component

Spring Component annotation is used to denote a class as Component. It means that Spring framework will autodetect these classes for dependency injection when annotation-based configuration and classpath scanning is used.

Spring Component注释用于将一个类表示为Component。 这意味着当使用基于注释的配置和类路径扫描时， Spring框架将自动检测这些类以进行依赖项注入 。

弹簧组件 (Spring Component)

In layman terms, a Component is responsible for some operations. Spring framework provides three other specific annotations to be used when marking a class as Component.

用外行术语来说，组件负责某些操作。 当将一个类标记为Component时，Spring框架提供了其他三个特定的注释。

1. Service: Denotes that the class provides some services. Our utility classes can be marked as Service classes.Service ：表示该类提供了一些服务。 我们的实用程序类可以标记为服务类。
2. Repository: This annotation indicates that the class deals with CRUD operations, usually it’s used with DAO implementations that deal with database tables.Repository ：此批注表明该类处理CRUD操作，通常与用于处理数据库表的DAO实现一起使用。
3. Controller: Mostly used with web applications or REST web services to specify that the class is a front controller and responsible to handle user request and return appropriate response.Controller ：通常与Web应用程序或REST Web服务一起使用，以指定该类为前端控制器，并负责处理用户请求并返回适当的响应。

Note that all these four annotations are in package org.springframework.stereotype and part of spring-context jar.

请注意，所有这四个注释都在org.springframework.stereotype包中，并且是spring-context jar的一部分。

Most of the time our component classes will fall under one of its three specialized annotations, so you may not use @Component annotation a lot.

大多数时候，我们的组件类将属于其三个专用注释之一，因此您可能不会大量使用@Component注释。

弹簧组件示例 (Spring Component Example)

Let’s create a very simple Spring maven application to showcase the use of Spring Component annotation and how Spring autodetects it with annotation-based configuration and classpath scanning.

让我们创建一个非常简单的Spring Maven应用程序，以展示Spring Component注释的用法以及Spring如何通过基于注释的配置和类路径扫描自动检测到它。

Create a maven project and add following spring core dependency.

创建一个maven项目并添加以下spring核心依赖项。

<dependency><groupId>org.springframework</groupId><artifactId>spring-context</artifactId><version>5.0.6.RELEASE</version>
</dependency>

That’s all we need to get the spring framework core features.

这就是我们获取Spring框架核心功能所需要的。

Let’s create a simple component class and mark it with @Component annotation.

让我们创建一个简单的组件类，并使用@Component批注对其进行标记。

package com.journaldev.spring;import org.springframework.stereotype.Component;@Component
public class MathComponent {public int add(int x, int y) {return x + y;}
}

Now we can create an annotation based spring context and get the MathComponent bean from it.

现在，我们可以创建一个基于注释的spring上下文，并从中获取MathComponent bean。

package com.journaldev.spring;import org.springframework.context.annotation.AnnotationConfigApplicationContext;public class SpringMainClass {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();context.scan("com.journaldev.spring");context.refresh();MathComponent ms = context.getBean(MathComponent.class);int result = ms.add(1, 2);System.out.println("Addition of 1 and 2 = " + result);context.close();}}

Just run the above class as normal java application and you should get following output in the console.

只需将上述类作为普通的Java应用程序运行，您将在控制台中获得以下输出。

Jun 05, 2018 12:49:26 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@ff5b51f: startup date [Tue Jun 05 12:49:26 IST 2018]; root of context hierarchy
Addition of 1 and 2 = 3
Jun 05, 2018 12:49:26 PM org.springframework.context.support.AbstractApplicationContext doClose
INFO: Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@ff5b51f: startup date [Tue Jun 05 12:49:26 IST 2018]; root of context hierarchy

Did you realized the power of Spring, we didn’t have to do anything to inject our component to spring context.

您是否意识到Spring的强大功能，我们无需执行任何操作即可将组件注入Spring上下文。

Below image shows the directory structure of our Spring Component example project.

下图显示了我们的Spring Component示例项目的目录结构。

We can also specify the component name and then get it from spring context using the same name.

我们还可以指定组件名称，然后使用相同的名称从spring上下文中获取它。

@Component("mc")
public class MathComponent {
}

MathComponent ms = (MathComponent) context.getBean("mc");

Reference: API Doc

参考： API文档

翻译自: https://www.journaldev.com/21429/spring-component

component