如何使用Jackson的ContextualDeserializer作为根值？

Question

我正在尝试为从某个抽象类扩展或实现某个接口的所有类实现通用反序列化器.在这个例子中,我使用的是界面StringConvertible.我需要确定具体类型,以便我可以创建一个实例.

程序员布鲁斯的一篇旧论坛帖子让我使用了ContextDeserializer,当StringConvertible是另一个类中的属性时它正在工作.

但是当我想直接反序列化StringConvertible时,我无法找到获取具体类型的方法,因为beanProperty参数是null.根据Jackson JSON用户组的这个问题/回答,显然这是预期的: The only case where property should be null is when serializing a "root value", meaning the object instance passed directly to ObjectMapper's (or ObjectWriter's) writeValue() method -- in this case there simply isn't a referring property. But otherwise it should always be passed.

有关两种情况的示例,请参阅下面的主要方法:

@JsonDeserialize(using = StringConvertibleDeserializer.class)
public final class SomeStringConvertible implements StringConvertible {

    private final String value;

    public SomeStringConvertible(final String value) {

        this.value = value;
    }

    @Override
    @JsonValue
    public String stringValue() {

        return value;
    }
}

public final class SomeWrapper {

    public SomeStringConvertible stringConvertible;

    public SomeWrapper() {

    }
}


public class StringConvertibleDeserializer extends StdDeserializer<StringConvertible> implements ContextualDeserializer {

    private final Class<? extends StringConvertible>    targetClass;

    StringConvertibleDeserializer() {

        super(StringConvertible.class);

        this.targetClass = null;
    }

    StringConvertibleDeserializer(final Class<? extends StringConvertible> targetClass) {

        super(StringConvertible.class);

        this.targetClass = targetClass;
    }

    @Override
    public JsonDeserializer<?> createContextual(final DeserializationContext deserializationContext, @Nullable final BeanProperty beanProperty)
                                                                                                                                                throws JsonMappingException {

        final StringConvertibleDeserializer contextualDeserializer;

        // ====  Determine target type  =====
        final Class<? extends StringConvertible> targetClass;
        JavaType type = beanProperty.getType(); // -> beanProperty is null when the StringConvertible type is a root value
        targetClass = (Class<? extends StringConvertible>) type.getRawClass();

        // ====  Create contextual deserializer  =====
        contextualDeserializer = new StringConvertibleDeserializer(targetClass);

        // ====  Return  =====
        return contextualDeserializer;
    }

    @Override
    public StringConvertible deserialize(JsonParser jsonParser, DeserializationContext context) throws IOException, JsonProcessingException {

        final StringConvertible value;

        // ====  Create instance using the target type  =====
        if (targetClass.equals(SomeStringConvertible.class))
            value = new SomeStringConvertible(jsonParser.getText());
        else {
            throw new RuntimeException();
        }

        // ====  Return  =====
        return value;
    }

}

public final class JacksonModule extends SimpleModule {

    public JacksonModule() {

        super();

        addDeserializer(StringConvertible.class, new StringConvertibleDeserializer());
    }
}

public final class Main {

    public static void main(String[] args) {

        final ObjectMapper mapper = new ObjectMapper();
        mapper.registerModule(new JacksonModule());

        final String wrappedValueJSON = "{\"stringConvertible\":\"hello world\"}";
        final String rootValueJSON = "\"hello world\"";

        try {
            mapper.readValue(wrappedValueJSON, SomeWrapper.class);  // This works fine
            mapper.readValue(rootValueJSON, SomeStringConvertible.class);   // This causes a NPE in createContextual(...) because beanProperty is null

        } catch (JsonProcessingException e) {
            throw new RuntimeException(e);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

问题:如果是根值,我如何获得类型具体类型？或者,如果有一个比这更好的解决方案,你会建议什么呢？

Answer 1

在研究StaxMan提出的解决方案时,我偶然发现了这个Github问题,jackson-databind该问题解决了完全相同的问题.作为回应,维护者DeserializationContext在版本中添加了一个方法2.5.0:

This turned out relatively easy to implement, so now there is:

class DeserializationContext {
   public JavaType getContextualType() { ... }
}
which will give expected type during call to createContextual(), including case of deserializers that are directly added via annotation.

因此,为了使我的工作在我的情况下,我只需要改变createContextual(...)方法中的一些代码.我改变了这个:

   // ====  Determine target type  =====
    final Class<? extends StringConvertible> targetClass;
    JavaType type = beanProperty.getType(); // -> beanProperty is null when the StringConvertible type is a root value
    targetClass = (Class<? extends StringConvertible>) type.getRawClass();

对此:

// ====  Determine target type  =====
final Class<? extends StringConvertible> targetClass;
{
    // ====  Get the contextual type info  =====
    final JavaType type; 
    if (beanProperty != null) 
        type = beanProperty.getType();  // -> beanProperty is null when the StringConvertible type is a root value

    else {
        type = deserializationContext.getContextualType();
    }

    // ====  Get raw Class from type info  =====
    targetClass = (Class<? extends StringConvertible>) type.getRawClass();
}