Many times, Java applications are bound to a single database schema. But, it is possible for a Java application to be bound to many schemas across database types, which leads to different types of scenarios. Confused? Let me explain with an example.
Let’s say, if multiple applications happen to share the same set of database entities, then you might define a single schema and bind each of the corresponding applications to it. Or you might define one or more schemas to store reusable entities and then create custom ones for specific requirements in your application.
Working with Java Persistence API
In SAP HCP documentation, JPA is considered the standard approach for developing applications for SAP HCP. At a high level, JPA is Java API that makes it easy for developers to implement object-relational mapping (ORM).
Now, you might be thinking, what’s ORM?
Well, ORM is a process for synchronizing data stored in objects with relational database tables. Here, Java objects and database rows become equivalent representations of the same data, and the operations performed on a Java object are automatically reflected in database and vice versa.
The advantage of using an ORM tool over performing such translations by hand is that developers don’t have to cross over from object-oriented world into the procedural world of SQL and relational databases. Usually JPA consists of two parts:
- Interface/API layer: At this layer, JPA provides Java developers with a generic library that can be used to define, search for, and manipulate entities. Classes and interfaces that make up the library are stored in the javax.persistence package, which is again available in both Java SE and Java EE APIs.
- Implementation/Provider layer: Low-level database translations and operations are performed within this layer. Here, external JPA persistence provider delivers functionality required to synchronize data from relational database tables into Java objects and vice versa.
Object-Oriented Mapping Concepts in JPA
Within JPA, Java classes which correspond with database tables are called entities. The instance attributes of entity classes are mapped to table columns. When it comes to creating entities within JPA, any Java class can be used. You don’t have to inherit from API-specific class or implement an interface with call-back methods.
Once your entity classes are defined, next step is to map them to relational database tables. For this purpose, you can either describe attribute-to-column mappings using annotations, or you can specify mapping outside the code using XML deployment descriptors.
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