I am developing a project at Java fresher level.
I had to implement database interaction using Hibernate.
At the first stage, I started using HQL query language. But, later I come to know about Criteria queries through my previous questions.
But, still after learning Criteria, I am not getting what are the steps I should follow to fill and fetch data to and from database.
In fact, what are the packages and classes I need to develop manually and the script or query I need to write to fill/fetch data if I am given a database and table in it, using Criteria ?
Please also tell me, where would be difference when I use different database like PostGresql or MySQL within steps.
In fact, what are the packages and classes I need to develop manually and the script or query I need to write to fill/fetch data if I am given a database and table in it, using Crieteria?
Create an object model and map it to the tables using either annotations or xml mapping files. Classes that can be persisted are called Entities. Using the reverse engineering module of Hibernate Tools, it is possible to generate them.
Create instances of entities, set their attributes and use session.persist(Object) to persist them to the database to "fill data". Use the Criteria API to read from the database and fetch data. Data access is typically done in a data access layer using the DAOs pattern. DAOs expose finders and CRUD methods (Create, Read, Update, Delete).
If all this is new to you, I'd suggest to use Spring, it provides useful support classes and will help you to structure your application following the above pattern. Have a look at the Chapter 12. Object Relational Mapping (ORM) data access.
Please also tell me, where would be difference when I use different database like PostGres or MySQL within steps.
If their physical model differs, you may have to change the mapping of entities. Apart from that, switching from one database to another would require using the appropriate JDBC driver, changing the connection string and the Hibernate dialect (i.e. this is more a matter of configuration).
Related
We have this situation:
One backend written with Spring.
One native Android app.
They share many of the models.
What it has been done is writing models in a package on the backend and then export it as jar in android.
On android we are using OrmLite to interact with these models on the DB. The models have their proper annotations to achieve this. On the backend we basically write raw crud queries for each model, this is getting crazy as every time we add a field we need to update all the relevant queries.
What we would like to achieve is to use another orm on the server. The problem is that server and tablet for old bad design choices have different column and table names even though the models are the same. Renaming columns and tables cannot be done because it involves too much work.
We need to get things smarter, but we would also like to avoid duplicating the models just to remap the models to a different database schema.
Do you have any idea on a smart way on how to achieve this?
Unfortunately, "Renaming columns and tables cannot be done because it involves too much work" is the best solution to have consistent and managebale code base in the long term.
For the short term, what could be smart is to use XML based configuration on the server and stick to annotations for the tablet app.
Update
You want to look at JPA specification, it defines the persistence mechanism in java. There are multiple implementation providers for it, here is the list for providers of the specificaions latest version.
I only have experience with Hibernate ORM which along with its native API also provides implementation for JPA. It has XML based configuration option in addition to annotations. You will have evaluate which provider suits your requirment.
If the annotation of ORMlite differ from JPA then you can even use the JPA annotation in the entity classes besides the ORMLite annotation. But beware this will make your entity model classes messy and you will need both your selected JPA implemantion library and ORMLite library on the classpath of entity model.
I am writing a java application in which I am using Spring Boot and JPA in order to map classes to my database tables.
However, due to a somewhat complex database structure I also have the need of creating custom queries that are not mapped to any specific POJOs / Entities.
Therefore I am using PreparedStatement together with a DataSource with #Autowired annotation.
It hit me that using both of these DB Access methods might not be suitable to use together?
So far everything has worked out in my dev environment, but are there any pitfalls that I should look out for when using both of these together or is there a preferred way of doing custom queries when using JPA?
It should be noted that my database calls are fairly short and happen in a stateless manner, so there should hopefully not be any problems with interfering sessions (?)
JPA EntityManager will not know anything about your changes made with PreparedStatement. This will cause issues with JPA built-in caching, maybe with versioning and also with transaction support.
Though you may need to check this question: Is it OK to use both JPA (for normal CRUDs) and JDBC (for batch update & call stored proc) in the same project
Invan's answer makes a clear point.
On the other hand your fine when:
you need complex queries to SHOW data (read only).
you infrequently need to do some batch updates and do a clear cache entityManager.getEntityManagerFactory().getCache().evictAll()
Answer says, not to trust hibernate.hbm2ddl.auto setting for production.
My understanding of using ORM:
1) To avoid designing & normalising DB schema at database layer(say RDBMS). In mongoDB world, ODM is used.
2) To avoid embedding SQL query language in code(say java).
3) To just think about storing and retrieving objects(in OOP sense)
Running DDL scripts breaks the purpose of using ORM tool and looks similar to JDBC approach except it provides the SQL dialect for vendor specific database.
For production, Can running of DDL scripts mandatory for safety?
Running DDL scripts manually breaks the purpose of using ORM tool.
No, it does not.
An Object-Relational Mapping tool is tool that helps translate data from your tables into objects that you can use in your object-orianted programming language - it has nothing to do with database administration.
Hibernate can generate a DDL based on what your classes look like right now, but it has no sense of history.
If all you're doing is adding new columns or tables you'll probably be fine but the minute you rename a column you're out of luck because Hibernate will see the old column and won't find a mapping to it so it will remove it and then create a new column using the new name. If you have a non-null requirement on that column you're screwed because you can't tell Hibernate what the default value is (well, there's a hack but please don't do this.)
You're also very limited in how you can change the types of columns - if the contents of the column can't be translated automatically by the database you're out of luck.
As an example we switched our databases from storing UUIDs in binary to storing it as a VARCHAR a while back and we had to manually convert them from binary to hexadecimal notation becasue MySQL can't do that automatically - you'd be properly screwed if you tried to do that with Hibernate's auto-DDL.
There's also no way of telling Hibernate where to create indexes - you'll get an index on each primary key column but if you want extra indexes you'll have to add these manually.
The DDL auto-generation of Hibernate is good for validating that your classes map correctly to your tables, but it should never be used to alter your production databases.
So to answer your question:
For production, does manual run of DDL scripts mandatory for safety?
Yes! And I recommend you use a management tool like Liquibase or Flyway to aid with it.
Yes, they are required. If you want to work efficiently that is.
Running DDL scripts manually breaks the purpose of using ORM tool
No it doesn't. ORM stands for Object Relational Mapping, meaning it maps the relational data of the RDBMS to Objects. Nowhere does it imply that the database schema must be changed by the ORM, even though the possibility exists (and works in very simple cases).
Besides you're not going to be running anything manually. There are database migration/refactoring products like Flyway and Liquibase that attempt to solve the problem of a database schema changing over time. They're also separate products, so you don't need to care whether you're using Hibernate or some other method of data access. They also try to provide some amount of transactionality, meaning you can revert a change to the schema in some cases.
In any non-trivial project one would try to make sure they can improve the database without being permanently locked into a legacy schema, as well as making incredibly sure that the data stays safe. A proper tool designed for that purpose makes it a lot easier, an ORM's half-baked mechanism does not.
Assuming an existing application which deals with lets say 20+ tables, needs to be rewritten using java/jpa/hibernate and if all the tables already existed, is it the usual/best practice to use hibernate reverse engineering to generate hibernate mapping files?
and if the table structure gets changed, say added/modifed 10 fields in 3 different tables, do the mapping files get edited by hand to reflect the changes in the database?
also if its a brand new application (with new tables), is it the usual/best practice to create the database objects using the ddl generated by hbm2ddl?
Blindly using hbm2dll on production database could lead to data loss.
Take a look at specialized database migration tool like Liquibase or Flyway.
See Hibernate using JPA (annotated Entities) and liquibase.
I'm currently working on a desktop application using JPA/Hibernate to persist data in a H2 database. I'm curious what my options are if I need to make changes to the database schema in the future for some reason. Maybe I'll have to introduce new entities, remove them or just change the types of properties in an entity.
Is there support in JPA/Hibernate to do this?
Would I have to manually script a solution?
I usually let Hibernate generate the DDL during development and then create a manual SQL migration script when deploying to the test server (which I later use for UAT and live servers as well).
The DDL generation in Hibernate does not offer support for data migration at all, if you only do as much as adding a non-null field, DDL generation cannot help you.
I have yet to find any truely useful migration abstraction to help with this.
There are a number of libraries (have a look at this SO question for examples), but when you're doing something like splitting an existing entity into a hierarchy using joined inheritance, you're always back to plain SQL.
Maybe I'll have to introduce new entities, remove them or just change the types of properties in an entity.
I don't have any experience with it but Liquibase provides some Hibernate Integration and can compare your mappings against a database and generate the appropriate change log:
The LiquiBase-Hibernate integration records the database changes required by your current Hibernate mapping to a change log file which you can then inspect and modify as needed before executing.
Still looking for an opportunity to play with it and find some answers to my pending questions:
does it work when using annotations?
does it require an hibernate.cfg.xml file (although this wouldn't be a big impediment)?
Update: Ok, both questions are covered by Nathan Voxland in this response and the answers are:
yes it works when using annotations
yes it requires an hibernate.cfg.xml (for now)
There are two options:
db-to-hibernate - mirror DB changes to your entities manually. This means your DB is "leading"
hibernate-to-db - either use hibernate.hbm2ddl.auto=update, or manually change the DB after changing your entity - here your object model is "leading"