Spring Boot 动手写一个 Start

我们在使用SpringBoot 项目时，引入一个springboot start依赖，只需要很少的代码，或者不用任何代码就能直接使用默认配置，再也不用那些繁琐的配置了，感觉特别神奇。我们自己也动手写一个start.

一、新建一个 Start 的 Maven 项目

1. pom 文件如下

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-dependencies</artifactId>
            <version>2.1.0.RELEASE</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-autoconfigure</artifactId>
        <scope>compile</scope>
    </dependency>

    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>1.18.6</version>
        <optional>true</optional>
        <scope>provided</scope>
    </dependency>

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

</dependencies>

• spring-boot-autoconfigure springboot 自动配置的核心依赖
• spring-boot-starter-test 测试包
• lombok 省去 getter/setter 等简化代码

1. 演示代码

DemoService:

public interface DemoService {

    String getMessage();

    Integer getCode();
}

DemoServiceImpl:

public class DemoServiceImpl implements DemoService {

    @Override
    public String getMessage() {
        return "Hello!";
    }

    @Override
    public Integer getCode() {
        return 123;
    }
}

DemoAutoConfiguration:

@Configuration
public class DemoAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean(DemoService.class)
    public DemoService demoService() {
        return new DemoServiceImpl();
    }
}

• @Configuration 标注该类为一个配置类
• ConditionalOnMissingBean(DemoService.class) 条件注解

1. 让springboot 识别自动自动配置的代码

需要在resources下新建文件META-INF/spring.factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.jiuxian.DemoAutoConfiguration

SpringBoot 中的注解 @EnableAutoConfiguration 在项目启动的时候会通过 SpringFactoriesLoader.loadFactoryNames 方法获取 spring.factories 文件下的配置类

1. 测试类

• 首先需要再包下建 Application run 的main 方法

@SpringBootApplication
public class StartDemoApplication {

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

• 测试类

@RunWith(SpringRunner.class)
@SpringBootTest
public class StartDemoApplicationTests {

    @Resource
    private DemoService demoService;

    @Test
    public void test() {
        String message = demoService.getMessage();
        System.out.println(message);
        Assert.assertEquals("Hello!", message);

        Integer code = demoService.getCode();
        System.out.println(code);
        Assert.assertEquals(123, (int) code);
    }
}

二、新建 springboot 项目引入刚写的start项目

1. service

@Service
public class TestService {

    @Resource
    private DemoService demoService;

    public void message() {
        System.out.println("code:" + demoService.getCode());
        System.out.println("message:" + demoService.getMessage());
    }
}

1. 测试

    @Resource
    private TestService testService;

    @Test
    public void test() {
        testService.message();
    }

结果：

code:123
message:Hello!

1. 重写DemoService方法

@Service
public class DemoServiceImpl implements DemoService {

    @Override
    public String getMessage() {
        return "Hello!";
    }

    @Override
    public Integer getCode() {
        return 123;
    }
}

1. 测试结果

code:123
message:Hello!

之所以这样的结果，是因为在start项目中的DemoService 实现类中有一个 @ConditionalOnMissingBean(DemoService.class) 的注解，如果不存在则使用默认的

三、Demo 源码

GitHub源码地址