一文了解 Spring Starter 原理

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简介

Spring Starter相当于模块，它能将模块所需的依赖整合起来并对模块内的Bean根据环境（ 条件）进行自动配置。使用者只需要依赖相应功能的Starter，无需做过多的配置和依赖，Spring Boot就能自动扫描并加载相应的模块。

Quick Start

我们以创建一个钉钉机器人 starter 为例

  1. 创建项目，添加必选依赖
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<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.0.6.RELEASE</version>
</parent>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
        <version>2.0.6.RELEASE</version>
    </dependency>
</dependencies>
  1. 创建配置类，用于读取配置信息
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@ConfigurationProperties(prefix = "dingding.chatbot")
@Data
public class DingdingProperties {
    private String secret;
    private String accessToken;
    private String webHook = "https://oapi.dingtalk.com/robot/send?access_token=";
    private Double rate = 1D/6D;
}

@ConfigurationProperties 注解用于设置配置信息的前缀

例如：accessToken 在配置文件中dingding.chatbot.access-token = 123456

  1. 创建自动装配类，创建想要生成的对象并装配到容器中
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@Slf4j
@Configuration
@EnableConfigurationProperties(DingdingProperties.class)
public class DingdingAutoConfiguration {

    @Resource
    private DingdingProperties dingdingProperties;

    @Bean
    @ConditionalOnMissingBean(DingChatBot.class)
    public DingChatBot createDingdingBean() {
        return new DingChatBot(
                dingdingProperties.getSecret(),
                dingdingProperties.getAccessToken(),
                dingdingProperties.getWebHook() + dingdingProperties.getAccessToken(),
                dingdingProperties.getRate());
    }
}
  1. 创建 spring.factories 文件，用于 SpringBoot 扫描

内容如下：

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org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.souche.chatbot.dingding.DingdingAutoConfiguration

spring.factories 放在 resources/META-INF 目录下。
容器启动时，SpringFactoriesLoader 类会扫描该文件，将相应的类实例化装配到容器中。
关键代码如下：

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   //解析成 CLASS 并实例化
public static <T> List<T> loadFactories(Class<T> factoryClass, @Nullable ClassLoader classLoader) {
	//获取 ClassLoader
	ClassLoader classLoaderToUse = classLoader;
	//解析成 CLASS
	List<String> factoryNames = loadFactoryNames(factoryClass, classLoaderToUse);

	List<T> result = new ArrayList<>(factoryNames.size());
	for (String factoryName : factoryNames) {
	    //依次实例化
		result.add(instantiateFactory(factoryName, factoryClass, classLoaderToUse));
	}
	//排序后返回
	AnnotationAwareOrderComparator.sort(result);
	return result;
}

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
	String factoryClassName = factoryClass.getName();
	return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}


private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
    //按 classLoader 保存已经解析出的类
	MultiValueMap<String, String> result = cache.get(classLoader);
	if (result != null) {
		return result;
	}

	try {
	    //读取所有的 "META-INF/spring.factories" 文件
		Enumeration<URL> urls = (classLoader != null ?
				classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
				ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
		result = new LinkedMultiValueMap<>();
		//按行读取 CLASS 绝对路径
		while (urls.hasMoreElements()) {
			URL url = urls.nextElement();
			UrlResource resource = new UrlResource(url);
			//spring.factories 文件 解析成 KEY VALUE 形式
			Properties properties = PropertiesLoaderUtils.loadProperties(resource);
			for (Map.Entry<?, ?> entry : properties.entrySet()) {
				List<String> factoryClassNames = Arrays.asList(
						StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
				result.addAll((String) entry.getKey(), factoryClassNames);
			}
		}
		cache.put(classLoader, result);
		return result;
	}
	catch (IOException ex) {
		throw new IllegalArgumentException("Unable to load factories from location [" +
				FACTORIES_RESOURCE_LOCATION + "]", ex);
	}
}
  1. 添加 spring-boot-configuration-processor 依赖，可以在引用 starter 后自动读取相应的配置文件
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<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-configuration-processor</artifactId>
    <optional>true</optional>
    <version>2.0.6.RELEASE</version>
</dependency>
  1. 使用

1）添加上面的依赖

2）添加配置

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# 钉钉
dingding.chatbot.secret= 1231231123121
dingding.chatbot.access-token = 1231231123121

3）使用demo

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@Autowired
private DingChatBot chatBot;

public void method() {
    try {
        chatBot.sendMsg(errorMsg);
    } catch (RateLimiterException rateLimiterException) {
        log.error(rateLimiterException.getMessage(), rateLimiterException);
    }
}

源码参考