Plain Old Java Object
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POJO is an acronym for Plain Old Java Object, and is favoured by advocates of the idea that the simpler the design, the better. The name is used to emphasize that the object in question is an ordinary Java Object not a special object, and in particular not an Enterprise JavaBean (especially before EJB 3). The term was coined by Martin Fowler, Rebecca Parsons and Josh MacKenzie in September 2000. "We wondered why people were so against using regular objects in their systems and concluded that it was because simple objects lacked a fancy name. So we gave them one, and it's caught on very nicely."[1] The term continues the pattern of older terms for technologies that do not use fancy new features, such as POTS (Plain Old Telephone Service) in telephony, and PODS (Plain Old Data Structures) that are defined in C++ but use only C language features.
The term has most likely gained widespread acceptance because of the need for a common and easily understood term that contrasts with complicated object frameworks. A JavaBean is a POJO that is serializable, has a no-argument constructor, and allows access to properties using getter and setter methods. An Enterprise JavaBean is not a single class but an entire component model (again, EJB 3 reduces the complexity of Enterprise JavaBeans).
The concept of a POJO predates the term POJO because the natural condition of an object class is that it is not special. The term serves as a reminder that simpler designs can be better, rather than incorporating a complicated framework in the architecture of a system without sufficient reason. As designs using POJOs have become more commonly-used, systems have arisen that give POJOs some of the functionality used in frameworks and more choice about which areas of functionality are actually needed. Hibernate and Spring are examples.
[edit] Contextual Variations
As of November 2005, the term "POJO" is mainly used to denote a Java object which does not follow any of the (major) Java object models, conventions, or frameworks such as EJB.
All Java objects are POJOs, therefore ideally speaking a POJO is a Java object not bound by any restriction other than those forced by the Java Language Specification. I.e., a POJO should not have to
1. extend prespecified classes, as in
public class Foo extends javax.servlet.http.HttpServlet{ ...
2. implement prespecified interfaces, as in
public class Bar implements javax.ejb.EntityBean{ ...
3. contain prespecified annotations, as in
@javax.ejb.Entity
public class Baz{ ...
However, due to technical difficulties and other reasons, many software products or frameworks described as POJO-compliant actually still require the use of prespecified annotations for features such as persistence to work properly.
