java - 树结构的Jackson序列化/反序列化

Question

我有以下 POJO 类:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "Folder", propOrder = {
    "id",
    "parent"   
})
@XmlSeeAlso({
    FolderTree.class
})
public class Folder
{    
    protected int id;
    protected int parent;

    getters, setters...
}

和

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "FolderTree", propOrder = {
    "subFolders"
})
@XmlRootElement(name = "folderTree")
public class FolderTree extends Folder
{
    @XmlElement(name = "subFolder", type = Folder.class)
    protected List<Folder> subFolders;

    getter...
}

和包装器:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "Folders", propOrder = {
    "folders"
})
@XmlRootElement(name = "folders")
public class Folders
{
    @XmlElement(name = "folder", type = Folder.class)
    protected List<Folder> folders;

    getter...
}

我需要在 XML 和 JSON 中对它们进行序列化和反序列化。 XML 工作正常，但 JSON 序列化为以下结构:

{
    "folder": [
        {
            "subFolder": [
                {
                    "subFolder": [],
                    "id": 2,
                    "parent": 1

                },
                {
                    "subFolder": [],
                    "id": 3,
                    "parent": 1
                }
            ],
            "id": 1,
            "parent": null            
        }
    ]
}

反序列化器无法识别 subFolder 对象(“subFolder”:[])内的第二个字段 subFolder。有没有办法让它只在父文件夹中包含子文件夹字段？或者可以做些什么来让 Jackson 按原样反序列化该对象？

我无法更改 POJO 类，因为它们是由 jaxb 创建的，并且一切都可以使用 XML 正常工作。

Answer 1

解决方案。

为了在没有子项的情况下摆脱空的 subFiolder 数组，我在序列化功能中禁用了 WRITE_EMPTY_JSON_ARRAYS。我使用 Spring，bean 看起来像这样:

<bean id="objectMapper" class="org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean">
    <property name="annotationIntrospector" ref="jaxbAnnotationIntrospector" />
    <property name="featuresToDisable">
        <list>
            <value type="com.fasterxml.jackson.databind.SerializationFeature">WRITE_EMPTY_JSON_ARRAYS</value>
        </list>
    </property>
</bean>

对于反序列化，因为我无法修改 POJO 类，所以我为 Folder 类添加了一个 MixIn:

@JsonTypeInfo(  
        use = JsonTypeInfo.Id.NAME,  
        include = JsonTypeInfo.As.PROPERTY,  
        property = "folder",
        defaultImpl = FolderTree.class)
public class FolderMixin {

}

最后，我扩展了 ObjectMapper 以设置 MixInAnnotations:

public class CustomObjectMapper extends ObjectMapper
{
    public CustomObjectMapper()
    {
        super();
        customize();
    }

    protected void customize()
    {
        registerModule(new SimpleModule() {
            private static final long serialVersionUID = 1L;

            public void setupModule(SetupContext context) {
                super.setupModule(context);
                context.setMixInAnnotations(Folder.class, FolderMixin.class);
            }         
        });
    }
}

并且我添加了两个 AnnotationIntrospectors 用于反序列化:

<bean id="customObjectMapper" class="path.to.my.CustomObjectMapper"> 
    <property name="annotationIntrospector" ref="introspectors" />
</bean> 

<!-- introspectors  -->
<bean id="xmlAnnotation" class="com.fasterxml.jackson.module.jaxb.JaxbAnnotationIntrospector">
    <constructor-arg value="#{customObjectMapper.getTypeFactory()}" />
</bean>

<bean id="jsonAnnotation" class="com.fasterxml.jackson.databind.introspect.JacksonAnnotationIntrospector" />

<bean id="introspectors" class="com.fasterxml.jackson.databind.introspect.AnnotationIntrospectorPair">
    <constructor-arg ref="xmlAnnotation" />
    <constructor-arg ref="jsonAnnotation" />
</bean>
<!--  end of introspectors  -->

<bean id="jsonConverter" class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
    <property name="objectMapper" ref="customObjectMapper"/>
</bean> 

我使用 Jackson 2.4.0。