How to fix org.hibernate.MappingException: Unknown entity Exception in Java

If you have used Hibernate with JPA and using annotation to declare your entity bean then you might have seen this confusing error called "org.hibernate.MappingException: Unknown entity". This error message is so misleading that you could easily lose anywhere between few minutes to few hours looking at wrong places. I was using Spring 3 and Hibernate 3.6 when I got this error,which occurs when addEntity() method was executed. I checked everything, from Spring configuration file applicationContext.xml, Hibernate config file, my Entity class and DAO class to see whether my Entity class is annotated or not, but I was still getting this error message. After some goggling I eventually find that, it was an incorrect import which was causing this error. Both hibernate and JPA has @Entity annotation and some how Eclipse was automatically importing org.hibernate.annotations.Entity instead of javax.persistence.Entity annotation. Once I fixed this import issue, everything went smooth. Unfortunately, this error is not easy to spot, the org.hibernate.annotations.Entity import seemed completely appropriate to many programmers. For a newbie Java or hibernate developer it�s really hard to understand which Entity annotation to use, shouldn't be in org.hibernate.annotations.Entity, but instead it's javax.persistence.Entity. By the way, if you are using auto-complete functionality of Eclipse IDE, then you can blame it them, alternatively you can configure your preferred import in Eclipse.

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Dealing with org.hibernate.LazyInitializationException: could not initialize proxy - no Session in Hibernate Java

If you are working in Hibernate framework, then you know that one of the key feature of Hibernate is "lazy initialization", which allows framework to lazily initialize dependencies, relationship or association lazily from database on need basis. For example, if you are dealing with User object, which has relationship with Permission object like one user can have multiple permissions, then Hibernate may choose not to initialize the collection which holds all permissions at the time it initialized User object and instead returns a proxy object. At this point, if you close your session and letter tries to access an attribute from Permission object, you will get "org.hibernate.LazyInitializationException: could not initialize proxy - no Session in Hibernate". Why this error comes, because hibernate needs to go database to initialize the proxy object, and connection is already closed. If you remember, what we discussed in difference between get vs load in hibernate that Proxy object is only initialized in Hibernate if you access any attribute other than id itself, that's why you would only see LazyInitializationException if you try to access an attribute other than id. In this article, we will see different scenarios on which you could possibly get "org.hibernate.LazyInitializationException: could not initialize proxy - no Session in Hibernate" and how to solve them appropriately. I have tried to explain reasons which caused the error, and explained the solution as why it will work, but if you still face issues, then feel free to post it here. By the way, good understanding of lazy initialization is also a good Hibernate interview question, so this not only help you to solve this error but also to do well during interviews.

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Hibernate NullPointerException due to Space in HQL named queries

If you are using Hibernate for implementing persistence layer in Java and JEE application from couple of years then you would have seen this notorious NullPointerException while executing HQL named queries, Exception in thread �main� java.lang.NullPointerException at org.hibernate.hql.ast.ParameterTranslationsImpl .getNamedParameterExpectedType (ParameterTranslationsImpl.java:63). Hibernate has some poor logging in case of Exception, which has caused me hours to debug a simple problem. By looking at NullPointerException below (look full stacktrace below), I had no clue that it's coming because of a missing space on HQL (Hibernate Query language) query. You can also take a look if you can figure this out :

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