Fastjson2基礎使用以及底層序列化/反序列化實現探究

1 Fastjson2簡介

Fastjson2是Fastjson的升級版，特徵：

• 協議支援：支援JSON/JSONB兩種協議
• 部分解析：可以使用JSONPath進行部分解析獲取需要的值
• 語言支援：Java/Kotlin
• 場景支援：Android8+/服務端
• 其他特性支援：Graal Native-Image、JSON Schema

2 基礎使用

2.1 測試環境

環境：

• JDK版本：1.8.0_341
• Fastjson2版本：2.0.19

測試類：

@Builder
@Data
@ToString
public class Entity {
    private String field1;
    private Integer field2;
}

2.2 JSON序列化

System.out.println(JSON.toJSONString(entity));

2.3 JSON反序列化

System.out.println(JSON.parseObject(str,Entity.class));

2.4 JSONB序列化

byte[] bytes = JSONB.toBytes(entity);

2.5 JSONB反序列化

System.out.println(JSONB.parseObject(bytes,Entity.class));

2.6 JSON Schema

JSON Schema可用於反序列化時對JSON欄位進行驗證使用，配置Schema可以透過@JSONField/@JSONType，這裡以@JSONField為例：

public class Entity {
    private String field1;
    @JSONField(schema = "{'minimum':0,'maximum':100}")
    private Integer field2;
}

測試程式碼：

Entity entity1 = Entity.builder().field2(-1).build();
Entity entity2 = Entity.builder().field2(50).build();
Entity entity3 = Entity.builder().field2(101).build();

String str1 = JSON.toJSONString(entity1);
String str2 = JSON.toJSONString(entity2);
String str3 = JSON.toJSONString(entity3);

try {
    JSON.parseObject(str1, Entity.class);
} catch (Exception e) {
    e.printStackTrace();
}

JSON.parseObject(str2, Entity.class);

try {
    JSON.parseObject(str3, Entity.class);
} catch (Exception e) {
    e.printStackTrace();
}

輸出：

2.7 JSONPath

JSONPath可用於部分解析JSON字串，示例：

Entity entity = Entity.builder().field1("a").field2(2).build();
// $符號表示根物件
// $.field1表示根物件的field1屬性
System.out.println(JSONPath.eval(entity,"$.field1"));
System.out.println(JSONPath.eval(entity,"$.field2"));

List<Entity> list = new ArrayList<>();
list.add(Entity.builder().field1("entity1").field2(1).build());
list.add(Entity.builder().field1("entity2").field2(2).build());
// 如果傳的是List，支援透過下標解析
// 此處是返回[0,0]下標範圍內的值
List<Entity> names = (List<Entity>)JSONPath.eval(list,"[0,0]");
System.out.println(names.get(0));

詳細的解析語法以及更多例子請參考官方文件。

2.8 AutoType

AutoType是在序列化的時候帶上型別的一種機制，這樣在反序列化時就不需要傳入型別，實現型別自動識別，例子：

Entity entity = Entity.builder().field1("a").field2(2).build();
String str = JSON.toJSONString(entity, JSONWriter.Feature.WriteClassName);
System.out.println(str);
System.out.println(JSON.parseObject(str, Object.class, JSONReader.Feature.SupportAutoType));

輸出：

由於在Fastjson1中出現過AutoType漏洞，因此官方提供了一個JVM引數完全禁止（safeMode功能）：

-Dfastjson2.parser.safeMode=true

3 底層實現探究

3.1 序列化

3.1.1 概覽

序列化的實現可以參考官方的一張類圖：

大概流程如下：

• 獲取ObjectWriter
• 如果從ObjectWriterProvider快取有ObjectWriter，直接提取
• 如果ObjectWriterProvider快取沒有ObjectWriter，構造對應的ObjectWriter，並快取
• 獲取到ObjectWriter後，將JavaBean物件寫入JSONWriter
• JSONWriter對基礎型別進行寫入
• 返回結果

3.1.2 入口

這裡的序列化探究以JSON.toJSONString(Object object)作為入口：

static String toJSONString(Object object) {
    // 初始化ObjectWriterProvider
    JSONWriter.Context writeContext = new JSONWriter.Context(JSONFactory.defaultObjectWriterProvider);
    // 格式化控制
    boolean pretty = (writeContext.features & com.alibaba.fastjson2.JSONWriter.Feature.PrettyFormat.mask) != 0L;
    Object jsonWriter;
    // 預設有三個JSONWriter，JDK8一個，針對JDK9之後的byte[]實現的字串最佳化也有一個，還有一個基於char[]實現的UTF16
    if (JDKUtils.JVM_VERSION == 8) {
        jsonWriter = new JSONWriterUTF16JDK8(writeContext);
    } else if ((writeContext.features & com.alibaba.fastjson2.JSONWriter.Feature.OptimizedForAscii.mask) != 0L) {
        jsonWriter = new JSONWriterUTF8JDK9(writeContext);
    } else {
        jsonWriter = new JSONWriterUTF16(writeContext);
    }

    try {
        // 格式化控制
        JSONWriter writer = pretty ? new JSONWriterPretty((JSONWriter)jsonWriter) : jsonWriter;

        String var12;
        try {
            if (object == null) {
                // null的話直接寫入"null"字串
                ((JSONWriter)writer).writeNull();
            } else {
                // 設定根物件
                ((JSONWriter)writer).setRootObject(object);
                Class<?> valueClass = object.getClass();
                if (valueClass == JSONObject.class) {
                    // 如果目標類是JSNOObject，直接呼叫writer的write方法
                    ((JSONWriter)writer).write((JSONObject)object);
                } else {
                    // 如果不是
                    JSONWriter.Context context = ((JSONWriter)writer).context;
                    boolean fieldBased = (context.features & com.alibaba.fastjson2.JSONWriter.Feature.FieldBased.mask) != 0L;
                    // 透過Provider獲取ObjectWriter
                    ObjectWriter<?> objectWriter = context.provider.getObjectWriter(valueClass, valueClass, fieldBased);
                    // ObjectWriter將資料寫入JSONWriter
                    objectWriter.write((JSONWriter)writer, object, (Object)null, (Type)null, 0L);
                }
            }
            // 結果
            var12 = writer.toString();
            // 下面的程式碼與序列化關係不大，可以不看
        } catch (Throwable var10) {
            if (writer != null) {
                try {
                    ((JSONWriter)writer).close();
                } catch (Throwable var9) {
                    var10.addSuppressed(var9);
                }
            }

            throw var10;
        }

        if (writer != null) {
            ((JSONWriter)writer).close();
        }

        return var12;
    } catch (NumberFormatException | NullPointerException var11) {
        throw new JSONException("JSON#toJSONString cannot serialize '" + object + "'", var11);
    }
}

3.1.3 獲取ObjectWriterProvider

JSON.toJSONString()入口：

JSONWriter.Context writeContext = new JSONWriter.Context(JSONFactory.defaultObjectWriterProvider);

其中會呼叫預設的構造方法初始化ObjectWriterProvider：

public ObjectWriterProvider() {
    this.init();
    ObjectWriterCreator creator = null;
    switch (JSONFactory.CREATOR) {
        case "reflect":
            creator = ObjectWriterCreator.INSTANCE;
            break;
        case "lambda":
            creator = ObjectWriterCreatorLambda.INSTANCE;
            break;
        case "asm":
        default:
            try {
                creator = ObjectWriterCreatorASM.INSTANCE;
            } catch (Throwable var5) {
            }

            if (creator == null) {
                creator = ObjectWriterCreatorLambda.INSTANCE;
            }
    }

    this.creator = (ObjectWriterCreator)creator;
}

ObjectWriterCreator採取的是單例模式，預設採用ASM動態位元組碼實現。

3.1.4 獲取ObjectWriter

有了ObjectWriterProvider後，下一步就是獲取ObjectWriter，也就是JSON.toJSONString()中的：

JSONWriter.Context context = ((JSONWriter)writer).context;
boolean fieldBased = (context.features & com.alibaba.fastjson2.JSONWriter.Feature.FieldBased.mask) != 0L;
ObjectWriter<?> objectWriter = context.provider.getObjectWriter(valueClass, valueClass, fieldBased);

getObjectWriter()如下：

public ObjectWriter getObjectWriter(Type objectType, Class objectClass, boolean fieldBased) {
    // fieldBased是基於欄位序列化的意思
    // false的話表示基於getter序列化
    // 根據不同的型別從不同的快取map中獲取
    ObjectWriter objectWriter = fieldBased ? (ObjectWriter)this.cacheFieldBased.get(objectType) : (ObjectWriter)this.cache.get(objectType);
    // 首次獲取應該為null
    if (objectWriter != null) {
        return (ObjectWriter)objectWriter;
    } else {
        // 這個useModules布林變數筆者不太瞭解
        // 這裡的邏輯是 基於欄位反序列化 並且 目標class不為空 並且 目標class可以賦值給Iterable 並且 目標class不能賦值給class
        boolean useModules = true;
        if (fieldBased && objectClass != null && Iterable.class.isAssignableFrom(objectClass) && !Collection.class.isAssignableFrom(objectClass)) {
            useModules = false;
        }
        
        // 這裡的例子是true
        if (useModules) {
            for(int i = 0; i < this.modules.size(); ++i) {
                // 獲取ObjectWriterModule
                ObjectWriterModule module = (ObjectWriterModule)this.modules.get(i);
                objectWriter = module.getObjectWriter(objectType, objectClass);
                // 為null
                if (objectWriter != null) {
                    ObjectWriter previous = fieldBased ? (ObjectWriter)this.cacheFieldBased.putIfAbsent(objectType, objectWriter) : (ObjectWriter)this.cache.putIfAbsent(objectType, objectWriter);
                    if (previous != null) {
                        objectWriter = previous;
                    }

                    return (ObjectWriter)objectWriter;
                }
            }
        }

        // 第一次執行暫時還拿不到ObjectWriter，這裡的條件全部符合
        if (objectWriter == null && objectClass != null && !fieldBased) {
            switch (objectClass.getName()) {
                // 針對Guava庫的map內建了ObjectWriter
                case "com.google.common.collect.HashMultimap":
                case "com.google.common.collect.LinkedListMultimap":
                case "com.google.common.collect.LinkedHashMultimap":
                case "com.google.common.collect.ArrayListMultimap":
                case "com.google.common.collect.TreeMultimap":
                    objectWriter = GuavaSupport.createAsMapWriter(objectClass);
                    break;
                // 不是JSONObject類
                case "com.alibaba.fastjson.JSONObject":
                    objectWriter = ObjectWriterImplMap.of(objectClass);
            }
        }
        
        // ObjectWriter還沒拿到
        if (objectWriter == null) {
            // 第一次拿需要透過ObjectWriterCreator()去建立
            ObjectWriterCreator creator = this.getCreator();
            if (objectClass == null) {
                objectClass = TypeUtils.getMapping(objectType);
            }
            
            // 建立ObjectWriter
            // ObjectWriter的裡面會建立FieldWriter，這裡面的邏輯很長，經過一些列複雜邏輯的判斷，再針對欄位獲取
            objectWriter = creator.createObjectWriter(objectClass, fieldBased ? Feature.FieldBased.mask : 0L, this);
            // 放入快取
            ObjectWriter previous = fieldBased ? (ObjectWriter)this.cacheFieldBased.putIfAbsent(objectType, objectWriter) : (ObjectWriter)this.cache.putIfAbsent(objectType, objectWriter);
            if (previous != null) {
                objectWriter = previous;
            }
        }

        return (ObjectWriter)objectWriter;
    }
}

其中FiledWriter獲取邏輯如下：

// ObjectWriterCreatorASM.java 236行左右的位置
BeanUtils.fields(objectClass, (field) -> {
    if (fieldBased || Modifier.isPublic(field.getModifiers())) {
        fieldInfo.init();
        // 建立FieldWriter
        FieldWriter fieldWriter = this.creteFieldWriter(objectClass, writerFeatures, provider.modules, beanInfo, fieldInfo, field);
        if (fieldWriter != null) {
            // 放入快取
            fieldWriterMap.putIfAbsent(fieldWriter.fieldName, fieldWriter);
        }

    }
});

3.1.5 write()操作

獲取到ObjectWriter之後，就可以進行write()操作了，JSON.toJSONString()入口：

objectWriter.write((JSONWriter)writer, object, (Object)null, (Type)null, 0L);

由於自定義類的ObjectWriter是執行時拿到的，無法透過除錯獲取到，但是可以透過內建的ObjectWriter來判斷大概的write()流程：

比如LocalDateWriter的write()如下：

public void write(JSONWriter jsonWriter, Object object, Object fieldName, Type fieldType, long features) {
    try {
        int year = (Integer)this.getYear.invoke(object);
        int monthOfYear = (Integer)this.getMonthOfYear.invoke(object);
        int dayOfMonth = (Integer)this.getDayOfMonth.invoke(object);
        Object chronology = this.getChronology.invoke(object);
        if (chronology != this.utc && chronology != null) {
            jsonWriter.startObject();
            // 寫入key
            jsonWriter.writeName("year");
            // 寫入value
            jsonWriter.writeInt32(year);
            jsonWriter.writeName("month");
            jsonWriter.writeInt32(monthOfYear);
            jsonWriter.writeName("day");
            jsonWriter.writeInt32(dayOfMonth);
            jsonWriter.writeName("chronology");
            jsonWriter.writeAny(chronology);
            jsonWriter.endObject();
        } else {
            LocalDate localDate = LocalDate.of(year, monthOfYear, dayOfMonth);
            DateTimeFormatter formatter = this.getDateFormatter();
            if (formatter == null) {
                formatter = jsonWriter.getContext().getDateFormatter();
            }

            if (formatter == null) {
                jsonWriter.writeLocalDate(localDate);
            } else {
                String str = formatter.format(localDate);
                jsonWriter.writeString(str);
            }
        }
    } catch (InvocationTargetException | IllegalAccessException var14) {
        throw new JSONException("write LocalDateWriter error", var14);
    }
}

根據getter獲取欄位值並呼叫對應的Writer去寫JSON。

3.2 反序列化

3.2.1 概覽

反序列化也可以參考官方的一張圖：

大概流程與序列化類似：

• 獲取ObjectReader
• 如果ObjectReaderProvider有快取，從快取提取
• 如果ObjectReaderProvider沒有快取，建立ObjectReader並且快取到ObjectReaderProvider中
• 透過JSONReader得到Object
• 返回結果

3.2.2 入口

Entity entity = Entity.builder().field1("a").field2(2).build();
String str = JSON.toJSONString(entity);
System.out.println(JSON.parseObject(str,Entity.class));

其中parseObject()如下：

static <T> T parseObject(String text, Class<T> clazz) {
    if (text != null && !text.isEmpty()) {
        // 獲取JSONReader
        JSONReader reader = JSONReader.of(text);

        Object var7;
        try {
            JSONReader.Context context = reader.context;
            // 判斷是否基於欄位反序列化
            boolean fieldBased = (context.features & Feature.FieldBased.mask) != 0L;
            // 透過ObjectReaderProvider獲取ObjectReader
            // 這個context看起來和ObjectReaderProvider無關，實際上內部實現Context包含了ObjectReaderProvider
            ObjectReader<T> objectReader = context.provider.getObjectReader(clazz, fieldBased);
            // 反序列化核心方法
            T object = objectReader.readObject(reader, (Type)null, (Object)null, 0L);
            if (reader.resolveTasks != null) {
                reader.handleResolveTasks(object);
            }

            if (reader.ch != 26 && (reader.context.features & Feature.IgnoreCheckClose.mask) == 0L) {
                throw new JSONException(reader.info("input not end"));
            }

            var7 = object;
        } catch (Throwable var9) {
            if (reader != null) {
                try {
                    reader.close();
                } catch (Throwable var8) {
                    var9.addSuppressed(var8);
                }
            }

            throw var9;
        }

        if (reader != null) {
            reader.close();
        }

        return var7;
    } else {
        return null;
    }
}

3.2.3 獲取JSONReader

JSON.parseObject()入口：

JSONReader reader = JSONReader.of(text);

其中JSONReader.of()實現如下：

public static JSONReader of(String str) {
    if (str == null) {
        throw new NullPointerException();
    } else {
        // 建立Context
        // Context內部包含了ObjectReaderProvider
        Context context = JSONFactory.createReadContext();
        int length;
        // 測試環境JDK8，此處if不成立
        if (JDKUtils.JVM_VERSION > 8 && JDKUtils.UNSAFE_SUPPORT) {
            try {
                length = JDKUtils.STRING_CODER != null ? JDKUtils.STRING_CODER.applyAsInt(str) : UnsafeUtils.getStringCoder(str);
                if (length == 0) {
                    byte[] bytes = JDKUtils.STRING_VALUE != null ? (byte[])JDKUtils.STRING_VALUE.apply(str) : UnsafeUtils.getStringValue(str);
                    return new JSONReaderASCII(context, str, bytes, 0, bytes.length);
                }
            } catch (Exception var4) {
                throw new JSONException("unsafe get String.coder error");
            }
        }

        length = str.length();
        char[] chars;
        // 測試環境JDK8
        if (JDKUtils.JVM_VERSION == 8) {
            // 獲取char array
            chars = JDKUtils.getCharArray(str);
        } else {
            chars = str.toCharArray();
        }
        
        // 由於JDK8的String還是使用char[]實現的，所以返回UTF16的JSONReader
        return new JSONReaderUTF16(context, str, chars, 0, length);
    }
}

3.2.4 獲取ObjectReader

JSON.parseObject()入口：

ObjectReader<T> objectReader = context.provider.getObjectReader(clazz, fieldBased);

getObjectReader()如下：

public ObjectReader getObjectReader(Type objectType, boolean fieldBased) {
    if (objectType == null) {
        objectType = Object.class;
    }

    // 有快取直接從快取提取
    ObjectReader objectReader = fieldBased ? (ObjectReader)this.cacheFieldBased.get(objectType) : (ObjectReader)this.cache.get(objectType);
    // 第一次獲取ObjectReader為null
    if (objectReader != null) {
        return objectReader;
    } else {
        Iterator var4 = this.modules.iterator();

        ObjectReader previous;
        while(var4.hasNext()) {
            ObjectReaderModule module = (ObjectReaderModule)var4.next();
            // 獲取到的ObjectReader為null
            objectReader = module.getObjectReader(this, (Type)objectType);
            if (objectReader != null) {
                previous = fieldBased ? (ObjectReader)this.cacheFieldBased.putIfAbsent(objectType, objectReader) : (ObjectReader)this.cache.putIfAbsent(objectType, objectReader);
                if (previous != null) {
                    objectReader = previous;
                }

                return objectReader;
            }
        }

        Type rawType;
        // 條件不符合
        if (objectType instanceof TypeVariable) {
            Type[] bounds = ((TypeVariable)objectType).getBounds();
            if (bounds.length > 0) {
                rawType = bounds[0];
                if (rawType instanceof Class) {
                    previous = this.getObjectReader(rawType, fieldBased);
                    if (previous != null) {
                        ObjectReader previous = this.getPreviousObjectReader(fieldBased, (Type)objectType, previous);
                        if (previous != null) {
                            previous = previous;
                        }

                        return previous;
                    }
                }
            }
        }
        
        // 條件不符合
        if (objectType instanceof ParameterizedType) {
            ParameterizedType parameterizedType = (ParameterizedType)objectType;
            rawType = parameterizedType.getRawType();
            Type[] typeArguments = parameterizedType.getActualTypeArguments();
            if (rawType instanceof Class) {
                Class rawClass = (Class)rawType;
                boolean generic = false;

                for(Class clazz = rawClass; clazz != Object.class; clazz = clazz.getSuperclass()) {
                    if (clazz.getTypeParameters().length > 0) {
                        generic = true;
                        break;
                    }
                }

                if (typeArguments.length == 0 || !generic) {
                    ObjectReader rawClassReader = this.getObjectReader(rawClass, fieldBased);
                    if (rawClassReader != null) {
                        ObjectReader previous = this.getPreviousObjectReader(fieldBased, (Type)objectType, rawClassReader);
                        if (previous != null) {
                            rawClassReader = previous;
                        }

                        return rawClassReader;
                    }
                }
            }
        }

        Class<?> objectClass = TypeUtils.getMapping((Type)objectType);
        String className = objectClass.getName();
        if (objectReader == null && !fieldBased) {
            switch (className) {
                // 針對Guava的MultiMap，這裡的條件不符合
                case "com.google.common.collect.ArrayListMultimap":
                    objectReader = ObjectReaderImplMap.of((Type)null, objectClass, 0L);
            }
        }
        
        // ObjectReader為null
        if (objectReader == null) {
            // 獲取ObjectReaderCreator
            ObjectReaderCreator creator = this.getCreator();
            // 建立ObjectReader
            objectReader = creator.createObjectReader(objectClass, (Type)objectType, fieldBased, this);
        }

        previous = this.getPreviousObjectReader(fieldBased, (Type)objectType, objectReader);
        if (previous != null) {
            objectReader = previous;
        }

        return objectReader;
    }
}

在建立ObjectReader的同時，會在其中建立FieldReader：

// ObjectReaderCreator.java 453
FieldReader[] fieldReaderArray = this.createFieldReaders(objectClass, objectType, beanInfo, fieldBased, provider);

3.2.5 read()操作

有了ObjectReader之後就可以進行具體的read()操作了，JSON.parseObject()入口：

T object = objectReader.readObject(reader, (Type)null, (Object)null, 0L);

此處的readObject()如下：

// ObjectReaderNoneDefaultConstructor.java 171行
public T readObject(JSONReader jsonReader, Type fieldType, Object fieldName, long features) {
    // 判斷是否帶有JSONReader.Feature.ErrorOnNoneSerializable
    if (!this.serializable) {
        jsonReader.errorOnNoneSerializable(this.objectClass);
    }

    // 是否JSONB，不符合
    if (jsonReader.isJSONB()) {
        return this.readJSONBObject(jsonReader, fieldType, fieldName, 0L);
    } else if (jsonReader.isArray() && jsonReader.isSupportBeanArray(features | this.features)) {
        // 是陣列而且支援JSONReader.Feature.SupportArrayToBean，不符合
        jsonReader.next();
        LinkedHashMap<Long, Object> valueMap = null;

        for(int i = 0; i < this.fieldReaders.length; ++i) {
            Object fieldValue = this.fieldReaders[i].readFieldValue(jsonReader);
            if (valueMap == null) {
                valueMap = new LinkedHashMap();
            }

            long hash = this.fieldReaders[i].fieldNameHash;
            valueMap.put(hash, fieldValue);
        }

        if (!jsonReader.nextIfMatch(']')) {
            throw new JSONException(jsonReader.info("array not end, " + jsonReader.current()));
        } else {
            jsonReader.nextIfMatch(',');
            return this.createInstanceNoneDefaultConstructor((Map)(valueMap == null ? Collections.emptyMap() : valueMap));
        }
    } else {
        // 讀取字元
        boolean objectStart = jsonReader.nextIfObjectStart();
        // 條件為false，進入else
        if (!objectStart && !jsonReader.isTypeRedirect() && jsonReader.nextIfEmptyString()) {
            return null;
        } else {
            // 獲取Context，讀取其中的Features
            JSONReader.Context context = jsonReader.getContext();
            long featuresAll = this.features | features | context.getFeatures();
            // 此處儲存物件的值
            // key是欄位的long雜湊值，雜湊採取的是fnv1a 64演算法生成的
            // value是欄位的具體值
            LinkedHashMap<Long, Object> valueMap = null;

            // 讀取
            for(int i = 0; !jsonReader.nextIfMatch('}'); ++i) {
                // 讀取欄位名對應的hashCode
                long hashCode = jsonReader.readFieldNameHashCode();
                if (hashCode != 0L) {
                    if (hashCode == this.typeKeyHashCode && i == 0) {
                        long typeHash = jsonReader.readTypeHashCode();
                        // 此處if條件不符合
                        if (typeHash != this.typeNameHash) {
                            boolean supportAutoType = (featuresAll & Feature.SupportAutoType.mask) != 0L;
                            ObjectReader autoTypeObjectReader;
                            String typeName;
                            if (supportAutoType) {
                                autoTypeObjectReader = context.getObjectReaderAutoType(typeHash);
                                if (autoTypeObjectReader == null) {
                                    typeName = jsonReader.getString();
                                    autoTypeObjectReader = context.getObjectReaderAutoType(typeName, this.objectClass, this.features);
                                }
                            } else {
                                typeName = jsonReader.getString();
                                autoTypeObjectReader = context.getObjectReaderAutoType(typeName, this.objectClass);
                            }

                            if (autoTypeObjectReader == null) {
                                typeName = jsonReader.getString();
                                autoTypeObjectReader = context.getObjectReaderAutoType(typeName, this.objectClass, this.features);
                            }

                            if (autoTypeObjectReader != null) {
                                Object object = autoTypeObjectReader.readObject(jsonReader, fieldType, fieldName, 0L);
                                jsonReader.nextIfMatch(',');
                                return object;
                            }
                        }
                    } else {
                        // 獲取欄位名雜湊對應的FieldReader
                        FieldReader fieldReader = this.getFieldReader(hashCode);
                           // 條件不符合
                        if (fieldReader == null && (featuresAll & Feature.SupportSmartMatch.mask) != 0L) {
                            long hashCodeLCase = jsonReader.getNameHashCodeLCase();
                            if (hashCodeLCase != hashCode) {
                                fieldReader = this.getFieldReaderLCase(hashCodeLCase);
                            }
                        }
                        
                        // fieldReader不為null
                        if (fieldReader == null) {
                            this.processExtra(jsonReader, (Object)null);
                        } else {
                            // 讀取欄位值
                            Object fieldValue = fieldReader.readFieldValue(jsonReader);
                            if (valueMap == null) {
                                valueMap = new LinkedHashMap();
                            }

                            long hash;
                            if (fieldReader instanceof FieldReaderObjectParam) {
                                hash = ((FieldReaderObjectParam)fieldReader).paramNameHash;
                            } else {
                                hash = fieldReader.fieldNameHash;
                            }
                            // 寫入
                            valueMap.put(hash, fieldValue);
                        }
                    }
                }
            }

            // 構造物件
            T object = this.createInstanceNoneDefaultConstructor((Map)(valueMap == null ? Collections.emptyMap() : valueMap));
            if (this.setterFieldReaders != null && valueMap != null) {
                for(int i = 0; i < this.setterFieldReaders.length; ++i) {
                    FieldReader fieldReader = this.setterFieldReaders[i];
                    // 讀取欄位值
                    Object fieldValue = valueMap.get(fieldReader.fieldNameHash);
                    if (fieldValue != null) {
                        // 透過setter注入
                        fieldReader.accept(object, fieldValue);
                    }
                }
            }

            jsonReader.nextIfMatch(',');
            return object;
        }
    }
}

4 結尾

其實文章中很多細節的地方由於篇幅的限制無法過於詳細的解釋，比如內建的各型別的Reader/Writer具體是如何獲取值進行序列化/反序列操作的，想要深入探究這部分就只能自己去挖原始碼了。另外需要注意的是，文章的環境是在JDK8下的，由於Fastjson2在不同的JDK下會有不同的序列化實現，因此僅供參考。

最後，關於效能的比較，可以參考官方的比較基準。

5 參考

• Fastjson2 Github
• Fastjson2 Wiki