {
            if (prefix) {
                int start = col;
                if (!string()) return error("string", start);
                skipWhiteSpace();
                if (c != ':') return error("colon", col);
                nextCharacter();
                skipWhiteSpace();
            }
            if (value()) {
                skipWhiteSpace();
                if (c == ',') {
                    nextCharacter();
                } else if (c == exitCharacter) {
                    break;
                } else {
                    return error("comma or " + exitCharacter, col);
                }
            } else {
                return error("value", col);
            }
            skipWhiteSpace();
        }

        nextCharacter();
        return true;
    }

    private boolean number() {
        if (!Character.isDigit(c) && c != '-') return false;
        int start = col;
        if (c == '-') nextCharacter();
        if (c == '0') {
            nextCharacter();
        } else if (Character.isDigit(c)) {
            while (Character.isDigit(c))
                nextCharacter();
        } else {
            return error("number", start);
        }
        if (c == '.') {
            nextCharacter();
            if (Character.isDigit(c)) {
                while (Character.isDigit(c))
                    nextCharacter();
            } else {
                return error("number", start);
            }
        }
        if (c == 'e' || c == 'E') {
            nextCharacter();
            if (c == '+' || c == '-') {
                nextCharacter();
            }
            if (Character.isDigit(c)) {
                while (Character.isDigit(c))
                    nextCharacter();
            } else {
                return error("number", start);
            }
        }
        return true;
    }

    private boolean string() {
        if (c != '"') return false;

        int start = col;
        boolean escaped = false;
        for (nextCharacter(); c != CharacterIterator.DONE; nextCharacter()) {
            if (!escaped && c == '\\') {
                escaped = true;
            } else if (escaped) {
                if (!escape()) {
                    return false;
                }
                escaped = false;
            } else if (c == '"') {
                nextCharacter();
                return true;
            }
        }
        return error("quoted string", start);
    }

    private boolean escape() {
        int start = col - 1;
        if (" \\\"/bfnrtu".indexOf(c) < 0) {
            return error("escape sequence  \\\",\\\\,\\/,\\b,\\f,\\n,\\r,\\t  or  \\uxxxx ", start);
        }
        if (c == 'u') {
            if (!ishex(nextCharacter()) || !ishex(nextCharacter()) || !ishex(nextCharacter())
                    || !ishex(nextCharacter())) {
                return error("unicode escape sequence  \\uxxxx ", start);
            }
        }
        return true;
    }

    private boolean ishex(char d) {
        return "0123456789abcdefABCDEF".indexOf(c) >= 0;
    }

    private char nextCharacter() {
        c = it.next();
        ++col;
        return c;
    }

    private void skipWhiteSpace() {
        while (Character.isWhitespace(c)) {
            nextCharacter();
        }
    }

    private boolean error(String type, int col) {
        System.out.printf("type: %s, col: %s%s", type, col, System.getProperty("line.separator"));
        return false;
    }

    public static void main(String[] args) {
//        String jsonStr = "{\"website\":\"oschina.net\"}";
        String jsonStr = "[\"BIG_PIC\",\"BIG_PLUS_PIC\",\"READ_PIC\"]";
//        String jsonStr = "{"
//                + " \"ccobjtypeid\": \"1001\","
//                + " \"fromuser\": \"李四\","
//                + " \"touser\": \"张三\","
//                + "  \"desc\": \"描述\","
//                + "  \"subject\": \"主题\","
//                + "  \"attach\": \"3245,3456,4345,4553\","
//                + " \"data\": {"
//                + "    \"desc\": \"测试对象\","
//                + "     \"dataid\": \"22\","
//                + "    \"billno\": \"TEST0001\","
//                + "    \"datarelation\":["
//                + " {"
//                + "  \"dataname\": \"关联对象1\","
//                + "  \"data\": ["
//                + "      {"
//                + "    \"dataid\": \"22\","
//                + "          \"datalineid\": \"1\","
//                + "          \"content1\": \"test1\","
//                + "          \"content2\": \"test2\""
//                + "      }"
//                + "  ]"
//                + " }"
//                + " ]"
//                + "  }"
//                + " }";
        System.out.println(jsonStr+":"+new JsonValidator().validate(jsonStr));
    }
}