Why jackson is serializing transient member also?

A new way to stop Jackson from serializing and deserializing is to call mapper.configure(MapperFeature.PROPAGATE_TRANSIENT_MARKER, true).

I can't make comments so complete the previous response here, changing the (now) deprecated method setVisibilityChecker and adding a missing clause for booleans:

mapper.setVisibility(
    mapper.getSerializationConfig().
    getDefaultVisibilityChecker().
    withFieldVisibility(JsonAutoDetect.Visibility.ANY).
    withGetterVisibility(JsonAutoDetect.Visibility.NONE).
    withIsGetterVisibility(JsonAutoDetect.Visibility.NONE)
);

The reason Jackson serializes the transient member is because the getters are used to determine what to serialize, not the member itself - and since y has a public getter, that gets serialized. If you want to change that default and have Jackson use fields - simply do:

om.setVisibilityChecker(
  om.getSerializationConfig()
    .getDefaultVisibilityChecker()
    .withFieldVisibility(JsonAutoDetect.Visibility.ANY)
    .withGetterVisibility(JsonAutoDetect.Visibility.NONE)
);

Another way to ignore a property on serialization is to do it directly on the class:

@JsonIgnoreProperties(value = { "y" })
public class TestElement {
...

And another way is directly on the field:

public class TestElement {

    @JsonIgnore
    private String y;
...

Hope this helps.

You can configure it with springs properties

spring.jackson.mapper.propagate-transient-marker=true