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camel
 Post subject: NullPointerException while deserializing object graph
PostPosted: Tue Dec 07, 2004 6:10 am 
Newbie

Joined: Fri Nov 12, 2004 4:33 am
Posts: 7
Location: Austin, Texas
I'm using Hibernate in a (remote) stateless session bean, and I'm getting a NullPointerException as the client tries to deserialize the object graph.

The two relevant persistent Objects are a Section, which has a bidirectional one-to-many to SectionBooks. (A SectionBook then has a many-to-one with a Book, but I don't believe the Book is relevant here).
The NullPointerException is occurring during deserialization when HashMap calls SectionBook.hashCode(), which calls Section.hashCode(). Here's the shortened stack trace (the full one is further down):

Code:
java.lang.NullPointerException
   at com.classgrabber.model.Section.hashCode(Section.java:259)
   at com.classgrabber.model.SectionBook.hashCode(SectionBook.java:90)
   at java.util.HashMap.hash(HashMap.java:261)
   at java.util.HashMap.putForCreate(HashMap.java:404)
   at java.util.HashMap.readObject(HashMap.java:1007)


The SectionBook object (and table) is really just a many-to-many link table, but I'm using two one-to-many mappings as suggested in HIA. The hashCode() method of SectionBook delegates to the Objects it references on the one- side (a Section and a Book):

Code:
public int hashCode() {
  int result = getSection().hashCode();
  result = 29 * result + getBook().hashCode();
  return result;
}


I've read HIA, and all the FAQs on the web site and I am stumped. I've seen the FAQ http://www.hibernate.org/117.html but I don't think that applies here, because everything works fine on the server-side (the object graph can be traversed); it's only when deserializing that the problem happens. Let me know if additional information is required. Thanks for your help,

-Chris

---------------------------------------------------------

Hibernate version:
2.1.6 under JBoss 4.0.0

Mapping documents:

Code:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping
    PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping package="com.classgrabber.model">
   <class name="Section" table="section">
      <id name="id" type="integer" unsaved-value="null" column="id">
         <generator class="native"/>
      </id>
      <property name="key" column="coursekey" />
      <property name="instructor" />
      <property name="seating" />
      <property name="valid" />
      <property name="open" />
      <property name="waitlisted" />
      <property name="flags" type="integer" />
      <many-to-one name="semester" column="semesterid" class="Semester"/>
      <many-to-one name="course" column="courseid" class="Course"/>
      <set name="sectionBooks" table="sectionbook" cascade="delete" inverse="true" order-by="id asc" lazy="true">
         <key column="sectionid"/>
         <one-to-many class="SectionBook"/>
      </set>
   </class>
</hibernate-mapping>

<hibernate-mapping package="com.classgrabber.model">
   <class name="SectionBook" table="sectionbook">
      <id name="id" type="integer" unsaved-value="null" column="id">
         <generator class="native"/>
      </id>
      <property name="authorityURL" column="authorityURL" />
      <property name="status" column="book_status" />
      <property name="required" type="boolean" />
      <many-to-one name="section" column="sectionid" class="Section" />
      <many-to-one name="book" column="bookid" class="Book"/>
   </class>
</hibernate-mapping>


Code between sessionFactory.openSession() and session.close():
// This method is in a remote stateless session bean:

Code:
List sections = new ArrayList();
for (Iterator it = keys.iterator(); it.hasNext();) {
   String key = (String) it.next();
   Integer semesterId = new Integer(14);      // hardcoded for test
   // Next line calls a query "from Section section where section.semester.id = :semesterid and section.key = :key"
   Section sv = getSection(key, semesterId);
   Hibernate.initialize(sv.getSectionBooks());   // *** This causes the exception below ****
   // I need the above call, otherwise I get a LazyInitializationException
   // on the client side
   sections.add(sv);
}
return sections;


Full stack trace of any exception that occurs:
// This happens on the client. No exception occurs in the session bean.

Code:
java.lang.NullPointerException
   at com.classgrabber.model.Section.hashCode(Section.java:259)
   at com.classgrabber.model.SectionBook.hashCode(SectionBook.java:90)
   at java.util.HashMap.hash(HashMap.java:261)
   at java.util.HashMap.putForCreate(HashMap.java:404)
   at java.util.HashMap.readObject(HashMap.java:1007)
   at sun.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
   at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:838)
   at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1746)
   at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
   at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
   at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1845)
   at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1769)
   at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
   at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
   at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1845)
   at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1769)
   at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
   at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
   at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1845)
   at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1769)
   at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
   at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
   at java.io.ObjectInputStream.readObject(ObjectInputStream.java:324)
   at java.util.ArrayList.readObject(ArrayList.java:547)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
   at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:838)
   at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1746)
   at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
   at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
   at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1845)
   at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1769)
   at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
   at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
   at java.io.ObjectInputStream.readObject(ObjectInputStream.java:324)
   at java.rmi.MarshalledObject.get(MarshalledObject.java:135)
   at org.jboss.invocation.jrmp.interfaces.JRMPInvokerProxy.invoke(JRMPInvokerProxy.java:119)
   at org.jboss.invocation.InvokerInterceptor.invoke(InvokerInterceptor.java:96)
   at org.jboss.proxy.TransactionInterceptor.invoke(TransactionInterceptor.java:46)
   at org.jboss.proxy.SecurityInterceptor.invoke(SecurityInterceptor.java:55)
   at org.jboss.proxy.ejb.StatelessSessionInterceptor.invoke(StatelessSessionInterceptor.java:97)
   at org.jboss.proxy.ClientContainer.invoke(ClientContainer.java:86)
   at $Proxy3.getBookDetails(Unknown Source)
   at com.classgrabber.hibernate.BookManagerHibernateDelegate.getBookDetails(BookManagerHibernateDelegate.java:133)
   at com.classgrabber.persistence.TestBookManager.testGetSectionBookDetails(TestBookManager.java:168)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
   at com.intellij.rt.execution.junit2.JUnitStarter.main(JUnitStarter.java:31)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
   at com.intellij.rt.execution.application.AppMain.main(AppMain.java:78)


Name and version of the database you are using:
MySQL 4.1.7

The generated SQL (show_sql=true):
(IF this is relevant tell me and I'll post it)

Debug level Hibernate log excerpt:
(It's over 1000 lines long, if it's relevant I'll post it)
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kkoster
 Post subject: NullPointerException while deserializing object graph
PostPosted: Sat Jan 15, 2005 2:25 pm 
Newbie

Joined: Fri Jan 14, 2005 12:36 am
Posts: 3
I have gotten this problem as well. It seems that a copy of the objects are being placed in a map before they are completely deserialized. It looks like only the key is available at this time. So, if you implement your hash code from anything but the key, (which is recommended by the way) you have to take into consideration null values of objects that contribute to your hash code.
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