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。
Readernext() 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
- char next()：读取下一个字符，移动
Reader
- char peek()：读取下一个字符，不移动
Reader
- 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。
peek()
- {：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。
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。
CharReader 进一步封装为 TokenReader
- 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。
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_ARRAYEND_DOCUMENTJsonReader 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
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);
}
}
}
}
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);
}
}
}
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