自定义Jackson对象映射器

Question

我有一个用例，其中我需要维护两组JSON输出，一组具有JSON属性的漂亮名称，另一组没有。所以我决定自定义ObjectMapper，以便它忽略字段上的@JSONProperty（“pretty name”）注释，而使用字段属性名。在本例中，希望得到以下JSON输出

{"result":{"data":[{"totalUsers":12345,"totalBooks":883}]}}

具有漂亮名称的JSON输出如下所示

{"result":{"data":[{"Total Users":12345,"Total Books":883}]}}

我的ObjectMapper配置代码如下所示

ObjectMapper mapper = new ObjectMapper();
mapper.setSerializationInclusion(Include.NON_EMPTY);
mapper.setSerializationInclusion(Include.NON_NULL);
mapper.configure(MapperFeature.USE_WRAPPER_NAME_AS_PROPERTY_NAME, true);
mapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
mapper.configure(MapperFeature.USE_ANNOTATIONS, false);
mapper.setVisibility(PropertyAccessor.FIELD, Visibility.NONE)
        .setVisibility(PropertyAccessor.GETTER, Visibility.ANY);

我看了一些其他的答案，但对我不起作用。我得到了一个NPE。请参阅以下stacktrace

java.lang.NullPointerException
at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector._renameWithWrappers(POJOPropertiesCollector.java:728)
at com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector.collect(POJOPropertiesCollector.java:264)
at com.fasterxml.jackson.databind.introspect.BasicClassIntrospector.collectProperties(BasicClassIntrospector.java:142)
at com.fasterxml.jackson.databind.introspect.BasicClassIntrospector.forSerialization(BasicClassIntrospector.java:68)
at com.fasterxml.jackson.databind.introspect.BasicClassIntrospector.forSerialization(BasicClassIntrospector.java:11)
at com.fasterxml.jackson.databind.SerializationConfig.introspect(SerializationConfig.java:490)
at com.fasterxml.jackson.databind.ser.BeanSerializerFactory.createSerializer(BeanSerializerFactory.java:131)
at com.fasterxml.jackson.databind.ser.SerializerFactory.createSerializer(SerializerFactory.java:53)
at com.fasterxml.jackson.databind.SerializerProvider._createUntypedSerializer(SerializerProvider.java:935)
at com.fasterxml.jackson.databind.SerializerProvider._createAndCacheUntypedSerializer(SerializerProvider.java:892)
at com.fasterxml.jackson.databind.SerializerProvider.findValueSerializer(SerializerProvider.java:429)
at com.fasterxml.jackson.databind.SerializerProvider.findTypedValueSerializer(SerializerProvider.java:520)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:99)
at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:457)
at com.fasterxml.jackson.jaxrs.json.JacksonJsonProvider.writeTo(JacksonJsonProvider.java:583)
at org.apache.cxf.jaxrs.utils.JAXRSUtils.writeMessageBody(JAXRSUtils.java:1173)
at org.apache.cxf.jaxrs.interceptor.JAXRSOutInterceptor.serializeMessage(JAXRSOutInterceptor.java:259)
at org.apache.cxf.jaxrs.interceptor.JAXRSOutInterceptor.processResponse(JAXRSOutInterceptor.java:155)
at org.apache.cxf.jaxrs.interceptor.JAXRSOutInterceptor.handleMessage(JAXRSOutInterceptor.java:86)
at org.apache.cxf.phase.PhaseInterceptorChain.doIntercept(PhaseInterceptorChain.java:271)
at org.apache.cxf.interceptor.OutgoingChainInterceptor.handleMessage(OutgoingChainInterceptor.java:77)
at org.apache.cxf.phase.PhaseInterceptorChain.doIntercept(PhaseInterceptorChain.java:271)
at org.apache.cxf.transport.ChainInitiationObserver.onMessage(ChainInitiationObserver.java:121)
at org.apache.cxf.transport.http.AbstractHTTPDestination.invoke(AbstractHTTPDestination.java:238)
at org.apache.cxf.transport.servlet.ServletController.invokeDestination(ServletController.java:222)
at org.apache.cxf.transport.servlet.ServletController.invoke(ServletController.java:163)
at org.apache.cxf.transport.servlet.ServletController.invoke(ServletController.java:137)
at org.apache.cxf.transport.servlet.CXFNonSpringServlet.invoke(CXFNonSpringServlet.java:158)
at org.apache.cxf.transport.servlet.AbstractHTTPServlet.handleRequest(AbstractHTTPServlet.java:239)
at org.apache.cxf.transport.servlet.AbstractHTTPServlet.doGet(AbstractHTTPServlet.java:164)

问题出在at中的这行代码中

com.fasterxml.jackson.databind.introspect.POJOPropertiesCollector._renameWithWrappers(POJOPropertiesCollector.java:728) 

其中_AnnotationIntrospector为null

PropertyName wrapperName = _annotationIntrospector.findWrapperName(member);

Answer 1

一个小示例来展示您的用例中的解决方案和mixin：

class Data {
    int totalUsers;
    int totalBooks;
}

…

class DataMixin {
    @JsonProperty("Total Users")
    int totalUsers;
    @JsonProperty("Total Books")
    int totalBooks;
}

…

ObjectMapper mapper = new ObjectMapper();
if (pretty) {
    mapper.addMixInAnnotations(Data.class, DataMixin.class);
}

ObjectMapper mapper = new ObjectMapper();
if (pretty) {
    mapper.addMixin(Data.class, DataMixin.class);
}