我问了一个常见的Spring问题:自动转换Spring bean,很多人回答说应该尽可能避免调用Spring的ApplicationContext.getBean()。为什么呢?

我还应该如何访问我配置Spring创建的bean呢?

我在一个非web应用程序中使用Spring,并计划访问LiorH所描述的共享ApplicationContext对象。

修正案

我接受下面的答案,但这里有Martin Fowler的另一种观点,他讨论了依赖注入与使用服务定位器(本质上与调用包装的ApplicationContext.getBean()相同)的优点。

In part, Fowler states, "With service locator the application class asks for it [the service] explicitly by a message to the locator. With injection there is no explicit request, the service appears in the application class - hence the inversion of control. Inversion of control is a common feature of frameworks, but it's something that comes at a price. It tends to be hard to understand and leads to problems when you are trying to debug. So on the whole I prefer to avoid it [Inversion of Control] unless I need it. This isn't to say it's a bad thing, just that I think it needs to justify itself over the more straightforward alternative."


当前回答

选择服务定位器而不是控制反转(IoC)的原因是:

Service Locator is much, much easier for other people to following in your code. IoC is 'magic' but maintenance programmers must understand your convoluted Spring configurations and all the myriad of locations to figure out how you wired your objects. IoC is terrible for debugging configuration problems. In certain classes of applications the application will not start when misconfigured and you may not get a chance to step through what is going on with a debugger. IoC is primarily XML based (Annotations improve things but there is still a lot of XML out there). That means developers can't work on your program unless they know all the magic tags defined by Spring. It is not good enough to know Java anymore. This hinders less experience programmers (ie. it is actually poor design to use a more complicated solution when a simpler solution, such as Service Locator, will fulfill the same requirements). Plus, support for diagnosing XML problems is far weaker than support for Java problems. Dependency injection is more suited to larger programs. Most of the time the additional complexity is not worth it. Often Spring is used in case you "might want to change the implementation later". There are other ways of achieving this without the complexity of Spring IoC. For web applications (Java EE WARs) the Spring context is effectively bound at compile time (unless you want operators to grub around the context in the exploded war). You can make Spring use property files, but with servlets property files will need to be at a pre-determined location, which means you can't deploy multiple servlets of the same time on the same box. You can use Spring with JNDI to change properties at servlet startup time, but if you are using JNDI for administrator-modifiable parameters the need for Spring itself lessens (since JNDI is effectively a Service Locator). With Spring you can lose program Control if Spring is dispatching to your methods. This is convenient and works for many types of applications, but not all. You may need to control program flow when you need to create tasks (threads etc) during initialization or need modifiable resources that Spring didn't know about when the content was bound to your WAR.

Spring非常适合事务管理,并且有一些优点。只是IoC在许多情况下会过度设计,给维护者带来不必要的复杂性。不要在没有考虑不使用IoC的情况下自动使用它。

其他回答

Spring的前提之一是避免耦合。定义和使用接口,DI, AOP,避免使用ApplicationContext.getBean():-)

其他人指出了普遍的问题(并且是有效的答案),但我只想提供一个额外的评论:并不是说你永远不应该这样做,而是尽可能少地做。

通常这意味着它只执行一次:在引导期间。然后,它只是访问“根”bean,通过它可以解决其他依赖关系。这可以是可重用的代码,如基本servlet(如果开发web应用程序)。

使用@Autowired或ApplicationContext.getBean()实际上是一回事。通过这两种方式,您可以获得在您的上下文中配置的bean,并且通过这两种方式,您的代码都依赖于spring。 你唯一应该避免的是实例化你的ApplicationContext。只做一次!换句话说,一条线就像

ApplicationContext context = new ClassPathXmlApplicationContext("AppContext.xml");

在应用程序中只应使用一次。

我只发现了两种需要getBean()的情况:

其他人已经提到在main()中使用getBean()为独立程序获取“主”bean。

我使用getBean()的另一种情况是交互用户配置为特定情况确定bean组成。因此,例如,引导系统的一部分使用带scope='prototype' bean定义的getBean()循环遍历数据库表,然后设置其他属性。据推测,有一种调整数据库表的UI比试图(重新)编写应用程序上下文XML更友好。

There is another time when using getBean makes sense. If you're reconfiguring a system that already exists, where the dependencies are not explicitly called out in spring context files. You can start the process by putting in calls to getBean, so that you don't have to wire it all up at once. This way you can slowly build up your spring configuration putting each piece in place over time and getting the bits lined up properly. The calls to getBean will eventually be replaced, but as you understand the structure of the code, or lack there of, you can start the process of wiring more and more beans and using fewer and fewer calls to getBean.