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List:       xmlbeans-dev
Subject:    [jira] Created: (XMLBEANS-300) Threading issue leads to NPE in
From:       "Gordon Eastman (JIRA)" <xmlbeans-dev () xml ! apache ! org>
Date:       2006-12-05 18:49:24
Message-ID: 23889068.1165344564987.JavaMail.jira () brutus
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Threading issue leads to NPE in SchemaComponent$Ref.getComponent
----------------------------------------------------------------

                 Key: XMLBEANS-300
                 URL: http://issues.apache.org/jira/browse/XMLBEANS-300
             Project: XMLBeans
          Issue Type: Bug
          Components: XmlObject
    Affects Versions: Version 2, Version 2.1, Version 2.2
            Reporter: Gordon Eastman


We have seen this NPE during normal use of our application on a MP Solaris system \
using XmlBeans 2.0.0. I tested and confirmed it is still a problem in 2.2.0:

java.lang.NullPointerException
        at org.apache.xmlbeans.SchemaComponent$Ref.getComponent(SchemaComponent.java:104)
  at org.apache.xmlbeans.SchemaType$Ref.get(SchemaType.java:872)
        at org.apache.xmlbeans.impl.schema.SchemaPropertyImpl.getType(SchemaPropertyImpl.java:92)
                
        at org.apache.xmlbeans.impl.schema.SchemaTypeImpl.createElementType(SchemaTypeImpl.java:965)
                
        at org.apache.xmlbeans.impl.values.XmlObjectBase.create_element_user(XmlObjectBase.java:895)
  at org.apache.xmlbeans.impl.store.Xobj.getUser(Xobj.java:1592)
        at org.apache.xmlbeans.impl.store.Cur.getUser(Cur.java:2627)
        at org.apache.xmlbeans.impl.store.Cur.getObject(Cur.java:2620)
        at org.apache.xmlbeans.impl.store.Cursor._getObject(Cursor.java:983)
        at org.apache.xmlbeans.impl.store.Cursor.getObject(Cursor.java:2896)

Here is how to reproduce with the debugger:

- start two threads, each of which loads a different xml document with the same \
                schema type
- each thread will traverse its socument, calling getObject at each element
- break before traversal starts
- insert breakpoint at SchemaComponent.java:104, and let both threads run to this \
                point
- resume one thread, and then the other
- the second thread fails with the NPE

Here is the code fragment from  SchemaComponent $Ref:

        public final SchemaComponent getComponent()
        {
            if (_schemaComponent == null && _handle != null)
            {
                _schemaComponent = _schemaTypeSystem.resolveHandle(_handle); // this \
is line 104  _schemaTypeSystem = null;
            }

            return _schemaComponent;
        }

Both  threads use the same Ref object, and there is no protection between use of \
_schemaTypeSystem on line 104 and setting it null on the next line

I do not know enough about how thread protection of schema types is designed to know \
if a localized fix to synchronize the lazy instantiation is appropriate, or if \
synchronization at a higher level is appropriate.



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