What is meaning of Plain Old Java Object (POJO)?

Solution 1:

Plain Old Java Object The name is used to emphasize that a given object is an ordinary Java Object, not a special object such as those defined by the EJB 2 framework.

class A {}
class B extends/implements C {}

Note: B is non POJO when C is kind of distributed framework class or ifc. e.g. javax.servlet.http.HttpServlet, javax.ejb.EntityBean or J2EE extn and not serializable/comparable. Since serializable/comparable are valid for POJO.

Here A is simple object which is independent. B is a Special obj since B is extending/implementing C. So B object gets some more meaning from C and B is restrictive to follow the rules from C. and B is tightly coupled with distributed framework. Hence B object is not POJO from its definition.

Code using class A object reference does not have to know anything about the type of it, and It can be used with many frameworks.

So a POJO should not have to 1) extend prespecified classes and 2) Implement prespecified interfaces.

JavaBean is a example of POJO that is serializable, has a no-argument constructor, and allows access to properties using getter and setter methods that follow a simple naming convention.

POJO purely focuses on business logic and has no dependencies on (enterprise) frameworks. It means it has the code for business logic but how this instance is created, Which service(EJB..) this object belongs to and what are its special characteristics( Stateful/Stateless) it has will be decided by the frameworks by using external xml file.

Example 1: JAXB is the service to represent java object as XML; These java objects are simple and come up with default constructor getters and setters.

Example 2: Hibernate where simple java class will be used to represent a Table. columns will be its instances.

Example 3: REST services. In REST services we will have Service Layer and Dao Layer to perform some operations over DB. So Dao will have vendor specific queries and operations. Service Layer will be responsible to call Which DAO layer to perform DB operations. Create or Update API(methods) of DAO will be take POJOs as arguments, and update that POJOs and insert/update in to DB. These POJOs (Java class) will have only states(instance variables) of each column and its getters and setters.

In practice, some people find annotations elegant, while they see XML as verbose, ugly and hard to maintain, yet others find annotations pollute the POJO model. Thus, as an alternative to XML, many frameworks (e.g. Spring, EJB and JPA) allow annotations to be used instead or in addition to XML:

Advantages:
Decoupling the application code from the infrastructure frameworks is one of the many benefits of using POJOs. Using POJOs future proofs your application's business logic by decoupling it from volatile, constantly evolving infrastructure frameworks. Upgrading to a new version or switching to a different framework becomes easier and less risky. POJOs also make testing easier, which simplifies and accelerates development. Your business logic will be clearer and simpler because it won't be tangled with the infrastructure code

References : wiki source2

Solution 2:

According to Martin Fowler, he and some others came up with it as a way to describe something which was a standard class as opposed to an EJB etc.

Solution 3:

Usage of the term implies what it's supposed to tell you. If, for example, a dependency injection framework tells you that you can inject a POJO into any other POJO they want to say that you do not have to do anything special: there is no need to obey any contracts with your object, implement any interfaces or extend special classes. You can just use whatever you've already got.

UPDATE To give another example: while Hibernate can map any POJO (any object you created) to SQL tables, in Core Data (Objective C on the iPhone) your objects have to extend NSManagedObject in order for the system to be able to persist them to a database. In that sense, Core Data cannot work with any POJO (or rather POOCO=PlainOldObjectiveCObject) while Hibernate can. (I might not by 100% correct re Core Data since I just started picking it up. Any hints / corrections are welcome :-) ).