最近在一个需要高并发的项目中使用到json，最开始使用jsoncpp进行解析和序列化，但是在json数据比较大时jsoncpp会抛异常，在多线程的情况下抛异常的概率更加频繁！后来发现是因为jsoncpp版本比较老，老版本的jsoncpp不是多线程安全的（参考 https://github.com/open-source-parsers/jsoncpp/issues/823） 更新到最新的版本后，处理性能达不到预期，于是查找原因，经过hotspots分析，发现大部分时间消耗在jsoncpp的readValue函数，在一些存在大量float数值的json中解析数值的操作耗时较大。

于是找到了rapidjson，根据benchmark结果看起来不错，经过实际测试发现在40线程的压力测试下相比jsoncpp性能提升10倍左右，一些存在大量float数值的case性能提升更明显！

如下，整理一下rapidjson使用的方法：

1. 通用解析方法放在rapidjson_utils.hpp文件中

#ifndef rapidjson_utils_h__
#define rapidjson_utils_h__
#include "rapidjson/rapidjson.h"
#include "rapidjson/document.h"
#include "rapidjson/pointer.h"
#include "rapidjson/writer.h"
#include "rapidjson/prettywriter.h"
#include "rapidjson/error/en.h"
#include <string>
namespace utils
{
    using std::string;
    static int get_rapidjson_string(const rapidjson::Value& js_in,
        std::string key,
        std::string& value) {
        rapidjson::Pointer pointer(key.c_str());
        if (!pointer.IsValid()) {
            return -1;
        }
        const rapidjson::Value *pValue = rapidjson::GetValueByPointer(js_in, pointer);
        if (NULL == pValue || !pValue->IsString()) {
            return -1;
        }
        value = pValue->GetString();
        return 0;
    }
    static int get_rapidjson_int(const rapidjson::Value& js_in,
        std::string key,
        int& value) {
        rapidjson::Pointer pointer(key.c_str());
        if (!pointer.IsValid()) {
            return -1;
        }
        const rapidjson::Value *pValue = rapidjson::GetValueByPointer(js_in, pointer);
        if (NULL == pValue || !pValue->IsNumber()) {
            return -1;
        }
        value = pValue->GetInt();
        return 0;
    }
    ///< get float number
    static int get_rapidjson_number(const rapidjson::Value& js_in,
        std::string key,
        float& value) {
        rapidjson::Pointer pointer(key.c_str());
        if (!pointer.IsValid()) {
            return -1;
        }
        const rapidjson::Value *pValue = rapidjson::GetValueByPointer(js_in, pointer);
        if (NULL == pValue || !pValue->IsNumber()) {
            return -1;
        }
        rapidjson::Type valueType = pValue->GetType();
        switch (valueType) {
        case rapidjson::Type::kNumberType:
            value = pValue->GetDouble();
            break;
        default:
            return -1;
        }
        return 0;
    }
    static int get_rapidjson_bool(const rapidjson::Value& js_in,
        std::string key,
        bool& value) {
        rapidjson::Pointer pointer(key.c_str());
        if (!pointer.IsValid()) {
            return -1;
        }
        const rapidjson::Value *pValue = rapidjson::GetValueByPointer(js_in, pointer);
        if (NULL == pValue || !pValue->IsBool()) {
            return -1;
        }
        value = pValue->GetBool();
        return 0;
    }
    static void add_member_string(rapidjson::Value& node, string key, string value, rapidjson::Document::AllocatorType &alloc) {
        node.AddMember(rapidjson::Value(key.c_str(), key.length(), alloc).Move(),
            rapidjson::Value(value.c_str(), value.length(), alloc).Move(), alloc);
    }
    static void add_member_int(rapidjson::Value& node, string key, int value,
        rapidjson::Document::AllocatorType &alloc) {
        node.AddMember(rapidjson::Value(key.c_str(), key.length(), alloc).Move(),
            rapidjson::Value(value).Move(), alloc);
    }
    static void add_member_float(rapidjson::Value& node, string key, double value,
        rapidjson::Document::AllocatorType &alloc) {
        node.AddMember(rapidjson::Value(key.c_str(), key.length(), alloc).Move(),
            rapidjson::Value(value).Move(), alloc);
    }
    static void add_member_bool(rapidjson::Value& node, string key, bool value,
        rapidjson::Document::AllocatorType &alloc) {
        node.AddMember(rapidjson::Value(key.c_str(), key.length(), alloc).Move(),
            rapidjson::Value(value).Move(), alloc);
    }
    static string to_string(const rapidjson::Value& v) {
        rapidjson::StringBuffer sb;
        rapidjson::Writer<rapidjson::StringBuffer> writer(sb);
        v.Accept(writer);
        std::string sResult = sb.GetString();
        return sResult;
    }
    static string to_styled_string(const rapidjson::Value& v) {
        rapidjson::StringBuffer sb;
        rapidjson::PrettyWriter<rapidjson::StringBuffer> writer(sb);
        v.Accept(writer);
        std::string s = sb.GetString();
        return s;
    }
} //namespace utils
#endif // rapidjson_utils_h__

2. 测试代码放在rapidjson_example.cpp中

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include "rapidjson_utils.hpp"
using std::string;
int main(int argc, char** argv)
{
    string data = "{\"user\":\"hello\",\"movies\":[{\"name\":\"TENET\",\"like\":10000}],\"song\":{\"暗里着迷\":\"已经播放了3遍\"},\"ratio\":0.98}";
    rapidjson::Document doc;
    rapidjson::ParseResult ok = doc.Parse(data.c_str());
    if (!ok) {
        printf("parse json failure id=%d offset=%d msg=%s\n", doc.GetParseError(), doc.GetErrorOffset(), rapidjson::GetParseError_En(doc.GetParseError()));
        return -1;
    }
    string user;
    utils::get_rapidjson_string(doc, "/user", user);
    printf("user=%s\n", user.c_str());
    ///< 使用迭代器遍历数组
    const rapidjson::Value* pMovies = rapidjson::GetValueByPointer(doc, "/movies");
    if (nullptr != pMovies && pMovies->IsArray() && !pMovies->Empty()) {
        for (rapidjson::Value::ConstValueIterator iter = pMovies->Begin(); iter != pMovies->End(); iter++) {
            string name;
            int like;
            utils::get_rapidjson_string(*iter, "/name", name);
            utils::get_rapidjson_int(*iter, "/like", like);
            printf("movie=%s like=%d\n", name.c_str(), like);
        }
    }
    const rapidjson::Value* pSong = rapidjson::GetValueByPointer(doc, "/song");
    if (nullptr != pSong && pSong->IsObject() && !pSong->ObjectEmpty()) {
        ///< 使用迭代器遍历object
        const rapidjson::Value& song = *pSong;
        for (rapidjson::Value::ConstMemberIterator miter = song.MemberBegin(); miter != song.MemberEnd(); miter++) {
            const rapidjson::Value& name = miter->name;
            const rapidjson::Value& value = miter->value;
            printf("song=%s message=%s\n", name.GetString(), value.GetString());
        }
    }
    float ratio;
    utils::get_rapidjson_number(doc, "/ratio", ratio);
    printf("ratio=%f\n", ratio);
    ///< 生成一个新的json
    rapidjson::Document out(rapidjson::kObjectType);
    rapidjson::Document::AllocatorType &a = out.GetAllocator();
    out.CopyFrom(doc, a);
    rapidjson::Value* pNewMovies = rapidjson::GetValueByPointer(out, "/movies");
    rapidjson::Value movie(rapidjson::kObjectType);
    utils::add_member_string(movie, "name", "Memento", a);
    utils::add_member_int(movie, "like", 12366, a);
    utils::add_member_float(movie, "score", 0.95, a);
    pNewMovies->PushBack(movie.Move(), a);
    ///< SetValueByPointer可以新增节点
    rapidjson::SetValueByPointer(out, "/version", 2.0, a);
    string s = utils::to_styled_string(out);
    printf("after modify json=%s\n", s.c_str());
    return 0;
}

参考：
rapidjson官网：http://rapidjson.org/zh-cn/
git仓库：https://github.com/Tencent/rapidjson.git
一些json解析器的性能对比：https://github.com/miloyip/nativejson-benchmark
jsoncpp和rapidjson哪个好用？：https://www.zhihu.com/question/23654513