I am working on a Java project for the university and I am trying to learn about framework, technologies and best practices for creating a well structured software that follows software engineering principles. I decided to use Spring (Spring-data-jpa), and Hibernate for the standalone software and now I am having some trouble in understanding how to use the Java Persistence Api to create an abstraction layer above the Hibernate implementation provider. What I am trying to figure out is in which part (configuration file or java class) I can switch from an ORM tool to another one. I saw that I have to use the persistence.xml file to specify the persistence unit and persistence context and also the DB parameters, but it's not clear how the EntityManager bind itself with the underlying ORM tool and in which properties this bind is set. Is the "provider" properties inside persistence.xml file that create this binding? Any link/references/examples or guide will be appreciated, thanks in advance and excuse me for my english ;)
Each persistence unit in he persistence.xml file is associated to a provider. For example, with Hibernate:
<provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
If you want to use another provider, you'll specify another value for this element.
Note that this is usually unnecessary, because you typically use only one provider in an application. If a single provider is available in the classpath, JPA will use that one. So switching from Hibernate to EclipseLink for example just consists in having the EclipseLink jars in the classpath rather than the Hibernate jars.
Hibernate works on the JPA API so you don't have to explicitly configure JPA if you are using hibernate. The ORM integration happens in the DAO "Data Access Object" layer of your application.
Related
My CRM application supports multiple clients where clients are allowed to create/update schema at runtime.
So, the question is:- how to create ORM entities at runtime without restarting the server?
Kindly suggest if any framework has already solved this use-case with multi-tenant support.
Is it possible by some dynamic programming like groovy, grails and gorm?
Is it possible by some dynamic programming like groovy, grails and
gorm?
It may be possible with some ORM. If you are using Hibernate, as a practical matter I think the answer is "no". The session factory is immutable. From https://docs.jboss.org/hibernate/orm/5.4/javadocs/org/hibernate/SessionFactory.html:
The internal state of a SessionFactory is immutable. Once it is
created this internal state is set. This internal state includes all
of the metadata about Object/Relational Mapping.
If you are using another ORM tool then the details will depend on which ORM tool you are using.
I have built a small application. Now I’m looking for the right way to deal with the JPA configuration.
My app use JPA 2.0 to connect to a database. Right now the configuration is hardcoded in my persistence.xml file. When I built the jar file, my persistence.xml is included in the jar.
So my questions are:
How do I configure my app/JPA to work with any JPA implementation.
And how can I use the same jar as a standalone application and as a component to deploy on a server (eg. Glassfish)
First of all JPA is a specification, which is being implemented by the ORM framework providers such as Hibernate, Eclipselink etc.
To make your application compatible with any implementation of JPA, you should strictly follow JPA specifications, you should not be using any ORM vendor specific features, this will allow your application to work with any ORM framework, however you have to specify the dialect class by your self to interact with a particular database.
One thing you can do here is write a properties file which will have all the database name with corrosponding dialect class for that particular database, you can ask user to choose the database and can fetch the related class name(Dialect class name) from the properties file.
About using the same application as standalone and using it as a component to deploy on a server, I am sorry but I am not getting your point.
I need to add a new datasource to my grails project wich uses the musicbrainz postgresql database. http://musicbrainz.org/doc/MusicBrainz_Database
I found a project on github where the data bindings are ready to use for a spring project:
https://github.com/lastfm/musicbrainz-data
Am i able to use these data bindings in a grails 2.2.3 project? If yes, how can i do this? (because there is no hibernate xml as needed by grails (regarding to the grails documentation: Hibernate Mapped Domain Classes))
I don't think it will be. Just setup the additional datasource and model out the tables or objects you need.
How do you access two databases in Grails
Once you do that, you can use all of the GORM methods and dynamic finders to get your data. Plus validation criteria, transactions, etc. Unless there is some very specialized criteria that make it necessary to bypass GORM, I would suggest leveraging it.
Stepwise, what would be a good way of integrating Spring and Hibernate into an existing JSF application that doesn't use ORM?
1) You would need to design the domain model first and then map it to the database. You could use hibernate reverse engineering tools for this.
2) Other option is to manually map your current objects(DTO) to database tables. Since you use JSF, I assume you'd be having some objects currently.
3) Design the Service Layer around the objects.
Spring -
1) You could use Spring to provide hibernate template, provide common services through bean.
2) You can manage the transaction in Spring.
I would recommend first to write tests to check your code of your previous persistent mechanism. This code could be used to check the correct behavior of our ORM integration.
As mentioned by other answers, having a clear DAO defined by interface helps to bound the DAO code.
Map the domain objects first, then write your DAO, then your service objects (which take care of large atomic suite of operations by enclosing its in a transaction).
Use persistence mechanism which is vendor-agnostic (JPA is the good and only choice).
Start with one kind of database and stick with it during all the migration. In very uncommon cases, you can meet subtle differences between databases which could be very hard to solve, especially if you're a beginner.
When starting, use automatic generation of database (generateDdl for hibernate subsystem) and then, when things starts to be stabilized, force #Table and #Column annotations to fix name of each column. At this point, write a SQL script which generate the database with empty tables. The reason if to fix your architecture and be sure you're controlling the database organization.
If you're serious about ORM, please look at Java Persistence With Hibernate of Christian Bauer (Manning publications), this is "the bible" about hibernate and JPA on Java.
If you've written Spring properly, you should have DAO/repository interfaces. If that's the case, all you have to do is write a new implementation of that interface using Hibernate, test it, and change your Spring configuration to inject it.
ORM presumes that you have an object model in place to map to a relational schema. If you don't, I would advise against using ORM. Better to use iBatis or JDBC in that case.
I would like to know if there are any tools to automatically generate EJB3 Entity Beans(for JPA) from a database schema.
Thanks.
Dali supports top-down, bottom-up (this is what you're looking for), and meet-in-the-middle development approaches.
Some of IDs have such feature, for example NetBeans
OpenJPA has a tool which will generate your Entity definitions.
From the OpenJPA user manual:
OpenJPA includes a reverse mapping tool for generating persistent class definitions, complete with metadata, from an existing database schema. You do not have to use the reverse mapping tool to access an existing schema; you are free to write your classes and mappings yourself, as described in Section 3, “ Meet-in-the-Middle Mapping ”. The reverse mapping tool, however, can give you an excellent starting point from which to grow your persistent classes.
No IDE required!
-Rick
I have a solution for you i.e to create auto generate domain objects with all table relationship properly mapped in class ...Try Dal4j yes you can find it in sourceforge.net/p/dal4j/wiki/ DAL4j is a Command Line and Framework tool that can be used to reverse engineer a MySQL or SQLServer database schema into a set of JPA Entity Beans.
DAL4j can be useful for scenarios where there is an existing database schema but a technology other that JPA is used by applications to interact with the database. DAL4j can provide an easy way to migrate your code base from other technologies such as JDBC or Hibernate to JPA.
The beans generated can be 1 or two types: Simple or Framework. Simple beans are standard pojo classes managed by your application using JPA semantics. Framework generated pojos use the DAL4j framework DAO generic to simplify CRUD operations.
DAL4j provides optional hooks to allow you integrate encryption/decryption of data fields that must be encrypted in the database.
Last, DAL4j provides a set of Generic classes that can be used to simplify creation of Session Beans which perform CRUD operations using generated Entities.
I think you will find this article feasible....