如何获取Jackson看到的类的属性列表？ [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Jackson配置定义的类属性列表的代码。

例如，对于以下类：

  @JsonIgnoreProperties（value = {intValue}）
公共类MyDto {
 
 @JsonProperty（name）
 private String stringValue; 
 
 private int intValue; 
 
 private long longValue; 
 
 @JsonIgnore 
 private boolean booleanValue; 
 
 //标准制定者和获取者未显示
} 
  

我会得到 [name，longValue] 因为这是杰克逊在序列化时实际考虑的属性。

我不知道我认为编写一整段代码来寻找getter / setter并检查Jackson注释是要走的路，因为这将重新实现Jackson。

如果我能够获得用于序列化的Jackson ObjectMapper 的句柄，有没有办法获得 Class<的属性列表。 ？> 或输入对象？（尊重杰克逊注释和配置）

我挖了一下杰克逊的实现，找到了 POJOPropertiesCollector ，但我不确定如何从杰克逊以外的地方使用它（我们不认为要做到这一点，我相信）。

作为最后的手段，我可​​以创建一个我鼓励的类的实例使用ObjectMapper执行，序列化它，然后解析JSON以查找属性名称，但我不认为这也是干净的（它会带来它的整个问题：nulls可能没有序列化，在construtor中会发生什么等。）。

任何想法？

解决方案

与Jackson，您可以内省任意类以获取可用的JSON属性：

< pre class =lang-java prettyprint-override> //为您的类构造Jackson JavaType
JavaType javaType = mapper.getTypeFactory（）。constructType（MyDto.class）;

//内省给定类型
BeanDescription beanDescription = mapper.getSerializationConfig（）。introspect（javaType）;

//查找属性
列表< BeanPropertyDefinition> properties = beanDescription.findProperties（）;


BeanPropertyDefinition 列表应该为您提供有关的详细信息JSON属性。

---

@JsonIgnoreProperties 不考虑类级别注释上面提到的方法。但您可以使用 AnnotationIntrospector 在类级别上忽略属性：

  //获取类级忽略的属性
 Set< String> ignoredProperties = mapper.getSerializationConfig（）。getAnnotationIntrospector（）
 .findPropertyIgnorals（beanDescription.getClassInfo（））。getIgnored（）; 
  

然后过滤属性删除属性存在于 ignoredProperties ：

  //删除类级别的过滤属性
 List< BeanPropertyDefinition> availableProperties = properties.stream（）
 .filter（property  - >！ignoredProperties.contains（property.getName（）））
 .collect（Collectors.toList（））; 
  

即使你为你的班级定义了混音，这种方法仍然有效。

---

AnnotationIntrospector＃findPropertyIgnorals（Annotated） 方法在Jackson 2.8中引入。 AnnotationIntrospector＃findPropertiesToIgnore（Annotated，boolean） 方法可用于旧版本（但自Jackson 2.8以来已弃用） 。

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

For instance, for the following class:

@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
}

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

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.

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)

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).

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.).

Any ideas?

解决方案

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();

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

---

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();

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());

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

---

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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