Spring在没有@Autowired注解的构造函数中注入依赖 [英] Spring injects dependencies in constructor without @Autowired annotation

问题描述

我正在尝试使用这个官方 Spring 教程 中的示例，并且有一个对这段代码的依赖:
https://github.com/spring-guides/gs-async-方法/树/主/完成

I'm experimenting with examples from this official Spring tutorials and there is a dependency on this code:
https://github.com/spring-guides/gs-async-method/tree/master/complete

如果您查看 AppRunner.java 类上的代码，我有两个问题:

If you look at the code on AppRunner.java class, I have 2 questions:

1. 当服务器启动时，如果我在这个类的构造函数中放置一个断点，就像在构造函数中一样，GitHubLookupService 由 spring 提供，使用 @Service bean那是配置的.但是，构造函数上没有 @Autowired 注释，那么这个构造函数到底是如何使用正确的依赖项调用的呢?它应该是 null.

1. When server is starting, if I put a breakpoint in this class's constructor, seems like in the constructor, the GitHubLookupService is provided by spring, using the @Service bean that was configured. BUT, there was no @Autowired annotation on the constructor, so how in the world this constructor get called with the right dependency? It was supposed to be null.

是否是 Spring Boot 的自动假设?
Spring 是否看到私有字段 + 构造函数参数，并假定它应该寻找合适的 bean?
是 Spring 框架 还是 Spring 启动?
我错过了什么吗?

Is it an automatic assumption of Spring Boot?
Does Spring see "private field + constructor argument, and it assumes it should look for an appropriate bean?
Is it Spring Framework or Spring boot?
Am I missing something?

1. 我记得，必须为bean/服务等提供默认构造函数.为什么这个类(AppRunner)没有默认构造函数?Spring 如何知道它应该使用参数运行构造函数?是因为它是唯一的构造函数吗?

1. As I remember, it was mendatory to provide default constructor to beans / service etc. How come this class (AppRunner) doesn't have a default constructor? How does Spring knows that it should run the constructor with the argument? Is it because it is the only constructor?

推荐答案

从 Spring 4.3 开始，如果配置为 Spring bean 的类只有一个构造函数，@Autowired 注释可以省略，Spring 将使用该构造函数并注入所有必要的依赖项.

Starting with Spring 4.3, if a class, which is configured as a Spring bean, has only one constructor, the @Autowired annotation can be omitted and Spring will use that constructor and inject all necessary dependencies.

关于默认构造函数:当你有多个构造函数时，你要么需要默认构造函数，一个带有 @Autowired 注释的构造函数，或者你的类中只有一个构造函数或者没有 @Autowired 注释.

Regarding the default constructor: You either need the default constructor, a constructor with the @Autowired annotation when you have multiple constructors, or only one constructor in your class with or without the @Autowired annotation.

阅读@Autowired 来自官方 Spring 文档的章节以获取更多信息.

Read the @Autowired chapter from the official Spring documentation for more information.

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