用于自定义 NULL 值序列化的 Jackson 自定义注释 [英] Jackson custom annotation for custom NULL value serialization
问题描述
根据这个答案:https://stackoverflow.com/a/43342675/5810648
我写了这样的序列化程序:
I wrote such serializer:
public class CustomSerializer extends StdSerializer<Double> implements ContextualSerializer {
private final NAifNull annotation;
public CustomSerializer() {
super(Double.class);
this.annotation = null;
}
public CustomSerializer(NAifNull annotation) {
super(Double.class);
this.annotation = annotation;
}
@Override
public void serialize(Double value, JsonGenerator gen, SerializerProvider provider) throws IOException {
if (annotation != null && value == null) {
gen.writeString("N/A");
} else {
gen.writeNumber(value);
}
}
@Override
public JsonSerializer<?> createContextual(SerializerProvider prov, BeanProperty property) {
NAifNull annotation = property.getAnnotation(NAifNull.class);
return new CustomSerializer(annotation);
}
}
如果注释存在且字段为null
,Witch 应该写字符串N/A".但是方法 serialize
只对非空字段调用.
Witch supposed to write string "N/A" if the annotation is present and field is null
. But method serialize
is called only for not null fields.
另外,我尝试调用 setNullValueSerializer
:
@Override
public JsonSerializer<?> createContextual(SerializerProvider prov, BeanProperty property) {
NAifNull annotation = property.getAnnotation(NAifNull.class);
prov.setNullValueSerializer(new CustomNullSerializer(annotation));
return new CustomSerializer(annotation);
}
有了这样的实现:
private static class CustomNullSerializer extends JsonSerializer<Object> {
private final NAifNull annotation;
public CustomNullSerializer(NAifNull annotation) {
this.annotation = annotation;
}
@Override
public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
if (annotation != null) {
gen.writeString("N/A");
} else {
gen.writeNull();
}
}
}
但没有结果.
如何以这种方式处理空字段?
How to handle null fields in such way?
更新
根据讨论:https://github.com/FasterXML/jackson-databind/issues/2057
prov.setNullValueSerializer(new CustomNullSerializer(annotation));
不应从 CreateContextual
方法调用.
推荐答案
使用 BeanSerializerModifier
为特定属性自定义空序列化器:
Use a BeanSerializerModifier
to customize the null serializer for a particular property:
public class CustomBeanSerializerModifier extends BeanSerializerModifier {
@Override
public List<BeanPropertyWriter> changeProperties(SerializationConfig config,
BeanDescription beanDesc, List<BeanPropertyWriter> beanProperties) {
for (BeanPropertyWriter beanProperty : beanProperties) {
if (beanProperty.getAnnotation(NAifNull.class) != null) {
beanProperty.assignNullSerializer(new CustomNullSerializer());
}
}
return beanProperties;
}
}
其中 @NAifNull
和 CustomNullSerializer
的定义如下:
Where @NAifNull
and CustomNullSerializer
are define as follows:
public class CustomNullSerializer extends JsonSerializer<Object> {
@Override
public void serialize(Object value, JsonGenerator jgen,
SerializerProvider provider) throws IOException {
jgen.writeString("N/A");
}
}
@Target({ ElementType.FIELD })
@Retention(RetentionPolicy.RUNTIME)
@interface NAifNull {
}
然后使用如下:
ObjectMapper mapper = new ObjectMapper();
mapper.registerModule(new SimpleModule() {
@Override
public void setupModule(SetupContext context) {
super.setupModule(context);
context.addBeanSerializerModifier(new CustomBeanSerializerModifier());
}
});
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