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249 lines
8.7 KiB
Markdown
249 lines
8.7 KiB
Markdown
# SAX
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The term "SAX" originated from [Simple API for XML](http://en.wikipedia.org/wiki/Simple_API_for_XML). We borrowed this term for JSON parsing and generation.
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In RapidJSON, `Reader` (typedef of `GenericReader<...>`) is the SAX-style parser for JSON, and `Writer` (typedef of `GenericWriter<...>`) is the SAX-style generator for JSON.
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[TOC]
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# Reader {#Reader}
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`Reader` parses a JSON from a stream. While it reads characters from the stream, it analyze the characters according to the syntax of JSON, and publish events to a handler.
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For example, here is a JSON.
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~~~~~~~~~~js
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{
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"hello": "world",
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"t": true ,
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"f": false,
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"n": null,
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"i": 123,
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"pi": 3.1416,
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"a": [1, 2, 3, 4]
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}
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~~~~~~~~~~
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While a `Reader` parses the JSON, it will publish the following events to the handler sequentially:
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~~~~~~~~~~
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BeginObject()
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String("hello", 5, true)
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String("world", 5, true)
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String("t", 1, true)
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Bool(true)
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String("f", 1, true)
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Bool(false)
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String("n", 1, true)
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Null()
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String("i")
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UInt(123)
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String("pi")
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Double(3.1416)
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String("a")
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BeginArray()
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Uint(1)
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Uint(2)
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Uint(3)
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Uint(4)
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EndArray(4)
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EndObject(7)
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~~~~~~~~~~
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These events can be easily match up with the JSON, except some event parameters need further explanation. Let's see the code which produces exactly the same output as above:
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~~~~~~~~~~cpp
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#include "rapidjson/reader.h"
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#include <iostream>
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using namespace rapidjson;
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using namespace std;
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struct MyHandler {
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void Null() { cout << "Null()" << endl; }
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void Bool(bool b) { cout << "Bool(" << (b ? "true" : "false") << ")" << endl; }
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void Int(int i) { cout << "Int(" << i << ")" << endl; }
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void Uint(unsigned u) { cout << "Uint(" << u << ")" << endl; }
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void Int64(int64_t i) { cout << "Int64(" << i << ")" << endl; }
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void Uint64(uint64_t u) { cout << "Uint64(" << u << ")" << endl; }
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void Double(double d) { { cout << "Double(" << d << ")" << endl; }
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void String(const char* str, SizeType length, bool copy) {
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cout << "String(" << str << ", " << length << ", " << (b ? "true" : "false") << ")" << endl; }
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void StartObject() { cout << "StartObject()" << endl; }
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void EndObject(SizeType memberCount) { cout << "EndObject(" << memberCount << ")" << endl; }
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void StartArray() { cout << "StartArray()" << endl; }
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void EndArray(SizeType elementCount) { cout << "EndArray(" << elementCount << ")" << endl; }
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};
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void main() {
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const char* json = "...";
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MyHandler handler;
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Reader<MyHandler> reader;
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StringStream ss(json);
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reader.Parse(ss, handler);
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}
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~~~~~~~~~~
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Note that, RapidJSON uses template to statically bind the `Reader` type and the handler type, instead of using class with virtual functions. This paradigm can improve the performance by inlining functions.
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## Handler {#Handler}
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As the previous example showed, user needs to implement a handler, which consumes the events (function calls) from `Reader`. The handler concept has the following member type and member functions.
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~~~~~~~~~~cpp
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concept Handler {
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typename Ch;
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void Null();
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void Bool(bool b);
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void Int(int i);
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void Uint(unsigned i);
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void Int64(int64_t i);
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void Uint64(uint64_t i);
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void Double(double d);
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void String(const Ch* str, SizeType length, bool copy);
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void StartObject();
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void EndObject(SizeType memberCount);
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void StartArray();
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void EndArray(SizeType elementCount);
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};
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~~~~~~~~~~
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`Null()` is called when the `Reader` encounters a JSON null value.
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`Bool(bool)` is called when the `Reader` encounters a JSON true or false value.
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When the `Reader` encounters a JSON number, it chooses a suitable C++ type mapping. And then it calls *one* function out of `Int(int)`, `Uint(unsigned)`, `Int64(int64_t)`, `Uint64(uint64_t)` and `Double(double)`.
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`String(const char* str, SizeType length, bool copy)` is called when the `Reader` encounters a string. The first parameter is pointer to the string. The second parameter is the length of the string (excluding the null terminator). Note that RapidJSON supports null character `'\0'` inside a string. If such situation happens, `strlen(str) < length`. The last `copy` indicates whether the handler needs to make a copy of the string. For normal parsing, `copy = true`. Only when *insitu* parsing is used, `copy = false`. And beware that, the character type depends on the target encoding, which will be explained later.
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When the `Reader` encounters the beginning of an object, it calls `StartObject()`. An object in JSON is a set of name-value pairs. If the object contains members it first calls `String()` for the name of member, and then calls functions depending on the type of the value. These calls of name-value pairs repeats until calling `EndObject(SizeType memberCount)`. Note that the `memberCount` parameter is just an aid for the handler, user may not need this parameter.
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Array is similar to object but simpler. At the beginning of an array, the `Reader` calls `BeginArary()`. If there is elements, it calls functions according to the types of element. Similarly, in the last call `EndArray(SizeType elementCount)`, the parameter `elementCount` is just an aid for the handler.
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## GenericReader {#GenericReader}
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As mentioned before, `Reader` is a typedef of a template class `GenericReader`:
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~~~~~~~~~~cpp
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namespace rapidjson {
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template <typename SourceEncoding, typename TargetEncoding, typename Allocator = MemoryPoolAllocator<> >
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class GenericReader {
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// ...
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};
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typedef GenericReader<UTF8<>, UTF8<> > Reader;
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} // namespace rapidjson
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~~~~~~~~~~
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The `Reader` uses UTF-8 as both source and target encoding. The source encoding means the encoding in the JSON stream. The target encoding means the encoding of the `str` parameter in `String()` calls. For example, to parse a UTF-8 stream and outputs UTF-16 string events, you can define a reader by:
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~~~~~~~~~~cpp
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GenericReader<UTF8<>, UTF16<> > reader;
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~~~~~~~~~~
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Note that, the default character type of `UTF16` is `wchar_t`. So this `reader`needs to call `String(const wchar_t*, SizeType, bool)` of the handler.
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The third template parameter `Allocator` is the allocator type for internal data structure (actually a stack).
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## Parsing {#Parsing}
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The one and only one function of `Reader` is to parse JSON.
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~~~~~~~~~~cpp
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template <unsigned parseFlags, typename InputStream, typename Handler>
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bool Parse(InputStream& is, Handler& handler);
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// with parseFlags = kDefaultParseFlags
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template <typename InputStream, typename Handler>
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bool Parse(InputStream& is, Handler& handler);
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~~~~~~~~~~
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If an error occurs during parsing, it will return `false`. User can also calls `bool HasParseEror()`, `ParseErrorCode GetParseErrorCode()` and `size_t GetErrorOffset()` to obtain the error states. Actually `Document` uses these `Reader` functions to obtain parse errors. Please refer to [DOM](doc/dom.md) for details about parse error.
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# Writer {#Writer}
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## PrettyWriter {#PrettyWriter}
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# Techniques {#Techniques}
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## Parsing JSON to Custom Data Structure {#CustomDataStructure}
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`Document`'s parsing capability is completely based on `Reader`. Actually `Document` is a handler which receives events from a reader to build a DOM during parsing.
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User may uses `Reader` to build other data structures directly. This eliminates building of DOM, thus reducing memory and improving performance.
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Example:
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~~~~~~~~~~cpp
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// Note: Ad hoc, not yet tested.
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using namespace std;
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using namespace rapidjson;
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typedef map<string, string> MessageMap;
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struct MessageHandler : public GenericBaseHandler<> {
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MessageHandler() : mState(kExpectStart) {
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}
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bool Default() {
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return false;
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}
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bool StartObject() {
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if (!kBeforeStart)
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return false;
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mState = mExpectName;
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return true;
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}
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bool String(const Ch* str, SizeType length, bool copy) {
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if (mState == kExpectName) {
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name_ = string(str, length);
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return true;
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}
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else if (mState == kExpectValue) {
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messages_.insert(MessageMap::value_type(name_, string(str, length)));
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return true;
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}
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else
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return false;
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}
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bool EndObject() {
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return mState == kExpectName;
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}
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MessageMap messages_;
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enum State {
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kExpectObjectStart,
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kExpectName,
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kExpectValue,
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}mState;
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std::string name_;
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};
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void ParseMessages(const char* json, MessageMap& messages) {
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Reader reader;
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MessageHandler handler;
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StringStream ss(json);
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if (reader.Parse(ss, handler))
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messages.swap(handler.messages_);
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}
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main() {
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MessageMap messages;
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ParseMessages("{ \"greeting\" : \"Hello!\", \"farewell\" : \"bye-bye!\" }", messages);
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}
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~~~~~~~~~~
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~~~~~~~~~~cpp
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// Parse a NxM array
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const char* json = "[3, 4, [1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]"
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~~~~~~~~~~
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## Filtering of JSON {#Filtering}
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