Spring Component Scanning

╰+攻爆jí腚メ 2024-03-25 18:00 14阅读 0赞

## @ComponentScan Without Arguments ##

### In a Spring Application ###

Use the `@ComponentScan` annotation along with the `@Configuration` annotation to specify the packages that we want to be scanned.

> **`@ComponentScan` without arguments tells Spring to scan the current package and all of its sub-packages.**

package com.componentscan.springapp.animals;
    @Component
    public class Cat {
        }
    
    package com.componentscan.springapp.animals;
    @Component
    public class Dog {
        }
    
    package com.componentscan.springapp.flowers;
    @Component
    public class Rose {
        }
    
    # ------------------------------------------------
    package com.componentscan.springapp
    @Configuration
    @ComponentScan
    public class SpringComponentScanApp {
        
        private static ApplicationContext applicationContext;
        @Bean
        public ExampleBean exampleBean() {
        
            return new ExampleBean();
        }
        public static void main(String[] args) {
        
            applicationContext = new AnnotationConfigApplicationContext(SpringComponentScanApp.class);
            for (String beanName : applicationContext.getBeanDefinitionNames()) {
        
                System.out.println(beanName);
            }
        }
    }
    
    springComponentScanApp
    cat
    dog
    rose
    exampleBean

--------------------

### In a Spring Boot Application ###

SpringBoot use `@SpringBootApplication` annotation, and it’s a combination of three annotations:

@Configuration
    @EnableAutoConfiguration
    @ComponentScan

package com.componentscan.springbootapp;
    
    @SpringBootApplication
    public class SpringBootComponentScanApp {
        
        private static ApplicationContext applicationContext;
        @Bean
        public ExampleBean exampleBean() {
        
            return new ExampleBean();
        }
        public static void main(String[] args) {
        
            applicationContext = SpringApplication.run(SpringBootComponentScanApp.class, args);
            checkBeansPresence( "cat", "dog", "rose", "exampleBean", "springBootComponentScanApp");
        }
        private static void checkBeansPresence(String... beans) {
        
            for (String beanName : beans) {
        
                System.out.println("Is " + beanName + " in ApplicationContext: " + applicationContext.containsBean(beanName));
            }
        }
    }
    
    Is cat in ApplicationContext: true
    Is dog in ApplicationContext: true
    Is rose in ApplicationContext: true
    Is exampleBean in ApplicationContext: true
    Is springBootComponentScanApp in ApplicationContext: true

> 这里检验存在而不是全部输出, is that the output would be too large.  
> **This is because of the implicit `@EnableAutoConfiguration` annotation, which makes Spring Boot create many beans automatically, relying on the dependencies in pom.xml file.**

--------------------

## @ComponentScan With Arguments ##

### In a Spring Application ###

package com.componentscan.springapp.animals;
    @Component
    public class Cat {
        }
    
    package com.componentscan.springapp.animals;
    @Component
    public class Dog {
        }
    
    package com.componentscan.springapp.flowers;
    @Component
    public class Rose {
        }
    
    @ComponentScan(basePackages = "com.componentscan.springapp.animals")
    @Configuration
    public class SpringComponentScanApp {
        
        private static ApplicationContext applicationContext;
        @Bean
        public ExampleBean exampleBean() {
        
            return new ExampleBean();
        }
        public static void main(String[] args) {
        
            applicationContext = new AnnotationConfigApplicationContext(SpringComponentScanApp.class);
            for (String beanName : applicationContext.getBeanDefinitionNames()) {
        
                System.out.println(beanName);
            }
        }
    }
    
    springComponentScanApp
    exampleBean
    cat
    dog

### In a Spring Boot Application ###

@SpringBootApplication
    @ComponentScan(basePackages = "com.componentscan.springbootapp.animals")

--------------------

## @ComponentScan with Multiple Packages ##

@ComponentScan(basePackages = {
        "com.componentscan.springapp.animals", "com.componentscan.springapp.flowers"})
    
    # since spring 4.1.1
    @ComponentScan(basePackages = "com.componentscan.springapp.animals;com.baeldung.componentscan.springapp.flowers")
    @ComponentScan(basePackages = "com.componentscan.springapp.animals,com.baeldung.componentscan.springapp.flowers")
    @ComponentScan(basePackages = "com.baeldung.componentscan.springapp.animals com.componentscan.springapp.flowers")

--------------------

## @ComponentScan with Exclusions/IncludeFilters ##

**参考：[Spring @ComponentScan – Filter Types][Spring _ComponentScan _ Filter Types]**

--------------------

**参考：[Spring Component Scanning][]**

[Spring _ComponentScan _ Filter Types]: https://blog.csdn.net/weixin_37646636/article/details/129601176
[Spring Component Scanning]: https://www.baeldung.com/spring-component-scanning