1

如果值为 0，我尝试为 XmlElement 返回 null。但如果我这样做，则会出现异常。我的意图是，如果它的值为 0，我生成的 JSON 中没有此列。那么我该如何实现呢？你能帮我么？

这是我在 District 班级中的 XmlElement。

@XmlElement(name="Car Quantity")
public long getCarsQuantity() {
      return carsQuantity == 0 ? null : carsQuantity;
}

这是例外：

java.lang.NullPointerException
    at at.opendata.entitys.District.getCarsQuantity(District.java:71)
    at at.opendata.entitys.District$JaxbAccessorM_getCarsQuantity_setCarsQuantity_long.get(MethodAccessor_Long.java:59)
    at com.sun.xml.bind.v2.runtime.property.SingleElementNodeProperty.serializeBody(SingleElementNodeProperty.java:131)
    at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:344)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsSoleContent(XMLSerializer.java:597)
    at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.serializeRoot(ClassBeanInfoImpl.java:328)
    at com.sun.xml.bind.v2.runtime.XMLSerializer.childAsRoot(XMLSerializer.java:498)
    at com.sun.xml.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:320)
    at com.sun.xml.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:177)
    at com.sun.jersey.json.impl.provider.entity.JSONListElementProvider.writeList(JSONListElementProvider.java:145)
    at com.sun.jersey.core.provider.jaxb.AbstractListElementProvider.writeTo(AbstractListElementProvider.java:264)
    at com.sun.jersey.spi.container.ContainerResponse.write(ContainerResponse.java:306)
    at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1479)
    at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1391)
    at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1381)
    at com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:416)
    at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:538)
    at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:716)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:395)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:250)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
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1 回答 1

2 
  return carsQuantity == 0 ? null : carsQuantity;

当返回类型很长时，您试图返回 null。当您尝试调用超级构造函数并为一个参数传递 null 时，您也可以得到这种 NPE。

long 不能为空 :)

您可以尝试改用 java.lang.Long 。

于 2013-02-20T17:10:19.160 回答