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问题:

I have a JAX-RS webservice that makes use of JPA entity classes. I have a resource class like this:

@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;
    }
}

When I run the service and do a get call on the entity, I get this error:

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

Nowhere in any of the stack traces are any of my classes referenced. Also, the "Exiting getMyEntity" statement is logged before the exception.

I have no idea what is throwing the NPE or how to debug this.

Before this error, I was getting a [com.sun.istack.SAXException2: A cycle is detected in the object graph. This will cause infinitely deep XML: from my JPA (EclipseLink) entity classes and I added the annotation MOXy @XmlInverseReference to my child class on the parent getter method.

Any thoughts on what could be throwing this exception?

回答1:

We have had some offline discussion about this issue, and for the benefit of others that find this post below is the correct setup of using @XmlInverseReference at multiple levels:

Entity 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;
    }

}

Entity 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;
    }
}

Entity C

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;
    }
}

Demo

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);
    }

}

Demo - Output

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

input.xml

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

This issue is also being handled on the EclipseLink forums, for more information see: