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。

我在另一个问题的评论中提到了这一点，但控制反转的整个思想是让你的任何类都不知道或关心它们如何获得它们所依赖的对象。这使得您可以随时更改所使用的给定依赖项的实现类型。它还使类易于测试，因为您可以提供依赖关系的模拟实现。最后，它使类更简单，更专注于它们的核心职责。

调用ApplicationContext.getBean()不是反转控制!虽然更改为给定bean名配置的实现仍然很容易，但类现在直接依赖Spring提供该依赖项，不能通过其他方式获得它。您不能只是在测试类中创建自己的模拟实现并将其传递给它自己。这基本上违背了Spring作为依赖注入容器的目的。

当你想说:

MyClass myClass = applicationContext.getBean("myClass");

相反，你应该声明一个方法:

public void setMyClass(MyClass myClass) {
   this.myClass = myClass;
}

然后在构型中

<bean id="myClass" class="MyClass">...</bean>

<bean id="myOtherClass" class="MyOtherClass">
   <property name="myClass" ref="myClass"/>
</bean>

Spring会自动将myClass注入到myOtherClass中。

以这种方式声明所有内容，并在其根源上有如下内容:

<bean id="myApplication" class="MyApplication">
   <property name="myCentralClass" ref="myCentralClass"/>
   <property name="myOtherCentralClass" ref="myOtherCentralClass"/>
</bean>

MyApplication是最核心的类，它至少间接地依赖于程序中的所有其他服务。当引导时，在你的主方法中，你可以调用applicationContext.getBean("myApplication")，但你不需要在其他任何地方调用getBean() !

其思想是依赖于依赖注入(控制反转，IoC)。也就是说，您的组件已经配置了所需的组件。这些依赖关系是注入的(通过构造函数或设置函数)——你不能自己得到。

getbean()要求您在组件中显式地命名bean。相反，通过使用IoC，您的配置可以确定将使用什么组件。

这让你可以轻松地用不同的组件实现重新连接应用程序，或者通过提供模拟变量(例如，模拟DAO，这样你就不会在测试期间碰到数据库)以一种直接的方式配置测试对象。

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.

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。

但是，仍然有一些情况需要使用服务定位器模式。 例如，我有一个控制器bean，这个控制器可能有一些默认的服务bean，这些服务bean可以通过配置注入依赖项。 虽然这个控制器现在或以后还可以调用许多附加的或新的服务，但这些服务需要服务定位器来检索服务bean。