每天一个SpringBoot注解之@Component和@ComponentScan

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每天一个SpringBoot注解之@Component和@ComponentScan

2024-07-17 01:40| 来源: 网络整理| 查看: 265

一句话，@Component实现的就是bean的注入，实例化到spring容器里，相当于SSM配置文件中的。打开idea，我们可以看到，常见的诸如@Controller，@Service，@Repository三个注解，里面的申明都加入了@Component注解和@Component注解配套的，就是@ComponentScan，我们光标记了不行，还需要一个东西来扫描我们加上@Component注解的类，我们要让spring知道哪些类，是需要以bean的形式注入到spring容器里。

常见用法就是在类上直接加上@ComponentScan，后面的参数就是我们要扫描的包名 @ComponentScan(basePackages = "com.example.demo.configuration")

下面举一个简单的例子：我们新建一个bean对象，加上@Component注解

@Component public class TestBean { private String username; private String url; private String password; public void sayHello() { System.out.println("TestBean sayHello..."); } public String toString() { return "username:" + this.username + ",url:" + this.url + ",password:" + this.password; } public void start() { System.out.println("TestBean 初始化。。。"); } public void cleanUp() { System.out.println("TestBean 销毁。。。"); } }

然后我们在项目自带的测试类里面去测试一下（1）当我不加@ComponentScan的时候代码

public class ConfigurationTest { public ConfigurationTest() { System.out.println("TestConfiguration容器启动初始化。。。"); } public static void main(String[] args) { ApplicationContext context = new AnnotationConfigApplicationContext(ConfigurationTest.class); TestBean testBean = context.getBean(TestBean.class); testBean.sayHello(); } }

运行后可以看到控制台报错了，因为我们没有加扫描，spring找不到这个bean对象

16:10:40.250 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'configurationTest' TestConfiguration容器启动初始化。。。 Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.example.demo.configuration.TestBean' available at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:351) at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:342) at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1177) at com.example.demo.configuration.ConfigurationTest.main(ConfigurationTest.java:18) Process finished with exit code 1

（2）加上@ComponentScan的时候代码

@ComponentScan(basePackages = "com.example.demo.configuration") public class ConfigurationTest { public ConfigurationTest() { System.out.println("TestConfiguration容器启动初始化。。。"); } public static void main(String[] args) { ApplicationContext context = new AnnotationConfigApplicationContext(ConfigurationTest.class); TestBean testBean = context.getBean(TestBean.class); testBean.sayHello(); } }

运行看看呢

16:14:03.290 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'configurationTest' TestConfiguration容器启动初始化。。。 16:14:03.296 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'testBean' TestBean sayHello... Process finished with exit code 0

这时候我们可以看到，正常注入bean，并且方法调用正常

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每天一个SpringBoot注解之@Component和@ComponentScan

每天一个SpringBoot注解之@Component和@ComponentScan

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