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."
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。
原因之一是可测试性。假设你有这样一个类:
interface HttpLoader {
String load(String url);
}
interface StringOutput {
void print(String txt);
}
@Component
class MyBean {
@Autowired
MyBean(HttpLoader loader, StringOutput out) {
out.print(loader.load("http://stackoverflow.com"));
}
}
如何测试这个bean?例如:
class MyBeanTest {
public void creatingMyBean_writesStackoverflowPageToOutput() {
// setup
String stackOverflowHtml = "dummy";
StringBuilder result = new StringBuilder();
// execution
new MyBean(Collections.singletonMap("https://stackoverflow.com", stackOverflowHtml)::get, result::append);
// evaluation
assertEquals(result.toString(), stackOverflowHtml);
}
}
容易,对吧?
当您仍然依赖于Spring(由于注释)时，您可以在不更改任何代码(只更改注释定义)的情况下删除对Spring的依赖，并且测试开发人员不需要了解Spring的工作原理(也许他应该知道，但是它允许将代码与Spring的工作分开检查和测试)。
在使用ApplicationContext时仍然可以做同样的事情。但是你需要模拟ApplicationContext，这是一个巨大的接口。你要么需要一个虚拟的实现，要么你可以使用一个mock框架，比如Mockito:
@Component
class MyBean {
@Autowired
MyBean(ApplicationContext context) {
HttpLoader loader = context.getBean(HttpLoader.class);
StringOutput out = context.getBean(StringOutput.class);
out.print(loader.load("http://stackoverflow.com"));
}
}
class MyBeanTest {
public void creatingMyBean_writesStackoverflowPageToOutput() {
// setup
String stackOverflowHtml = "dummy";
StringBuilder result = new StringBuilder();
ApplicationContext context = Mockito.mock(ApplicationContext.class);
Mockito.when(context.getBean(HttpLoader.class))
.thenReturn(Collections.singletonMap("https://stackoverflow.com", stackOverflowHtml)::get);
Mockito.when(context.getBean(StringOutput.class)).thenReturn(result::append);
// execution
new MyBean(context);
// evaluation
assertEquals(result.toString(), stackOverflowHtml);
}
}
这是很有可能的，但我认为大多数人会同意第一种选择更优雅，使测试更简单。
唯一真正有问题的选项是这个:
@Component
class MyBean {
@Autowired
MyBean(StringOutput out) {
out.print(new HttpLoader().load("http://stackoverflow.com"));
}
}
测试这个需要付出巨大的努力，否则您的bean将在每次测试时尝试连接到stackoverflow。一旦出现网络故障(或者stackoverflow的管理员由于访问速率过高而阻止了您)，您的测试就会随机失败。
因此，作为结论，我不会说直接使用ApplicationContext是自动错误的，应该不惜一切代价避免。然而，如果有更好的选择(大多数情况下都有)，那么就使用更好的选择。
