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Sometimes wrong classloader used for proxy interfaces


For some of our business classes we used a Mapping declaring the class to be lazy initialized and providing a proxy interface like e.g.:

<class name="TestClassImpl" proxy="TestClass" table="testTable" lazy="true"> ... </class>

Using classes mapped this way - e.g. as endpoint of a one to one relation - sporadically leads to exceptions like the following:

org.hibernate.PropertyAccessException: IllegalArgumentException occurred while calling setter of test.AbstractTestClass2Impl.testField
at org.hibernate.property.BasicPropertyAccessor$BasicSetter.set(BasicPropertyAccessor.java:104)
at org.hibernate.tuple.entity.AbstractEntityTuplizer.setPropertyValues(AbstractEntityTuplizer.java:337)
at org.hibernate.tuple.entity.PojoEntityTuplizer.setPropertyValues(PojoEntityTuplizer.java:200)
at org.hibernate.persister.entity.AbstractEntityPersister.setPropertyValues(AbstractEntityPersister.java:3514)
at org.hibernate.engine.TwoPhaseLoad.initializeEntity(TwoPhaseLoad.java:129)
at org.hibernate.loader.Loader.initializeEntitiesAndCollections(Loader.java:842)
at org.hibernate.loader.Loader.doQuery(Loader.java:717)
at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:224)
at org.hibernate.loader.Loader.doList(Loader.java:2211)
at org.hibernate.loader.Loader.listIgnoreQueryCache(Loader.java:2095)
at org.hibernate.loader.Loader.list(Loader.java:2090)
at org.hibernate.loader.hql.QueryLoader.list(QueryLoader.java:388)
at org.hibernate.hql.ast.QueryTranslatorImpl.list(QueryTranslatorImpl.java:338)
at org.hibernate.engine.query.HQLQueryPlan.performList(HQLQueryPlan.java:172)
at org.hibernate.impl.SessionImpl.list(SessionImpl.java:1121)
at org.hibernate.impl.QueryImpl.list(QueryImpl.java:79)
at org.hibernate.impl.AbstractQueryImpl.uniqueResult(AbstractQueryImpl.java:804)

Note that the application is residing in a different classloader than hibernate (and CGLIB...).

Examining the situation we see that the value passed to the setter was a CGLIB proxy. Internally the proxy stores an array of interfaces the proxy should implement. In our situation this interface contains the HibernateProxy interface and our business interface provided as proxy interface in the mapping. In this interface array we checked the classloader of the interfaces. As expected the HibernateProxy interface is loaded by the system classloader and our interface was loaded by our custom classloader. However examining the actual interfaces of the proxy (with proxy.getClass().getInterfaces()[i].getClassLoader() for all i) shows that all interfaces are loaded with the system classloader. This causes the exception above.

Doing some more experiments we experience that the problem does not occur all the time. Sometimes the actual proxy interfaces are as expected (the HibernateProxy interface is loaded by the system classloader and our interface was loaded by our custom classloader). We notice that each time the test failed the HibernateProxy was the first interface in the interface array stored in the proxy.

Some experiments with the CGLIB (the Enhancer class) shows us that (if there is no superclass given) they use the classloader of the first passed interface as default classloader (compare method net.sf.cglib.proxy.Enhancer.getDefaultClassLoader()).

Finally we find out that hibernate passes the proxy interfaces in an arbitrary order since they are using a HashSet for the proxy interfaces in the method org.hibernate.tuple.entity.PojoEntityTuplizer.buildProxyFactory(PersistentClass persistentClass, Getter idGetter, Setter idSetter). This Hash set is passed to the method org.hibernate.proxy.pojo.cglib.CGLIB_ProxyFactory.postInstantiate(...). There it is simply converted to an Array (leading to a randomized order).

As a workaround it is possible to patch the class org.hibernate.proxy.pojo.cglib.CGLIB_ProxyFactory and add reorganize the array if the HibernateProxy interface is the first one:

this.interfaces = (Class[]) interfaces.toArray(NO_CLASSES);
if(this.interfaces.length > 1) {
Class firstIfc = this.interfaces[0];
if(firstIfc.getName().startsWith("org.hibernate")) {
this.interfaces[0] = this.interfaces[1];
this.interfaces[1] = firstIfc;
System.err.println("Replace: " + firstIfc.getName() + " by " + this.interfaces[0].getName());

After applying this patch everything woks as expected.

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Jan Wiemer

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