diff --git a/amalgamation/xgboost-all0.cc b/amalgamation/xgboost-all0.cc
index 5f694a63a500..da2c80622ad3 100644
--- a/amalgamation/xgboost-all0.cc
+++ b/amalgamation/xgboost-all0.cc
@@ -1,5 +1,5 @@
 /*!
- * Copyright 2015 by Contributors.
+ * Copyright 2015-2019 by Contributors.
  * \brief XGBoost Amalgamation.
  *  This offers an alternative way to compile the entire library from this single file.
  *
@@ -66,6 +66,8 @@
 #include "../src/common/common.cc"
 #include "../src/common/host_device_vector.cc"
 #include "../src/common/hist_util.cc"
+#include "../src/common/json.cc"
+#include "../src/common/io.cc"
 
 // c_api
 #include "../src/c_api/c_api.cc"
diff --git a/include/xgboost/json.h b/include/xgboost/json.h
new file mode 100644
index 000000000000..46836f1add48
--- /dev/null
+++ b/include/xgboost/json.h
@@ -0,0 +1,530 @@
+/*!
+ * Copyright (c) by Contributors 2019
+ */
+#ifndef XGBOOST_JSON_H_
+#define XGBOOST_JSON_H_
+
+#include <xgboost/logging.h>
+
+#include <string>
+
+#include <map>
+#include <memory>
+#include <vector>
+#include <functional>
+#include <utility>
+
+namespace xgboost {
+
+class Json;
+class JsonReader;
+class JsonWriter;
+
+class Value {
+ public:
+  /*!\brief Simplified implementation of LLVM RTTI. */
+  enum class ValueKind {
+    String,
+    Number,
+    Integer,
+    Object,  // std::map
+    Array,   // std::vector
+    Raw,
+    Boolean,
+    Null
+  };
+
+  explicit Value(ValueKind _kind) : kind_{_kind} {}
+
+  ValueKind Type() const { return kind_; }
+  virtual ~Value() = default;
+
+  virtual void Save(JsonWriter* writer) = 0;
+
+  virtual Json& operator[](std::string const & key) = 0;
+  virtual Json& operator[](int ind) = 0;
+
+  virtual bool operator==(Value const& rhs) const = 0;
+  virtual Value& operator=(Value const& rhs) = 0;
+
+  std::string TypeStr() const;
+
+ private:
+  ValueKind kind_;
+};
+
+template <typename T>
+bool IsA(Value const* value) {
+  return T::isClassOf(value);
+}
+
+template <typename T, typename U>
+T* Cast(U* value) {
+  if (IsA<T>(value)) {
+    return dynamic_cast<T*>(value);
+  } else {
+    throw std::runtime_error(
+        "Invalid cast, from " + value->TypeStr() + " to " + T().TypeStr());
+  }
+}
+
+class JsonString : public Value {
+  std::string str_;
+ public:
+  JsonString() : Value(ValueKind::String) {}
+  JsonString(std::string const& str) :  // NOLINT
+      Value(ValueKind::String), str_{str} {}
+  JsonString(std::string&& str) :  // NOLINT
+      Value(ValueKind::String), str_{std::move(str)} {}
+
+  void Save(JsonWriter* writer) override;
+
+  Json& operator[](std::string const & key) override;
+  Json& operator[](int ind) override;
+
+  std::string const& getString() &&      { return str_; }
+  std::string const& getString() const & { return str_; }
+  std::string&       getString()       & { return str_; }
+
+  bool operator==(Value const& rhs) const override;
+  Value& operator=(Value const& rhs) override;
+
+  static bool isClassOf(Value const* value) {
+    return value->Type() == ValueKind::String;
+  }
+};
+
+class JsonArray : public Value {
+  std::vector<Json> vec_;
+
+ public:
+  JsonArray() : Value(ValueKind::Array) {}
+  JsonArray(std::vector<Json>&& arr) :  // NOLINT
+      Value(ValueKind::Array), vec_{std::move(arr)} {}
+  JsonArray(std::vector<Json> const& arr) :  // NOLINT
+      Value(ValueKind::Array), vec_{arr} {}
+  JsonArray(JsonArray const& that) = delete;
+  JsonArray(JsonArray && that);
+
+  void Save(JsonWriter* writer) override;
+
+  Json& operator[](std::string const & key) override;
+  Json& operator[](int ind) override;
+
+  std::vector<Json> const& getArray() &&      { return vec_; }
+  std::vector<Json> const& getArray() const & { return vec_; }
+  std::vector<Json>&       getArray()       & { return vec_; }
+
+  bool operator==(Value const& rhs) const override;
+  Value& operator=(Value const& rhs) override;
+
+  static bool isClassOf(Value const* value) {
+    return value->Type() == ValueKind::Array;
+  }
+};
+
+class JsonRaw : public Value {
+  std::string str_;
+
+ public:
+  explicit JsonRaw(std::string&& str) :
+      Value(ValueKind::Raw),
+      str_{std::move(str)}{}  // NOLINT
+  JsonRaw() : Value(ValueKind::Raw) {}
+
+  std::string const& getRaw() &&      { return str_; }
+  std::string const& getRaw() const & { return str_; }
+  std::string&       getRaw()       & { return str_; }
+
+  void Save(JsonWriter* writer) override;
+
+  Json& operator[](std::string const & key) override;
+  Json& operator[](int ind) override;
+
+  bool operator==(Value const& rhs) const override;
+  Value& operator=(Value const& rhs) override;
+
+  static bool isClassOf(Value const* value) {
+    return value->Type() == ValueKind::Raw;
+  }
+};
+
+class JsonObject : public Value {
+  std::map<std::string, Json> object_;
+
+ public:
+  JsonObject() : Value(ValueKind::Object) {}
+  JsonObject(std::map<std::string, Json>&& object);  // NOLINT
+  JsonObject(JsonObject const& that) = delete;
+  JsonObject(JsonObject && that);
+
+  void Save(JsonWriter* writer) override;
+
+  Json& operator[](std::string const & key) override;
+  Json& operator[](int ind) override;
+
+  std::map<std::string, Json> const& getObject() &&      { return object_; }
+  std::map<std::string, Json> const& getObject() const & { return object_; }
+  std::map<std::string, Json> &      getObject() &       { return object_; }
+
+  bool operator==(Value const& rhs) const override;
+  Value& operator=(Value const& rhs) override;
+
+  static bool isClassOf(Value const* value) {
+    return value->Type() == ValueKind::Object;
+  }
+  virtual ~JsonObject() = default;
+};
+
+class JsonNumber : public Value {
+ public:
+  using Float = float;
+
+ private:
+  Float number_;
+
+ public:
+  JsonNumber() : Value(ValueKind::Number) {}
+  JsonNumber(double value) : Value(ValueKind::Number) {  // NOLINT
+    number_ = value;
+  }
+
+  void Save(JsonWriter* writer) override;
+
+  Json& operator[](std::string const & key) override;
+  Json& operator[](int ind) override;
+
+  Float const& getNumber() &&      { return number_; }
+  Float const& getNumber() const & { return number_; }
+  Float&       getNumber()       & { return number_; }
+
+  bool operator==(Value const& rhs) const override;
+  Value& operator=(Value const& rhs) override;
+
+  static bool isClassOf(Value const* value) {
+    return value->Type() == ValueKind::Number;
+  }
+};
+
+class JsonNull : public Value {
+ public:
+  JsonNull() : Value(ValueKind::Null) {}
+  JsonNull(std::nullptr_t) : Value(ValueKind::Null) {}  // NOLINT
+
+  void Save(JsonWriter* writer) override;
+
+  Json& operator[](std::string const & key) override;
+  Json& operator[](int ind) override;
+
+  bool operator==(Value const& rhs) const override;
+  Value& operator=(Value const& rhs) override;
+
+  static bool isClassOf(Value const* value) {
+    return value->Type() == ValueKind::Null;
+  }
+};
+
+/*! \brief Describes both true and false. */
+class JsonBoolean : public Value {
+  bool boolean_;
+
+ public:
+  JsonBoolean() : Value(ValueKind::Boolean) {}  // NOLINT
+  // Ambigious with JsonNumber.
+  template <typename Bool,
+            typename std::enable_if<
+              std::is_same<Bool, bool>::value ||
+              std::is_same<Bool, bool const>::value>::type* = nullptr>
+  JsonBoolean(Bool value) :  // NOLINT
+      Value(ValueKind::Boolean), boolean_{value} {}
+
+  void Save(JsonWriter* writer) override;
+
+  Json& operator[](std::string const & key) override;
+  Json& operator[](int ind) override;
+
+  bool const& getBoolean() &&      { return boolean_; }
+  bool const& getBoolean() const & { return boolean_; }
+  bool&       getBoolean()       & { return boolean_; }
+
+  bool operator==(Value const& rhs) const override;
+  Value& operator=(Value const& rhs) override;
+
+  static bool isClassOf(Value const* value) {
+    return value->Type() == ValueKind::Boolean;
+  }
+};
+
+struct StringView {
+  char const* str_;
+  size_t size_;
+
+ public:
+  StringView() = default;
+  StringView(char const* str, size_t size) : str_{str}, size_{size} {}
+
+  char const& operator[](size_t p) const { return str_[p]; }
+  char const& at(size_t p) const {  // NOLINT
+    CHECK_LT(p, size_);
+    return str_[p];
+  }
+  size_t size() const { return size_; }  // NOLINT
+  // Copies a portion of string.  Since we don't have std::from_chars and friends here, so
+  // copying substring is necessary for appending `\0`.  It's not too bad since string by
+  // default has small vector optimization, which is enabled by most if not all modern
+  // compilers for numeric values.
+  std::string substr(size_t beg, size_t n) const {  // NOLINT
+    CHECK_LE(beg, size_);
+    return std::string {str_ + beg, n < (size_ - beg) ? n : (size_ - beg)};
+  }
+  char const* c_str() const { return str_; }  // NOLINT
+};
+
+/*!
+ * \brief Data structure representing JSON format.
+ *
+ * Limitation:  UTF-8 is not properly supported.  Code points above ASCII are
+ *              invalid.
+ *
+ * Examples:
+ *
+ * \code
+ *   // Create a JSON object.
+ *   Json object { Object() };
+ *   // Assign key "key" with a JSON string "Value";
+ *   object["key"] = String("Value");
+ *   // Assign key "arr" with a empty JSON Array;
+ *   object["arr"] = Array();
+ * \endcode
+ */
+class Json {
+  friend JsonWriter;
+
+ public:
+  /*! \brief Load a Json object from string. */
+  static Json Load(StringView str, bool ignore_specialization = false);
+  /*! \brief Pass your own JsonReader. */
+  static Json Load(JsonReader* reader);
+  /*! \brief Dump json into stream. */
+  static void Dump(Json json, std::ostream* stream,
+                   bool pretty = ConsoleLogger::ShouldLog(
+                       ConsoleLogger::LogVerbosity::kDebug));
+
+  Json() : ptr_{new JsonNull} {}
+
+  // number
+  explicit Json(JsonNumber number) : ptr_{new JsonNumber(number)} {}
+  Json& operator=(JsonNumber number) {
+    ptr_.reset(new JsonNumber(std::move(number)));
+    return *this;
+  }
+
+  // array
+  explicit Json(JsonArray list) :
+      ptr_ {new JsonArray(std::move(list))} {}
+  Json& operator=(JsonArray array) {
+    ptr_.reset(new JsonArray(std::move(array)));
+    return *this;
+  }
+
+  // raw
+  explicit Json(JsonRaw str) :
+      ptr_{new JsonRaw(std::move(str))} {}
+  Json& operator=(JsonRaw str) {
+    ptr_.reset(new JsonRaw(std::move(str)));
+    return *this;
+  }
+
+  // object
+  explicit Json(JsonObject object) :
+      ptr_{new JsonObject(std::move(object))} {}
+  Json& operator=(JsonObject object) {
+    ptr_.reset(new JsonObject(std::move(object)));
+    return *this;
+  }
+  // string
+  explicit Json(JsonString str) :
+      ptr_{new JsonString(std::move(str))} {}
+  Json& operator=(JsonString str) {
+    ptr_.reset(new JsonString(std::move(str)));
+    return *this;
+  }
+  // bool
+  explicit Json(JsonBoolean boolean) :
+      ptr_{new JsonBoolean(std::move(boolean))} {}
+  Json& operator=(JsonBoolean boolean) {
+    ptr_.reset(new JsonBoolean(std::move(boolean)));
+    return *this;
+  }
+  // null
+  explicit Json(JsonNull null) :
+      ptr_{new JsonNull(std::move(null))} {}
+  Json& operator=(JsonNull null) {
+    ptr_.reset(new JsonNull(std::move(null)));
+    return *this;
+  }
+
+  // copy
+  Json(Json const& other) : ptr_{other.ptr_} {}
+  Json& operator=(Json const& other);
+  // move
+  Json(Json&& other) : ptr_{std::move(other.ptr_)} {}
+  Json& operator=(Json&& other) {
+    ptr_ = std::move(other.ptr_);
+    return *this;
+  }
+
+  /*! \brief Index Json object with a std::string, used for Json Object. */
+  Json& operator[](std::string const & key) const { return (*ptr_)[key]; }
+  /*! \brief Index Json object with int, used for Json Array. */
+  Json& operator[](int ind)                 const { return (*ptr_)[ind]; }
+
+  /*! \Brief Return the reference to stored Json value. */
+  Value const& GetValue() const & { return *ptr_; }
+  Value const& GetValue() &&      { return *ptr_; }
+  Value&       GetValue() &       { return *ptr_; }
+
+  bool operator==(Json const& rhs) const {
+    return *ptr_ == *(rhs.ptr_);
+  }
+
+ private:
+  std::shared_ptr<Value> ptr_;
+};
+
+template <typename T>
+bool IsA(Json const j) {
+  auto const& v = j.GetValue();
+  return IsA<T>(&v);
+}
+
+namespace detail {
+
+// Number
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonNumber>::value>::type* = nullptr>
+JsonNumber::Float& GetImpl(T& val) {  // NOLINT
+  return val.getNumber();
+}
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonNumber const>::value>::type* = nullptr>
+double const& GetImpl(T& val) {  // NOLINT
+  return val.getNumber();
+}
+
+// String
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonString>::value>::type* = nullptr>
+std::string& GetImpl(T& val) {  // NOLINT
+  return val.getString();
+}
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonString const>::value>::type* = nullptr>
+std::string const& GetImpl(T& val) {  // NOLINT
+  return val.getString();
+}
+
+// Boolean
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonBoolean>::value>::type* = nullptr>
+bool& GetImpl(T& val) {  // NOLINT
+  return val.getBoolean();
+}
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonBoolean const>::value>::type* = nullptr>
+bool const& GetImpl(T& val) {  // NOLINT
+  return val.getBoolean();
+}
+
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonRaw>::value>::type* = nullptr>
+std::string& GetImpl(T& val) {  // NOLINT
+  return val.getRaw();
+}
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonRaw const>::value>::type* = nullptr>
+std::string const& GetImpl(T& val) {  // NOLINT
+  return val.getRaw();
+}
+
+// Array
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonArray>::value>::type* = nullptr>
+std::vector<Json>& GetImpl(T& val) {  // NOLINT
+  return val.getArray();
+}
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonArray const>::value>::type* = nullptr>
+std::vector<Json> const& GetImpl(T& val) {  // NOLINT
+  return val.getArray();
+}
+
+// Object
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonObject>::value>::type* = nullptr>
+std::map<std::string, Json>& GetImpl(T& val) {  // NOLINT
+  return val.getObject();
+}
+template <typename T,
+          typename std::enable_if<
+            std::is_same<T, JsonObject const>::value>::type* = nullptr>
+std::map<std::string, Json> const& GetImpl(T& val) {  // NOLINT
+  return val.getObject();
+}
+
+}  // namespace detail
+
+/*!
+ * \brief Get Json value.
+ *
+ * \tparam T One of the Json value type.
+ *
+ * \param json
+ * \return Value contained in Json object of type T.
+ */
+template <typename T, typename U>
+auto get(U& json) -> decltype(detail::GetImpl(*Cast<T>(&json.GetValue())))& { // NOLINT
+  auto& value = *Cast<T>(&json.GetValue());
+  return detail::GetImpl(value);
+}
+
+using Object  = JsonObject;
+using Array   = JsonArray;
+using Number  = JsonNumber;
+using Boolean = JsonBoolean;
+using String  = JsonString;
+using Null    = JsonNull;
+using Raw     = JsonRaw;
+
+// Utils tailored for XGBoost.
+
+template <typename Type>
+Object toJson(dmlc::Parameter<Type> const& param) {
+  Object obj;
+  for (auto const& kv : param.__DICT__()) {
+    obj[kv.first] = kv.second;
+  }
+  return obj;
+}
+
+inline std::map<std::string, std::string> fromJson(std::map<std::string, Json> const& param) {
+  std::map<std::string, std::string> res;
+  for (auto const& kv : param) {
+    res[kv.first] = get<String const>(kv.second);
+  }
+  return res;
+}
+
+}  // namespace xgboost
+#endif  // XGBOOST_JSON_H_
diff --git a/include/xgboost/json_io.h b/include/xgboost/json_io.h
new file mode 100644
index 000000000000..d0323354d1a3
--- /dev/null
+++ b/include/xgboost/json_io.h
@@ -0,0 +1,217 @@
+/*!
+ * Copyright (c) by Contributors 2019
+ */
+#ifndef XGBOOST_JSON_IO_H_
+#define XGBOOST_JSON_IO_H_
+#include <xgboost/json.h>
+
+#include <memory>
+#include <string>
+#include <cinttypes>
+#include <utility>
+#include <map>
+#include <limits>
+#include <sstream>
+#include <locale>
+
+namespace xgboost {
+
+template <typename Allocator>
+class FixedPrecisionStreamContainer : public std::basic_stringstream<
+  char, std::char_traits<char>, Allocator> {
+ public:
+  FixedPrecisionStreamContainer() {
+    this->precision(std::numeric_limits<Number::Float>::max_digits10);
+  }
+};
+
+using FixedPrecisionStream = FixedPrecisionStreamContainer<std::allocator<char>>;
+
+/*
+ * \brief An reader that can be specialised.
+ *
+ * Why specialization?
+ *
+ *   First of all, we don't like specialization.  This is purely for performance concern.
+ *   Distributed environment freqently serializes model so at some point this could be a
+ *   bottle neck for training performance.  There are many other techniques for obtaining
+ *   better performance, but all of them requires implementing thier own allocaltor(s),
+ *   using simd instructions.  And few of them can provide a easy to modify structure
+ *   since they assumes a fixed memory layout.
+ *
+ *   In XGBoost we provide specialized logic for parsing/writing tree models and linear
+ *   models, where dense numeric values is presented, including weights, node ids etc.
+ *
+ * Plan for removing the specialization:
+ *
+ *   We plan to upstream this implementaion into DMLC as it matures.  For XGBoost, most of
+ *   the time spent in load/dump is actually `sprintf`.
+ *
+ * To enable specialization, register a keyword that corresponds to
+ * key in Json object.  For example in:
+ *
+ * \code
+ * { "key": {...} }
+ * \endcode
+ *
+ * To add special logic for parsing {...}, one can call:
+ *
+ * \code
+ * JsonReader::registry("key", [](StringView str, size_t* pos){ ... return JsonRaw(...); });
+ * \endcode
+ *
+ * Where str is a view of entire input string, while pos is a pointer to current position.
+ * The function must return a raw object.  Later after obtaining a parsed object, say
+ * `Json obj`, you can obtain * the raw object by calling `obj["key"]' then perform the
+ * specialized parsing on it.
+ *
+ * See `LinearSelectRaw` and `LinearReader` in combination as an example.
+ */
+class JsonReader {
+ protected:
+  size_t constexpr static kMaxNumLength =
+      std::numeric_limits<double>::max_digits10 + 1;
+
+  struct SourceLocation {
+    size_t pos_;  // current position in raw_str_
+
+   public:
+    SourceLocation() : pos_(0) {}
+    explicit SourceLocation(size_t pos) : pos_{pos} {}
+    size_t  Pos()  const { return pos_; }
+
+    SourceLocation& Forward(char c = 0) {
+      pos_++;
+      return *this;
+    }
+  } cursor_;
+
+  StringView raw_str_;
+  bool ignore_specialization_;
+
+ protected:
+  void SkipSpaces();
+
+  char GetNextChar() {
+    if (cursor_.Pos() == raw_str_.size()) {
+      return -1;
+    }
+    char ch = raw_str_[cursor_.Pos()];
+    cursor_.Forward();
+    return ch;
+  }
+
+  char PeekNextChar() {
+    if (cursor_.Pos() == raw_str_.size()) {
+      return -1;
+    }
+    char ch = raw_str_[cursor_.Pos()];
+    return ch;
+  }
+
+  char GetNextNonSpaceChar() {
+    SkipSpaces();
+    return GetNextChar();
+  }
+
+  char GetChar(char c) {
+    char result = GetNextNonSpaceChar();
+    if (result != c) { Expect(c, result); }
+    return result;
+  }
+
+  void Error(std::string msg) const;
+
+  // Report expected character
+  void Expect(char c, char got) {
+    std::string msg = "Expecting: \"";
+    msg += c;
+    msg += "\", got: \"";
+    msg += std::string {got} + " \"";
+    Error(msg);
+  }
+
+  virtual Json ParseString();
+  virtual Json ParseObject();
+  virtual Json ParseArray();
+  virtual Json ParseNumber();
+  virtual Json ParseBoolean();
+  virtual Json ParseNull();
+
+  Json Parse();
+
+ private:
+  using Fn = std::function<Json (StringView, size_t*)>;
+
+ public:
+  explicit JsonReader(StringView str, bool ignore = false) :
+      raw_str_{str},
+      ignore_specialization_{ignore} {}
+  explicit JsonReader(StringView str, size_t pos, bool ignore = false) :
+      cursor_{pos},
+      raw_str_{str},
+      ignore_specialization_{ignore} {}
+
+  virtual ~JsonReader() = default;
+
+  Json Load();
+
+  static std::map<std::string, Fn>& getRegistry() {
+    static std::map<std::string, Fn> set;
+    return set;
+  }
+
+  static std::map<std::string, Fn> const& registry(
+      std::string const& key, Fn fn) {
+    getRegistry()[key] = fn;
+    return getRegistry();
+  }
+};
+
+class JsonWriter {
+  static constexpr size_t kIndentSize = 2;
+  FixedPrecisionStream convertor_;
+
+  size_t n_spaces_;
+  std::ostream* stream_;
+  bool pretty_;
+
+ public:
+  JsonWriter(std::ostream* stream, bool pretty) :
+      n_spaces_{0}, stream_{stream}, pretty_{pretty} {}
+
+  virtual ~JsonWriter() = default;
+
+  void NewLine() {
+    if (pretty_) {
+      *stream_ << u8"\n" << std::string(n_spaces_, ' ');
+    }
+  }
+
+  void BeginIndent() {
+    n_spaces_ += kIndentSize;
+  }
+  void EndIndent() {
+    n_spaces_ -= kIndentSize;
+  }
+
+  void Write(std::string str) {
+    *stream_ << str;
+  }
+  void Write(StringView str) {
+    stream_->write(str.c_str(), str.size());
+  }
+
+  void Save(Json json);
+
+  virtual void Visit(JsonArray  const* arr);
+  virtual void Visit(JsonObject const* obj);
+  virtual void Visit(JsonNumber const* num);
+  virtual void Visit(JsonRaw    const* raw);
+  virtual void Visit(JsonNull   const* null);
+  virtual void Visit(JsonString const* str);
+  virtual void Visit(JsonBoolean const* boolean);
+};
+}      // namespace xgboost
+
+#endif  // XGBOOST_JSON_IO_H_
diff --git a/src/common/io.cc b/src/common/io.cc
new file mode 100644
index 000000000000..025e70ffb780
--- /dev/null
+++ b/src/common/io.cc
@@ -0,0 +1,67 @@
+/*!
+ * Copyright (c) by Contributors 2019
+ */
+#if defined(__unix__)
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#endif  // defined(__unix__)
+#include <cstdio>
+#include <string>
+
+#include "xgboost/logging.h"
+
+namespace xgboost {
+namespace common {
+
+std::string LoadSequentialFile(std::string fname) {
+  auto OpenErr = [&fname]() {
+                   std::string msg;
+                   msg = "Opening " + fname + " failed: ";
+                   msg += strerror(errno);
+                   LOG(FATAL) << msg;
+                 };
+  auto ReadErr = [&fname]() {
+                   std::string msg {"Error in reading file: "};
+                   msg += fname;
+                   msg += ": ";
+                   msg += strerror(errno);
+                   LOG(FATAL) << msg;
+                 };
+
+  std::string buffer;
+#if defined(__unix__)
+  struct stat fs;
+  if (stat(fname.c_str(), &fs) != 0) {
+    OpenErr();
+  }
+
+  size_t f_size_bytes = fs.st_size;
+  buffer.resize(f_size_bytes+1);
+  int32_t fd = open(fname.c_str(), O_RDONLY);
+  posix_fadvise(fd, 0, 0, POSIX_FADV_SEQUENTIAL);
+  ssize_t bytes_read = read(fd, &buffer[0], f_size_bytes);
+  if (bytes_read < 0) {
+    close(fd);
+    ReadErr();
+  }
+  close(fd);
+#else
+  FILE *f = fopen(fname.c_str(), "r");
+  if (f == NULL) {
+    std::string msg;
+    OpenErr();
+  }
+  fseek(f, 0, SEEK_END);
+  auto fsize = ftell(f);
+  fseek(f, 0, SEEK_SET);
+
+  buffer.resize(fsize + 1);
+  fread(&buffer[0], 1, fsize, f);
+  fclose(f);
+#endif  // defined(__unix__)
+  return buffer;
+}
+
+}  // namespace common
+}  // namespace xgboost
diff --git a/src/common/io.h b/src/common/io.h
index 29d68abec09a..6dac70c3d79d 100644
--- a/src/common/io.h
+++ b/src/common/io.h
@@ -70,6 +70,10 @@ class PeekableInStream : public dmlc::Stream {
   /*! \brief internal buffer */
   std::string buffer_;
 };
+
+// Optimized for consecutive file loading in unix like systime.
+std::string LoadSequentialFile(std::string fname);
+
 }  // namespace common
 }  // namespace xgboost
 #endif  // XGBOOST_COMMON_IO_H_
diff --git a/src/common/json.cc b/src/common/json.cc
new file mode 100644
index 000000000000..917aa1748379
--- /dev/null
+++ b/src/common/json.cc
@@ -0,0 +1,624 @@
+/*!
+ * Copyright (c) by Contributors 2019
+ */
+#include <sstream>
+
+#include "xgboost/logging.h"
+#include "xgboost/json.h"
+#include "xgboost/json_io.h"
+#include "../common/timer.h"
+
+namespace xgboost {
+
+void JsonWriter::Save(Json json) {
+  json.ptr_->Save(this);
+}
+
+void JsonWriter::Visit(JsonArray const* arr) {
+  this->Write("[");
+  auto const& vec = arr->getArray();
+  size_t size = vec.size();
+  for (size_t i = 0; i < size; ++i) {
+    auto const& value = vec[i];
+    this->Save(value);
+    if (i != size-1) { Write(", "); }
+  }
+  this->Write("]");
+}
+
+void JsonWriter::Visit(JsonObject const* obj) {
+  this->Write("{");
+  this->BeginIndent();
+  this->NewLine();
+
+  size_t i = 0;
+  size_t size = obj->getObject().size();
+
+  for (auto& value : obj->getObject()) {
+    this->Write("\"" + value.first + "\": ");
+    this->Save(value.second);
+
+    if (i != size-1) {
+      this->Write(",");
+      this->NewLine();
+    }
+    i++;
+  }
+  this->EndIndent();
+  this->NewLine();
+  this->Write("}");
+}
+
+void JsonWriter::Visit(JsonNumber const* num) {
+  convertor_ << num->getNumber();
+  auto const& str = convertor_.str();
+  this->Write(StringView{str.c_str(), str.size()});
+  convertor_.str("");
+}
+
+void JsonWriter::Visit(JsonRaw const* raw) {
+  auto const& str = raw->getRaw();
+  this->Write(str);
+}
+
+void JsonWriter::Visit(JsonNull const* null) {
+  this->Write("null");
+}
+
+void JsonWriter::Visit(JsonString const* str) {
+  std::string buffer;
+  buffer += '"';
+  auto const& string = str->getString();
+  for (size_t i = 0; i < string.length(); i++) {
+    const char ch = string[i];
+    if (ch == '\\') {
+      if (i < string.size() && string[i+1] == 'u') {
+        buffer += "\\";
+      } else {
+        buffer += "\\\\";
+      }
+    } else if (ch == '"') {
+      buffer += "\\\"";
+    } else if (ch == '\b') {
+      buffer += "\\b";
+    } else if (ch == '\f') {
+      buffer += "\\f";
+    } else if (ch == '\n') {
+      buffer += "\\n";
+    } else if (ch == '\r') {
+      buffer += "\\r";
+    } else if (ch == '\t') {
+      buffer += "\\t";
+    } else if (static_cast<uint8_t>(ch) <= 0x1f) {
+      // Unit separator
+      char buf[8];
+      snprintf(buf, sizeof buf, "\\u%04x", ch);
+      buffer += buf;
+    } else {
+      buffer += ch;
+    }
+  }
+  buffer += '"';
+  this->Write(buffer);
+}
+
+void JsonWriter::Visit(JsonBoolean const* boolean) {
+  bool val = boolean->getBoolean();
+  if (val) {
+    this->Write(u8"true");
+  } else {
+    this->Write(u8"false");
+  }
+}
+
+// Value
+std::string Value::TypeStr() const {
+  switch (kind_) {
+    case ValueKind::String:  return "String";  break;
+    case ValueKind::Number:  return "Number";  break;
+    case ValueKind::Object:  return "Object";  break;
+    case ValueKind::Array:   return "Array";   break;
+    case ValueKind::Boolean: return "Boolean"; break;
+    case ValueKind::Null:    return "Null";    break;
+    case ValueKind::Raw:     return "Raw";     break;
+    case ValueKind::Integer: return "Integer"; break;
+  }
+  return "";
+}
+
+// Only used for keeping old compilers happy about non-reaching return
+// statement.
+Json& DummyJsonObject() {
+  static Json obj;
+  return obj;
+}
+
+// Json Object
+JsonObject::JsonObject(JsonObject && that) :
+    Value(ValueKind::Object), object_{std::move(that.object_)} {}
+
+JsonObject::JsonObject(std::map<std::string, Json>&& object)
+    : Value(ValueKind::Object), object_{std::move(object)} {}
+
+Json& JsonObject::operator[](std::string const & key) {
+  return object_[key];
+}
+
+Json& JsonObject::operator[](int ind) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by Integer.";
+  return DummyJsonObject();
+}
+
+bool JsonObject::operator==(Value const& rhs) const {
+  if (!IsA<JsonObject>(&rhs)) { return false; }
+  return object_ == Cast<JsonObject const>(&rhs)->getObject();
+}
+
+Value& JsonObject::operator=(Value const &rhs) {
+  JsonObject const* casted = Cast<JsonObject const>(&rhs);
+  object_ = casted->getObject();
+  return *this;
+}
+
+void JsonObject::Save(JsonWriter* writer) {
+  writer->Visit(this);
+}
+
+// Json String
+Json& JsonString::operator[](std::string const & key) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by string.";
+  return DummyJsonObject();
+}
+
+Json& JsonString::operator[](int ind) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by Integer."
+             << "  Please try obtaining std::string first.";
+  return DummyJsonObject();
+}
+
+bool JsonString::operator==(Value const& rhs) const {
+  if (!IsA<JsonString>(&rhs)) { return false; }
+  return Cast<JsonString const>(&rhs)->getString() == str_;
+}
+
+Value & JsonString::operator=(Value const &rhs) {
+  JsonString const* casted = Cast<JsonString const>(&rhs);
+  str_ = casted->getString();
+  return *this;
+}
+
+// FIXME: UTF-8 parsing support.
+void JsonString::Save(JsonWriter* writer) {
+  writer->Visit(this);
+}
+
+// Json Array
+JsonArray::JsonArray(JsonArray && that) :
+    Value(ValueKind::Array), vec_{std::move(that.vec_)} {}
+
+Json& JsonArray::operator[](std::string const & key) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by string.";
+  return DummyJsonObject();
+}
+
+Json& JsonArray::operator[](int ind) {
+  return vec_.at(ind);
+}
+
+bool JsonArray::operator==(Value const& rhs) const {
+  if (!IsA<JsonArray>(&rhs)) { return false; }
+  auto& arr = Cast<JsonArray const>(&rhs)->getArray();
+  return std::equal(arr.cbegin(), arr.cend(), vec_.cbegin());
+}
+
+Value & JsonArray::operator=(Value const &rhs) {
+  JsonArray const* casted = Cast<JsonArray const>(&rhs);
+  vec_ = casted->getArray();
+  return *this;
+}
+
+void JsonArray::Save(JsonWriter* writer) {
+  writer->Visit(this);
+}
+
+// Json raw
+Json& JsonRaw::operator[](std::string const & key) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by string.";
+  return DummyJsonObject();
+}
+
+Json& JsonRaw::operator[](int ind) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by Integer.";
+  return DummyJsonObject();
+}
+
+bool JsonRaw::operator==(Value const& rhs) const {
+  if (!IsA<JsonRaw>(&rhs)) { return false; }
+  auto& arr = Cast<JsonRaw const>(&rhs)->getRaw();
+  return std::equal(arr.cbegin(), arr.cend(), str_.cbegin());
+}
+
+Value & JsonRaw::operator=(Value const &rhs) {
+  auto const* casted = Cast<JsonRaw const>(&rhs);
+  str_ = casted->getRaw();
+  return *this;
+}
+
+void JsonRaw::Save(JsonWriter* writer) {
+  writer->Visit(this);
+}
+
+// Json Number
+Json& JsonNumber::operator[](std::string const & key) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by string.";
+  return DummyJsonObject();
+}
+
+Json& JsonNumber::operator[](int ind) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by Integer.";
+  return DummyJsonObject();
+}
+
+bool JsonNumber::operator==(Value const& rhs) const {
+  if (!IsA<JsonNumber>(&rhs)) { return false; }
+  return number_ == Cast<JsonNumber const>(&rhs)->getNumber();
+}
+
+Value & JsonNumber::operator=(Value const &rhs) {
+  JsonNumber const* casted = Cast<JsonNumber const>(&rhs);
+  number_ = casted->getNumber();
+  return *this;
+}
+
+void JsonNumber::Save(JsonWriter* writer) {
+  writer->Visit(this);
+}
+
+// Json Null
+Json& JsonNull::operator[](std::string const & key) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by string.";
+  return DummyJsonObject();
+}
+
+Json& JsonNull::operator[](int ind) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by Integer.";
+  return DummyJsonObject();
+}
+
+bool JsonNull::operator==(Value const& rhs) const {
+  if (!IsA<JsonNull>(&rhs)) { return false; }
+  return true;
+}
+
+Value & JsonNull::operator=(Value const &rhs) {
+  Cast<JsonNull const>(&rhs);  // Checking only.
+  return *this;
+}
+
+void JsonNull::Save(JsonWriter* writer) {
+  writer->Write("null");
+}
+
+// Json Boolean
+Json& JsonBoolean::operator[](std::string const & key) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by string.";
+  return DummyJsonObject();
+}
+
+Json& JsonBoolean::operator[](int ind) {
+  LOG(FATAL) << "Object of type "
+             << Value::TypeStr() << " can not be indexed by Integer.";
+  return DummyJsonObject();
+}
+
+bool JsonBoolean::operator==(Value const& rhs) const {
+  if (!IsA<JsonBoolean>(&rhs)) { return false; }
+  return boolean_ == Cast<JsonBoolean const>(&rhs)->getBoolean();
+}
+
+Value & JsonBoolean::operator=(Value const &rhs) {
+  JsonBoolean const* casted = Cast<JsonBoolean const>(&rhs);
+  boolean_ = casted->getBoolean();
+  return *this;
+}
+
+void JsonBoolean::Save(JsonWriter *writer) {
+  writer->Visit(this);
+}
+
+size_t constexpr JsonReader::kMaxNumLength;
+
+Json JsonReader::Parse() {
+  while (true) {
+    SkipSpaces();
+    char c = PeekNextChar();
+    if (c == -1) { break; }
+
+    if (c == '{') {
+      return ParseObject();
+    } else if ( c == '[' ) {
+      return ParseArray();
+    } else if ( c == '-' || std::isdigit(c) ) {
+      return ParseNumber();
+    } else if ( c == '\"' ) {
+      return ParseString();
+    } else if ( c == 't' || c == 'f' ) {
+      return ParseBoolean();
+    } else if (c == 'n') {
+      return ParseNull();
+    } else {
+      Error("Unknown construct");
+    }
+  }
+  return Json();
+}
+
+Json JsonReader::Load() {
+  Json result = Parse();
+  return result;
+}
+
+void JsonReader::Error(std::string msg) const {
+  // just copy it.
+  std::istringstream str_s(raw_str_.substr(0, raw_str_.size()));
+
+  msg += ", around character: " + std::to_string(cursor_.Pos());
+  msg += '\n';
+
+  constexpr size_t kExtend = 8;
+  auto beg = cursor_.Pos() - kExtend < 0 ? 0 : cursor_.Pos() - kExtend;
+  auto end = cursor_.Pos() + kExtend >= raw_str_.size() ?
+             raw_str_.size() : cursor_.Pos() + kExtend;
+
+  msg += "    ";
+  msg += raw_str_.substr(beg, end - beg);
+  msg += '\n';
+
+  msg += "    ";
+  for (size_t i = beg; i < cursor_.Pos() - 1; ++i) {
+    msg += '~';
+  }
+  msg += '^';
+  for (size_t i = cursor_.Pos(); i < end; ++i) {
+    msg += '~';
+  }
+  LOG(FATAL) << msg;
+}
+
+// Json class
+void JsonReader::SkipSpaces() {
+  while (cursor_.Pos() < raw_str_.size()) {
+    char c = raw_str_[cursor_.Pos()];
+    if (std::isspace(c)) {
+      cursor_.Forward(c);
+    } else {
+      break;
+    }
+  }
+}
+
+void ParseStr(std::string const& str) {
+  size_t end = 0;
+  for (size_t i = 0; i < str.size(); ++i) {
+    if (str[i] == '"' && i > 0 && str[i-1] != '\\') {
+      end = i;
+      break;
+    }
+  }
+  std::string result;
+  result.resize(end);
+}
+
+Json JsonReader::ParseString() {
+  char ch { GetChar('\"') };  // NOLINT
+  std::ostringstream output;
+  std::string str;
+  while (true) {
+    ch = GetNextChar();
+    if (ch == '\\') {
+      char next = static_cast<char>(GetNextChar());
+      switch (next) {
+        case 'r':  str += u8"\r"; break;
+        case 'n':  str += u8"\n"; break;
+        case '\\': str += u8"\\"; break;
+        case 't':  str += u8"\t"; break;
+        case '\"': str += u8"\""; break;
+        case 'u':
+          str += ch;
+          str += 'u';
+          break;
+        default: Error("Unknown escape");
+      }
+    } else {
+      if (ch == '\"') break;
+      str += ch;
+    }
+    if (ch == EOF || ch == '\r' || ch == '\n') {
+      Expect('\"', ch);
+    }
+  }
+  return Json(std::move(str));
+}
+
+Json JsonReader::ParseNull() {
+  char ch = GetNextNonSpaceChar();
+  std::string buffer{ch};
+  for (size_t i = 0; i < 3; ++i) {
+    buffer.push_back(GetNextChar());
+  }
+  if (buffer != "null") {
+    Error("Expecting null value \"null\"");
+  }
+  return Json{JsonNull()};
+}
+
+Json JsonReader::ParseArray() {
+  std::vector<Json> data;
+
+  char ch { GetChar('[') };  // NOLINT
+  while (true) {
+    if (PeekNextChar() == ']') {
+      GetChar(']');
+      return Json(std::move(data));
+    }
+    auto obj = Parse();
+    data.push_back(obj);
+    ch = GetNextNonSpaceChar();
+    if (ch == ']') break;
+    if (ch != ',') {
+      Expect(',', ch);
+    }
+  }
+
+  return Json(std::move(data));
+}
+
+Json JsonReader::ParseObject() {
+  char ch = GetChar('{');
+
+  std::map<std::string, Json> data;
+  if (ch == '}') return Json(std::move(data));
+
+  while (true) {
+    SkipSpaces();
+    ch = PeekNextChar();
+    if (ch != '"') {
+      Expect('"', ch);
+    }
+    Json key = ParseString();
+
+    ch = GetNextNonSpaceChar();
+
+    if (ch != ':') {
+      Expect(':', ch);
+    }
+
+    Json value;
+    if (!ignore_specialization_ &&
+        (getRegistry().find(get<String>(key)) != getRegistry().cend())) {
+      LOG(DEBUG) << "Using specialized parser for: " << get<String>(key);
+      value = getRegistry().at(get<String>(key))(raw_str_, &(cursor_.pos_));
+    } else {
+      value = Parse();
+    }
+
+    data[get<JsonString>(key)] = std::move(value);
+
+    ch = GetNextNonSpaceChar();
+
+    if (ch == '}') break;
+    if (ch != ',') {
+      Expect(',', ch);
+    }
+  }
+
+  return Json(std::move(data));
+}
+
+Json JsonReader::ParseNumber() {
+  std::string substr = raw_str_.substr(cursor_.Pos(), kMaxNumLength);
+  size_t pos = 0;
+
+  Number::Float number{0};
+  number = std::stof(substr, &pos);
+  for (size_t i = 0; i < pos; ++i) {
+    GetNextChar();
+  }
+  return Json(number);
+}
+
+Json JsonReader::ParseBoolean() {
+  bool result = false;
+  char ch = GetNextNonSpaceChar();
+  std::string const t_value = u8"true";
+  std::string const f_value = u8"false";
+  std::string buffer;
+
+  if (ch == 't') {
+    for (size_t i = 0; i < 3; ++i) {
+      buffer.push_back(GetNextNonSpaceChar());
+    }
+    if (buffer != u8"rue") {
+      Error("Expecting boolean value \"true\".");
+    }
+    result = true;
+  } else {
+    for (size_t i = 0; i < 4; ++i) {
+      buffer.push_back(GetNextNonSpaceChar());
+    }
+    if (buffer != u8"alse") {
+      Error("Expecting boolean value \"false\".");
+    }
+    result = false;
+  }
+  return Json{JsonBoolean{result}};
+}
+
+// This is an ad-hoc solution for writing numeric value in standard way.  We need to add
+// something locale independent way of writing stream.
+// FIXME(trivialfis): Remove this.
+class GlobalCLocale {
+  std::locale ori_;
+
+ public:
+  GlobalCLocale() : ori_{std::locale()} {
+    std::string const name {"C"};
+    try {
+      std::locale::global(std::locale(name.c_str()));
+    } catch (std::runtime_error const& e) {
+      LOG(FATAL) << "Failed to set locale: " << name;
+    }
+  }
+  ~GlobalCLocale() {
+    std::locale::global(ori_);
+  }
+};
+
+Json Json::Load(StringView str, bool ignore_specialization) {
+  GlobalCLocale guard;
+  LOG(WARNING) << "Json serialization is still experimental."
+      "  Output schema is subject to change in the future.";
+  JsonReader reader(str, ignore_specialization);
+  common::Timer t;
+  t.Start();
+  Json json{reader.Load()};
+  t.Stop();
+  t.PrintElapsed("Json::load");
+  return json;
+}
+
+Json Json::Load(JsonReader* reader) {
+  GlobalCLocale guard;
+  common::Timer t;
+  t.Start();
+  Json json{reader->Load()};
+  t.Stop();
+  t.PrintElapsed("Json::load");
+  return json;
+}
+
+void Json::Dump(Json json, std::ostream *stream, bool pretty) {
+  GlobalCLocale guard;
+  LOG(WARNING) << "Json serialization is still experimental."
+      "  Output schema is subject to change in the future.";
+  JsonWriter writer(stream, true);
+  common::Timer t;
+  t.Start();
+  writer.Save(json);
+  t.Stop();
+  t.PrintElapsed("Json::dump");
+}
+
+Json& Json::operator=(Json const &other) = default;
+}  // namespace xgboost
diff --git a/tests/cpp/common/test_json.cc b/tests/cpp/common/test_json.cc
new file mode 100644
index 000000000000..a599c3f19fbb
--- /dev/null
+++ b/tests/cpp/common/test_json.cc
@@ -0,0 +1,371 @@
+/*!
+ * Copyright (c) by Contributors 2019
+ */
+#include <gtest/gtest.h>
+#include <dmlc/filesystem.h>
+#include <fstream>
+#include <map>
+
+#include "xgboost/json.h"
+#include "xgboost/logging.h"
+#include "xgboost/json_io.h"
+#include "../../../src/common/io.h"
+
+namespace xgboost {
+
+std::string GetModelStr() {
+  std::string model_json = R"json(
+{
+  "model_parameter": {
+    "base_score": "0.5",
+    "num_class": "0",
+    "num_feature": "10"
+  },
+  "train_parameter": {
+    "debug_verbose": "0",
+    "disable_default_eval_metric": "0",
+    "dsplit": "auto",
+    "nthread": "0",
+    "seed": "0",
+    "seed_per_iteration": "0",
+    "test_flag": "",
+    "tree_method": "gpu_hist"
+  },
+  "configuration": {
+    "booster": "gbtree",
+    "n_gpus": "1",
+    "num_class": "0",
+    "num_feature": "10",
+    "objective": "reg:linear",
+    "predictor": "gpu_predictor",
+    "tree_method": "gpu_hist",
+    "updater": "grow_gpu_hist"
+  },
+  "objective": "reg:linear",
+  "booster": "gbtree",
+  "gbm": {
+    "GBTreeModelParam": {
+      "num_feature": "10",
+      "num_output_group": "1",
+      "num_roots": "1",
+      "size_leaf_vector": "0"
+    },
+    "trees": [{
+        "TreeParam": {
+          "num_feature": "10",
+          "num_roots": "1",
+          "size_leaf_vector": "0"
+        },
+        "num_nodes": "9",
+        "nodes": [
+          {
+            "depth": 0,
+            "gain": 31.8892,
+            "hess": 10,
+            "left": 1,
+            "missing": 1,
+            "nodeid": 0,
+            "right": 2,
+            "split_condition": 0.580717,
+            "split_index": 2
+          },
+          {
+            "depth": 1,
+            "gain": 1.5625,
+            "hess": 3,
+            "left": 5,
+            "missing": 5,
+            "nodeid": 2,
+            "right": 6,
+            "split_condition": 0.160345,
+            "split_index": 0
+          },
+          {
+            "depth": 2,
+            "gain": 0.25,
+            "hess": 2,
+            "left": 7,
+            "missing": 7,
+            "nodeid": 6,
+            "right": 8,
+            "split_condition": 0.62788,
+            "split_index": 0
+          },
+          {
+            "hess": 1,
+            "leaf": 0.375,
+            "nodeid": 8
+          },
+          {
+            "hess": 1,
+            "leaf": 0.075,
+            "nodeid": 7
+          },
+          {
+            "hess": 1,
+            "leaf": -0.075,
+            "nodeid": 5
+          },
+          {
+            "depth": 3,
+            "gain": 10.4866,
+            "hess": 7,
+            "left": 3,
+            "missing": 3,
+            "nodeid": 1,
+            "right": 4,
+            "split_condition": 0.238748,
+            "split_index": 1
+          },
+          {
+            "hess": 6,
+            "leaf": 1.54286,
+            "nodeid": 4
+          },
+          {
+            "hess": 1,
+            "leaf": 0.225,
+            "nodeid": 3
+          }
+        ],
+        "leaf_vector": []
+      }],
+    "tree_info": [0]
+  }
+}
+)json";
+  return model_json;
+}
+
+TEST(Json, TestParseObject) {
+  std::string str = R"obj({"TreeParam" : {"num_feature": "10"}})obj";
+  auto json = Json::Load(StringView{str.c_str(), str.size()});
+}
+
+TEST(Json, ParseNumber) {
+  std::string str = "31.8892";
+  auto json = Json::Load(StringView{str.c_str(), str.size()});
+  ASSERT_NEAR(get<JsonNumber>(json), 31.8892f, kRtEps);
+}
+
+TEST(Json, ParseArray) {
+  std::string str = R"json(
+{
+    "nodes": [
+        {
+	    "depth": 3,
+	    "gain": 10.4866,
+	    "hess": 7,
+	    "left": 3,
+	    "missing": 3,
+	    "nodeid": 1,
+	    "right": 4,
+	    "split_condition": 0.238748,
+	    "split_index": 1
+        },
+        {
+	    "hess": 6,
+	    "leaf": 1.54286,
+	    "nodeid": 4
+        },
+        {
+	    "hess": 1,
+	    "leaf": 0.225,
+	    "nodeid": 3
+        }
+    ]
+}
+)json";
+  auto json = Json::Load(StringView{str.c_str(), str.size()}, true);
+  json = json["nodes"];
+  std::vector<Json> arr = get<JsonArray>(json);
+  ASSERT_EQ(arr.size(), 3);
+  Json v0 = arr[0];
+  ASSERT_EQ(get<JsonNumber>(v0["depth"]), 3);
+}
+
+TEST(Json, Null) {
+  Json json {JsonNull()};
+  std::stringstream ss;
+  Json::Dump(json, &ss);
+  ASSERT_EQ(ss.str(), "null");
+
+  std::string null_input {R"null({"key":  null })null"};
+
+  json = Json::Load({null_input.c_str(), null_input.size()});
+  ASSERT_TRUE(IsA<Null>(json["key"]));
+}
+
+TEST(Json, EmptyArray) {
+  std::string str = R"json(
+{
+  "leaf_vector": []
+}
+)json";
+  std::istringstream iss(str);
+  auto json = Json::Load(StringView{str.c_str(), str.size()}, true);
+  auto arr = get<JsonArray>(json["leaf_vector"]);
+  ASSERT_EQ(arr.size(), 0);
+}
+
+TEST(Json, Boolean) {
+  std::string str = R"json(
+{
+  "left_child": true,
+  "right_child": false
+}
+)json";
+  Json j {Json::Load(StringView{str.c_str(), str.size()}, true)};
+  ASSERT_EQ(get<JsonBoolean>(j["left_child"]), true);
+  ASSERT_EQ(get<JsonBoolean>(j["right_child"]), false);
+}
+
+TEST(Json, Indexing) {
+  auto str = GetModelStr();
+  JsonReader reader(StringView{str.c_str(), str.size()}, true);
+  Json j {Json::Load(&reader)};
+  auto& value_1 = j["model_parameter"];
+  auto& value = value_1["base_score"];
+  std::string result = Cast<JsonString>(&value.GetValue())->getString();
+
+  ASSERT_EQ(result, "0.5");
+}
+
+TEST(Json, AssigningObjects) {
+  {
+    Json json;
+    json = JsonObject();
+    json["Okay"] = JsonArray();
+    ASSERT_EQ(get<JsonArray>(json["Okay"]).size(), 0);
+  }
+
+  {
+    std::map<std::string, Json> objects;
+    Json json_objects { JsonObject() };
+    std::vector<Json> arr_0 (1, Json(3.3));
+    json_objects["tree_parameters"] = JsonArray(arr_0);
+    std::vector<Json> json_arr = get<JsonArray>(json_objects["tree_parameters"]);
+    ASSERT_NEAR(get<JsonNumber>(json_arr[0]), 3.3f, kRtEps);
+  }
+
+  {
+    Json json_object { JsonObject() };
+    auto str = JsonString("1");
+    auto& k = json_object["1"];
+    k  = str;
+    auto& m = json_object["1"];
+    std::string value = get<JsonString>(m);
+    ASSERT_EQ(value, "1");
+    ASSERT_EQ(get<JsonString>(json_object["1"]), "1");
+  }
+}
+
+TEST(Json, AssigningArray) {
+  Json json;
+  json = JsonArray();
+  std::vector<Json> tmp_0 {Json(Number(1)), Json(Number(2))};
+  json = tmp_0;
+  std::vector<Json> tmp_1 {Json(Number(3))};
+  get<Array>(json) = tmp_1;
+  std::vector<Json> res = get<Array>(json);
+  ASSERT_EQ(get<Number>(res[0]), 3);
+}
+
+TEST(Json, AssigningNumber) {
+  {
+    // right value
+    Json json = Json{ Number(4) };
+    get<Number>(json) = 15;
+    ASSERT_EQ(get<Number>(json), 15);
+  }
+
+  {
+    // left value ref
+    Json json = Json{ Number(4) };
+    Number::Float& ref = get<Number>(json);
+    ref = 15;
+    ASSERT_EQ(get<Number>(json), 15);
+  }
+
+  {
+    // left value
+    Json json = Json{ Number(4) };
+    double value = get<Number>(json);
+    ASSERT_EQ(value, 4);
+    value = 15;  // NOLINT
+    ASSERT_EQ(get<Number>(json), 4);
+  }
+}
+
+TEST(Json, AssigningString) {
+  {
+    // right value
+    Json json = Json{ String("str") };
+    get<String>(json) = "modified";
+    ASSERT_EQ(get<String>(json), "modified");
+  }
+
+  {
+    // left value ref
+    Json json = Json{ String("str") };
+    std::string& ref = get<String>(json);
+    ref = "modified";
+    ASSERT_EQ(get<String>(json), "modified");
+  }
+
+  {
+    // left value
+    Json json = Json{ String("str") };
+    std::string value = get<String>(json);
+    value = "modified";
+    ASSERT_EQ(get<String>(json), "str");
+  }
+}
+
+TEST(Json, LoadDump) {
+  std::string buffer = GetModelStr();
+  Json origin {Json::Load(StringView{buffer.c_str(), buffer.size()}, true)};
+
+  dmlc::TemporaryDirectory tempdir;
+  auto const& path = tempdir.path + "test_model_dump";
+
+  std::ofstream fout (path);
+  Json::Dump(origin, &fout);
+  fout.close();
+
+  buffer = common::LoadSequentialFile(path);
+  Json load_back {Json::Load(StringView(buffer.c_str(), buffer.size()), true)};
+
+  ASSERT_EQ(load_back, origin);
+}
+
+// For now Json is quite ignorance about unicode.
+TEST(Json, CopyUnicode) {
+  std::string json_str = R"json(
+{"m": ["\ud834\udd1e", "\u20ac", "\u0416", "\u00f6"]}
+)json";
+  Json loaded {Json::Load(StringView{json_str.c_str(), json_str.size()}, true)};
+
+  std::stringstream ss_1;
+  Json::Dump(loaded, &ss_1);
+
+  std::string dumped_string = ss_1.str();
+  ASSERT_NE(dumped_string.find("\\u20ac"), std::string::npos);
+}
+
+TEST(Json, WrongCasts) {
+  {
+    Json json = Json{ String{"str"} };
+    ASSERT_ANY_THROW(get<Number>(json));
+  }
+  {
+    Json json = Json{ Array{ std::vector<Json>{ Json{ Number{1} } } } };
+    ASSERT_ANY_THROW(get<Number>(json));
+  }
+  {
+    Json json = Json{ Object{std::map<std::string, Json>{
+          {"key", Json{String{"value"}}}} } };
+    ASSERT_ANY_THROW(get<Number>(json));
+  }
+}
+}  // namespace xgboost