使用ejb的时机（请各位高手补充）-----摘自java程序设计：j2ee

zjkdc

使用ejb的时机
1、当所开发的应用系统可能扩展至更大的使用层面，并支持多用户实时访问；
2、当所开发的应用系统需要支持事务管理；
3、如果所开发的系统需要一个安全模型。

不适宜的时机
1、当所开发的应用系统无需考虑任何可扩展性、事务管理或安全问题；
2、所开发的应用系统无需考虑任何平台无关性，并且不但心会在先后的开发工作中依赖相同的支持厂商；
3、当相关的开发团体并无甚多EJB开发经验，并且不希望培训或引入相关经验的时候，选择EJB这项相对基本JAVA开发较复杂的技术，应该说是不明智的。

dennis2

From Rod Johnson's "Expert One-on-One J2EE Design and Development":

Implications of Using EJB:
- Using EJB makes applications harder to test
- Using EJB makes applications harder to deploy
- Using EJB with remote interfaces may hamper practicing OO design
- Using EJB may make simple things hard
- Reduced choice of application servers

Questionable Arguments for Using EJB:
- To ensure clean architecture by exposing business objects as EJBs
- To permit the use of entity beans for data access
- To develop scalable, robust applications

Compelling Arguments for Using EJB:
- To allow remote access to application components
- To allow application components to be spread across multiple physical servers
- To support multiple Java or CORBA client types
- To implement message consumers when an asynchronous model is appropriate

Important:
EJBs are a good solution to problems of distributed applications and complex transaction management. However, many applications don't encounter these problems. EJBs add unnecessary complexity in such applications. An EJB solution can be likened to a truck and a web application to a car. When we need to perform certain tasks, such as moving large objects, a truck will be far more effective than a car, but when a truck and a car can do the same job, the car will be faster, cheaper to run, more maneuverable and more fun to drive.