为什么要 starter ？

Spring Boot Starter 的 POM 文件中，引入了各种 jar 文件，当你想拥有某个功能的时候，引入 starter 即可。

比如 常见的 starter 有 spring-boot-starter-web、spring-boot-starter-test、spring-boot-starter-jdbc，将这些 starter 添加到 POM 文件中，就拥有了对应了能力。

自定义 starter

样例：自定义一个 starter，通过配置文件，输出姓名和年龄

添加依赖

<?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>2.5.3</version><relativePath/> <!-- lookup parent from repository --></parent><groupId>com.example</groupId><artifactId>CustomSpringBootStarter</artifactId><version>0.0.1-SNAPSHOT</version><name>CustomSpringBootStarter</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-autoconfigure</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-configuration-processor</artifactId><optional>true</optional></dependency></dependencies></project>

新建服务类

用于调用测试

public class TestService {private String name;private int age;public TestService(String name, int age) {this.name = name;this.age = age;}public void test(){System.out.println("Hello " + name + ", your age is " + age);}
}

新建属性配置类

读取配置文件

@ConfigurationProperties(prefix = "start")
public class TestProperties {private String name;private int age;public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}
}

编写自动配置类

@Configuration
@ConditionalOnClass(TestService.class)
@EnableConfigurationProperties(TestProperties.class)
public class TestServiceConfiguration {@Autowiredprivate TestProperties testProperties;@Bean@ConditionalOnMissingBeanpublic TestService testService(){return new TestService(testProperties.getName(),testProperties.getAge());}
}

添加配置管理

starter 配置分为被动生效和主动生效两种场景，被动生效就是当它被添加到依赖的项目中时，自动被装载到 Spring 容器中；而主动生效则需要主动声明 starter 才会将 starter 里面声明的 Bean 添加到 Spring 容器中。

• 被动生效，依赖时就生效
  在 resource/META-INF/ 目录下面，新建 spring.factories 文件

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\com.example.customspringbootstarter.configuration.TestServiceConfiguration

• 主动生效

自定义注解，通过 @Import 将配置类引入进来。

测试时需要在启动类上加上该注解才能生效

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(TestServiceConfiguration.class)
public @interface EnableTest {
}

测试验证 starter

仅验证 starter 被动生效场景

• 新建 Spring Boot 项目，添加上面 starter 依赖

  <dependency><groupId>com.example</groupId><artifactId>CustomSpringBootStarter</artifactId><version>0.0.1-SNAPSHOT</version>
  </dependency>
  

• application.properties 配置

  start.age = 18
  start.name=Answer
  

• 运行 Main 函数进行测试输出

@SpringBootApplication
public class CutomerSpringBootStarterTestApplication implements CommandLineRunner {public static void main(String[] args) {SpringApplication.run(CutomerSpringBootStarterTestApplication.class, args);}@Autowiredprivate TestService testService;@Overridepublic void run(String... args) throws Exception {testService.test();}
}输出:.   ____          _            __ _ _/\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \\\/  ___)| |_)| | | | | || (_| |  ) ) ) )'  |____| .__|_| |_|_| |_\__, | / / / /=========|_|==============|___/=/_/_/_/:: Spring Boot ::                (v2.5.3)2021-08-02 13:43:04.480  INFO 14400 --- [           main] .CutomerSpringBootStarterTestApplication : Starting CutomerSpringBootStarterTestApplication using Java 1.8.0_211 on Answer with PID 14400 (C:\Users\answer\Documents\CutomerSpringBootStarterTest\target\classes started by answer in C:\Users\answer\Documents\CutomerSpringBootStarterTest)
2021-08-02 13:43:04.483  INFO 14400 --- [           main] .CutomerSpringBootStarterTestApplication : No active profile set, falling back to default profiles: default
2021-08-02 13:43:05.115  INFO 14400 --- [           main] .CutomerSpringBootStarterTestApplication : Started CutomerSpringBootStarterTestApplication in 1.362 seconds (JVM running for 3.115)
Hello Answer, your age is 18

总结

starter 主动生效与被动生效：若在 META-INF/spring.factories 中指定了配置类，那么 starter 将自动被装载到 Spring 容器中；否则需要通过 @Import 方式将配置类装载到 Spring 容器中。

命名规范：自己写的 starter 命名规则建议为 {name}-spring-boot-starter，Spring 官方 starter 的命名通常为 spring-boot-starter-{name}

自动装配流程：Spring Boot 扫描依赖包中的 spring.factories 文件，加载 EnableAutoConfiguration 指定的配置类，将配置类里面的 Bean 加载到 Spring 容器中。