如何在 Jackson 视图中获取类的属性列表? [英] How to get the list of properties of a class as Jackson views it?

问题描述

我正在编写需要访问由 Jackson 配置定义的类的属性列表的代码.

I'm writing code that needs to access the list of properties of a class as defined by a Jackson configuration.

例如，对于以下类:

@JsonIgnoreProperties(value = { "intValue" })
public class MyDto {

    @JsonProperty("name")
    private String stringValue;

    private int intValue;

    private long longValue;

    @JsonIgnore
    private boolean booleanValue;

    // standard setters and getters are not shown
}

我会得到 [name,longValue] 因为这是 Jackson 在序列化时实际考虑的属性.

I would get [name,longValue] because that's the properties that Jackson actually considers when serializing.

我不认为编写一整段代码来查找 getter/setter 并检查 Jackson 注释是可行的方法，因为那会重新实现 Jackson.

I don't think writing a whole piece of code to look for getters/setters and inspect Jackson annotations is the way to go, since that would be reimplementing Jackson.

如果我能够获得用于序列化的 Jackson ObjectMapper 的句柄，是否有办法获取 Class 的属性列表? 还是 Type 对象?(尊重 Jackson 注释和配置)

If I'm able to get a handle on the Jackson ObjectMapper used for serialization, is there a way to get a list of properties of a Class<?> or Type object? (respecting Jackson annotations and config)

我深入研究了 Jackson 的实现，并找到了 POJOPropertiesCollector，但我不确定如何从 Jackson 外部使用它(我相信我们不应该这样做).

I dug a bit into Jackson's implementation, and found the POJOPropertiesCollector, but I'm not sure about how I can use it from outside Jackson (we're not supposed to do this I believe).

作为最后的手段，我可​​以创建我正在检查的类的实例，使用 ObjectMapper 对其进行序列化，然后解析 JSON 以查找属性名称，但我认为这也不干净(而且它会带来它的整套问题:空值可能不会被序列化，构造函数中会发生什么等).

As a last resort, I could create an instance of the class I'm inspecting, serialize it with the ObjectMapper, and then parse the JSON to find property names, but I don't think this is clean either (and it would bring its whole set of probelms: nulls might not be serialized, what happens in the construtor etc.).

有什么想法吗?

推荐答案

使用 Jackson，您可以内省任意类获取可用的 JSON 属性:

With Jackson, you can introspect an arbitrary class to get the available JSON properties:

// Construct a Jackson JavaType for your class
JavaType javaType = mapper.getTypeFactory().constructType(MyDto.class);

// Introspect the given type
BeanDescription beanDescription = mapper.getSerializationConfig().introspect(javaType);

// Find properties
List<BeanPropertyDefinition> properties = beanDescription.findProperties();

BeanPropertyDefinition 列表应该为您提供有关 JSON 属性所需的详细信息.

The BeanPropertyDefinition list should give you the details you need regarding the JSON properties.

@JsonIgnoreProperties 类级别的注释没有考虑到上述方法.但是您可以使用 AnnotationIntrospector 获取在类级别忽略的属性:

The @JsonIgnoreProperties class level annotation is not taken into account with the above mentioned approach. But you can use an AnnotationIntrospector to get the properties ignored on class level:

// Get class level ignored properties
Set<String> ignoredProperties = mapper.getSerializationConfig().getAnnotationIntrospector()
        .findPropertyIgnorals(beanDescription.getClassInfo()).getIgnored();

然后过滤 properties 删除出现在 ignoredProperties 中的属性:

Then filter properties removing the properties which are present in ignoredProperties:

// Filter properties removing the class level ignored ones
List<BeanPropertyDefinition> availableProperties = properties.stream()
        .filter(property -> !ignoredProperties.contains(property.getName()))
        .collect(Collectors.toList());

即使您为类定义了混入，这种方法也有效.

This approach works even if you have mix-ins defined for your class.

AnnotationIntrospector#findPropertyIgnorals(Annotated) 方法是在 Jackson 2.8 中引入的.AnnotationIntrospector#findPropertiesToIgnore(Annotated, boolean) 方法可用于旧版本(但自 Jackson 2.8 起已弃用).

The AnnotationIntrospector#findPropertyIgnorals(Annotated) method was introduced in Jackson 2.8. The AnnotationIntrospector#findPropertiesToIgnore(Annotated, boolean) method can be used for older versions (but it's deprecated since Jackson 2.8).

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