JAX-RS 中 NPE 抛出的编组实体

Question

我有一个使用 JPA 实体类的 JAX-RS Web 服务。我有一个这样的资源类：

@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;
    }
}

当我运行服务并对实体进行 get 调用时，我收到此错误：

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

在任何堆栈跟踪中都没有引用我的任何类。此外，"Exiting getMyEntity" 语句会在异常之前记录。

我不知道是什么引发了 NPE 或如何调试它。

在此错误之前，我从我的 JPA (EclipseLink) 实体类中获得了 [com.sun.istack.SAXException2: A cycle is detected in the object graph. This will cause infinitely deep XML:，并在父 getter 方法上将注释 MOXy @XmlInverseReference 添加到我的子类中。

有什么想法会引发这个异常吗？

Answer 1

我们已经就这个问题进行了一些离线讨论，为了其他人的利益，发现下面这篇文章是在多个级别使用 @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 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;
    }
}

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

输入.xml

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

EclipseLink 论坛也正在处理此问题，有关更多信息，请参阅：

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

下面是另一个使用 JPA 模型的 @XmlInverseReference 示例

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

Answer 2

您是否在实体类中包含了 jaxb.properties 文件？

查看 Blaise 对此问题的回答：JAXB Mapping cyclic references to XML

希望这会有所帮助。 RG