@SpringBootApplication 之 @ComponentScan

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原理

@SpringBootApplication 是一个复合注解，包括

• @SpringBootConfiguration
• @EnableAutoConfiguration
• @ComponentScan

Note：

@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })

• excludeFilters：Specifies which types are not eligible for component scanning
• @Filter：Filter annotation
• FilterType.CUSTOM：Filter candidates using a given custom implementation
• classes：Filter specific class, include TypeExcludeFilter, AutoConfigurationExcludeFilter

@ComponentScan 源码

Note：

• 通常自定义一个Spring Bean，一般在类上加上注解 @Component、@Service、@Controller，定义好后，需要使用@ComponentScan 来告诉Spring IoC 容器去哪里装载自定义的Spring Bean
• @SpringBootApplication 包含了@ComponentScan，所以@SpringBootApplication注解所在包及其子包目录下的Spring Bean都会被扫描并装载到Spring IoC容器。
• 如果自定义的一个Spring Bean，不在@SpringBootApplication 注解所在包及其子包目录下，需要手动加上@ComponentScan 注解并指定自定义Spring Bean所在的包。

示例

Project Directory

Maven Dependency

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.12.RELEASE</version>
        <relativePath/>
    </parent>

    <groupId>org.fool</groupId>
    <artifactId>hellospring</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

SRC

MockComponentScan.java

package org.fool.spring.component.scan;

import org.springframework.stereotype.Service;

@Service
public class MockComponentScan {

    public MockComponentScan() {
        System.out.println("MockComponentScan init...");
    }

    @Override
    public String toString() {
        return "MockComponentScan{}";
    }
}

MainTest.java

package org.fool.spring.component.core;

import org.fool.spring.component.scan.MockComponentScan;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.annotation.ComponentScan;

@SpringBootApplication
@ComponentScan("org.fool.spring.component.scan")
public class MainTest {
    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(MainTest.class, args);
        MockComponentScan bean = context.getBean(MockComponentScan.class);
        System.out.println(bean);
    }
}

Note:  MockComponentScan.java 和 打上@SpringBootApplication 注解的MainTest.java 不在同一个包中

@ComponentScan("org.fool.spring.component.scan")

Run

反之如果将 @ComponentScan 注释掉

再次运行MainTest.java，会抛出异常

Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.fool.spring.component.scan.MockComponentScan' 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:1127)
    at org.fool.spring.component.core.MainTest.main(MainTest.java:14)

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