spring-boot-starter简介 解决@Configuration中@value无值问题

spring-boot 在配置上相比spring要简单许多, 其核心在于spring-boot-starter, 在使用spring-boot来搭建一个项目时, 只需要引入官方提供的starter, 就可以直接使用, 免去了各种配置, 原因在于spring-boot的自动发现,比如当classpath下面有tomcat-embedded.jar 时,对应的bean就会被加载.下面会介绍如何自己写一个简单的starter,并在自己的工程中使用这个starter
新建一个maven project结构如下:

其中GreetorService是我们要提供给外部使用的bean, GreetorProperties包含了这个bean需要的信息, GreetorAutoConfiguration负责提供这个bean
下面依次看一下各个class

public class GreetorService {

    private GreetorProperties properties;

    public GreetorService(GreetorProperties properties) {
        this.properties = properties;
    }

    public String greet() {
        return "greet by" + properties.getName() + " msg " + properties.getMsg();
    }

}

GreetorService , 一个简单的service

@ConfigurationProperties(prefix="greetor")
public class GreetorProperties {

    private String name;
    
    private String msg;
    
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }
    
}

properties bean, 会从application.properties 或 yml中把对应的properties映射到bean中

@ConditionalOnClass(GreetorService.class)
@EnableConfigurationProperties(GreetorProperties.class)
@Configuration
public class GreetorAutoConfiguration {
    
    @Autowired
    private GreetorProperties greetorProperties;
    
    @Bean
    @ConditionalOnMissingBean(GreetorService.class)
    public GreetorService greetorService() {
        return new GreetorService(greetorProperties);
    };
    
}

最为重要的class, @ConditionalOnClass 只有当特定的class在classpath上时, BeanDefination才会被注册, @ConditionalOnMissingBean 同理, @EnableConfigurationProperties 这个用于激活PropertyBean
在META-INF中新建一个名为spring.factories的文件 (下文介绍其作用),内容为

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.iplay.springtest.configuration.GreetorAutoConfiguration

最后看一下依赖

<?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 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>2.1.6.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.iplay.springtest</groupId>
    <artifactId>greeter-starter</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>starter-greetor</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-actuator-autoconfigure</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-configuration-processor</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>
</project>

之后我们新建另一个maven project, 并依赖我们刚才建立的starter
测试类 如下

@SpringBootApplication
public class StarterGreetorUsageApplication implements CommandLineRunner {

    @Autowired
    private GreetorService greetorService;

    @Override
    public void run(String... args) throws Exception {
        System.out.println(greetorService.greet());
    }

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

}

在application.properties中增加如下内容

greetor.name=test
greetor.msg=hello

运行application 可以看到以下信息
greet bytest msg hello
下面我们看一下为什么GreetorAutoConfiguration会得到处理
@SpringBootApplication 包含了@EnableAutoConfiguration, @EnableAutoConfiguration包含了@Import(AutoConfigurationImportSelector.class)
@Import可以接受一个ImportSelector
而AutoAutoConfigurationImportSelector是ImportSelector的实现
其中核心代码为


决定了哪些configuration会被import

这里调用了getCandidateConfigurations

SpringFactoriesLoader会从META-INF/spring.factories中读取对应的factory, 所以当我们启动项目时,会检查META-INF/spring.factories key为org.springframework.boot.autoconfigure.EnableAutoConfiguration的值.
在spring-boot-autoconfigure这个包中也包含了spring.factories, 部分值为下

对应了各个starter的autoconfiguration, 这些autoconfiguration使我们只需引入各个starter就能够快速搭建一个spring-boot app