【Gson七】Gson预定义类型适配器
编程技术 / houtizong 发布于 3年前 337
Gson提供了丰富的预定义类型适配器，在对象和JSON串之间进行序列化和反序列化时，指定对象和字符串之间的转换方式，
DateTypeAdapter
public final class DateTypeAdapter extends TypeAdapter<Date> { public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() { @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) { return typeToken.getRawType() == Date.class ? (TypeAdapter<T>) new DateTypeAdapter() : null; } }; private final DateFormat enUsFormat = DateFormat.getDateTimeInstance(DateFormat.DEFAULT, DateFormat.DEFAULT, Locale.US);//美国Locale private final DateFormat localFormat = DateFormat.getDateTimeInstance(DateFormat.DEFAULT, DateFormat.DEFAULT);//本地Locale private final DateFormat iso8601Format = buildIso8601Format();//ISO8601时间格式 private static DateFormat buildIso8601Format() {//构造（ISO8601DateFormat DateFormat iso8601Format = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'", Locale.US); iso8601Format.setTimeZone(TimeZone.getTimeZone("UTC")); return iso8601Format; } @Override public Date read(JsonReader in) throws IOException {//JsonReader中包含着要转换为Date的字符串 if (in.peek() == JsonToken.NULL) {//如果字符串为null in.nextNull();//将这个null消费完 return null; } return deserializeToDate(in.nextString());//将字符串反序列化为日期对象 } private synchronized Date deserializeToDate(String json) { try { return localFormat.parse(json);//首先尝试本地Locale } catch (ParseException ignored) { } try { return enUsFormat.parse(json);//再次尝试美国英语Locale } catch (ParseException ignored) { } try { return iso8601Format.parse(json); //最后尝试ISO8601时间格式 } catch (ParseException e) { throw new JsonSyntaxException(json, e); } } @Override public synchronized void write(JsonWriter out, Date value) throws IOException { if (value == null) { out.nullValue();//设置null值 return; } String dateFormatAsString = enUsFormat.format(value);//直接使用美国英语Locale将Date转换成为字符串 out.value(dateFormatAsString); }}
ArrayTypeAdapter
public final class ArrayTypeAdapter<E> extends TypeAdapter<Object> {//为什么是Object而不是E public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() { @SuppressWarnings({"unchecked", "rawtypes"}) public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) { Type type = typeToken.getType(); //如果type不是Array，返回null if (!(type instanceof GenericArrayType || type instanceof Class && ((Class<?>) type).isArray())) { return null; } Type componentType = $Gson$Types.getArrayComponentType(type); TypeAdapter<?> componentTypeAdapter = gson.getAdapter(TypeToken.get(componentType)); return new ArrayTypeAdapter( gson, componentTypeAdapter, $Gson$Types.getRawType(componentType)); } }; private final Class<E> componentType; private final TypeAdapter<E> componentTypeAdapter; public ArrayTypeAdapter(Gson context, TypeAdapter<E> componentTypeAdapter, Class<E> componentType) { this.componentTypeAdapter = new TypeAdapterRuntimeTypeWrapper<E>(context, componentTypeAdapter, componentType); this.componentType = componentType; } public Object read(JsonReader in) throws IOException { if (in.peek() == JsonToken.NULL) { in.nextNull(); return null; } List<E> list = new ArrayList<E>(); in.beginArray();//开始读数组 while (in.hasNext()) { E instance = componentTypeAdapter.read(in); list.add(instance); } in.endArray();//读数组结束 Object array = Array.newInstance(componentType, list.size()); for (int i = 0; i < list.size(); i++) { Array.set(array, i, list.get(i)); } return array; } @SuppressWarnings("unchecked") @Override public void write(JsonWriter out, Object array) throws IOException { if (array == null) { out.nullValue(); return; } out.beginArray(); for (int i = 0, length = Array.getLength(array); i < length; i++) { E value = (E) Array.get(array, i); componentTypeAdapter.write(out, value); } out.endArray(); }}
看到ArrayTypeAdapter的实现，可以说有点晕菜了，原因是对泛型不熟悉了，回头再看看泛型吧！！
SqlDateTypeAdapter
public final class SqlDateTypeAdapter extends TypeAdapter<java.sql.Date> { public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() {//这个FACTORY干啥用的？ @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) { return typeToken.getRawType() == java.sql.Date.class ? (TypeAdapter<T>) new SqlDateTypeAdapter() : null; } }; private final DateFormat format = new SimpleDateFormat("MMM d, yyyy"); //java.sql.Date与字符串转换的DateFormat @Override public synchronized java.sql.Date read(JsonReader in) throws IOException { if (in.peek() == JsonToken.NULL) { in.nextNull(); return null; } try { final long utilDate = format.parse(in.nextString()).getTime(); return new java.sql.Date(utilDate); } catch (ParseException e) { throw new JsonSyntaxException(e); } } @Override public synchronized void write(JsonWriter out, java.sql.Date value) throws IOException { out.value(value == null ? null : format.format(value));//将java.sql.Date对象转换成字符串，例如"Aug 13, 2014"，所以默认的java.sql.Date转换为年月日 }}
ObjectTypeAdapter
import com.google.gson.Gson;import com.google.gson.TypeAdapter;import com.google.gson.TypeAdapterFactory;import com.google.gson.internal.LinkedTreeMap;import com.google.gson.reflect.TypeToken;import com.google.gson.stream.JsonReader;import com.google.gson.stream.JsonToken;import com.google.gson.stream.JsonWriter;import java.io.IOException;import java.util.ArrayList;import java.util.List;import java.util.Map;/** * Adapts types whose static type is only 'Object'. Uses getClass() on * serialization and a primitive/Map/List on deserialization. */public final class ObjectTypeAdapter extends TypeAdapter<Object> { public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() { @SuppressWarnings("unchecked") public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) { if (type.getRawType() == Object.class) { // return (TypeAdapter<T>) new ObjectTypeAdapter(gson); } return null; } }; private final Gson gson; private ObjectTypeAdapter(Gson gson) { this.gson = gson; } @Override public Object read(JsonReader in) throws IOException { JsonToken token = in.peek();//判断字符串的实际类型 switch (token) { case BEGIN_ARRAY://如果是Array或者Map或者List，解析为ArrayList List<Object> list = new ArrayList<Object>(); in.beginArray(); while (in.hasNext()) { list.add(read(in)); } in.endArray(); return list; case BEGIN_OBJECT: //如果是Object，那么解析为Map，Map为String和Object的映射 Map<String, Object> map = new LinkedTreeMap<String, Object>(); in.beginObject(); while (in.hasNext()) { map.put(in.nextName(), read(in)); } in.endObject(); return map; case STRING://如果是字符串类型 return in.nextString(); case NUMBER://如果是数字类型 return in.nextDouble(); case BOOLEAN://如果是布尔类型 return in.nextBoolean(); case NULL://如果是Null in.nextNull(); return null; default: throw new IllegalStateException(); } } @SuppressWarnings("unchecked") @Override public void write(JsonWriter out, Object value) throws IOException { if (value == null) { out.nullValue(); return; } TypeAdapter<Object> typeAdapter = (TypeAdapter<Object>) gson.getAdapter(value.getClass());//得到value的实际类型对应的TypeAdapter if (typeAdapter instanceof ObjectTypeAdapter) {//因为是Object，没有数据字段可以写 out.beginObject(); out.endObject(); return; } typeAdapter.write(out, value);//调用实际的TypeAdapter进行写操作 }}
说明：下面的代码片段没有使用上面的ObjectTypeAdapter进行转换那么ObjectTypeAdapter究竟如何使用？？
Map<String, String> map = new HashMap<String, String>(); map.put("IM1","QQ"); map.put("IM2","MSN"); Object obj = map; str = new Gson().toJson(obj); System.out.println(str);//{"IM2":"MSN","IM1":"QQ"} obj = new Integer(100); str = new Gson().toJson(obj); System.out.println(str);//100 obj = new Boolean(true); str = new Gson().toJson(obj); System.out.println(str);//true
常用数据类型适配器
如下的代码，对单个的obj进行序列化，得到的字符串是100，也能将"100"反序列化为Integer对象，这里并没有JSON串的格式，但是却实现了序列化和反序列化
Integer obj = new Integer(100); str = new Gson().toJson(obj); System.out.println(str);//100 obj = new Gson().fromJson(str,Integer.class); //obj是100
将基本数据类型的数据序列化为普通字符串，普通字符串序列化为基本数据类型，这是如何做到的呢？Gson在TypeAdapters中定义了一系列的简单数据类型的类型适配器，
Integer/Long/Short/Boolean/String/BigDecimal/BitSet/Byte/Double/Float/InetAddress/Locale/StringBuffer/StringBuilder/Timestamp/URI/URL/Enum
默认的Gson对象内置了哪些类型适配器
Gson构造方法表明默认的Gson对象已经将上面提到的预定义类型适配器参加到Gson对象中
public Gson() { this(Excluder.DEFAULT, FieldNamingPolicy.IDENTITY, Collections.<Type, InstanceCreator<?>>emptyMap(), false, false, DEFAULT_JSON_NON_EXECUTABLE, true, false, false, LongSerializationPolicy.DEFAULT, Collections.<TypeAdapterFactory>emptyList()); }Gson(final Excluder excluder, final FieldNamingStrategy fieldNamingPolicy, final Map<Type, InstanceCreator<?>> instanceCreators, boolean serializeNulls, boolean complexMapKeySerialization, boolean generateNonExecutableGson, boolean htmlSafe, boolean prettyPrinting, boolean serializeSpecialFloatingPointValues, LongSerializationPolicy longSerializationPolicy, List<TypeAdapterFactory> typeAdapterFactories) { this.constructorConstructor = new ConstructorConstructor(instanceCreators); this.serializeNulls = serializeNulls; this.generateNonExecutableJson = generateNonExecutableGson; this.htmlSafe = htmlSafe; this.prettyPrinting = prettyPrinting; List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>(); // built-in type adapters that cannot be overridden factories.add(TypeAdapters.JSON_ELEMENT_FACTORY); factories.add(ObjectTypeAdapter.FACTORY); // the excluder must precede all adapters that handle user-defined types factories.add(excluder); // user's type adapters factories.addAll(typeAdapterFactories); // type adapters for basic platform types factories.add(TypeAdapters.STRING_FACTORY); factories.add(TypeAdapters.INTEGER_FACTORY); factories.add(TypeAdapters.BOOLEAN_FACTORY); factories.add(TypeAdapters.BYTE_FACTORY); factories.add(TypeAdapters.SHORT_FACTORY); factories.add(TypeAdapters.newFactory(long.class, Long.class, longAdapter(longSerializationPolicy))); factories.add(TypeAdapters.newFactory(double.class, Double.class, doubleAdapter(serializeSpecialFloatingPointValues))); factories.add(TypeAdapters.newFactory(float.class, Float.class, floatAdapter(serializeSpecialFloatingPointValues))); factories.add(TypeAdapters.NUMBER_FACTORY); factories.add(TypeAdapters.CHARACTER_FACTORY); factories.add(TypeAdapters.STRING_BUILDER_FACTORY); factories.add(TypeAdapters.STRING_BUFFER_FACTORY); factories.add(TypeAdapters.newFactory(BigDecimal.class, TypeAdapters.BIG_DECIMAL)); factories.add(TypeAdapters.newFactory(BigInteger.class, TypeAdapters.BIG_INTEGER)); factories.add(TypeAdapters.URL_FACTORY); factories.add(TypeAdapters.URI_FACTORY); factories.add(TypeAdapters.UUID_FACTORY); factories.add(TypeAdapters.LOCALE_FACTORY); factories.add(TypeAdapters.INET_ADDRESS_FACTORY); factories.add(TypeAdapters.BIT_SET_FACTORY); factories.add(DateTypeAdapter.FACTORY); factories.add(TypeAdapters.CALENDAR_FACTORY); factories.add(TimeTypeAdapter.FACTORY); factories.add(SqlDateTypeAdapter.FACTORY); factories.add(TypeAdapters.TIMESTAMP_FACTORY); factories.add(ArrayTypeAdapter.FACTORY); factories.add(TypeAdapters.ENUM_FACTORY); factories.add(TypeAdapters.CLASS_FACTORY); // type adapters for composite and user-defined types factories.add(new CollectionTypeAdapterFactory(constructorConstructor)); factories.add(new MapTypeAdapterFactory(constructorConstructor, complexMapKeySerialization)); factories.add(new ReflectiveTypeAdapterFactory( constructorConstructor, fieldNamingPolicy, excluder)); this.factories = Collections.unmodifiableList(factories); }
注册自定义类型适配器
使用方法Gson gson = new GsonBuilder().registerTypeAdapter(Type type, Object typeAdapter),例如注册自定义的日期和Long的类型适配器
Gson gson = new GsonBuilder().registerTypeAdapter(Date.class, new DateLongTypeAdapter())
总结：
1. 泛型是理解Gson类型适配器乃至整个Gson的序列化和发序列化的根本，比如ArrayTypeAdapter
2. 什么样的对象会用到ObjectTypeAdapter进行序列化和反序列化
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