json parser

快放假了上班没啥事,参考https://segmentfault.com/a/1190000038551193摸了个json解析

public class JsonParser {
    // json文本
    private char[] source;
    // 当前解析到的位置
    private int i;
    // 当前需要解析的字符
    private char ch;

    private static JsonParser parser;

    public static JsonParser getInstance() {
        if (parser == null) {
            parser = new JsonParser();
        }
        return parser;
    }

    public static Object parse(String json) {
        parser = getInstance();
        parser.init(json);
        return parser.parse();
    }

    private void init(String json) {
        json += " ";
        this.source = json.toCharArray();
        this.i = -1;
        this.ch = ' ';
    }

    private char next(char... expected) {
        if (expected.length > 0 && expected[0] != ch) {
            throw new RuntimeException(String.format("期望是: '%s', 实际上是: '%s'", expected[0], ch));
        }
        ch = source[++i];
        return ch;
    }

    private void skipWhite() {
        while (ch == '\n' || ch == '\t' || ch == ' ') {
            next();
        }
    }

    public Object parse() {
        skipWhite();
        switch (ch) {
            case '{':
                return parseObject();
            case '"':
                return parseString();
            case '[':
                return parseArray();
            case '-':
                return parseNumber();
            default:
                return ch >= '0' && ch <= '9' ? parseNumber() : parseBoolAndNull();
        }
    }

    private Object parseBoolAndNull() {
        switch (ch) {
            case 't':
                next('t');
                next('r');
                next('u');
                next('e');
                return true;
            case 'f':
                next('f');
                next('a');
                next('l');
                next('s');
                next('e');
                return false;
            case 'n':
                next('n');
                next('u');
                next('l');
                next('l');
                return null;
        }
        throw new RuntimeException("解析bool或是null异常");
    }

    // "k": 10, "k2": -10.5
    private BigDecimal parseNumber() {
        String number = "";
        // 负数
        if (ch == '-') {
            next('-');
            number += "-";
        }
        while (ch >= '0' && ch <= '9') {
            number += ch;
            next();
        }
        // 小数
        if (ch == '.') {
            next('.');
            number += '.';
            while (ch >= '0' && ch <= '9') {
                number += ch;
                next();
            }
        }
        return new BigDecimal(number);
    }

    // [xx, xx, xx]
    private List<Object> parseArray() {
        List<Object> list = new ArrayList<>();
        next('[');
        skipWhite();
        if (ch == ']') {
            next(']');
            return list;
        }

        while (i < source.length) {
            // 解析元素
            list.add(parse());
            skipWhite();
            if (ch == ']') {
                next(']');
                return list;
            }
            next(',');
            skipWhite();
        }
        throw new RuntimeException("解析array异常");
    }

    // 此时 ch为第一个"
    // 解析 "..." 之间的内容
    private String parseString() {
        next('"');
        String str = "";
        while (ch != '"') {
            str += ch;
            next();
        }
        next('"');
        return str;
    }

    private Map<String, Object> parseObject() {
        Map<String, Object> result = new HashMap<>();

        next('{');
        skipWhite();
        // 空json {}
        if (ch == '}') {
            next('}');
            return new HashMap<>();
        }

        while (i < source.length) {
            // 解析key
            String key = parseString();
            skipWhite();
            next(':');
            // 冒号后面不确定是啥
            result.put(key, parse());
            skipWhite();
            if (ch == '}') {
                next('}');
                return result;
            }
            next(',');
            skipWhite();
        }
        throw new RuntimeException("解析object异常");
    }


    public static void print(Object result, String skip) {
        if (result instanceof Map) {
            Map<String, Object> map = (Map<String, Object>) result;
            for (Map.Entry<String, Object> entry : map.entrySet()) {
                System.out.print(skip + entry.getKey() + ": \n");
                print(entry.getValue(), skip + "=====");
            }
        } else if (result instanceof List) {
            List<Object> list = (List<Object>) result;
            for (Object o : list) {
                print(o, skip);
            }
        } else {
            System.out.println(skip + result);
        }
    }

    public static void main(String[] args) {
        String json = "{\"k1\" : \"zz\", \"k2\" : {\"kk1\": 10.5, \"kk2\": -9.9}, \"k3\": [614, \"b\", {\"kkk1\": \"nanase\", \"kkk2\": true, \"kkk3\": false, \"kkk4\": null, \"kkkk5\": []}]}";
        Object result = JsonParser.parse(json);
        System.out.println(result);
        JsonParser.print(result, "");
    }

}
image.png

推荐阅读更多精彩内容