【Gson】源码解析

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private FieldNamingStrategy fieldNamingPolicy = FieldNamingPolicy.IDENTITY;

public Gson create() {
  List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>();
  factories.addAll(this.factories);
  Collections.reverse(factories);
  factories.addAll(this.hierarchyFactories);
  addTypeAdaptersForDate(datePattern, dateStyle, timeStyle, factories);

  return new Gson(excluder, fieldNamingPolicy, instanceCreators,
      serializeNulls, complexMapKeySerialization,
      generateNonExecutableJson, escapeHtmlChars, prettyPrinting,
      serializeSpecialFloatingPointValues, longSerializationPolicy, factories);
}

当我们没有声明字段名称策略的时候，默认使用IDENTITY策略

//指定日期序列化反序列化的格式
  public GsonBuilder setDateFormat(String pattern) {
    // TODO(Joel): Make this fail fast if it is an invalid date format
    this.datePattern = pattern;
    return this;
  }

//Gson默认的date类型的序列化反序列化
  private String datePattern;
  private int dateStyle = DateFormat.DEFAULT;
  private int timeStyle = DateFormat.DEFAULT;

//创建Gson
public Gson create() {
    List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>();
    factories.addAll(this.factories);
    Collections.reverse(factories);
    factories.addAll(this.hierarchyFactories);
//创建Date类型转换器
    addTypeAdaptersForDate(datePattern, dateStyle, timeStyle, factories);

    return new Gson(excluder, fieldNamingPolicy, instanceCreators,
        serializeNulls, complexMapKeySerialization,
        generateNonExecutableJson, escapeHtmlChars, prettyPrinting,
        serializeSpecialFloatingPointValues, longSerializationPolicy, factories);
  }

  private void addTypeAdaptersForDate(String datePattern, int dateStyle, int timeStyle,
      List<TypeAdapterFactory> factories) {
   //默认的date类型转换器
    DefaultDateTypeAdapter dateTypeAdapter;
    if (datePattern != null && !"".equals(datePattern.trim())) {
      dateTypeAdapter = new DefaultDateTypeAdapter(datePattern);
    } else if (dateStyle != DateFormat.DEFAULT && timeStyle != DateFormat.DEFAULT) {
      dateTypeAdapter = new DefaultDateTypeAdapter(dateStyle, timeStyle);
    } else {
      return;
    }

//提供Date，Timestamp，Date的类型转换
    factories.add(TreeTypeAdapter.newFactory(TypeToken.get(Date.class), dateTypeAdapter));
    factories.add(TreeTypeAdapter.newFactory(TypeToken.get(Timestamp.class), dateTypeAdapter));
    factories.add(TreeTypeAdapter.newFactory(TypeToken.get(java.sql.Date.class), dateTypeAdapter));
  }

//通过构造方法创建对应的转换处理类
DefaultDateTypeAdapter() {
    this(DateFormat.getDateTimeInstance(DateFormat.DEFAULT, DateFormat.DEFAULT, Locale.US),
        DateFormat.getDateTimeInstance(DateFormat.DEFAULT, DateFormat.DEFAULT));
  }

  DefaultDateTypeAdapter(String datePattern) {
    this(new SimpleDateFormat(datePattern, Locale.US), new SimpleDateFormat(datePattern));
  }

  DefaultDateTypeAdapter(int style) {
    this(DateFormat.getDateInstance(style, Locale.US), DateFormat.getDateInstance(style));
  }

  public DefaultDateTypeAdapter(int dateStyle, int timeStyle) {
    this(DateFormat.getDateTimeInstance(dateStyle, timeStyle, Locale.US),
        DateFormat.getDateTimeInstance(dateStyle, timeStyle));
  }

  DefaultDateTypeAdapter(DateFormat enUsFormat, DateFormat localFormat) {
    this.enUsFormat = enUsFormat;
    this.localFormat = localFormat;
    this.iso8601Format = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'", Locale.US);
    this.iso8601Format.setTimeZone(TimeZone.getTimeZone("UTC"));
  }

默认/通过setDateFormat实现的序列化/反序列化源码
  public JsonElement serialize(Date src, Type typeOfSrc, JsonSerializationContext context) {
    synchronized (localFormat) {
      String dateFormatAsString = enUsFormat.format(src);
      return new JsonPrimitive(dateFormatAsString);
    }
  }

public Date deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
      throws JsonParseException {
    if (!(json instanceof JsonPrimitive)) {
      throw new JsonParseException("The date should be a string value");
    }
    Date date = deserializeToDate(json);
    if (typeOfT == Date.class) {
      return date;
    } else if (typeOfT == Timestamp.class) {
      return new Timestamp(date.getTime());
    } else if (typeOfT == java.sql.Date.class) {
      return new java.sql.Date(date.getTime());
    } else {
      throw new IllegalArgumentException(getClass() + " cannot deserialize to " + typeOfT);
    }
  }

  private Date deserializeToDate(JsonElement json) {
    synchronized (localFormat) {
      try {
        return localFormat.parse(json.getAsString());
      } catch (ParseException ignored) {
      }
      try {
        return enUsFormat.parse(json.getAsString());
      } catch (ParseException ignored) {
      }
      try {
        return iso8601Format.parse(json.getAsString());
      } catch (ParseException e) {
        throw new JsonSyntaxException(json.getAsString(), e);
      }
    }
  }

设置日期类型转换的方式，Gson使用SimpleDateFormat作为日期类型的序列化/发序列化方式

默认使用本地的日期格式作为日期类型的序列化/反序列化

下面是Gson的构造源码

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  我自己创建的类型转换器就是通过改方法传递过来的
    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.CLASS_FACTORY);

    // type adapters for composite and user-defined types
    factories.add(new CollectionTypeAdapterFactory(constructorConstructor));
    factories.add(new MapTypeAdapterFactory(constructorConstructor, complexMapKeySerialization));
    factories.add(new JsonAdapterAnnotationTypeAdapterFactory(constructorConstructor));
    factories.add(TypeAdapters.ENUM_FACTORY);
    factories.add(new ReflectiveTypeAdapterFactory(
        constructorConstructor, fieldNamingPolicy, excluder));

    this.factories = Collections.unmodifiableList(factories);
  }

Gson默认提供大量的类型转换器，包含了基本所有的java数据类型，同时对于需要特殊处理的数据类型数据我们可以通过GsonBuilder来构建，实现数据的序列化与反序列化

Gson博文：

https://www.jianshu.com/p/0e40a52c0063

https://www.jianshu.com/p/8cc857583ff4