Spring在没有@Autowired注解的情况下将依赖项注入构造函数中 [英] Spring injects dependencies in constructor without @Autowired annotation

问题描述

我正在尝试使用此官方Spring的示例教程，其中有一个依赖于此代码:
https://github.com/spring-guides/gs-async -method/tree/master/complete

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2个问题:

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

1. 服务器启动时，如果我在此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使用配置的@Service bean提供.但是，构造函数上没有@Autowired批注，那么该构造函数在世界范围内如何以正确的依赖关系进行调用?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.

这是 Spring Boot 的自动假设吗?
Spring是否看到私有字段+构造函数参数"，并且假定它应该寻找合适的bean?
是 Spring Framework 还是 Spring boot ?
我想念什么吗?

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.此类(AppRunner)为何没有默认构造函数? Spring如何知道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，则@Autowired注解可以省略，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:您需要默认构造函数，如果您有多个构造函数，则需要带有@Autowired批注的构造函数，或者您的类中只有一个构造函数，带有或不带有@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 <有关更多信息，请参见Spring官方文档中的c3> 章节.

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