用JAXB保持干爽
我正在开发许多必须按以下格式序列化为XML的Java类：
一开始， Foo.java看起来像这样：
@XmlRootElement @XmlAccessorType(XmlAccessType.FIELD) class Foo { @XmlAttribute String value; // snip constructors // snip methods // getValue // equals, hashCode, toString // static valueOf(String), static valueOf(int) }
猜测Bar.java和Baz.java可能是什么Bar.java并不需要太多想象力。 这些是非常简单的包装类（在本例中为int ， String和boolean 。 到目前为止，一切都是笨拙的。 写完第九轮之后
class WhoCares { @XmlAttribute Whatever value; /* 2 constructors */ /* 6 methods found in every one of these classes */ }
我想： “嘿，我知道如何解决这个问题 – 我只需要一个层次的抽象！” 介绍Wrapper ：
abstract class Wrapper { @XmlAttribute private T value; Wrapper() {} // default ctor for JAXB Wrapper(T value) { this.value = value; } T getValue() { return value; } // snip equals, hashCode, toString }
现在，要实现Foo ，我需要做的就是扩展Wrapper ，实现我需要的工厂方法：
class Foo extends Wrapper { // snip constructors static Foo valueOf(String value) { return new Foo(value); } }
并在Wrapper的声明中添加@XmlSeeAlso({... Foo.class ...}) 。
- 亲：这样更好。
- Con： JAXB不会序列化这些对象。
任何对JAXBContext.getInstance(Wrapper.class)或JAXBContext.getInstance(Foo.class) JAXBContext.getInstance(Wrapper.class)因为在JAXB的内部深处抛出NullPointerException而失败：
17:06:03,706 ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[localhost].[/Redacted].[JAX-RS Servlet]] Servlet.service() for servlet JAX-RS Servlet threw exception: java.lang.NullPointerException at com.sun.xml.bind.v2.runtime.reflect.TransducedAccessor.get(TransducedAccessor.java:165) [:2.2] at com.sun.xml.bind.v2.runtime.property.AttributeProperty.(AttributeProperty.java:87) [:2.2] at com.sun.xml.bind.v2.runtime.property.PropertyFactory.create(PropertyFactory.java:104) [:2.2] at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.(ClassBeanInfoImpl.java:179) [:2.2] at com.sun.xml.bind.v2.runtime.JAXBContextImpl.getOrCreate(JAXBContextImpl.java:515) [:2.2] at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.(ClassBeanInfoImpl.java:166) [:2.2] at com.sun.xml.bind.v2.runtime.JAXBContextImpl.getOrCreate(JAXBContextImpl.java:515) [:2.2] at com.sun.xml.bind.v2.runtime.JAXBContextImpl.getOrCreate(JAXBContextImpl.java:534) [:2.2] at com.sun.xml.bind.v2.runtime.property.SingleElementNodeProperty.(SingleElementNodeProperty.java:101) [:2.2] at sun.reflect.GeneratedConstructorAccessor49.newInstance(Unknown Source) [:1.6.0_24] at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27) [:1.6.0_24] at java.lang.reflect.Constructor.newInstance(Constructor.java:513) [:1.6.0_24] at com.sun.xml.bind.v2.runtime.property.PropertyFactory.create(PropertyFactory.java:124) [:2.2] at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.(ClassBeanInfoImpl.java:179) [:2.2] at com.sun.xml.bind.v2.runtime.JAXBContextImpl.getOrCreate(JAXBContextImpl.java:515) [:2.2] at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.(ClassBeanInfoImpl.java:166) [:2.2] at com.sun.xml.bind.v2.runtime.JAXBContextImpl.getOrCreate(JAXBContextImpl.java:515) [:2.2] at com.sun.xml.bind.v2.runtime.JAXBContextImpl.(JAXBContextImpl.java:330) [:2.2] at com.sun.xml.bind.v2.runtime.JAXBContextImpl$JAXBContextBuilder.build(JAXBContextImpl.java:1140) [:2.2] at com.sun.xml.bind.v2.ContextFactory.createContext(ContextFactory.java:154) [:2.2] at com.sun.xml.bind.v2.ContextFactory.createContext(ContextFactory.java:121) [:2.2] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [:1.6.0_24] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) [:1.6.0_24] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) [:1.6.0_24] at java.lang.reflect.Method.invoke(Method.java:597) [:1.6.0_24] at javax.xml.bind.ContextFinder.newInstance(ContextFinder.java:201) [:1.0.0.Final] at javax.xml.bind.ContextFinder.find(ContextFinder.java:362) [:1.0.0.Final] at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:618) [:1.0.0.Final] at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:565) [:1.0.0.Final] at com.sun.jersey.core.provider.jaxb.AbstractJAXBProvider.getStoredJAXBContext(AbstractJAXBProvider.java:189) [:1.6] at com.sun.jersey.core.provider.jaxb.AbstractJAXBProvider.getJAXBContext(AbstractJAXBProvider.java:182) [:1.6] at com.sun.jersey.core.provider.jaxb.AbstractJAXBProvider.getMarshaller(AbstractJAXBProvider.java:160) [:1.6] at com.sun.jersey.core.provider.jaxb.AbstractJAXBProvider.getMarshaller(AbstractJAXBProvider.java:139) [:1.6] at com.sun.jersey.core.provider.jaxb.AbstractRootElementProvider.writeTo(AbstractRootElementProvider.java:151) [:1.6] at com.sun.jersey.spi.container.ContainerResponse.write(ContainerResponse.java:306) [:1.6] at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1310) [:1.6] at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1223) [:1.6] at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1213) [:1.6] at com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:414) [:1.6] at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:537) [:1.6] at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:699) [:1.6] at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) [:1.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:324) [:6.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:242) [:6.0.0.Final] at com.example.mgp.filter.RetryFilter.doFilter(RetryFilter.java:109) [:] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:274) [:6.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:242) [:6.0.0.Final] at com.example.mgp.filter.AuthFilter.doFilter(AuthFilter.java:57) [:] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:274) [:6.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:242) [:6.0.0.Final] at org.jboss.weld.servlet.ConversationPropagationFilter.doFilter(ConversationPropagationFilter.java:67) [:6.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:274) [:6.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:242) [:6.0.0.Final] at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275) [:6.0.0.Final] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191) [:6.0.0.Final] at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:181) [:6.0.0.Final] at org.jboss.modcluster.catalina.CatalinaContext$RequestListenerValve.event(CatalinaContext.java:285) [:1.1.0.Final] at org.jboss.modcluster.catalina.CatalinaContext$RequestListenerValve.invoke(CatalinaContext.java:261) [:1.1.0.Final] at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:88) [:6.0.0.Final] at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.invoke(SecurityContextEstablishmentValve.java:100) [:6.0.0.Final] at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127) [:6.0.0.Final] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) [:6.0.0.Final] at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:158) [:6.0.0.Final] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) [:6.0.0.Final] at org.jboss.web.tomcat.service.request.ActiveRequestResponseCacheValve.invoke(ActiveRequestResponseCacheValve.java:53) [:6.0.0.Final] at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:362) [:6.0.0.Final] at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:877) [:6.0.0.Final] at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:654) [:6.0.0.Final] at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:951) [:6.0.0.Final] at java.lang.Thread.run(Thread.java:662) [:1.6.0_24]
我已经尝试将我的JAXB实现从Metro切换到MOXy（我确实检查了实现是否已更改 ）但是MOXy会抛出类似的NPE：
17:46:48,481 SEVERE [com.sun.jersey.spi.container.ContainerResponse] Mapped exception to response: 500 (Internal Server Error): javax.ws.rs.WebApplicationException: javax.xml.bind.MarshalException - with linked exception: [Exception [EclipseLink-25003] (Eclipse Persistence Services - 2.2.0.v20110202-r8913): org.eclipse.persistence.exceptions.XMLMarshalException Exception Description: An error occurred marshalling the object Internal Exception: java.lang.NullPointerException] at com.sun.jersey.core.provider.jaxb.AbstractRootElementProvider.writeTo(AbstractRootElementProvider.java:159) [:1.6] at com.sun.jersey.spi.container.ContainerResponse.write(ContainerResponse.java:306) [:1.6] at com.sun.jersey.server.impl.application.WebApplicationImpl._handleRequest(WebApplicationImpl.java:1310) [:1.6] at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1223) [:1.6] at com.sun.jersey.server.impl.application.WebApplicationImpl.handleRequest(WebApplicationImpl.java:1213) [:1.6] at com.sun.jersey.spi.container.servlet.WebComponent.service(WebComponent.java:414) [:1.6] at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:537) [:1.6] at com.sun.jersey.spi.container.servlet.ServletContainer.service(ServletContainer.java:699) [:1.6] at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) [:1.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:324) [:6.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:242) [:6.0.0.Final] at com.example.mgp.filter.RetryFilter.doFilter(RetryFilter.java:109) [:] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:274) [:6.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:242) [:6.0.0.Final] at com.example.mgp.filter.AuthFilter.doFilter(AuthFilter.java:57) [:] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:274) [:6.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:242) [:6.0.0.Final] at org.jboss.weld.servlet.ConversationPropagationFilter.doFilter(ConversationPropagationFilter.java:67) [:6.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:274) [:6.0.0.Final] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:242) [:6.0.0.Final] at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275) [:6.0.0.Final] at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191) [:6.0.0.Final] at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:181) [:6.0.0.Final] at org.jboss.modcluster.catalina.CatalinaContext$RequestListenerValve.event(CatalinaContext.java:285) [:1.1.0.Final] at org.jboss.modcluster.catalina.CatalinaContext$RequestListenerValve.invoke(CatalinaContext.java:261) [:1.1.0.Final] at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:88) [:6.0.0.Final] at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.invoke(SecurityContextEstablishmentValve.java:100) [:6.0.0.Final] at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127) [:6.0.0.Final] at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) [:6.0.0.Final] at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:158) [:6.0.0.Final] at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) [:6.0.0.Final] at org.jboss.web.tomcat.service.request.ActiveRequestResponseCacheValve.invoke(ActiveRequestResponseCacheValve.java:53) [:6.0.0.Final] at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:362) [:6.0.0.Final] at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:877) [:6.0.0.Final] at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:654) [:6.0.0.Final] at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:951) [:6.0.0.Final] at java.lang.Thread.run(Thread.java:662) [:1.6.0_24] Caused by: javax.xml.bind.MarshalException - with linked exception: [Exception [EclipseLink-25003] (Eclipse Persistence Services - 2.2.0.v20110202-r8913): org.eclipse.persistence.exceptions.XMLMarshalException Exception Description: An error occurred marshalling the object Internal Exception: java.lang.NullPointerException] at org.eclipse.persistence.jaxb.JAXBMarshaller.marshal(JAXBMarshaller.java:314) [:2.2.0.v20110202-r8913] at com.sun.jersey.core.provider.jaxb.AbstractRootElementProvider.writeTo(AbstractRootElementProvider.java:179) [:1.6] at com.sun.jersey.core.provider.jaxb.AbstractRootElementProvider.writeTo(AbstractRootElementProvider.java:157) [:1.6] ... 36 more Caused by: Exception [EclipseLink-25003] (Eclipse Persistence Services - 2.2.0.v20110202-r8913): org.eclipse.persistence.exceptions.XMLMarshalException Exception Description: An error occurred marshalling the object Internal Exception: java.lang.NullPointerException at org.eclipse.persistence.exceptions.XMLMarshalException.marshalException(XMLMarshalException.java:78) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.oxm.XMLMarshaller.marshal(XMLMarshaller.java:516) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.jaxb.JAXBMarshaller.marshal(JAXBMarshaller.java:312) [:2.2.0.v20110202-r8913] ... 38 more Caused by: java.lang.NullPointerException at org.eclipse.persistence.oxm.record.OutputStreamRecord.outputStreamWrite(OutputStreamRecord.java:511) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.oxm.record.OutputStreamRecord.openStartElement(OutputStreamRecord.java:159) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XPathNode.startElement(XPathNode.java:330) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XMLCompositeObjectMappingNodeValue.marshalSingleValue(XMLCompositeObjectMappingNodeValue.java:185) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XMLCompositeObjectMappingNodeValue.marshal(XMLCompositeObjectMappingNodeValue.java:116) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.NodeValue.marshal(NodeValue.java:104) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.record.ObjectMarshalContext.marshal(ObjectMarshalContext.java:60) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XPathNode.marshal(XPathNode.java:322) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.TreeObjectBuilder.marshalAttributes(TreeObjectBuilder.java:338) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XPathNode.startElement(XPathNode.java:344) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XMLCompositeObjectMappingNodeValue.marshalSingleValue(XMLCompositeObjectMappingNodeValue.java:167) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XMLCompositeObjectMappingNodeValue.marshal(XMLCompositeObjectMappingNodeValue.java:116) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.NodeValue.marshal(NodeValue.java:104) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.record.ObjectMarshalContext.marshal(ObjectMarshalContext.java:60) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XPathNode.marshal(XPathNode.java:322) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.TreeObjectBuilder.buildRow(TreeObjectBuilder.java:325) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XMLAnyObjectMappingNodeValue.marshalSingleValue(XMLAnyObjectMappingNodeValue.java:154) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XMLAnyObjectMappingNodeValue.marshal(XMLAnyObjectMappingNodeValue.java:70) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.NodeValue.marshal(NodeValue.java:104) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.record.ObjectMarshalContext.marshal(ObjectMarshalContext.java:60) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.XPathNode.marshal(XPathNode.java:322) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.internal.oxm.TreeObjectBuilder.buildRow(TreeObjectBuilder.java:325) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.oxm.XMLMarshaller.marshal(XMLMarshaller.java:932) [:2.2.0.v20110202-r8913] at org.eclipse.persistence.oxm.XMLMarshaller.marshal(XMLMarshaller.java:486) [:2.2.0.v20110202-r8913] ... 39 more
有没有办法生成所需的XML，同时最小化样板和保持DRY ？
您看到的问题是由于Metro（参考实现）和EclipseLink MOXy JAXB实现中的错误。 相关的MOXy错误是：
关于您的用例需要注意的一点是，由于类型擦除，JAXB实现将把value属性视为Object类型。 这意味着编组操作对您有用，但是解组操作会将值作为String返回。 这就是为什么如果使用@XmlElement注释值字段，则将包含xsi：type信息以保留类型信息：
- @XmlRootElement和
抛出IllegalAnnotationExceptions
MOXy解决方法
您可以通过利用以下XmlAdapter来解决MOXy错误：
import javax.xml.bind.annotation.adapters.XmlAdapter; public class ObjectAdapter extends XmlAdapter { @Override public Object unmarshal(Object v) throws Exception { return v; } @Override public Object marshal(Object v) throws Exception { return v; } }
XML适配器已在您的媒体资源上注册，如下所示：
import javax.xml.bind.annotation.XmlAccessType; import javax.xml.bind.annotation.XmlAccessorType; import javax.xml.bind.annotation.XmlAttribute; import javax.xml.bind.annotation.XmlSeeAlso; import javax.xml.bind.annotation.XmlTransient; import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter; @XmlTransient @XmlAccessorType(XmlAccessType.FIELD) @XmlSeeAlso(Foo.class) abstract class Wrapper { @XmlAttribute @XmlJavaTypeAdapter(ObjectAdapter.class) private T value; Wrapper() {} // default ctor for JAXB public Wrapper(T t) { value = t; } }