1 Fastjson2简介
Fastjson2是Fastjson的升级版，特色：
- 协定反对：反对
JSON/JSONB两种协定 - 局部解析：能够应用
JSONPath进行局部解析获取须要的值 - 语言反对：
Java/Kotlin - 场景反对：
Android8+/服务端 - 其余个性反对：
Graal Native-Image、JSON Schema
2 根底应用
2.1 测试环境
环境：
JDK版本：1.8.0_341Fastjson2版本：2.0.19
测试类：
@Builder@Data@ToStringpublic 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可用于反序列化时对JSON字段进行验证应用，配置Schema能够通过@JSONField/@JSONType，这里以@JSONField为例：
public class Entity { private String field1; @JSONField(schema = "{'minimum':0,'maximum':100}") private Integer field2;}测试代码：
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();}输入：
2.7 JSONPath
JSONPath可用于局部解析JSON字符串，示例：
Entity entity = Entity.builder().field1("a").field2(2).build();// $符号示意根对象// $.field1示意根对象的field1属性System.out.println(JSONPath.eval(entity,"$.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());// 如果传的是List，反对通过下标解析// 此处是返回[0,0]下标范畴内的值List<Entity> names = (List<Entity>)JSONPath.eval(list,"[0,0]");System.out.println(names.get(0));具体的解析语法以及更多例子请参考官网文档。
2.8 AutoType
AutoType是在序列化的时候带上类型的一种机制，这样在反序列化时就不须要传入类型，实现类型自动识别，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));输入：
因为在Fastjson1中呈现过AutoType破绽，因而官网提供了一个JVM参数齐全禁止（safeMode性能）：
-Dfastjson2.parser.safeMode=true3 底层实现探索
3.1 序列化
3.1.1 概览
序列化的实现能够参考官网的一张类图：
大略流程如下：
- 获取
ObjectWriter - 如果从
ObjectWriterProvider缓存有ObjectWriter，间接提取 - 如果
ObjectWriterProvider缓存没有ObjectWriter，结构对应的ObjectWriter，并缓存 - 获取到
ObjectWriter后，将JavaBean对象写入JSONWriter JSONWriter对根底类型进行写入- 返回后果
3.1.2 入口
这里的序列化探索以JSON.toJSONString(Object object)作为入口：
static String toJSONString(Object object) { // 初始化ObjectWriterProvider JSONWriter.Context writeContext = new JSONWriter.Context(JSONFactory.defaultObjectWriterProvider); // 格式化管制 boolean pretty = (writeContext.features & com.alibaba.fastjson2.JSONWriter.Feature.PrettyFormat.mask) != 0L; Object jsonWriter; // 默认有三个JSONWriter，JDK8一个，针对JDK9之后的byte[]实现的字符串优化也有一个，还有一个基于char[]实现的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 { // 格式化管制 JSONWriter writer = pretty ? new JSONWriterPretty((JSONWriter)jsonWriter) : jsonWriter; String var12; try { if (object == null) { // null的话间接写入"null"字符串 ((JSONWriter)writer).writeNull(); } else { // 设置根对象 ((JSONWriter)writer).setRootObject(object); Class<?> valueClass = object.getClass(); if (valueClass == JSONObject.class) { // 如果指标类是JSNOObject，间接调用writer的write办法 ((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; // 通过Provider获取ObjectWriter ObjectWriter<?> objectWriter = context.provider.getObjectWriter(valueClass, valueClass, fieldBased); // ObjectWriter将数据写入JSONWriter objectWriter.write((JSONWriter)writer, object, (Object)null, (Type)null, 0L); } } // 后果 var12 = writer.toString(); // 上面的代码与序列化关系不大，能够不看 } 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 获取ObjectWriterProvider
JSON.toJSONString()入口：
JSONWriter.Context writeContext = new JSONWriter.Context(JSONFactory.defaultObjectWriterProvider);其中会调用默认的构造方法初始化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采取的是单例模式，默认采纳ASM动静字节码实现。
3.1.4 获取ObjectWriter
有了ObjectWriterProvider后，下一步就是获取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是基于字段序列化的意思 // false的话示意基于getter序列化 // 依据不同的类型从不同的缓存map中获取 ObjectWriter objectWriter = fieldBased ? (ObjectWriter)this.cacheFieldBased.get(objectType) : (ObjectWriter)this.cache.get(objectType); // 首次获取应该为null if (objectWriter != null) { return (ObjectWriter)objectWriter; } else { // 这个useModules布尔变量笔者不太理解 // 这里的逻辑是 基于字段反序列化 并且 指标class不为空 并且 指标class能够赋值给Iterable 并且 指标class不能赋值给class boolean useModules = true; if (fieldBased && objectClass != null && Iterable.class.isAssignableFrom(objectClass) && !Collection.class.isAssignableFrom(objectClass)) { useModules = false; } // 这里的例子是true if (useModules) { for(int i = 0; i < this.modules.size(); ++i) { // 获取ObjectWriterModule ObjectWriterModule module = (ObjectWriterModule)this.modules.get(i); objectWriter = module.getObjectWriter(objectType, objectClass); // 为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; } } } // 第一次执行临时还拿不到ObjectWriter，这里的条件全副合乎 if (objectWriter == null && objectClass != null && !fieldBased) { switch (objectClass.getName()) { // 针对Guava库的map内置了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类 case "com.alibaba.fastjson.JSONObject": objectWriter = ObjectWriterImplMap.of(objectClass); } } // ObjectWriter还没拿到 if (objectWriter == null) { // 第一次拿须要通过ObjectWriterCreator()去创立 ObjectWriterCreator creator = this.getCreator(); if (objectClass == null) { objectClass = TypeUtils.getMapping(objectType); } // 创立ObjectWriter // ObjectWriter的外面会创立FieldWriter，这外面的逻辑很长，通过一些列简单逻辑的判断，再针对字段获取 objectWriter = creator.createObjectWriter(objectClass, fieldBased ? Feature.FieldBased.mask : 0L, this); // 放入缓存 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获取逻辑如下：
// ObjectWriterCreatorASM.java 236行左右的地位BeanUtils.fields(objectClass, (field) -> { if (fieldBased || Modifier.isPublic(field.getModifiers())) { fieldInfo.init(); // 创立FieldWriter FieldWriter fieldWriter = this.creteFieldWriter(objectClass, writerFeatures, provider.modules, beanInfo, fieldInfo, field); if (fieldWriter != null) { // 放入缓存 fieldWriterMap.putIfAbsent(fieldWriter.fieldName, fieldWriter); } }});3.1.5 write()操作
获取到ObjectWriter之后，就能够进行write()操作了，JSON.toJSONString()入口：
objectWriter.write((JSONWriter)writer, object, (Object)null, (Type)null, 0L);因为自定义类的ObjectWriter是运行时拿到的，无奈通过调试获取到，然而能够通过内置的ObjectWriter来判断大略的write()流程：
比方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(); // 写入key jsonWriter.writeName("year"); // 写入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); }}依据getter获取字段值并调用对应的Writer去写JSON。
3.2 反序列化
3.2.1 概览
反序列化也能够参考官网的一张图：
大略流程与序列化相似：
- 获取
ObjectReader - 如果
ObjectReaderProvider有缓存，从缓存提取 - 如果
ObjectReaderProvider没有缓存，创立ObjectReader并且缓存到ObjectReaderProvider中 - 通过
JSONReader失去Object - 返回后果
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()) { // 获取JSONReader JSONReader reader = JSONReader.of(text); Object var7; try { JSONReader.Context context = reader.context; // 判断是否基于字段反序列化 boolean fieldBased = (context.features & Feature.FieldBased.mask) != 0L; // 通过ObjectReaderProvider获取ObjectReader // 这个context看起来和ObjectReaderProvider无关，实际上外部实现Context蕴含了ObjectReaderProvider ObjectReader<T> objectReader = context.provider.getObjectReader(clazz, fieldBased); // 反序列化外围办法 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 获取JSONReader
JSON.parseObject()入口：
JSONReader reader = JSONReader.of(text);其中JSONReader.of()实现如下：
public static JSONReader of(String str) { if (str == null) { throw new NullPointerException(); } else { // 创立Context // Context外部蕴含了ObjectReaderProvider Context context = JSONFactory.createReadContext(); int length; // 测试环境JDK8，此处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; // 测试环境JDK8 if (JDKUtils.JVM_VERSION == 8) { // 获取char array chars = JDKUtils.getCharArray(str); } else { chars = str.toCharArray(); } // 因为JDK8的String还是应用char[]实现的，所以返回UTF16的JSONReader return new JSONReaderUTF16(context, str, chars, 0, length); }}3.2.4 获取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; } // 有缓存间接从缓存提取 ObjectReader objectReader = fieldBased ? (ObjectReader)this.cacheFieldBased.get(objectType) : (ObjectReader)this.cache.get(objectType); // 第一次获取ObjectReader为null if (objectReader != null) { return objectReader; } else { Iterator var4 = this.modules.iterator(); ObjectReader previous; while(var4.hasNext()) { ObjectReaderModule module = (ObjectReaderModule)var4.next(); // 获取到的ObjectReader为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; // 条件不合乎 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; } } } } // 条件不合乎 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) { // 针对Guava的MultiMap，这里的条件不合乎 case "com.google.common.collect.ArrayListMultimap": objectReader = ObjectReaderImplMap.of((Type)null, objectClass, 0L); } } // ObjectReader为null if (objectReader == null) { // 获取ObjectReaderCreator ObjectReaderCreator creator = this.getCreator(); // 创立ObjectReader objectReader = creator.createObjectReader(objectClass, (Type)objectType, fieldBased, this); } previous = this.getPreviousObjectReader(fieldBased, (Type)objectType, objectReader); if (previous != null) { objectReader = previous; } return objectReader; }}在创立ObjectReader的同时，会在其中创立FieldReader：
// ObjectReaderCreator.java 453FieldReader[] fieldReaderArray = this.createFieldReaders(objectClass, objectType, beanInfo, fieldBased, provider);3.2.5 read()操作
有了ObjectReader之后就能够进行具体的read()操作了，JSON.parseObject()入口：
T object = objectReader.readObject(reader, (Type)null, (Object)null, 0L);此处的readObject()如下：
// ObjectReaderNoneDefaultConstructor.java 171行public T readObject(JSONReader jsonReader, Type fieldType, Object fieldName, long features) { // 判断是否带有JSONReader.Feature.ErrorOnNoneSerializable if (!this.serializable) { jsonReader.errorOnNoneSerializable(this.objectClass); } // 是否JSONB，不合乎 if (jsonReader.isJSONB()) { return this.readJSONBObject(jsonReader, fieldType, fieldName, 0L); } else if (jsonReader.isArray() && jsonReader.isSupportBeanArray(features | this.features)) { // 是数组而且反对JSONReader.Feature.SupportArrayToBean，不合乎 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 { // 读取字符 boolean objectStart = jsonReader.nextIfObjectStart(); // 条件为false，进入else if (!objectStart && !jsonReader.isTypeRedirect() && jsonReader.nextIfEmptyString()) { return null; } else { // 获取Context，读取其中的Features JSONReader.Context context = jsonReader.getContext(); long featuresAll = this.features | features | context.getFeatures(); // 此处存储对象的值 // key是字段的long哈希值，哈希采取的是fnv1a 64算法生成的 // value是字段的具体值 LinkedHashMap<Long, Object> valueMap = null; // 读取 for(int i = 0; !jsonReader.nextIfMatch('}'); ++i) { // 读取字段名对应的hashCode long hashCode = jsonReader.readFieldNameHashCode(); if (hashCode != 0L) { if (hashCode == this.typeKeyHashCode && i == 0) { long typeHash = jsonReader.readTypeHashCode(); // 此处if条件不合乎 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 { // 获取字段名哈希对应的FieldReader FieldReader fieldReader = this.getFieldReader(hashCode); // 条件不合乎 if (fieldReader == null && (featuresAll & Feature.SupportSmartMatch.mask) != 0L) { long hashCodeLCase = jsonReader.getNameHashCodeLCase(); if (hashCodeLCase != hashCode) { fieldReader = this.getFieldReaderLCase(hashCodeLCase); } } // fieldReader不为null if (fieldReader == null) { this.processExtra(jsonReader, (Object)null); } else { // 读取字段值 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; } // 写入 valueMap.put(hash, fieldValue); } } } } // 结构对象 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]; // 读取字段值 Object fieldValue = valueMap.get(fieldReader.fieldNameHash); if (fieldValue != null) { // 通过setter注入 fieldReader.accept(object, fieldValue); } } } jsonReader.nextIfMatch(','); return object; } }}4 结尾
其实文章中很多细节的中央因为篇幅的限度无奈过于具体的解释，比方内置的各类型的Reader/Writer具体是如何获取值进行序列化/反序列操作的，想要深刻探索这部分就只能本人去挖源码了。另外须要留神的是，文章的环境是在JDK8下的，因为Fastjson2在不同的JDK下会有不同的序列化实现，因而仅供参考。
最初，对于性能的比拟，能够参考官网的比拟基准。
5 参考
- Fastjson2 Github
- Fastjson2 Wiki