java - 使用 Jackson 序列化通用接口(interface)子类

Question

我有一个带有多个实现类的通用接口(interface)，我需要通过 Json 对其进行序列化和反序列化。我正在尝试开始使用 Jackson，使用完整的数据绑定(bind)，但运气不佳。

示例代码说明了问题:

import org.codehaus.jackson.map.*;
import org.codehaus.jackson.map.type.TypeFactory;
import org.codehaus.jackson.type.JavaType;

public class Test {

    interface Result<T> {}

    static class Success<T> implements Result<T> {
        T value;
        T getValue() {return value;}
        Success(T value) {this.value = value;}
    }

    public static void main(String[] args) {
        Result<String> result = new Success<String>("test");
        JavaType type = TypeFactory.defaultInstance().constructParametricType(Result.class, String.class);
        ObjectMapper mapper = new ObjectMapper().enableDefaultTyping();
        ObjectWriter writer = mapper.writerWithType(type);
        ObjectReader reader = mapper.reader(type);

        try {
            String json = writer.writeValueAsString(result);
            Result<String> result2 = reader.readValue(json);
            Success<String> success = (Success<String>)result2;
        } catch (Throwable ex) {
            System.out.print(ex);
        }
    }
}

调用 writeValueAsString 导致以下异常:

org.codehaus.jackson.map.JsonMappingException:未找到类 Test$Success 的序列化器，也未发现创建 BeanSerializer 的属性(为避免异常，禁用 SerializationConfig.Feature.FAIL_ON_EMPTY_BEANS))

为什么 Jackson 希望我注册一个序列化器——我虽然完全数据绑定(bind)的目的是我不需要这样做？

上述做法是否正确？

Answer 1

首先，您需要使用工厂方法 TypeFactory.constructSpecializedType 注册专用类型以将其用于 Jackson。然后，专用类型应该是一个 bean(它应该有一个默认构造函数、getter 和 setter)来反序列化它。

看看这些测试说明。

@Test
public void canSerializeParametricInterface() throws IOException {
    final ObjectMapper mapper = new ObjectMapper().enableDefaultTyping();
    final JavaType baseInterface = TypeFactory.defaultInstance().constructParametricType(Result.class, String.class);
    final JavaType subType = TypeFactory.defaultInstance().constructSpecializedType(baseInterface, Success.class);
    final ObjectWriter writer = mapper.writerWithType(subType);
    final String json = writer.writeValueAsString(Success.create("test"));
    Assert.assertEquals("{\"value\":\"test\"}", json);
}

@Test
public void canDeserializeParametricInterface() throws IOException {
    final ObjectMapper mapper = new ObjectMapper().enableDefaultTyping();
    final JavaType baseInterface = TypeFactory.defaultInstance().constructParametricType(Result.class, String.class);
    final JavaType subType = TypeFactory.defaultInstance().constructSpecializedType(baseInterface, Success.class);
    final ObjectReader reader = mapper.reader(subType);
    final Success<String> success = reader.readValue("{\"value\":\"test\"}");
    Assert.assertEquals("test", success.getValue());
}

public static interface Result<T> {
}

public static class Success<T> implements Result<T> {

    private T value;

    public static <T> Success<T> create(T value) {
        final Success<T> success = new Success<T>();
        success.value = value;
        return success;
    }

    public T getValue() {
        return value;
    }

    public void setValue(T value) {
        this.value = value;
    }
}