每天一个SpringBoot注解之@Component和@ComponentScan

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<bean class = >。
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