public class Test {
public static void main(String[] args) {

TestModel testModel = new TestModel();
    testModel.setDesc(null);

    System.out.println(JSON.toJSONString(testModel, SerializerFeature.WriteNullStringAsEmpty));
}

static class TestModel {
    String desc;

    public String getDesc() {
        return desc;
    }

    public void setDesc(String desc) {
        this.desc = desc;
    }
}

}

实验输出结果为：{}