前面的博客演示了Spring一些注解的使用，实验过程中遇到了一个问题：Spring容器重复创建示例并注入容器。前面一篇博客中也提出了猜想：@SpringBootApplication注解使容器自动扫描当前启动类下子包内类和平行类所有注解，并创建相应的Bean再注入容器。这里的错误的造成是因为在创建maven项目时，导入了Spring boot模块而并非Spring context模块。

在这里需要澄清！！！

现在通过做一个实验来澄清这个错误。

• 当前maven配置文件：

<?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 https://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>2.2.5.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.michael.annotation</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>demo</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

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

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    </dependencies>

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

• 当前项目目录结构
  

*这里我们并不通过加载配置类来获取应用（容器）上下文，而是直接获取SpringApplication.run(DemoApplication.class, args)方法返回的上下文实例。

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ApplicationContext;

import static java.lang.System.out;
@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        ApplicationContext applicationContext = SpringApplication.run(DemoApplication.class, args);
        for(String name : applicationContext.getBeanDefinitionNames()){
            out.println(name);
        }
    }
}

• 与启动类平行并被@Service注解的Test类

import org.springframework.stereotype.Service;

@Service
public class Test {
}

• 在子包下被@Configuration注解的配置类

import com.michael.annotation.demo.POJO.Person;
import org.springframework.context.annotation.*;

@Configuration
public class MyConfig {

    @Bean
    public Person person(){
        return new Person("Michael", 19);
    }

}

输出：

org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.annotation.internalCommonAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory
demoApplication
org.springframework.boot.autoconfigure.internalCachingMetadataReaderFactory
test
myConfig
personService
person
org.springframework.boot.autoconfigure.AutoConfigurationPackages
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration
propertySourcesPlaceholderConfigurer
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration
mbeanExporter
objectNamingStrategy
mbeanServer
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration
springApplicationAdminRegistrar
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration$ClassProxyingConfiguration
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration
org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor
org.springframework.boot.context.internalConfigurationPropertiesBinderFactory
org.springframework.boot.context.internalConfigurationPropertiesBinder
org.springframework.boot.context.properties.ConfigurationPropertiesBeanDefinitionValidator
org.springframework.boot.context.properties.ConfigurationBeanFactoryMetadata
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration
spring.info-org.springframework.boot.autoconfigure.info.ProjectInfoProperties
org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration
taskExecutorBuilder
applicationTaskExecutor
spring.task.execution-org.springframework.boot.autoconfigure.task.TaskExecutionProperties
org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration
taskSchedulerBuilder
spring.task.scheduling-org.springframework.boot.autoconfigure.task.TaskSchedulingProperties
org.springframework.aop.config.internalAutoProxyCreator

• 从输出我们可以看到被@Service注解的平行类的实例和子包下符合注解的实例都被注入到了容器里。

*现在我们修改一下maven文件

<?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 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.michael.annotation</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>demo</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>5.2.4.RELEASE</version>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
    </dependencies>

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

• 修改启动类，通过加载配置类获取容器上下文示例

import com.michael.annotation.demo.configuration.MyConfig;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

import static java.lang.System.out;
public class DemoApplication {
    public static void main(String[] args) {
        ApplicationContext applicationContext = new AnnotationConfigApplicationContext(MyConfig.class);
        for(String name : applicationContext.getBeanDefinitionNames()){
            out.println(name);
        }
    }
}

注意，这里我们把@SpringBootApplication注解删除了，其他不变。

输出：

org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.annotation.internalCommonAnnotationProcessor
org.springframework.context.event.internalEventListenerProcessor
org.springframework.context.event.internalEventListenerFactory
myConfig
person

通过观察输出，可以发现除配置文件外被注解类的实例没有被创建并注入容器中。

• 总结：Springboot项目的最大的优点是方便了开发人员不用去做更多的配置；恰好这也是其缺点，如果使用者对其原理不够理解往往会囫囵吞枣。