java - Jackson 多态反序列化为特定子类型失败，错误代码为 "InvalidTypeIdException"

Question

我有一个多态数据类型，我试图将其直接反序列化为它的子类型(为了向后兼容)，但如果没有类型信息，我无法反序列化，即使我在对象映射器上的 readValue 调用中直接引用子类型。我可以使用 jackson 魔法来解决这个问题吗？

class SerializationTest {
    @Test
    fun serializationTest() {
        val serializedFoo = """ {"base":"blah", "foo": "something"} """
        val foo = ObjectMapper().registerModule(KotlinModule()).readValue(serializedFoo, Foo::class.java)
        println(foo)
    }

    @JsonTypeInfo(use = JsonTypeInfo.Id.CLASS)
    @JsonSubTypes(
            JsonSubTypes.Type(value = Foo::class),
            JsonSubTypes.Type(value = Bar::class)
    )
    internal abstract class Base {
        abstract val base: String
    }

    internal class Foo(@JsonProperty("base") override val base: String,
                       @JsonProperty("foo") val foo: String): Base()

    internal class Bar(@JsonProperty("base") override val base: String,
                       @JsonProperty("foo") val bar: String): Base()
}

失败

com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Missing type id when trying to resolve subtype of [simple type, class com.foo.bar.SerializationTest$Foo]: missing type id property '@class'

我目前使用的是 jackson 2.9.8

Answer 1

如果您可以在 objectmapper.readValue 中指定具体类，则可以使用 JsonTypeInfo.Id.NONE。

为了完整起见，来自NONE的javadoc:

This means that no explicit type metadata is included, and typing ispurely done using contextual information possibly augmented with other annotations.

这是一个 Java 测试用例:

public class SerializationTest {

    @Test
    public void serializationTest() throws JsonParseException, JsonMappingException, IOException {
        final String json = getSerializedFoo();
        final ObjectMapper om = getObjectMapper();

        final Foo value = om.readValue(json, Foo.class);

        assertTrue(value != null);
    }

    private ObjectMapper getObjectMapper() {
        final ObjectMapper om = new ObjectMapper();
        return om;
    }

    private String getSerializedFoo() {
        return "{\"base\": \"blah\", \"foo\": \"something\"}";
    }
}

@JsonTypeInfo(use = JsonTypeInfo.Id.NONE)
@JsonSubTypes({
        @Type(value = Foo.class),
        @Type(value = Bar.class)
})
abstract class Base {

    @JsonProperty("base")
    protected String base;

    public String getBase() {
        return base;
    }

    public void setBase(final String base) {
        this.base = base;
    }
}

class Bar extends Base {

    @JsonProperty("bar")
    private String bar;

    public String getBar() {
        return bar;
    }

    public void setBar(final String foo) {
        bar = foo;
    }
}

class Foo extends Base {

    @JsonProperty("foo")
    private String foo;

    public String getFoo() {
        return foo;
    }

    public void setFoo(final String foo) {
        this.foo = foo;
    }
}