"ConstraintViolationException" occurs in Hibernate

By: pencha*** On: Tue Jan 29 11:11:33 IST 2013     0 0 0	0
Exception in thread "main" org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:71) at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43) at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:253) at org.hibernate.jdbc.AbstractBatcher.prepareStatement(AbstractBatcher.java:92) at org.hibernate.jdbc.AbstractBatcher.prepareStatement(AbstractBatcher.java:87) at org.hibernate.jdbc.AbstractBatcher.prepareBatchStatement(AbstractBatcher.java:222) at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2224) at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2660) at org.hibernate.action.EntityInsertAction.execute(EntityInsertAction.java:56) at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:250) at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:234) at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:141) at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:298) at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:27) at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1000) at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:338) at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:106) at com.penchal.store.Main.main(Main.java:33) Caused by: java.sql.BatchUpdateException: ORA-01400: cannot insert NULL into ("SYSTEM"."EMPLOYEE"."ID") at oracle.jdbc.driver.DatabaseError.throwBatchUpdateException(DatabaseError.java:343) at oracle.jdbc.driver.OraclePreparedStatement.executeBatch(OraclePreparedStatement.java:10657) at org.hibernate.jdbc.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:48) at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:246) ... 15 more	0