杰克逊json反序列化,忽略了json的根元素

Question

如何忽略json中的父标记？

这是我的json

String str = "{\"parent\": {\"a\":{\"id\": 10, \"name\":\"Foo\"}}}";

这是从json映射的类.

public class RootWrapper {
  private List<Foo> foos;

  public List<Foo> getFoos() {
    return foos;
  }

  @JsonProperty("a")
  public void setFoos(List<Foo> foos) {
    this.foos = foos;
  }
 }

这是测试公共类JacksonTest {

@Test
public void wrapRootValue() throws Exception {
    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(DeserializationConfig.Feature.UNWRAP_ROOT_VALUE, true);
    mapper.configure(DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);

    String str = "{\"parent\": {\"a\":{\"id\": 10, \"name\":\"Foo\"}}}";

    RootWrapper root = mapper.readValue(str, RootWrapper.class);

    Assert.assertNotNull(root);
}

我收到错误::

 org.codehaus.jackson.map.JsonMappingException: Root name 'parent' does not match expected ('RootWrapper') for type [simple type, class MavenProjectGroup.mavenProjectArtifact.RootWrapper]

我找到了Jackson注释给出的解决方案::

  (a) Annotate you class as below

  @JsonRootName(value = "parent")
  public class RootWrapper {

  (b) It will only work if and only if ObjectMapper is asked to wrap.
    ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationConfig.Feature.UNWRAP_ROOT_VALUE, true);

任务完成!!

杰克逊反序列化的另一种打嗝:(

如果'DeserializationConfig.Feature.UNWRAP_ROOT_VALUE配置',它解包所有jsons,虽然我的类没有注释@JsonRootName(value ="rootTagInJson"),但是没有被安装.

我只想在使用@JsonRootName注释类时解包根标记,否则不要解包.

以下是解包根标记的用例.

  ###########################################################
     Unwrap only if the class is annotated with @JsonRootName.
  ############################################################

我对Jackson源代码的ObjectMapper进行了一些小改动,并创建了一个新版本的jar.1.将此方法放在ObjectMapper中

// Ash:: Wrap json if the class being deserialized, are annotated
// with @JsonRootName else do not wrap.
private boolean hasJsonRootName(JavaType valueType) {
    if (valueType.getRawClass() == null)
        return false;

    Annotation rootAnnotation =  valueType.getRawClass().getAnnotation(JsonRootName.class);
    return rootAnnotation != null;
}


    2. Edit ObjectMapper method :: 
    Replace 
       cfg.isEnabled(DeserializationConfig.Feature.UNWRAP_ROOT_VALUE)
    with
       hasJsonRootName(valueType)

    3. Build your jar file and use it.

Answer 1

从https://github.com/FasterXML/jackson-databind中的 TestRootName.java中获取的示例 可能会提供更好的方法.特别是使用withRootName(""):

private ObjectMapper rootMapper()
{
    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
    mapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
    return mapper;
}

public void testRootUsingExplicitConfig() throws Exception
{
    ObjectMapper mapper = new ObjectMapper();
    ObjectWriter writer = mapper.writer().withRootName("wrapper");
    String json = writer.writeValueAsString(new Bean());
    assertEquals("{\"wrapper\":{\"a\":3}}", json);

    ObjectReader reader = mapper.reader(Bean.class).withRootName("wrapper");
    Bean bean = reader.readValue(json);
    assertNotNull(bean);

    // also: verify that we can override SerializationFeature as well:
    ObjectMapper wrapping = rootMapper();
    json = wrapping.writer().withRootName("something").writeValueAsString(new Bean());
    assertEquals("{\"something\":{\"a\":3}}", json);
    json = wrapping.writer().withRootName("").writeValueAsString(new Bean());
    assertEquals("{\"a\":3}", json);

    bean = wrapping.reader(Bean.class).withRootName("").readValue(json);
    assertNotNull(bean);
}

Answer 2

我在Spring中开发了一个类似的问题.我必须支持一个非常异构的API,其中一些有根元素,另一个没有.我找不到比实时配置此属性更好的解决方案.遗憾的是,杰克逊不支持每类根元素展开.无论如何,有人可能会觉得这很有帮助.

@Component
public class ObjectMapper extends com.fasterxml.jackson.databind.ObjectMapper {
    private void autoconfigureFeatures(JavaType javaType) {
        Annotation rootAnnotation = javaType.getRawClass().getAnnotation(JsonRootName.class);
        this.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, rootAnnotation != null);
    }

    @Override
    protected Object _readMapAndClose(JsonParser jsonParser, JavaType javaType) throws IOException, JsonParseException, JsonMappingException {
        autoconfigureFeatures(javaType);
        return super._readMapAndClose(jsonParser, javaType);
    }

}