java - Jackson Kotlin/Java 中泛型类型的自定义反序列化

Question

有了这个数据模型...

TestClass.kt

data class TestClass (val bar: Optional<Double>?)

我的目标是反序列化以下 json 值:

{"foo": 3.5}  --> foo = 3.5
{"foo": null} --> foo = Optional.empty() // This is currently my problem. foo is null and I can't seem to fix it
{}            --> foo = null

我已经看到了solution here并尝试了这个，但我的“反序列化”方法中的断点似乎从未命中。

OptionalDeserializer.java

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.deser.ContextualDeserializer;
import java.io.IOException;
import java.util.Optional;

public class OptionalDeserializer extends JsonDeserializer<Optional<?>> implements ContextualDeserializer {
    private JavaType valueType;

    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext context, BeanProperty property) {
        this.valueType = property.getType().containedType(0);
        return this;
    }

    @Override
    public Optional<?> deserialize(final JsonParser parser, final DeserializationContext context) throws IOException {
        final JsonNode node = parser.getCodec().readTree(parser);

        return node.isNull()
                ? Optional.empty()
                : Optional.of(context.readValue(parser, valueType));
    }

}

TestDeserialization.kt

fun main(): {
    val objectMapper = ObjectMapper().registerModule(KotlinModule())

    val module = SimpleModule()
    module.addDeserializer(Optional::class.java, OptionalDeserializer())
    objectMapper.registerModule(module)

    objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL)

    objectMapper.configure(JsonGenerator.Feature.WRITE_BIGDECIMAL_AS_PLAIN, true)
    objectMapper.configure(DeserializationFeature.USE_BIG_DECIMAL_FOR_FLOATS, true)
    objectMapper.nodeFactory = JsonNodeFactory.withExactBigDecimals(true)

    val inputJson = """{"foo" : null}"""

    val expectedObject = TestClass(foo = Optional.empty())

    val actualObject = objectMapper.readValue(inputJson, TestClassBravo::class.java)

    assertEquals(expectedObject, actualObject)
}

build.gradle(用于版本信息)

compile 'com.fasterxml.jackson.core:jackson-databind:2.9.8'
compile 'com.fasterxml.jackson.module:jackson-module-kotlin:2.9.8'

Answer 1

有两种方法可以解决这个问题。

1. 不使用自定义序列化器会更容易，而是依赖 jackson 默认的 Jdk8Module (请参阅 here )

2. 如果您想要自定义反序列化器，则需要指定 null 访问模式，否则不会针对 null 值调用反序列化器:

2 的代码:

// In OptionalDeserializer

@Override
public AccessPattern getNullAccessPattern() {
    return AccessPattern.CONSTANT;
}