Java Spring MVC - @Autowired 如何工作？
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Spring MVC - How does @Autowired work?
提问by COBOL
I am working on a web application using Java and Spring. I'm new at Spring, so as an opportunity to learn, I was basically given an existing application and was told to extend it. I am having trouble understanding how the @Autowired work. I understand the high-level concept of dependency injection, but I want to know how the @Autowired annotation knows which concrete implementation of an interface to inject?
我正在使用 Java 和 Spring 开发 Web 应用程序。我是 Spring 的新手，所以作为学习的机会，我基本上得到了一个现有的应用程序，并被告知要扩展它。我无法理解@Autowired 的工作原理。我理解依赖注入的高级概念，但我想知道@Autowired 注释如何知道要注入的接口的具体实现？
To put my question into context, I will explain the problem I am having:
为了将我的问题放在上下文中，我将解释我遇到的问题：
I have an interfacecalled PostDao, and a classcalled PostDaoImplwhich implements PostDao. I then have another classcalled PostDaoPublicImplwhich extends PostDaoImpl. These classes exist in my persistence layer.
I then have an interfacecalled PostService, and a classcalled called PostServiceImplwhich implement PostService. I then have another classcalled PostServicePublicImplwhich extends PostServiceImpl. These classes exist in my services layer.
我有一个名为PostDao的接口和一个名为PostDaoImpl的类，它实现了PostDao。然后我有另一个名为PostDaoPublicImpl 的类，它扩展了PostDaoImpl。这些类存在于我的持久层中。
然后我有一个名为PostService的接口和一个名为PostServiceImpl的类，它实现了PostService。然后我有另一个名为PostServicePublicImpl 的类，它扩展了PostServiceImpl。这些类存在于我的服务层中。
In PostServiceImplthe following line injects the following object:
在PostServiceImpl 中，以下行注入以下对象：
@Autowired private PostDao postDao;
//This injects an object of the class PostDaoImpl
My problem is, in PostServicePublicImplhow do I have the same declaration as above, but have it inject an object of the class PostDaoPublicImpl:
我的问题是，在PostServicePublicImpl 中，我如何具有与上述相同的声明，但让它注入类PostDaoPublicImpl的对象：
@Autowired private PostDao postDao;
//This injects an object of the class PostDaoPublicImpl
I feel as though if I understand how the @Autowired annotation works, then I will be able to solve this problem. Any help and suggestions to solve the problem would be greatly appreciated.
我觉得如果我了解 @Autowired 注释的工作原理，那么我就能够解决这个问题。任何解决问题的帮助和建议将不胜感激。
回答by sanbhat
First of all you need to understand how Autowiring works
首先你需要了解自动装配的工作原理
then in the bean definition, define autowire strategy- let it be byNamefor our case
然后在 bean 定义中，定义自动装配策略-对于我们的情况，让它成为byName
<bean id="postDaoPublicImpl" class="com.yourpackage.PostDaoPublicImpl" autowire="byName" />
that means, if any class has a property with name "postDaoPublicImpl", which is the bean id of above bean, will be automatically injected with the bean instance
这意味着，如果任何类具有名为“postDaoPublicImpl”的属性，即上述 bean 的 bean id，将自动注入 bean 实例
after that in your class PostServicePublicImpldefine property like this
之后在你的班级中PostServicePublicImpl定义这样的属性
class PostServicePublicImpl {
@Autowired private PostDaoPublicImpl postDaoPublicImpl;
//here Spring will auto inject PostDaoPublicImpl implementation, since this property name matches the bean id and (auto wiring strategy is "byName")
}
回答by COBOL
I found an easier solution to my problem. Using the @Qualifier tag I can give a unique name to each implementation of the interface. Then when auto-wiring the object, I just provide a qualifier and the object of the implementation matching the qualifier is injected.
我找到了一个更简单的方法来解决我的问题。使用@Qualifier 标签，我可以为接口的每个实现指定一个唯一的名称。然后在自动连接对象时，我只提供一个限定符，并注入与该限定符匹配的实现对象。
回答by David Ruan
Add this to your applicationContext
将此添加到您的 applicationContext
<bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor"/>
