1概述
在这个教程中,我们将探讨如何利用jackson库中的推断式多态功能。
2. 基于名称的多态性
设想我们有一个如下面图像所示的类结构:
首先,NamedCharacter
和 ImperialSpy
类实现了 Character
接口。其次,King
和 Knight
类实现了 NamedCharacter
类。最后,我们有一个 ControlledCharacter
类,它包含玩家控制的角色引用。
我们的目标是将JSON对象解析为Java对象,而无需修改接收到的JSON结构。
让我们看看类的定义。请注意,对于基接口,我们需要使用Jackson注解来声明我们将使用的推断,同时还需要添加@JsonSubTypes
注解来声明我们想要推断的类。
@JsonTypeInfo(use = Id.NAME)
@JsonSubTypes({ @Type(ImperialSpy.class), @Type(King.class), @Type(Knight.class) })
public interface Character {
}
此外,我们还可以在Character
接口和King
、Knight
类之间有一个中介类。因此,Jackson也知道在这种情况下如何推断多态:
public class NamedCharacter implements Character {
private String name;
// standard setters and getters
}
接下来,我们将实现Character
接口的子类。我们在前面的代码示例中已经声明了这些子类作为子类型,所以实现中并不依赖于Jackson库:
public class ImperialSpy implements Character {
}
public class King extends NamedCharacter {
private String land;
// standard setters and getters
}
public class Knight extends NamedCharacter {
private String weapon;
// standard setters and getters
}
我们希望映射的一个JSON示例如下:
{
"name": "Old King Allant",
"land": "Boletaria",
}
首先,如果我们尝试读取上述JSON结构,Jackson会抛出一个运行时异常,消息为:“无法解析[简单类型,com.baeldung.jackson.deductionbasedpolymorphism.Character]的子类型:缺少'type'属性的类型标识。”
@Test
void givenAKingWithoutType_whenMapping_thenExpectAnError() {
String kingJson = formatJson("{'name': 'Old King Allant', 'land':'Boletaria'}");
assertThrows(InvalidTypeIdException.class, () -> objectMapper.readValue(kingJson, Character.class));
}
此外,formatJson
工具方法通过将引号转换为双引号,帮助我们在测试代码中保持简洁,因为JSON需要这样:
public static String formatJson(String input) {
return input.replaceAll("'", "\"");
}
因此,为了能够根据类型进行多态推断,我们必须修改JSON结构,并显式添加对象的类型。这就意味着我们将多态行为与JSON结构耦合在一起:
{
"@type": "King"
"name": "Old King Allant",
"land": "Boletaria",
}
@Test
void givenAKing_whenMapping_thenExpectAKingType() throws Exception {
String kingJson = formatJson("{'name': 'Old King Allant', 'land':'Boletaria', '@type':'King'}");
Character character = objectMapper.readValue(kingJson, Character.class);
assertTrue(character instanceof King);
assertSame(character.getClass(), King.class);
King king = (King) character;
assertEquals("Boletaria", king.getLand());
}
3. 推断式多态性
要启用推断式多态性,我们唯一需要做的就是使用@JsonTypeInfo(use = Id.DEDUCTION)
:
@JsonTypeInfo(use = Id.DEDUCTION)
@JsonSubTypes({ @Type(ImperialSpy.class), @Type(King.class), @Type(Knight.class) })
public interface Character {
}
4. 简单推断
现在,让我们探索如何以多态方式读取JSON,使用简单的推断。我们要读取的对象如下:
{
"name": "Ostrava, of Boletaria",
"weapon": "Rune Sword",
}
首先,我们将值读入Character
对象,然后测试Jackson是否正确推断了JSON的类型:
@Test
void givenAKnight_whenMapping_thenExpectAKnightType() throws Exception {
String knightJson = formatJson("{'name':'Ostrava, of Boletaria', 'weapon':'Rune Sword'}");
Character character = objectMapper.readValue(knightJson, Character.class);
assertTrue(character instanceof Knight);
assertSame(character.getClass(), Knight.class);
Knight king = (Knight) character;
assertEquals("Ostrava, of Boletaria", king.getName());
assertEquals("Rune Sword", king.getWeapon());
}
此外,如果JSON是一个空对象,Jackson将把它解释为没有属性的ImperialSpy
类:
@Test
void givenAnEmptyObject_whenMapping_thenExpectAnImperialSpy() throws Exception {
String imperialSpyJson = "{}";
Character character = objectMapper.readValue(imperialSpyJson, Character.class);
assertTrue(character instanceof ImperialSpy);
}
而且,Jackson还会将空的JSON对象推断为null对象:
@Test
void givenANullObject_whenMapping_thenExpectANullObject() throws Exception {
Character character = objectMapper.readValue("null", Character.class);
assertNull(character);
}
5. 不区分大小写的推断
Jackson也可以在属性名称大小写不匹配的情况下推断多态性。首先,我们需要创建一个启用ACCEPT_CASE_INSENSITIVE_PROPERTIES
的ObjectMapper:
ObjectMapper objectMapper = JsonMapper.builder().configure(MapperFeature.ACCEPT_CASE_INSENSITIVE_PROPERTIES, true).build();
然后,使用已实例化的objectMapper
,我们可以测试多态性是否正确推断:
{
"NaMe": "Ostrava, of Boletaria",
"WeaPON": "Rune Sword",
}
@Test
void givenACaseInsensitiveKnight_whenMapping_thenExpectKnight() throws Exception {
String knightJson = formatJson("{'NaMe':'Ostrava, of Boletaria', 'WeaPON':'Rune Sword'}");
Character character = objectMapper.readValue(knightJson, Character.class);
assertTrue(character instanceof Knight);
assertSame(character.getClass(), Knight.class);
Knight knight = (Knight) character;
assertEquals("Ostrava, of Boletaria", knight.getName());
assertEquals("Rune Sword", knight.getWeapon());
}
6. 包含的推断
我们还可以推断嵌套在其他对象中的对象的多态性。我们将使用ControlledCharacter
类的定义来演示以下JSON的映射:
{
"character": {
"name": "Ostrava, of Boletaria",
"weapon": "Rune Sword"
}
}
@Test
void givenAKnightControlledCharacter_whenMapping_thenExpectAControlledCharacterWithKnight() throws Exception {
String controlledCharacterJson = formatJson("{'character': {'name': 'Ostrava, of Boletaria', 'weapon': 'Rune Sword'}}");
ControlledCharacter controlledCharacter = objectMapper.readValue(controlledCharacterJson, ControlledCharacter.class);
Character character = controlledCharacter.getCharacter();
assertTrue(character instanceof Knight);
assertSame(character.getClass(), Knight.class);
Knight knight = (Knight) character;
assertEquals("Ostrava, of Boletaria", knight.getName());
assertEquals("Rune Sword", knight.getWeapon());
}
7. 总结
在这个教程中,我们探讨了如何使用Jackson库实现推断式多态性。
本文的源代码可以在GitHub上找到。