如何编写一个JSON解析器

廖雪峰
资深软件开发工程师,业余马拉松选手。

编写一个JSON解析器实际上就是一个函数,它的输入是一个表示JSON的字符串,输出是结构化的对应到语言本身的数据结构。

和XML相比,JSON本身结构非常简单,并且仅有几种数据类型,以Java为例,对应的数据结构是:

  • "string":Java的String
  • number:Java的LongDouble
  • true/false:Java的Boolean
  • null:Java的null
  • [array]:Java的List<Object>Object[]
  • {"key":"value"}:Java的Map<String, Object>

解析JSON和解析XML类似,最终都是解析为内存的一个对象。出于效率考虑,使用流的方式几乎是唯一选择,也就是解析器只从头扫描一遍JSON字符串,就完整地解析出对应的数据结构。

本质上解析器就是一个状态机,只要按照JSON定义的格式(参考https://www.json.org,正确实现状态转移即可。但是为了简化代码,我们也没必要完整地实现一个字符一个字符的状态转移。

解析器的输入应该是一个字符流,所以,第一步是获得Reader,以便能不断地读入下一个字符。

在解析的过程中,我们经常要根据下一个字符来决定状态跳转,此时又涉及到回退的问题,就是某些时候不能用next()取下一个字符,而是用peek()取下一个字符,但字符流的指针不移动。所以,Reader接口不能满足这个需求,应当进一步封装一个CharReader,它可以实现:

  • char next():读取下一个字符,移动Reader指针;
  • char peek():读取下一个字符,不移动Reader指针;
  • String next(int size):读取指定的N个字符并移动指针;
  • boolean hasMore():判断流是否结束。

JSON解析比其他文本解析要简单的地方在于,任何JSON数据类型,只需要根据下一个字符即可确定,仔细总结可以发现,如果peek()返回的字符是某个字符,就可以期望读取的数据类型:

  • {:期待一个JSON object;
  • ::期待一个JSON object的value;
  • ,:期待一个JSON object的下一组key-value,或者一个JSON array的下一个元素;
  • [:期待一个JSON array;
  • t:期待一个true;
  • f:期待一个false;
  • n:期待一个null;
  • ":期待一个string;
  • 0~9:期待一个number。

但是单个字符要匹配的状态太多了,需要进一步把字符流变为Token,可以总结出如下几种Token

  • END_DOCUMENT:JSON文档结束;
  • BEGIN_OBJECT:开始一个JSON object;
  • END_OBJECT:结束一个JSON object;
  • BEGIN_ARRAY:开始一个JSON array;
  • END_ARRAY:结束一个JSON array;
  • SEP_COLON:读取一个冒号;
  • SEP_COMMA:读取一个逗号;
  • STRING:一个String;
  • BOOLEAN:一个true或false;
  • NUMBER:一个number;
  • NULL:一个null。

然后,将CharReader进一步封装为TokenReader,提供以下接口:

  • Token readNextToken():读取下一个Token;
  • boolean readBoolean():读取一个boolean;
  • Number readNumber():读取一个number;
  • String readString():读取一个string;
  • void readNull():读取一个null。

由于JSON的ObjectArray可以嵌套,在读取过程中,使用一个栈来存储ObjectArray是必须的。每当我们读到一个BEGIN_OBJECT时,就创建一个Map并压栈;每当读到一个BEGIN_ARRAY时,就创建一个List并压栈;每当读到一个END_OBJECTEND_ARRAY时,就弹出栈顶元素,并根据新的栈顶元素判断是否压栈。此外,读到Object的Key也必须压栈,读到后面的Value后将Key-Value压入栈顶的Map

如果读到END_DOCUMENT时,栈恰好只剩下一个元素,则读取正确,将该元素返回,读取结束。如果栈剩下不止一个元素,则JSON文档格式不正确。

最后,JsonReader的核心解析代码parse()就是负责从TokenReader中不断读取Token,根据当前状态操作,然后设定下一个Token期望的状态,如果与期望状态不符,则JSON的格式无效。起始状态被设定为STATUS_EXPECT_SINGLE_VALUE | STATUS_EXPECT_BEGIN_OBJECT | STATUS_EXPECT_BEGIN_ARRAY,即期望读取到单个value、{[。循环的退出点是读取到END_DOCUMENT时。

public class JsonReader {
    TokenReader reader;

    public Object parse() {
        Stack stack = new Stack();
        int status = STATUS_EXPECT_SINGLE_VALUE | STATUS_EXPECT_BEGIN_OBJECT | STATUS_EXPECT_BEGIN_ARRAY;
        for (;;) {
            Token currentToken = reader.readNextToken();
            switch (currentToken) {
            case BOOLEAN:
                if (hasStatus(STATUS_EXPECT_SINGLE_VALUE)) {
                    // single boolean:
                    Boolean bool = reader.readBoolean();
                    stack.push(StackValue.newJsonSingle(bool));
                    status = STATUS_EXPECT_END_DOCUMENT;
                    continue;
                }
                if (hasStatus(STATUS_EXPECT_OBJECT_VALUE)) {
                    Boolean bool = reader.readBoolean();
                    String key = stack.pop(StackValue.TYPE_OBJECT_KEY).valueAsKey();
                    stack.peek(StackValue.TYPE_OBJECT).valueAsObject().put(key, bool);
                    status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_OBJECT;
                    continue;
                }
                if (hasStatus(STATUS_EXPECT_ARRAY_VALUE)) {
                    Boolean bool = reader.readBoolean();
                    stack.peek(StackValue.TYPE_ARRAY).valueAsArray().add(bool);
                    status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_ARRAY;
                    continue;
                }
                throw new JsonParseException("Unexpected boolean.", reader.reader.readed);

            case NULL:
                if (hasStatus(STATUS_EXPECT_SINGLE_VALUE)) {
                    // single null:
                    reader.readNull();
                    stack.push(StackValue.newJsonSingle(null));
                    status = STATUS_EXPECT_END_DOCUMENT;
                    continue;
                }
                if (hasStatus(STATUS_EXPECT_OBJECT_VALUE)) {
                    reader.readNull();
                    String key = stack.pop(StackValue.TYPE_OBJECT_KEY).valueAsKey();
                    stack.peek(StackValue.TYPE_OBJECT).valueAsObject().put(key, null);
                    status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_OBJECT;
                    continue;
                }
                if (hasStatus(STATUS_EXPECT_ARRAY_VALUE)) {
                    reader.readNull();
                    stack.peek(StackValue.TYPE_ARRAY).valueAsArray().add(null);
                    status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_ARRAY;
                    continue;
                }
                throw new JsonParseException("Unexpected null.", reader.reader.readed);

            case NUMBER:
                if (hasStatus(STATUS_EXPECT_SINGLE_VALUE)) {
                    // single number:
                    Number number = reader.readNumber();
                    stack.push(StackValue.newJsonSingle(number));
                    status = STATUS_EXPECT_END_DOCUMENT;
                    continue;
                }
                if (hasStatus(STATUS_EXPECT_OBJECT_VALUE)) {
                    Number number = reader.readNumber();
                    String key = stack.pop(StackValue.TYPE_OBJECT_KEY).valueAsKey();
                    stack.peek(StackValue.TYPE_OBJECT).valueAsObject().put(key, number);
                    status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_OBJECT;
                    continue;
                }
                if (hasStatus(STATUS_EXPECT_ARRAY_VALUE)) {
                    Number number = reader.readNumber();
                    stack.peek(StackValue.TYPE_ARRAY).valueAsArray().add(number);
                    status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_ARRAY;
                    continue;
                }
                throw new JsonParseException("Unexpected number.", reader.reader.readed);

            case STRING:
                if (hasStatus(STATUS_EXPECT_SINGLE_VALUE)) {
                    // single string:
                    String str = reader.readString();
                    stack.push(StackValue.newJsonSingle(str));
                    status = STATUS_EXPECT_END_DOCUMENT;
                    continue;
                }
                if (hasStatus(STATUS_EXPECT_OBJECT_KEY)) {
                    String str = reader.readString();
                    stack.push(StackValue.newJsonObjectKey(str));
                    status = STATUS_EXPECT_COLON;
                    continue;
                }
                if (hasStatus(STATUS_EXPECT_OBJECT_VALUE)) {
                    String str = reader.readString();
                    String key = stack.pop(StackValue.TYPE_OBJECT_KEY).valueAsKey();
                    stack.peek(StackValue.TYPE_OBJECT).valueAsObject().put(key, str);
                    status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_OBJECT;
                    continue;
                }
                if (hasStatus(STATUS_EXPECT_ARRAY_VALUE)) {
                    String str = reader.readString();
                    stack.peek(StackValue.TYPE_ARRAY).valueAsArray().add(str);
                    status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_ARRAY;
                    continue;
                }
                throw new JsonParseException("Unexpected char \'\"\'.", reader.reader.readed);

            case SEP_COLON: // :
                if (status == STATUS_EXPECT_COLON) {
                    status = STATUS_EXPECT_OBJECT_VALUE | STATUS_EXPECT_BEGIN_OBJECT | STATUS_EXPECT_BEGIN_ARRAY;
                    continue;
                }
                throw new JsonParseException("Unexpected char \':\'.", reader.reader.readed);

            case SEP_COMMA: // ,
                if (hasStatus(STATUS_EXPECT_COMMA)) {
                    if (hasStatus(STATUS_EXPECT_END_OBJECT)) {
                        status = STATUS_EXPECT_OBJECT_KEY;
                        continue;
                    }
                    if (hasStatus(STATUS_EXPECT_END_ARRAY)) {
                        status = STATUS_EXPECT_ARRAY_VALUE | STATUS_EXPECT_BEGIN_ARRAY | STATUS_EXPECT_BEGIN_OBJECT;
                        continue;
                    }
                }
                throw new JsonParseException("Unexpected char \',\'.", reader.reader.readed);

            case END_ARRAY:
                if (hasStatus(STATUS_EXPECT_END_ARRAY)) {
                    StackValue array = stack.pop(StackValue.TYPE_ARRAY);
                    if (stack.isEmpty()) {
                        stack.push(array);
                        status = STATUS_EXPECT_END_DOCUMENT;
                        continue;
                    }
                    int type = stack.getTopValueType();
                    if (type == StackValue.TYPE_OBJECT_KEY) {
                        // key: [ CURRENT ] ,}
                        String key = stack.pop(StackValue.TYPE_OBJECT_KEY).valueAsKey();
                        stack.peek(StackValue.TYPE_OBJECT).valueAsObject().put(key, array.value);
                        status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_OBJECT;
                        continue;
                    }
                    if (type == StackValue.TYPE_ARRAY) {
                        // xx, xx, [CURRENT] ,]
                        stack.peek(StackValue.TYPE_ARRAY).valueAsArray().add(array.value);
                        status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_ARRAY;
                        continue;
                    }
                }
                throw new JsonParseException("Unexpected char: \']\'.", reader.reader.readed);

            case END_OBJECT:
                if (hasStatus(STATUS_EXPECT_END_OBJECT)) {
                    StackValue object = stack.pop(StackValue.TYPE_OBJECT);
                    if (stack.isEmpty()) {
                        // root object:
                        stack.push(object);
                        status = STATUS_EXPECT_END_DOCUMENT;
                        continue;
                    }
                    int type = stack.getTopValueType();
                    if (type == StackValue.TYPE_OBJECT_KEY) {
                        String key = stack.pop(StackValue.TYPE_OBJECT_KEY).valueAsKey();
                        stack.peek(StackValue.TYPE_OBJECT).valueAsObject().put(key, object.value);
                        status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_OBJECT;
                        continue;
                    }
                    if (type == StackValue.TYPE_ARRAY) {
                        stack.peek(StackValue.TYPE_ARRAY).valueAsArray().add(object.value);
                        status = STATUS_EXPECT_COMMA | STATUS_EXPECT_END_ARRAY;
                        continue;
                    }
                }
                throw new JsonParseException("Unexpected char: \'}\'.", reader.reader.readed);

            case END_DOCUMENT:
                if (hasStatus(STATUS_EXPECT_END_DOCUMENT)) {
                    StackValue v = stack.pop();
                    if (stack.isEmpty()) {
                        return v.value;
                    }
                }
                throw new JsonParseException("Unexpected EOF.", reader.reader.readed);

            case BEGIN_ARRAY:
                if (hasStatus(STATUS_EXPECT_BEGIN_ARRAY)) {
                    stack.push(StackValue.newJsonArray(this.jsonArrayFactory.createJsonArray()));
                    status = STATUS_EXPECT_ARRAY_VALUE | STATUS_EXPECT_BEGIN_OBJECT | STATUS_EXPECT_BEGIN_ARRAY | STATUS_EXPECT_END_ARRAY;
                    continue;
                }
                throw new JsonParseException("Unexpected char: \'[\'.", reader.reader.readed);

            case BEGIN_OBJECT:
                if (hasStatus(STATUS_EXPECT_BEGIN_OBJECT)) {
                    stack.push(StackValue.newJsonObject(this.jsonObjectFactory.createJsonObject()));
                    status = STATUS_EXPECT_OBJECT_KEY | STATUS_EXPECT_BEGIN_OBJECT | STATUS_EXPECT_END_OBJECT;
                    continue;
                }
                throw new JsonParseException("Unexpected char: \'{\'.", reader.reader.readed);
            }
        }
    }
}

详细源码请参考:https://github.com/michaelliao/jsonstream



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