今日立春，SpringBoot！ 简单springboot项目搭建开始。

准备新建springboot项目

jar 文件类型，jdk版本为8，启用lombok和开发测试工具，

生成的pom文件里，增加maven的版本信息否则pom第一行报错

<properties>
        <java.version>1.8</java.version>
         <maven-jar-plugin.version>3.1.1</maven-jar-plugin.version>   
    </properties>

开始：

1，新建 domain 类 User，

// 安装lombok后，启用lombok注解
@Data
@AllArgsConstructor
@NoArgsConstructor
@ToString
public class User {

    private int id;
    private String name;
    private String address;
    
}

2，新建config类，并且用@Configuration，标识为一个配置类。且实例化一个User

@Configuration
public class UserConfig {

    @Bean
    public User user() {
        return new User(1,"xiaoming","beijing");
    }
}

3. 运行测试类

@SpringBootTest
class DemoApplicationTests {

    @Autowired
    private User user;
    
    @Test
    void contextLoads() {
        System.out.println(user);
    }

}

4、结果

2021-02-03 09:59:20.443  INFO 5644 --- [           main] cn.taotao.DemoApplicationTests           : Started DemoApplicationTests in 2.97 seconds (JVM running for 5.125)
User(id=1, name=xiaoming, address=beijing)
2021-02-03 09:59:20.829  INFO 5644 --- [extShutdownHook] o.s.s.concurrent.ThreadPoolTaskExecutor  : Shutting down ExecutorService 'applicationTaskExecutor'

 5、报错信息，

2021-02-03 11:57:09.301 ERROR 9872 --- [           main] o.s.test.context.TestContextManager      : Caught exception while allowing TestExecutionListener [org.springframework.boot.test.autoconfigure.SpringBootDependencyInjectionTestExecutionListener@4de4b452] to prepare test instance [com.example.demo.DemoApplicationTests@2d746ce4]

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'com.example.demo.DemoApplicationTests': Unsatisfied dependency expressed through field 'user'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.example.domain.User' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:643) ~[spring-beans-5.3.2.jar:5.3.2]

提示：expected at least 1 bean which qualifies as autowire candidate. 至少有一个bean被实例化

无法实例化bean，原因往往目录结构不对，导致不自动扫描

自动扫描的默认配置是，主程序application入口，必须在各个包之上，各个子包不是继承自入口包，需要在application的注解文件中加入scanBasePackages的注解

即使是Test的测试程序，也必须在application的入口处，加入scanbasepackages的注解

package com.example.demo;   //这个入口在demo包下，但是配置文件和实体bean在example下

//指明scan哪个父包，否则不会实例化差异包下的bean，甚至连Test也运行不正常
@SpringBootApplication(scanBasePackages = "com.example")
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

}

或者，用@ComponentScan 注解，扫描父包，默认扫描执行入口包  及其子包。

@SpringBootApplication
@ComponentScan(basePackages = "com.example")
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

}

另外还有常用的注解；

@Import (value ={configA.class,configB.class})  ，引入其他的配置文件

@Qulifier ("user1")  合格者，当注入的bean不是一个的时候，用来标识唯一。