昨天跟友人一起大塊歡飲之后,問了一個比較好玩的問題? JSON.parse 是怎么實現?當時草草的中規中矩的回答了一番,但終究對自己無知不是很滿意
今天上午想起來之后,便迅速翻出node的源碼來一探究竟!
對js的解析無疑是 v8的專屬,但由於v8是由c++編寫,我對c++的各種語法了解比較一般,如果有描述錯誤的地方,還請各位大神斧正.
v8的git地址 https://chromium.googlesource.com/v8/v8.git 需要翻牆
node的官網 https://nodejs.org , 如果速度慢可以直接訪問 http://nodejs.cn/
我是在node源碼的deps找到的v8的相關代碼
在v8的src文件夾中 直接全局搜索 JSON::Parse ( c++的語法 ) 可以找到對應代碼編寫在 api.cc文件中,具體代碼如下:
在剛剛的搜索結果中,找到一個簡單的json parse,我們就從這里入手
這個例子里面關鍵的代碼不多,主要是
ASSIGN_RETURN_ON_EXCEPTION(isolate, result,
JsonParser(isolate, source).ParseJson(), Object);
對string做處理的是 JSONParse對象的ParseJson函數,然后繼續全局搜索 JSONParse,在 json-parser.cc中找到了ParseJson的相關代碼

template <bool seq_one_byte> MaybeHandle<Object> JsonParser<seq_one_byte>::ParseJson() { // Advance to the first character (possibly EOS) AdvanceSkipWhitespace(); Handle<Object> result = ParseJsonValue(); if (result.is_null() || c0_ != kEndOfString) { // Some exception (for example stack overflow) is already pending. if (isolate_->has_pending_exception()) return Handle<Object>::null(); // Parse failed. Current character is the unexpected token. Factory* factory = this->factory(); MessageTemplate::Template message; Handle<Object> arg1 = Handle<Smi>(Smi::FromInt(position_), isolate()); Handle<Object> arg2; switch (c0_) { case kEndOfString: message = MessageTemplate::kJsonParseUnexpectedEOS; break; case '-': case '0': case '1': case '2': case '3': case '4': case '5': case '6': case '7': case '8': case '9': message = MessageTemplate::kJsonParseUnexpectedTokenNumber; break; case '"': message = MessageTemplate::kJsonParseUnexpectedTokenString; break; default: message = MessageTemplate::kJsonParseUnexpectedToken; arg2 = arg1; arg1 = factory->LookupSingleCharacterStringFromCode(c0_); break; } Handle<Script> script(factory->NewScript(source_)); // We should sent compile error event because we compile JSON object in // separated source file. isolate()->debug()->OnCompileError(script); MessageLocation location(script, position_, position_ + 1); Handle<Object> error = factory->NewSyntaxError(message, arg1, arg2); return isolate()->template Throw<Object>(error, &location); } return result; }
很明顯,這個函數的代碼中最關鍵的就是ParseJsonValue這個函數

// Parse any JSON value. template <bool seq_one_byte> Handle<Object> JsonParser<seq_one_byte>::ParseJsonValue() { StackLimitCheck stack_check(isolate_); if (stack_check.HasOverflowed()) { isolate_->StackOverflow(); return Handle<Object>::null(); } if (stack_check.InterruptRequested() && isolate_->stack_guard()->HandleInterrupts()->IsException(isolate_)) { return Handle<Object>::null(); } if (c0_ == '"') return ParseJsonString(); if ((c0_ >= '0' && c0_ <= '9') || c0_ == '-') return ParseJsonNumber(); if (c0_ == '{') return ParseJsonObject(); if (c0_ == '[') return ParseJsonArray(); if (c0_ == 'f') { if (AdvanceGetChar() == 'a' && AdvanceGetChar() == 'l' && AdvanceGetChar() == 's' && AdvanceGetChar() == 'e') { AdvanceSkipWhitespace(); return factory()->false_value(); } return ReportUnexpectedCharacter(); } if (c0_ == 't') { if (AdvanceGetChar() == 'r' && AdvanceGetChar() == 'u' && AdvanceGetChar() == 'e') { AdvanceSkipWhitespace(); return factory()->true_value(); } return ReportUnexpectedCharacter(); } if (c0_ == 'n') { if (AdvanceGetChar() == 'u' && AdvanceGetChar() == 'l' && AdvanceGetChar() == 'l') { AdvanceSkipWhitespace(); return factory()->null_value(); } return ReportUnexpectedCharacter(); } return ReportUnexpectedCharacter(); }
string在解析之前,進行了一次字符串格式的整理,這樣可以保證c0_ 是string的第一個有效字符,而不是空格之類的無用字符.
接下來就比較簡單了,根據不同的第一個字符進行不同的處理,其他都會報未期待的字符錯誤
字符 | 調用函數 |
" |
ParseJsonString |
數字或 - | ParseJsonNumber |
{ | ParseJsonObject |
[ | ParseJsonArray |
f | 判斷是否是false |
t | 判斷是否是true |
n | 判斷是否是null |
ParseJsonString 繼續驗證string的有效性,因為本身就string類型,所以不需要類型轉換

template <bool seq_one_byte> bool JsonParser<seq_one_byte>::ParseJsonString(Handle<String> expected) { int length = expected->length(); if (source_->length() - position_ - 1 > length) { DisallowHeapAllocation no_gc; String::FlatContent content = expected->GetFlatContent(); if (content.IsOneByte()) { DCHECK_EQ('"', c0_); const uint8_t* input_chars = seq_source_->GetChars() + position_ + 1; const uint8_t* expected_chars = content.ToOneByteVector().start(); for (int i = 0; i < length; i++) { uint8_t c0 = input_chars[i]; if (c0 != expected_chars[i] || c0 == '"' || c0 < 0x20 || c0 == '\\') { return false; } } if (input_chars[length] == '"') { position_ = position_ + length + 1; AdvanceSkipWhitespace(); return true; } } } return false; }
ParseJsonNumber 驗證有效性后,利用 StringToDouble 把string轉化為數字

template <bool seq_one_byte> Handle<Object> JsonParser<seq_one_byte>::ParseJsonNumber() { bool negative = false; int beg_pos = position_; if (c0_ == '-') { Advance(); negative = true; } if (c0_ == '0') { Advance(); // Prefix zero is only allowed if it's the only digit before // a decimal point or exponent. if (IsDecimalDigit(c0_)) return ReportUnexpectedCharacter(); } else { int i = 0; int digits = 0; if (c0_ < '1' || c0_ > '9') return ReportUnexpectedCharacter(); do { i = i * 10 + c0_ - '0'; digits++; Advance(); } while (IsDecimalDigit(c0_)); if (c0_ != '.' && c0_ != 'e' && c0_ != 'E' && digits < 10) { SkipWhitespace(); return Handle<Smi>(Smi::FromInt((negative ? -i : i)), isolate()); } } if (c0_ == '.') { Advance(); if (!IsDecimalDigit(c0_)) return ReportUnexpectedCharacter(); do { Advance(); } while (IsDecimalDigit(c0_)); } if (AsciiAlphaToLower(c0_) == 'e') { Advance(); if (c0_ == '-' || c0_ == '+') Advance(); if (!IsDecimalDigit(c0_)) return ReportUnexpectedCharacter(); do { Advance(); } while (IsDecimalDigit(c0_)); } int length = position_ - beg_pos; double number; if (seq_one_byte) { Vector<const uint8_t> chars(seq_source_->GetChars() + beg_pos, length); number = StringToDouble(isolate()->unicode_cache(), chars, NO_FLAGS, // Hex, octal or trailing junk. std::numeric_limits<double>::quiet_NaN()); } else { Vector<uint8_t> buffer = Vector<uint8_t>::New(length); String::WriteToFlat(*source_, buffer.start(), beg_pos, position_); Vector<const uint8_t> result = Vector<const uint8_t>(buffer.start(), length); number = StringToDouble(isolate()->unicode_cache(), result, NO_FLAGS, // Hex, octal or trailing junk. 0.0); buffer.Dispose(); } SkipWhitespace(); return factory()->NewNumber(number, pretenure_); }
ParseJsonObject 先建立了一個空對象,對整個字符串開始循環檢查語法匹配,並將對應的key,value全部找到后添加到之前的空對象中

// Parse a JSON object. Position must be right at '{'. template <bool seq_one_byte> Handle<Object> JsonParser<seq_one_byte>::ParseJsonObject() { HandleScope scope(isolate()); Handle<JSObject> json_object = factory()->NewJSObject(object_constructor(), pretenure_); Handle<Map> map(json_object->map()); int descriptor = 0; ZoneList<Handle<Object> > properties(8, zone()); DCHECK_EQ(c0_, '{'); bool transitioning = true; AdvanceSkipWhitespace(); if (c0_ != '}') { do { if (c0_ != '"') return ReportUnexpectedCharacter(); int start_position = position_; Advance(); if (IsDecimalDigit(c0_)) { ParseElementResult element_result = ParseElement(json_object); if (element_result == kNullHandle) return Handle<Object>::null(); if (element_result == kElementFound) continue; } // Not an index, fallback to the slow path. position_ = start_position; #ifdef DEBUG c0_ = '"'; #endif Handle<String> key; Handle<Object> value; // Try to follow existing transitions as long as possible. Once we stop // transitioning, no transition can be found anymore. DCHECK(transitioning); // First check whether there is a single expected transition. If so, try // to parse it first. bool follow_expected = false; Handle<Map> target; if (seq_one_byte) { key = TransitionArray::ExpectedTransitionKey(map); follow_expected = !key.is_null() && ParseJsonString(key); } // If the expected transition hits, follow it. if (follow_expected) { target = TransitionArray::ExpectedTransitionTarget(map); } else { // If the expected transition failed, parse an internalized string and // try to find a matching transition. key = ParseJsonInternalizedString(); if (key.is_null()) return ReportUnexpectedCharacter(); target = TransitionArray::FindTransitionToField(map, key); // If a transition was found, follow it and continue. transitioning = !target.is_null(); } if (c0_ != ':') return ReportUnexpectedCharacter(); AdvanceSkipWhitespace(); value = ParseJsonValue(); if (value.is_null()) return ReportUnexpectedCharacter(); if (transitioning) { PropertyDetails details = target->instance_descriptors()->GetDetails(descriptor); Representation expected_representation = details.representation(); if (value->FitsRepresentation(expected_representation)) { if (expected_representation.IsHeapObject() && !target->instance_descriptors() ->GetFieldType(descriptor) ->NowContains(value)) { Handle<FieldType> value_type( value->OptimalType(isolate(), expected_representation)); Map::GeneralizeField(target, descriptor, details.constness(), expected_representation, value_type); } DCHECK(target->instance_descriptors() ->GetFieldType(descriptor) ->NowContains(value)); properties.Add(value, zone()); map = target; descriptor++; continue; } else { transitioning = false; } } DCHECK(!transitioning); // Commit the intermediate state to the object and stop transitioning. CommitStateToJsonObject(json_object, map, &properties); JSObject::DefinePropertyOrElementIgnoreAttributes(json_object, key, value) .Check(); } while (transitioning && MatchSkipWhiteSpace(',')); // If we transitioned until the very end, transition the map now. if (transitioning) { CommitStateToJsonObject(json_object, map, &properties); } else { while (MatchSkipWhiteSpace(',')) { HandleScope local_scope(isolate()); if (c0_ != '"') return ReportUnexpectedCharacter(); int start_position = position_; Advance(); if (IsDecimalDigit(c0_)) { ParseElementResult element_result = ParseElement(json_object); if (element_result == kNullHandle) return Handle<Object>::null(); if (element_result == kElementFound) continue; } // Not an index, fallback to the slow path. position_ = start_position; #ifdef DEBUG c0_ = '"'; #endif Handle<String> key; Handle<Object> value; key = ParseJsonInternalizedString(); if (key.is_null() || c0_ != ':') return ReportUnexpectedCharacter(); AdvanceSkipWhitespace(); value = ParseJsonValue(); if (value.is_null()) return ReportUnexpectedCharacter(); JSObject::DefinePropertyOrElementIgnoreAttributes(json_object, key, value) .Check(); } } if (c0_ != '}') { return ReportUnexpectedCharacter(); } } AdvanceSkipWhitespace(); return scope.CloseAndEscape(json_object); }
ParseJsonArray 大致上跟ParseJsonObject比較相同,

// Parse a JSON array. Position must be right at '['. template <bool seq_one_byte> Handle<Object> JsonParser<seq_one_byte>::ParseJsonArray() { HandleScope scope(isolate()); ZoneList<Handle<Object> > elements(4, zone()); DCHECK_EQ(c0_, '['); ElementKindLattice lattice; AdvanceSkipWhitespace(); if (c0_ != ']') { do { Handle<Object> element = ParseJsonValue(); if (element.is_null()) return ReportUnexpectedCharacter(); elements.Add(element, zone()); lattice.Update(element); } while (MatchSkipWhiteSpace(',')); if (c0_ != ']') { return ReportUnexpectedCharacter(); } } AdvanceSkipWhitespace(); // Allocate a fixed array with all the elements. Handle<Object> json_array; const ElementsKind kind = lattice.GetElementsKind(); switch (kind) { case PACKED_ELEMENTS: case PACKED_SMI_ELEMENTS: { Handle<FixedArray> elems = factory()->NewFixedArray(elements.length(), pretenure_); for (int i = 0; i < elements.length(); i++) elems->set(i, *elements[i]); json_array = factory()->NewJSArrayWithElements(elems, kind, pretenure_); break; } case PACKED_DOUBLE_ELEMENTS: { Handle<FixedDoubleArray> elems = Handle<FixedDoubleArray>::cast( factory()->NewFixedDoubleArray(elements.length(), pretenure_)); for (int i = 0; i < elements.length(); i++) { elems->set(i, elements[i]->Number()); } json_array = factory()->NewJSArrayWithElements(elems, kind, pretenure_); break; } default: UNREACHABLE(); } return scope.CloseAndEscape(json_array); }
這幾個方法中對ParseJsonValue進行的遞歸,來保證一次循環中讀取的值是一個完整的值,這個想法非常棒,從而保證了對象的屬性放置對象,數字,數組,字符串,null...等等的兼容.
學而時習之,不亦說乎.
第一次寫技術博客,難免有所欠缺,
歡迎江湖大俠前來指導.