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_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。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
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);
            }
        }
    }
}
上一篇:JavaScript遍历集合(for...of/for...in/forEach)


下一篇:ArcGIS API for Silverlight 地图元素点闪烁,线流动显示的处理方式