什么是POJO，JavaBean？

總結：

POJO：

一個簡單的Java類，這個類沒有實現/繼承任何特殊的java接口或者類，不遵循任何主要java模型，約定或者框架的java對象。在理想情況下，POJO不應該有注解。

JavaBean：

JavaBean是可序列化的，實現了serializable接口

具有一個無參構造器

有按照命名規范的set和gett，is(可以用于訪問布爾類型的屬性)方法

pojo

POJO的創始人(martinfowler)博客：

The term was coined while Rebecca Parsons, Josh MacKenzie and I were preparing for a talk at a conference in September 2000. In the talk we were pointing out the many benefits of encoding business logic into regular java objects rather than using Entity Beans. 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.

...在談話中我們指出，編寫業務邏輯的時候，使用常規的java對象要比實體bean要好的多。我們懷疑為什么一些人極力反對在他們的代碼中使用常規對象，還辯解稱因為這些常規對象沒有一個花哨的名字，所以我們給他們起了一個非常好聽的名字。(Plain Old Java Object)

維基百科原文————(以下代碼和示例均來源于此)

The term "POJO" initially denoted a Java object which does not follow any of the major Java object models, conventions, or frameworks; nowadays "POJO" may be used as an acronym for "Plain Old JavaScript Object" as well, in which case the term denotes a JavaScript object of similar pedigree.[2]

術語POJO起初表示為不遵任何主要的java模型，約定或者框架的java對象，現在，pojo也可以用作'Plain Old JavaScript Object'的縮寫，這樣的話和javascript對象有著相似的淵源。

理想狀態下，pojo應該是一個不受Java語言規范限制的java對象。換句話說，pojo不應該：

1.繼承預先設定的類，如：

public class Foo extends javax.servlet.http.HttpServlet {

// ...

}

2.實現預先設定的接口，如：

public class Bar implements javax.ejb.EntityBean {

// ...

}

3.包含預先指定的注解，如：

@javax.persistence.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. The idea is that if the object (actually class) was a POJO before any annotations were added, and would return to POJO status if the annotations are removed then it can still be considered a POJO. Then the basic object remains a POJO in that it has no special characteristics (such as an implemented interface) that makes it a "Specialized Java Object" (SJO or (sic) SoJO).

然而，由于技術和其他原因，很多被稱之為POJO標準的軟件產品或框架仍然需要使用特定的注解來保證持久化等功能。這個想法是，如果對象(類)在任何注解添加之前是一個pojo的話，并且注解移除之后仍然是pojo。所以最基礎的pojo解釋是沒有特別的特征(尤其是實現接口之類的)，使其稱之為“專用java對象”

JavaBean

A JavaBean is a 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. Because of this convention, simple declarative references can be made to the properties of arbitrary JavaBeans. Code using such a declarative reference does not have to know anything about the type of the bean, and the bean can be used with many frameworks without these frameworks having to know the exact type of the bean. The JavaBeans specification, if fully implemented, slightly breaks the POJO model as the class must implement the Serializable interface to be a true JavaBean. Many POJO classes still called JavaBeans do not meet this requirement. Since Serializable is a marker (method-less) interface, this is not much of a burden.

JavaBean是一個可序列化的POJO，具有一個無參構造器，并且允許使用遵循簡單命名約定的getter和setter方法來訪問屬性。由于這個慣例，可以對任意JavaBean屬性進行簡單的聲明引用。使用這種聲明引用的代碼不需要知道bean的具體類型。并且，這個bean還可以被很多框架使用，這些java框架也不需要知道bean的類型。由于java.io.Serializable是一個標記接口(無方法)，所以這并不是一個多大的負擔。如果JavaBean完全實現的話，稍微打破了一些POJO模型。很多被稱之為JavaBean的POJO類并不符合這個要求，因為JavaBean必須實現Serializable接口才能成為真正的JavaBean。

JavaBean的優點：

The properties, events, and methods of a bean can be exposed to another application.

A bean may register to receive events from other objects and can generate events that are sent to those other objects.

Auxiliary software can be provided to help configure a bean.

The configuration settings of a bean can be saved to persistent storage and restored.

bean中的屬性，事件和方法可以暴露給另一個應用程序

一個bean可以注冊來自于其他對象的事件，也可以產生事件并發送給其他對象

輔助代碼可以提供javabean的配置

一個bean的配置設置可以永遠被存儲和恢復

一些疑問？

什么是事件？有什么作用？bean如何注冊來自于其他對象的事件？

配置設置如何被存儲和恢復？

serializable接口是做什么的？

總結

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