【Gson七】Gson预定义类型适配器

最新推荐文章于 2023-03-26 19:06:39 发布

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最新推荐文章于 2023-03-26 19:06:39 发布

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分类专栏： JSON Gson 文章标签： java json runtime

本文链接：https://blog.youkuaiyun.com/axxbc123/article/details/84625357

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Gson

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本文详细介绍了Gson库中预定义的各种类型适配器，包括DateTypeAdapter、ArrayTypeAdapter、SqlDateTypeAdapter等，并探讨了它们在序列化和反序列化过程中的作用。还提到了Gson默认内置的类型适配器，以及如何注册自定义类型适配器，强调了泛型在理解Gson工作原理中的关键角色。

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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);
  }