NPE在JAX-RS中抛出编组实体

Question

我有一个使用JPA实体类的JAX-RS web服务。我有这样的资源类：

@Path("/entity") 
public class MyEntityResource 
{ 
    @GET 
    @Produces(MediaType.APPLICATION_XML) 
    @Path("/{entity}") 
    public MyEntity getMyEntity(@PathParam("entity") String entity) 
    { 
     log.debug("Entering getMyEntity with param: " + entity); 
     MyEntity entityObject = genericService.find(MyEntity.class, entity); 

     if (entityObject == null) 
     { 
      log.debug("Entity not found."); 
      throw new WebApplicationException(Response.Status.NOT_FOUND); 
     } 

     log.debug("Exiting getMyEntity"); 

     return entityObject; 
    } 
} 

当我运行的服务，并做实体get调用，我得到这个错误：

SEVERE: The response of the WebApplicationException cannot be utilized as the response is already committed. Re-throwing to the HTTP container 
javax.ws.rs.WebApplicationException: javax.xml.bind.MarshalException 
- with linked exception: 
[Exception [EclipseLink-25003] (Eclipse Persistence Services - 2.0.1.v20100213-r6600): 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:151) 
... 
<snip> 
... 
Caused by: javax.xml.bind.MarshalException 
- with linked exception: 
[Exception [EclipseLink-25003] (Eclipse Persistence Services - 2.0.1.v20100213-r6600): 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:271) 
    at com.sun.jersey.core.provider.jaxb.AbstractRootElementProvider.writeTo(AbstractRootElementProvider.java:171) 
    at com.sun.jersey.core.provider.jaxb.AbstractRootElementProvider.writeTo(AbstractRootElementProvider.java:149) 
    ... 32 more 
Caused by: Exception [EclipseLink-25003] (Eclipse Persistence Services - 2.0.1.v20100213-r6600): 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:76) 
    at org.eclipse.persistence.oxm.XMLMarshaller.marshal(XMLMarshaller.java:502) 
    at org.eclipse.persistence.jaxb.JAXBMarshaller.marshal(JAXBMarshaller.java:269) 
    ... 34 more 
Caused by: java.lang.NullPointerException 
    at org.eclipse.persistence.exceptions.ConversionException.couldNotBeConverted(ConversionException.java:69) 
... 
<snip> 
... 
    at org.eclipse.persistence.oxm.XMLMarshaller.marshal(XMLMarshaller.java:916) 
    at org.eclipse.persistence.oxm.XMLMarshaller.marshal(XMLMarshaller.java:468) 
    ... 35 more 

在任何堆栈跟踪

无处任何我的课程被引用。此外，"Exiting getMyEntity"语句在异常之前被记录。

我不知道什么是抛NPE或如何调试。

在此错误发生之前，我从JPA（EclipseLink）实体类获取[com.sun.istack.SAXException2: A cycle is detected in the object graph. This will cause infinitely deep XML:，并且将注释MOXy @XmlInverseReference添加到父类getter方法的子类。

有什么想法可以抛出这个异常？

Answer 1

我们已经收到了这个问题的一些讨论下线，以及其他人找到下面这个职位的好处是在多个层面使用@XmlInverseReference的正确设置：

实体A

import java.io.Serializable; 
import javax.persistence.*; 
import javax.xml.bind.annotation.XmlAttribute; 
import javax.xml.bind.annotation.XmlElement; 
import javax.xml.bind.annotation.XmlRootElement; 

import java.util.Set; 

@Entity 
@XmlRootElement 
public class EntityA implements Serializable { 
    private static final long serialVersionUID = 1L; 
    private String name; 
    private Set<EntityB> entityBs; 

    @Id 
    @XmlAttribute 
    public String getName() { 
     return this.name; 
    } 

    public void setName(String name) { 
     this.name = name; 
    } 

    @OneToMany(mappedBy = "entityABean") 
    @XmlElement 
    public Set<EntityB> getEntityBs() { 
     return this.entityBs; 
    } 

    public void setEntityBs(Set<EntityB> entityBs) { 
     this.entityBs = entityBs; 
    } 

} 

实体B

import java.io.Serializable; 
import javax.persistence.*; 
import javax.xml.bind.annotation.XmlAttribute; 
import javax.xml.bind.annotation.XmlElement; 
import javax.xml.bind.annotation.XmlRootElement; 

import org.eclipse.persistence.oxm.annotations.XmlInverseReference; 

import java.util.Set; 

@Entity 
@XmlRootElement 
public class EntityB implements Serializable { 
    private static final long serialVersionUID = 1L; 
    private String name; 
    private Set<EntityC> entityCs; 
    private EntityA entityABean; 

    @Id 
    @XmlAttribute 
    public String getName() { 
     return this.name; 
    } 

    public void setName(String name) { 
     this.name = name; 
    } 

    @ManyToOne 
    @JoinColumn(name = "EntityA") 
    @XmlInverseReference(mappedBy = "entityBs") 
    public EntityA getEntityABean() { 
     return this.entityABean; 
    } 

    public void setEntityABean(EntityA entityABean) { 
     this.entityABean = entityABean; 
    } 

    @OneToMany(mappedBy = "entityBBean") 
    @XmlElement 
    public Set<EntityC> getEntityCs() { 
     return this.entityCs; 
    } 

    public void setEntityCs(Set<EntityC> entityCs) { 
     this.entityCs = entityCs; 
    } 
} 

实体ç

import java.io.Serializable; 
import javax.persistence.*; 

import org.eclipse.persistence.oxm.annotations.XmlInverseReference; 

@Entity 
public class EntityC implements Serializable { 
    private static final long serialVersionUID = 1L; 
    private String name; 
    private EntityB entityBBean; 

    @Id 
    public String getName() { 
     return this.name; 
    } 

    public void setName(String name) { 
     this.name = name; 
    } 

    @ManyToOne 
    @JoinColumn(name = "EntityB") 
    @XmlInverseReference(mappedBy = "entityCs") 
    public EntityB getEntityBBean() { 
     return this.entityBBean; 
    } 

    public void setEntityBBean(EntityB entityBBean) { 
     this.entityBBean = entityBBean; 
    } 
} 

演示

import java.io.FileInputStream; 
import javax.xml.bind.JAXBContext; 
import javax.xml.bind.Marshaller; 
import javax.xml.bind.Unmarshaller; 

public class Demo { 

    public static void main(String[] args) throws Exception { 
     JAXBContext jc = JAXBContext.newInstance(EntityA.class); 

     Unmarshaller unmarshaller = jc.createUnmarshaller(); 
     FileInputStream xml = new FileInputStream("src/test/jaxb/input.xml"); 
     EntityA a = (EntityA) unmarshaller.unmarshal(xml); 

     for(EntityB b : a.getEntityBs()) { 
      System.out.println(b.getEntityABean()); 
      for(EntityC c : b.getEntityCs()) { 
       System.out.println(c.getEntityBBean()); 
      } 
     } 
     Marshaller marshaller = jc.createMarshaller(); 
     marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true); 
     marshaller.marshal(a, System.out); 
    } 

} 

演示 - 输出

test.jaxb.EntityA@1292d26 
test.jaxb.EntityB@196c1b0 
test.jaxb.EntityB@196c1b0 
test.jaxb.EntityA@1292d26 
test.jaxb.EntityB@1e13d52 
test.jaxb.EntityB@1e13d52 

的input.xml

<?xml version="1.0" encoding="UTF-8"?> 
<entityA> 
    <entityBs> 
     <entityCs/> 
     <entityCs/> 
    </entityBs> 
    <entityBs> 
     <entityCs/> 
     <entityCs/> 
    </entityBs> 
</entityA> 

这个问题也正在对论坛的EclipseLink处理，以获取更多信息，请参见：

• http://www.eclipse.org/forums/index.php?t=msg&th=172544&start=0&

下面是一个使用@XmlInverseReference与JPA模型

• http://bdoughan.blogspot.com/2010/07/jpa-entities-to-xml-bidirectional.html

Answer 2

您是否在您的实体类中包含jaxb.properties文件？

退房Blaise的回答这个问题：JAXB Mapping cyclic references to XML

希望这有助于。 RG