Does Hibernate have to drive database design?

Question

I spent all of yesterday reading various articles/tutorials on Hibernate and although I am blown-away by how powerful it is, I have one major concern with it.

It seems that the standard practice is to allow Hibernate to design/generate your DB schema for you, which is a new and scary concept that I am choking on. From the tutorials I read, you just add a new entity to your hibernate.cfg.xml config file, annotate any POJO you want with @Entity, and voila - Hibernate creates the tables for you. Although this is very cool, it has me wondering about a handful of scenarios:

• What if you already have a DB schema and the one Hibernate wants to generate for you does not conform to it? What if you have a crazy DBA that refuses to budge on the pre-defined (non-Hibernate) schema?
• What if you have reference tables with tens of thousands of records in it (like all the cities in the world)? Would you have to instantiate and save() tens of thousands of unique POJOs or is there a way to configure Hibernate so it will honor and not overwrite data already existing in your tables?
• What if you want to do perf tuning on your schema/tables? This includes indexing, normalizing above and beyond what Hibernate creates automatically?
• What if you want to add constraints or triggers to your tables? Indexes?

I guess at the root of this is the following:

It looks like Hibernate creates and forces a particular schema/config on your DB. I am wondering how this agenda will conflict with our platform standards, our DBA philosophies, and our ability to perf tune/tweak tables that Hibernate interacts with.

Thanks in advance.

Answer 1

I think you're attributing too much power to Hibernate.

Hibernate does have an idiom that may influence database implementation.

Hibernate does not generate a schema for you unless you ask it to do so. It's possible to start with an existing schema and map it to Java objects using Hibernate. But it might not be possible or optimal if the schema conflicts with Hibernate requirements.

If the DBA won't budge - as they shouldn't - or Hibernate can't accomodate you, then you have your answer: you can't use Hibernate.

Your DBA might consent, but your app might find that the dynamic SQL that's generated for you by Hibernate isn't what you want.

Fortunately for you, it's not the only game in town.

I don't think implementations have to be all or none. If you use simple JDBC to access reference data, what's the harm?

Database design considerations should be independent of Hibernate. Constraints, triggers, normalization, and indexes should be driven by business needs, not your middleware choices.

If you don't have a solid object model, or the schema can't accomodate it, then you should reconsider Hibernate. There's straight JDBC, stored procedures, Spring JDBC, and iBatis as alternatives.

Answer 2

Hibernate comes with a default way to map objects to tables - like several tools/libraries, it favours convention over configuration for simplicity.

However, if you want to map the entities to database tables differently, you can explicitly tell Hibernate how these are mapped (from simple attributes such as changing the table name, through to redefining the foreign-key relationships between related entities and how this is persisted).

If you do this correctly, you don't need to instantiate and save existing data, as this would be pointless - the database already contains the information about the entities in exactly the form that Hibernate understands. (Think about it - to load and then immediately save an entity should always be a no-op, and so can be skipped altogether.)

So the short answer to your question is "no". If you don't care for designing tables, you can let Hibernate adopt a reasonable default. If you do want to design your schema explicitly though, you can do this and then describe that exact schema to Hibernate.

Answer 3

As someone who's worked on java and hibernate in the enterprise for a long time, I have seen very few projects which use this capability. You'll see some build tools and other things do this, but for a real enterprise app, i've never seen this.

Most DBA's won't let the application user create tables. They rely on a privileged user to do those things, and the user that the app connects as would have r/w privs on the data but not the schema itself.

As a result, you write the SQL yourself, and you do the hibernate mappings to match. It doesn't mean your object design won't influence your SQL, but you should still always create your schema upfront.

Answer 4

No. You can use hibernate tools to generate the entities from existing database.

There are 2 ways you can go about in using Hibernate. If you have good DBA or database designer, then it is better to design the database and then map it into hibernate. On the other hand if you don't have DBA and have good developer then let Hibernate generate Database for you.

The concept behind Hibernate is to map Database and the Objects. So it is called as ORM (Object-Relational Mapping) tool.
Read here for Object Relational Impedance.

Answer 5

This is the preferred way for a quick'n dirty prototype or a simple tutorial, but it's far from being the preferred way for any production application. I largely prefer designing the database independently, using scripts to generate the schema, tables, views, indexes, etc., and map the schema to entities.

As long as the mapping finds the tables and columns in the database, everything is fine.

As soon as you have data in your database and the schema must change, you'll have to write migration scripts anyway. You can't just drop everything and restart from scratch. The tutorials are written for developers starting with Hibernate and who must discover Hibernate as quick as possible, without dealing with complex SQL scripts.

Answer 6

• What if you already have a DB schema ...

I don't know where you get that impression. Hibernate can use existing schema. It is quite flexible.

• What if you have reference tables ...

Make the relationship LAZY, and it won't load automatically. Only changed object will be saved.

• What if you want to do perf tuning ...

Just don't use the generated schema. It is just a starting point. You can customize as you need.

• What if you want to add constraints or triggers to your tables? Indexes?

Some as above.

Answer 7

You can use hibernate with an existing database schema. You can use various annotations to map to existing tables and columns, for example:

@Table(name = "dbschema.dbTable") - should be placed before your class file to map it @Column(name = "colName") - to map a column

Just be sure that the hibernate is configured with this option: hibernate.hbm2ddl.auto=update

If you set this to create it will create the schema, so do not do this in your case.

Answer 8

Use hibernate/jpa when appropiate. A common practice when designing apps is to extract the draft and alter it manually after needs (indices etc). However, it will be a pain for you if you change the db layout from hibernate way to do things. Lots of the beauty of JPA will be lost. For tasks which require heavy performance tuning and full control - just go for reguar jdbc.

Answer 9

Some answers:
A. It is possible to add an index annotation : see the table annotation.

B. If you have reference tables, you may choose to have lazy fetching or eager fetching (i.e - if your tables represent a person and a its books - whether to load a person without its book, or with its books)

C. Hibernate can be used to work on existing schema. The schema might not be trivial to work with , but as other have said, you should design db only according to business needs, and not according to framework conventions

D. I would like to encourage you also to read what hibernate does "under the hood" - it uses lots of usage of proxies, which hurts performance, you must understand well the scope of session , and the usages of 1st level and 2nd level cache

. E. Following what I wrote at section D - working with triggers will cause your DB to change "under the hood" when it comes to hibernate. Consider a case where updating a record will create (using a trigger) an entry in some archiving table , and let's say this table is also annotated via hibernate
- your hibernate caching will not be aware of the change that happend outside of the application scope.

F. It is important to me to state that I'm not against Hibernate, but you should not use it for all solutions, this is a mistake I did in the past. I now work with Spring-JDBC and I'm quite pleased (for our application needs it will be hard to use Hibernate, and I assume we will consider this only in the case we need to support more than one DB flavor).