如何编写一个JSON解析器
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Long或Double;
- true/false:Java的Boolean;
- null:Java的null;
- [array]:Java的List<Object>或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http://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Object和Array可以嵌套,在读取过程中,使用一个栈来存储Object和Array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_OBJECT和END_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、{或[。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。
来自:http://www.liaoxuefeng.com/article/0014211269349633dda29ee3f29413c91fa65c372585f23000