Json 为什么杰克逊还要序列化瞬态成员

I am serializing a POJO into JSON using Jackson 2.1.4 but I want to ignore a particular field from getting serialized. I used transient but still it is serializing that element.

public class TestElement {

    int x;

    private transient String y;

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public String getY() {
        return y;
    }

    public void setY(String y) {
        this.y = y;
    }
}

I am serializing as following:

public static void main(String[] args) throws JsonProcessingException {
    TestElement testElement = new TestElement();
    testElement.setX(10);
    testElement.setY("adasd");
    ObjectMapper om = new ObjectMapper();
    String serialized = om.writeValueAsString(testElement);
    System.err.println(serialized);
}

Please don't suggest @JsonIgnore as I don't want to tie my model to jackson specific annotations. Can it be done using transient only? Is there any API on objectmapper for visibility settings?