Object provides an abstract interface to some type of database or other persistence mechanism.
Real world example
There's a set of customers that need to be persisted to database. Additionally we need the whole set of CRUD (create/read/update/delete) operations so we can operate on customers easily.
Ensures that each object gets loaded only once by keeping every loaded object in a map. Looks up objects using the map when referring to them.
Real world example
We are writing a program which the user may use to find the records of a given person in a database.
Holds details of object-relational mapping in the metadata.
Real world example
Hibernate ORM Tool uses Metadata Mapping Pattern to specify the mapping between classes and tables either using XML or annotations in code.
Private Class Data design pattern seeks to reduce exposure of attributes by limiting their visibility. It reduces the number of class attributes by encapsulating them in single Data object.
Real world example
Imagine you are cooking a stew for your family for dinner. You want to prevent your family members from consuming the stew by tasting it while you are cooking, otherwise there will be no more stew for dinner later.
Repository layer is added between the domain and data mapping layers to isolate domain objects from details of the database access code and to minimize scattering and duplication of query code. The Repository pattern is especially useful in systems where number of domain classes is large or heavy querying is utilized.
Resource Acquisition Is Initialization pattern can be used to implement exception safe resource management.
Use the Resource Acquisition Is Initialization pattern when
To easily persist Java objects to the database.
Java serialization allow us to convert the object to a set of bytes. We can store these bytes into database as BLOB(binary long objects) and read them at any time and reconstruct them into Java objects.
Service Layer is an abstraction over domain logic. It defines application's boundary with a layer of services that establishes a set of available operations and coordinates the application's response in each operation.
Typically applications require different kinds of interfaces to the data they store and the logic they implement. Despite their different purposes, these interfaces often need common interactions with the application to access and manipulate its data and invoke its business logic. Encoding the logic of the interactions separately in each module causes a lot of duplication. It's better to centralize building the business logic inside single Service Layer to avoid these pitfalls.
Filter, Criteria
Specification pattern separates the statement of how to match a candidate, from the candidate object that it is matched against. As well as its usefulness in selection, it is also valuable for validation and for building to order.
Table Module organizes domain logic with one class per table in the database, and a single instance of a class contains the various procedures that will act on the data.
Real world example
When dealing with a user system, we need some operations on the user table. We can use the table module pattern in this scenario. We can create a class named UserTableModule and initialize a instance of that class to handle the business logic for all rows in the user table.
