无法获得基本的Jackson Mixin工作

Question

我正在尝试使用Jackson 2.0 mixins序列化一个没有注释的类.

简化的源代码如下.请注意,我没有使用getter/setter,但似乎我仍然可以根据一些相当粗略的文档使用mixins .

public class NoAnnotation {
   private Date created;
   private String name;
   private int id;
   private List<URL> urls = new ArrayList();

   // make one with some data in it for the test
   static NoAnnotation make() {
      NoAnnotation na= new NoAnnotation();
      na.created = new Date();
      na.name = "FooBear";
      na.id = 23456;

      try {
         na.urls.add(new URL("http://www.eclipse.org/eclipselink/moxy.php"));
         na.urls.add(new URL("http://jaxb.java.net"));
      } catch (MalformedURLException e) {
         throw new RuntimeException(e);
      }
      return na;
   }

   // my Mixin "Class"
   static class JacksonMixIn {
      JacksonMixIn(@JsonProperty("created") Date created,
                   @JsonProperty("name") String name,
                   @JsonProperty("id") int id, 
                   @JsonProperty("urls") List<URL> urls)
         { /* do nothing */ }
   }

   // test code
   public static void main(String[] args) throws Exception {
      NoAnnotation na = NoAnnotation.make();
      ObjectMapper objectMapper = new ObjectMapper();
      objectMapper.addMixInAnnotations(NoAnnotation.class, JacksonMixIn.class);
      String jsonText = objectMapper.writeValueAsString(na);
      System.out.println(jsonText);
   }
}

当我跑主要我得到

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: No serializer found for class com.flyingspaniel.so.NoAnnotation and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationConfig.SerializationFeature.FAIL_ON_EMPTY_BEANS) )
    at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.failForEmpty(UnknownSerializer.java:51)
    at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.serialize(UnknownSerializer.java:25)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:108)
    at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:2407)
    at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:1983)
    at com.flyingspaniel.so.NoAnnotation.main(NoAnnotation.java:49)

我猜我在某个地方遗漏了一个基本的"setThis"步骤,但不知道是什么.谢谢!

编辑:当我按照例外中的说明并添加一行

objectMapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);

我不再获得异常,但结果是一个空的JSON字符串,"{}".

编辑2:如果我将字段公开,它可以工作,但这不是一个合理的对象设计.

Answer 1

如果你想以正确的方式使用注释混合来声明它是:

static class JacksonMixIn {
    @JsonProperty Date created;
    @JsonProperty String name;
    @JsonProperty int id;
    @JsonProperty List<URL> urls;
}

通过这种方式,您可以控制要序列化的字段,只需在混合中包含/排除它们.

Answer 2

我想到了.如果要访问私有字段,则需要通过添加以下行来使用"可见性":

 objectMapper.setVisibilityChecker(VisibilityChecker.Std.defaultInstance().withFieldVisibility(Visibility.ANY));

对于受保护的字段,您也可以使用 Visibility.PROTECTED_AND_PUBLIC