I have an abstract class describing a mongo document. There could be different implementations of this class that need to override an abstract method. Here is a simplified example:
@Document
@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS)
public abstract class Entity {
@Id
private ObjectId id;
abstract String getSomething();
}
I want getSomething() to be written to the document as a string field. But I don't want to read it back.
I've tried to use the @AccessType annotation:
@AccessType(AccessType.Type.PROPERTY)
abstract String getSomething();
But when I'm reading this document from the db, spring throws UnsupportedOperationException: No accessor to set property. It is trying to find a setter for this field, but I don't want to define a setter for it - the method may return a calculated value and there should be no ability to change it. Although an empty setter could help, it looks more like a workaround, and I would try to avoid it.
So I'm wondering if there is a way to skip this particular property when reading from the db? Something opposite to the @Transient annotation. Or similar to @JsonIgnore in the Jackson library.
Surprisingly, there is no easy solution to make a field write only.
Although creating an empty setter would solve the problem, I feel that it breaks the least surprise principle: if you have a setter, you expect it to set a value.
So I decided to create my own @WriteOnly annotation and use it to ignore fields I don't want to read from the db:
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.FIELD, ElementType.METHOD})
public @interface WriteOnly {
}
To use it you will need to extend an AbstractMongoEventListener:
@Service
public class WriteOnlyListener extends AbstractMongoEventListener<Entity> {
@Override
public void onAfterLoad(AfterLoadEvent<Entity> event) {
Document doc = event.getDocument();
if (doc == null) return;
for (Field field: getWriteOnly(event.getType(), Class::getDeclaredFields)) {
doc.remove(field.getName());
}
for (Method method: getWriteOnly(event.getType(), Class::getDeclaredMethods)) {
doc.remove(getFieldName(method.getName()));
}
}
private <T extends AccessibleObject> List<T> getWriteOnly(Class<?> type,
Function<Class<?>, T[]> extractor) {
List<T> list = new ArrayList<>();
for (Class<?> c = type; c != null; c = c.getSuperclass()) {
list.addAll(Arrays.stream(extractor.apply(c))
.filter(o -> o.isAnnotationPresent(WriteOnly.class))
.collect(Collectors.toList()));
}
return list;
}
private static String getFieldName(String methodName) {
return Introspector.decapitalize(methodName.substring(methodName.startsWith("is") ? 2 :
methodName.startsWith("get") ? 3 : 0));
}
}