杰克逊泛型与变量JsonProperty（用泛型） [英] Jackson Generics with variable JsonProperty (usage with generics)

问题描述

  public class Data< U> {
 
 @JsonProperty（困难）
私人U []数据; 
 
 // ... geter setter构造函数
} 
  

我不想创建10个更类似的类，因为我只需要更改一行代码（在这种情况下@JsonProperty（困难））。属性值取决于类型。是否可以在一个类中编写它？

解决方案

基于MichałZiober在这里我能够改变默认值字段名称通过重写PropertyNamingStrategy值
：

这些是我收到的JSON示例（简化版）：

  {status：OK，error：null，data：{
困难：[{value：easy}，{ value：medium}] 
}} 
 
 {status：ok，error：null，data：{
countries ：[{code：AT}，{code：BE}] 
}} 
  

查看第二行的差异，其中数据对象包含难度
或国家（或许多其他基于上下文的名称）。

基于JSON响应的响应类：

  public class Response< T> {
 private String status; 
私有字符串错误; 
私人资料< T>数据; 
 // Getters Setters构造函数
} 
  

基于JSON响应的数据类：

  public class Data< T> {
 //属性名称，将被更改
 @JsonProperty（DataNamingStrategy.DATA_FIELD）
 private T [] data; 
 // Getters Setters构造函数
} 
  

这是命名策略，将默认值更改为运行时指定值

  public class DataNamingStrategy extends PropertyNamingStrategy {
 
 //由其他类（这将是应该更改的默认字段名称）
 public static final String DATA_FIELD =variable：data; 
 private String fieldName; 
 
 public DataNamingStrategy（String fieldName）{
 this.fieldName = fieldName; 
} 
 
 //在这种情况下，使用它来改变不需要的字段名称（格式variable：value）
 @Override 
 public String nameForField MapperConfig<？> config，AnnotatedField field，
 String defaultName）{
 return（defaultName.equals（DATA_FIELD））？ 
 fieldName：
 super.nameForField（config，field，defaultName）; 
} 
 
 //使用它来更改setter方法字段名称（JSON  - >格式为variable的对象：{}）
 @Override 
 public String nameForSetterMethod（MapperConfig<？> config，
 AnnotatedMethod方法，String defaultName）{
 return（defaultName.equals（DATA_FIELD））？ 
 fieldName：
 super.nameForGetterMethod（config，method，defaultName）; 
} 
 
 //使用它来改变getter方法字段名称（Object  - >格式为variable的JSON：{}）
 //应该和nameForSetterMethod相同
 @Override 
 public String nameForGetterMethod（MapperConfig<？> config，
 AnnotatedMethod method，String defaultName）{
 return nameForSetterMethod（config，method，defaultName）; 
} 
} 
  

使用应该如下所示：

  ObjectMapper mapper = new ObjectMapper（）; 
 mapper.setPropertyNamingStrategy（new DataNamingStrategy（tableName））; 
 JavaType type = mapper.getTypeFactory（）。constructParametricType（Response.class，dataClass）; 
回应< U> u = mapper.readValue（result，type）; 
  

其中 result 是Json as String， code> tableName 是将用于JSON而不是默认值的字符串， dataClass 是 U的类（例如Difficulty.class）。

更好地使用 PropertyNamingStrategy 应该是 Map 而不是一个字符串。但我只需要改变一个特定的值。

另请参阅 PropertyNamingStrategy 文档或在米奇尔齐奥贝的回答

I have class that looks like this:

public class Data<U> {

    @JsonProperty("difficulties")
    private U[] data;

    // ... geter setter constructor
}

And I don't want to create 10 more similar classes just because I need to change only one line of code (@JsonProperty("difficulties") in this case). The property value depends on Type. Is it possible to write it in one class?

解决方案

Based on response of Jackson - Modify an attribute at runtime without annotation by Michał Ziober here I was able to change default field name values by overriding PropertyNamingStrategy:

These are my received JSON examples (simplified):

{"status": "OK","error": null,"data": {
    "difficulties": [{"value":"easy"},{"value":"medium"}]
}}

{"status": "ok", "error": null, "data": {
    "countries": [{"code": "AT"},{"code": "BE"}]
}}

see the difference in second line where data object contains either difficulties or countries (or many other names based on context).

Response class based on JSON response:

public class Response<T>{
    private String status;
    private String error;
    private Data<T> data;
    // Getters Setters Constructors
}

Data class based on JSON response:

public class Data<T> {
    // property name, that will be changed
    @JsonProperty(DataNamingStrategy.DATA_FIELD)
    private T[] data;
    // Getters Setters Constructors
}

And this is Naming strategy, that changes default value to runtime specified value

public class DataNamingStrategy extends PropertyNamingStrategy{

    // used by other classes (this will be default field name that should be changed)
    public static final String DATA_FIELD = "variable:data";
    private String fieldName;

    public DataNamingStrategy(String fieldName) {
        this.fieldName = fieldName;
    }

    // use this to change field name (format "variable":"value") not needed in my case
    @Override
    public String nameForField(MapperConfig<?> config, AnnotatedField field,
            String defaultName) {
        return (defaultName.equals(DATA_FIELD))?
            fieldName :
            super.nameForField(config, field, defaultName);
    }

    // use this to change setter method field name (JSON -> Object with format "variable":{})
    @Override
    public String nameForSetterMethod(MapperConfig<?> config,
            AnnotatedMethod method, String defaultName) {
        return (defaultName.equals(DATA_FIELD))?
            fieldName :
            super.nameForGetterMethod(config, method, defaultName);
    }

    // use this to change getter method field name (Object -> JSON with format "variable":{})
    // should be same as nameForSetterMethod
    @Override
    public String nameForGetterMethod(MapperConfig<?> config,
            AnnotatedMethod method, String defaultName) {
        return nameForSetterMethod(config, method, defaultName);
    }
}

And usage should look like this:

ObjectMapper mapper = new ObjectMapper();
mapper.setPropertyNamingStrategy(new DataNamingStrategy(tableName));
JavaType type = mapper.getTypeFactory().constructParametricType(Response.class, dataClass);
Response<U> u = mapper.readValue(result, type);

Where result is Json as String, tableName is String that will be used in JSON instead of default value and dataClass is class for U (for example Difficulty.class).

Better usage of PropertyNamingStrategy should be Map instead of one String. But I just needed to change one particular value.

Also have a look at PropertyNamingStrategy documentation or again at Michał Ziober's answer

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