487 lines
20 KiB
Markdown
487 lines
20 KiB
Markdown
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# 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 this JSON, it publishes the following events to the handler sequentially:
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~~~~~~~~~~
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StartObject()
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Key("hello", 5, true)
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String("world", 5, true)
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Key("t", 1, true)
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Bool(true)
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Key("f", 1, true)
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Bool(false)
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Key("n", 1, true)
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Null()
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Key("i")
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UInt(123)
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Key("pi")
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Double(3.1416)
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Key("a")
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StartArray()
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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 matched with the JSON, except some event parameters need further explanation. Let's see the `simplereader` example 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 : public BaseReaderHandler<UTF8<>, MyHandler> {
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bool Null() { cout << "Null()" << endl; return true; }
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bool Bool(bool b) { cout << "Bool(" << boolalpha << b << ")" << endl; return true; }
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bool Int(int i) { cout << "Int(" << i << ")" << endl; return true; }
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bool Uint(unsigned u) { cout << "Uint(" << u << ")" << endl; return true; }
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bool Int64(int64_t i) { cout << "Int64(" << i << ")" << endl; return true; }
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bool Uint64(uint64_t u) { cout << "Uint64(" << u << ")" << endl; return true; }
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bool Double(double d) { cout << "Double(" << d << ")" << endl; return true; }
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bool String(const char* str, SizeType length, bool copy) {
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cout << "String(" << str << ", " << length << ", " << boolalpha << copy << ")" << endl;
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return true;
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}
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bool StartObject() { cout << "StartObject()" << endl; return true; }
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bool Key(const char* str, SizeType length, bool copy) {
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cout << "Key(" << str << ", " << length << ", " << boolalpha << copy << ")" << endl;
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return true;
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}
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bool EndObject(SizeType memberCount) { cout << "EndObject(" << memberCount << ")" << endl; return true; }
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bool StartArray() { cout << "StartArray()" << endl; return true; }
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bool EndArray(SizeType elementCount) { cout << "EndArray(" << elementCount << ")" << endl; return true; }
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};
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void main() {
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const char json[] = " { \"hello\" : \"world\", \"t\" : true , \"f\" : false, \"n\": null, \"i\":123, \"pi\": 3.1416, \"a\":[1, 2, 3, 4] } ";
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MyHandler handler;
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Reader 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 must contain the following member functions.
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~~~~~~~~~~cpp
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class Handler {
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bool Null();
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bool Bool(bool b);
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bool Int(int i);
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bool Uint(unsigned i);
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bool Int64(int64_t i);
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bool Uint64(uint64_t i);
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bool Double(double d);
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bool RawNumber(const Ch* str, SizeType length, bool copy);
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bool String(const Ch* str, SizeType length, bool copy);
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bool StartObject();
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bool Key(const Ch* str, SizeType length, bool copy);
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bool EndObject(SizeType memberCount);
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bool StartArray();
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bool 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)`. If `kParseNumbersAsStrings` is enabled, `Reader` will always calls `RawNumber()` instead.
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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 `Key()` 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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Every handler functions returns a `bool`. Normally it should returns `true`. If the handler encounters an error, it can return `false` to notify event publisher to stop further processing.
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For example, when we parse a JSON with `Reader` and the handler detected that the JSON does not conform to the required schema, then the handler can return `false` and let the `Reader` stop further parsing. And the `Reader` will be in error state with error code `kParseErrorTermination`.
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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 {#SaxParsing}
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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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`Reader` converts (parses) JSON into events. `Writer` does exactly the opposite. It converts events into JSON.
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`Writer` is very easy to use. If your application only need to converts some data into JSON, it may be a good choice to use `Writer` directly, instead of building a `Document` and then stringifying it with a `Writer`.
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In `simplewriter` example, we do exactly the reverse of `simplereader`.
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~~~~~~~~~~cpp
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#include "rapidjson/writer.h"
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#include "rapidjson/stringbuffer.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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void main() {
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StringBuffer s;
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Writer<StringBuffer> writer(s);
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writer.StartObject();
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writer.Key("hello");
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writer.String("world");
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writer.Key("t");
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writer.Bool(true);
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writer.Key("f");
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writer.Bool(false);
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writer.Key("n");
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writer.Null();
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writer.Key("i");
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writer.Uint(123);
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writer.Key("pi");
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writer.Double(3.1416);
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writer.Key("a");
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writer.StartArray();
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for (unsigned i = 0; i < 4; i++)
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writer.Uint(i);
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writer.EndArray();
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writer.EndObject();
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cout << s.GetString() << endl;
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}
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~~~~~~~~~~
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~~~~~~~~~~
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{"hello":"world","t":true,"f":false,"n":null,"i":123,"pi":3.1416,"a":[0,1,2,3]}
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~~~~~~~~~~
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There are two `String()` and `Key()` overloads. One is the same as defined in handler concept with 3 parameters. It can handle string with null characters. Another one is the simpler version used in the above example.
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Note that, the example code does not pass any parameters in `EndArray()` and `EndObject()`. An `SizeType` can be passed but it will be simply ignored by `Writer`.
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You may doubt that, why not just using `sprintf()` or `std::stringstream` to build a JSON?
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There are various reasons:
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1. `Writer` must output a well-formed JSON. If there is incorrect event sequence (e.g. `Int()` just after `StartObject()`), it generates assertion fail in debug mode.
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2. `Writer::String()` can handle string escaping (e.g. converting code point `U+000A` to `\n`) and Unicode transcoding.
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3. `Writer` handles number output consistently.
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4. `Writer` implements the event handler concept. It can be used to handle events from `Reader`, `Document` or other event publisher.
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5. `Writer` can be optimized for different platforms.
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Anyway, using `Writer` API is even simpler than generating a JSON by ad hoc methods.
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## Template {#WriterTemplate}
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`Writer` has a minor design difference to `Reader`. `Writer` is a template class, not a typedef. There is no `GenericWriter`. The following is the declaration.
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~~~~~~~~~~cpp
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namespace rapidjson {
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template<typename OutputStream, typename SourceEncoding = UTF8<>, typename TargetEncoding = UTF8<>, typename Allocator = CrtAllocator<>, unsigned writeFlags = kWriteDefaultFlags>
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class Writer {
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public:
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Writer(OutputStream& os, Allocator* allocator = 0, size_t levelDepth = kDefaultLevelDepth)
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// ...
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};
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} // namespace rapidjson
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~~~~~~~~~~
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The `OutputStream` template parameter is the type of output stream. It cannot be deduced and must be specified by user.
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The `SourceEncoding` template parameter specifies the encoding to be used in `String(const Ch*, ...)`.
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The `TargetEncoding` template parameter specifies the encoding in the output stream.
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The `Allocator` is the type of allocator, which is used for allocating internal data structure (a stack).
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The `writeFlags` are combination of the following bit-flags:
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Parse flags | Meaning
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------------------------------|-----------------------------------
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`kWriteNoFlags` | No flag is set.
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`kWriteDefaultFlags` | Default write flags. It is equal to macro `RAPIDJSON_WRITE_DEFAULT_FLAGS`, which is defined as `kWriteNoFlags`.
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`kWriteValidateEncodingFlag` | Validate encoding of JSON strings.
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`kWriteNanAndInfFlag` | Allow writing of `Infinity`, `-Infinity` and `NaN`.
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Besides, the constructor of `Writer` has a `levelDepth` parameter. This parameter affects the initial memory allocated for storing information per hierarchy level.
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## PrettyWriter {#PrettyWriter}
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While the output of `Writer` is the most condensed JSON without white-spaces, suitable for network transfer or storage, it is not easily readable by human.
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Therefore, RapidJSON provides a `PrettyWriter`, which adds indentation and line feeds in the output.
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The usage of `PrettyWriter` is exactly the same as `Writer`, expect that `PrettyWriter` provides a `SetIndent(Ch indentChar, unsigned indentCharCount)` function. The default is 4 spaces.
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## Completeness and Reset {#CompletenessReset}
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A `Writer` can only output a single JSON, which can be any JSON type at the root. Once the singular event for root (e.g. `String()`), or the last matching `EndObject()` or `EndArray()` event, is handled, the output JSON is well-formed and complete. User can detect this state by calling `Writer::IsComplete()`.
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When a JSON is complete, the `Writer` cannot accept any new events. Otherwise the output will be invalid (i.e. having more than one root). To reuse the `Writer` object, user can call `Writer::Reset(OutputStream& os)` to reset all internal states of the `Writer` with a new output stream.
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# Techniques {#SaxTechniques}
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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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In the following `messagereader` example, `ParseMessages()` parses a JSON which should be an object with key-string pairs.
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~~~~~~~~~~cpp
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#include "rapidjson/reader.h"
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#include "rapidjson/error/en.h"
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#include <iostream>
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#include <string>
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#include <map>
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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
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: public BaseReaderHandler<UTF8<>, MessageHandler> {
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MessageHandler() : state_(kExpectObjectStart) {
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}
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bool StartObject() {
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switch (state_) {
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case kExpectObjectStart:
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state_ = kExpectNameOrObjectEnd;
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return true;
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default:
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return false;
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}
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}
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bool String(const char* str, SizeType length, bool) {
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switch (state_) {
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case kExpectNameOrObjectEnd:
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name_ = string(str, length);
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state_ = kExpectValue;
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return true;
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case kExpectValue:
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messages_.insert(MessageMap::value_type(name_, string(str, length)));
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state_ = kExpectNameOrObjectEnd;
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return true;
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default:
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return false;
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}
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}
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|
|
||
|
|
bool EndObject(SizeType) { return state_ == kExpectNameOrObjectEnd; }
|
||
|
|
|
||
|
|
bool Default() { return false; } // All other events are invalid.
|
||
|
|
|
||
|
|
MessageMap messages_;
|
||
|
|
enum State {
|
||
|
|
kExpectObjectStart,
|
||
|
|
kExpectNameOrObjectEnd,
|
||
|
|
kExpectValue,
|
||
|
|
}state_;
|
||
|
|
std::string name_;
|
||
|
|
};
|
||
|
|
|
||
|
|
void ParseMessages(const char* json, MessageMap& messages) {
|
||
|
|
Reader reader;
|
||
|
|
MessageHandler handler;
|
||
|
|
StringStream ss(json);
|
||
|
|
if (reader.Parse(ss, handler))
|
||
|
|
messages.swap(handler.messages_); // Only change it if success.
|
||
|
|
else {
|
||
|
|
ParseErrorCode e = reader.GetParseErrorCode();
|
||
|
|
size_t o = reader.GetErrorOffset();
|
||
|
|
cout << "Error: " << GetParseError_En(e) << endl;;
|
||
|
|
cout << " at offset " << o << " near '" << string(json).substr(o, 10) << "...'" << endl;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
int main() {
|
||
|
|
MessageMap messages;
|
||
|
|
|
||
|
|
const char* json1 = "{ \"greeting\" : \"Hello!\", \"farewell\" : \"bye-bye!\" }";
|
||
|
|
cout << json1 << endl;
|
||
|
|
ParseMessages(json1, messages);
|
||
|
|
|
||
|
|
for (MessageMap::const_iterator itr = messages.begin(); itr != messages.end(); ++itr)
|
||
|
|
cout << itr->first << ": " << itr->second << endl;
|
||
|
|
|
||
|
|
cout << endl << "Parse a JSON with invalid schema." << endl;
|
||
|
|
const char* json2 = "{ \"greeting\" : \"Hello!\", \"farewell\" : \"bye-bye!\", \"foo\" : {} }";
|
||
|
|
cout << json2 << endl;
|
||
|
|
ParseMessages(json2, messages);
|
||
|
|
|
||
|
|
return 0;
|
||
|
|
}
|
||
|
|
~~~~~~~~~~
|
||
|
|
|
||
|
|
~~~~~~~~~~
|
||
|
|
{ "greeting" : "Hello!", "farewell" : "bye-bye!" }
|
||
|
|
farewell: bye-bye!
|
||
|
|
greeting: Hello!
|
||
|
|
|
||
|
|
Parse a JSON with invalid schema.
|
||
|
|
{ "greeting" : "Hello!", "farewell" : "bye-bye!", "foo" : {} }
|
||
|
|
Error: Terminate parsing due to Handler error.
|
||
|
|
at offset 59 near '} }...'
|
||
|
|
~~~~~~~~~~
|
||
|
|
|
||
|
|
The first JSON (`json1`) was successfully parsed into `MessageMap`. Since `MessageMap` is a `std::map`, the printing order are sorted by the key. This order is different from the JSON's order.
|
||
|
|
|
||
|
|
In the second JSON (`json2`), `foo`'s value is an empty object. As it is an object, `MessageHandler::StartObject()` will be called. However, at that moment `state_ = kExpectValue`, so that function returns `false` and cause the parsing process be terminated. The error code is `kParseErrorTermination`.
|
||
|
|
|
||
|
|
## Filtering of JSON {#Filtering}
|
||
|
|
|
||
|
|
As mentioned earlier, `Writer` can handle the events published by `Reader`. `condense` example simply set a `Writer` as handler of a `Reader`, so it can remove all white-spaces in JSON. `pretty` example uses the same relationship, but replacing `Writer` by `PrettyWriter`. So `pretty` can be used to reformat a JSON with indentation and line feed.
|
||
|
|
|
||
|
|
Actually, we can add intermediate layer(s) to filter the contents of JSON via these SAX-style API. For example, `capitalize` example capitalize all strings in a JSON.
|
||
|
|
|
||
|
|
~~~~~~~~~~cpp
|
||
|
|
#include "rapidjson/reader.h"
|
||
|
|
#include "rapidjson/writer.h"
|
||
|
|
#include "rapidjson/filereadstream.h"
|
||
|
|
#include "rapidjson/filewritestream.h"
|
||
|
|
#include "rapidjson/error/en.h"
|
||
|
|
#include <vector>
|
||
|
|
#include <cctype>
|
||
|
|
|
||
|
|
using namespace rapidjson;
|
||
|
|
|
||
|
|
template<typename OutputHandler>
|
||
|
|
struct CapitalizeFilter {
|
||
|
|
CapitalizeFilter(OutputHandler& out) : out_(out), buffer_() {
|
||
|
|
}
|
||
|
|
|
||
|
|
bool Null() { return out_.Null(); }
|
||
|
|
bool Bool(bool b) { return out_.Bool(b); }
|
||
|
|
bool Int(int i) { return out_.Int(i); }
|
||
|
|
bool Uint(unsigned u) { return out_.Uint(u); }
|
||
|
|
bool Int64(int64_t i) { return out_.Int64(i); }
|
||
|
|
bool Uint64(uint64_t u) { return out_.Uint64(u); }
|
||
|
|
bool Double(double d) { return out_.Double(d); }
|
||
|
|
bool RawNumber(const char* str, SizeType length, bool copy) { return out_.RawNumber(str, length, copy); }
|
||
|
|
bool String(const char* str, SizeType length, bool) {
|
||
|
|
buffer_.clear();
|
||
|
|
for (SizeType i = 0; i < length; i++)
|
||
|
|
buffer_.push_back(std::toupper(str[i]));
|
||
|
|
return out_.String(&buffer_.front(), length, true); // true = output handler need to copy the string
|
||
|
|
}
|
||
|
|
bool StartObject() { return out_.StartObject(); }
|
||
|
|
bool Key(const char* str, SizeType length, bool copy) { return String(str, length, copy); }
|
||
|
|
bool EndObject(SizeType memberCount) { return out_.EndObject(memberCount); }
|
||
|
|
bool StartArray() { return out_.StartArray(); }
|
||
|
|
bool EndArray(SizeType elementCount) { return out_.EndArray(elementCount); }
|
||
|
|
|
||
|
|
OutputHandler& out_;
|
||
|
|
std::vector<char> buffer_;
|
||
|
|
};
|
||
|
|
|
||
|
|
int main(int, char*[]) {
|
||
|
|
// Prepare JSON reader and input stream.
|
||
|
|
Reader reader;
|
||
|
|
char readBuffer[65536];
|
||
|
|
FileReadStream is(stdin, readBuffer, sizeof(readBuffer));
|
||
|
|
|
||
|
|
// Prepare JSON writer and output stream.
|
||
|
|
char writeBuffer[65536];
|
||
|
|
FileWriteStream os(stdout, writeBuffer, sizeof(writeBuffer));
|
||
|
|
Writer<FileWriteStream> writer(os);
|
||
|
|
|
||
|
|
// JSON reader parse from the input stream and let writer generate the output.
|
||
|
|
CapitalizeFilter<Writer<FileWriteStream> > filter(writer);
|
||
|
|
if (!reader.Parse(is, filter)) {
|
||
|
|
fprintf(stderr, "\nError(%u): %s\n", (unsigned)reader.GetErrorOffset(), GetParseError_En(reader.GetParseErrorCode()));
|
||
|
|
return 1;
|
||
|
|
}
|
||
|
|
|
||
|
|
return 0;
|
||
|
|
}
|
||
|
|
~~~~~~~~~~
|
||
|
|
|
||
|
|
Note that, it is incorrect to simply capitalize the JSON as a string. For example:
|
||
|
|
~~~~~~~~~~
|
||
|
|
["Hello\nWorld"]
|
||
|
|
~~~~~~~~~~
|
||
|
|
|
||
|
|
Simply capitalizing the whole JSON would contain incorrect escape character:
|
||
|
|
~~~~~~~~~~
|
||
|
|
["HELLO\NWORLD"]
|
||
|
|
~~~~~~~~~~
|
||
|
|
|
||
|
|
The correct result by `capitalize`:
|
||
|
|
~~~~~~~~~~
|
||
|
|
["HELLO\nWORLD"]
|
||
|
|
~~~~~~~~~~
|
||
|
|
|
||
|
|
More complicated filters can be developed. However, since SAX-style API can only provide information about a single event at a time, user may need to book-keeping the contextual information (e.g. the path from root value, storage of other related values). Some processing may be easier to be implemented in DOM than SAX.
|