Briefly introduce several java objects

1. PO: (persistant object), persistent object

can be regarded as a java object that maps to a table in the database. Using Hibernate to generate PO is a good choice.

2. VO: (value object), value object

is usually used for data transfer between business layers. Like PO, it only contains data. But it should be an abstracted business object, which may or may not correspond to a table, depending on the needs of the business.

PO can only be used in the data layer, and VO is used in the business logic layer and presentation layer. Each layer operates its own data objects, which reduces the coupling between layers and facilitates future system maintenance and expansion.

3. DAO: (Data Access Objects), data access Object interface
DAO is the Data Access Object data access interface, data access: as the name suggests, it is to deal with the database. Sandwiched between business logic and database resources.

J2EE developers use Data Access Objects (DAO)Design PatternSeparate the underlying data access logic and high-level business logic. Implementing the DAO pattern can focus more on writing data access code.

The DAO pattern is one of the standard J2EE design patterns. Developers use this pattern to separate the underlying data access operations from the upper-level business logic. A typical DAO implementation has the following components:
1. A DAO factory class;
2. A DAO interface;
3. A concrete class that implements the DAO interface;
4. Data transfer object (sometimes called a value object).
Specific DAO class Contains the logic for accessing data from a specific data source.

4. BO: (Business Object), the business object layer
represents all entity classes of "things" in the application domain. These entity classes reside on the server and utilize service classes to assist in their responsibilities.

5. POJO: (Plain Old Java Objects), simple Java objects
are actually ordinary JavaBeans. The POJO name is used to avoid confusion with EJB, and the abbreviation is more direct.
Among them are Some classes with attributes and their getter and setter methods can sometimes be used as value objects or dto (Data Transform Object). Of course, if you have a simple operation attribute, it is also possible, but business methods are not allowed and cannot be carried. There are methods like connection.

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