在看本节文章之前，建议大家先去了一下java的SPI机制，因为Spring的Factories就是Spring版本的Java Spi，我在关于java基础系列文章中有详细介绍Java SPI机制。 Spring Factories的最重要的功能就是：可以通过配置文件指定Spring容器加载一些特定的组件。

基本概念

什么是Spring Factories

Spring Factories是一种类似于Java SPI的机制，它在META-INF/spring.factories文件中配置接口的实现类名称，然后在程序中读取这些配置文件并实例化。

引用自博客：Java SPI 主要是应用于厂商自定义组件或插件中。在java.util.ServiceLoader的文档里有比较详细的介绍。简单的总结下java SPI机制的思想：我们系统里抽象的各个模块，往往有很多不同的实现方案，比如日志模块、xml解析模块、jdbc模块等方案。面向的对象的设计里，我们一般推荐模块之间基于接口编程，模块之间不对实现类进行硬编码。一旦代码里涉及具体的实现类，就违反了可拔插的原则，如果需要替换一种实现，就需要修改代码。为了实现在模块装配的时候能不在程序里动态指明，这就需要一种服务发现机制。 Java SPI就是提供这样的一个机制：为某个接口寻找服务实现的机制。有点类似IOC的思想，就是将装配的控制权移到程序之外，在模块化设计中这个机制尤其重要。

为什么要有Spring Factories

Spring Factories机制提供了一种解耦容器注入的方式，帮助外部包（独立于spring-boot项目）注册Bean到spring boot项目容器中。

问题：如果想要被Spring容器管理的Bean的路径不再Spring Boot 项目的扫描路径下，那该怎么办呢？ 普通解法1：在Spring Boot 项目中配置ComponentScan注解的扫描路径，添加需要被扫描的方法。 普通解法2：通过在Spring Boot 项目中添加@EnableAutoConfiguration注解，并自定义@EnableXXXXConfiguration的注解，通过注解中的方法注入Bean。 普通解法3：在Spring Boot项目中xxxx.................。

这些解法主要思想都如下图所示，虽然已经实现了帮助外部包（独立于spring-boot项目）注册Bean到spring boot项目容器中的功能，但是Spring Boot 项目都需要知道外部包的信息，并在其代码中引入外部包的相关逻辑，如果外部包变更的情况下，Spring Boot 项目也需要跟着变更，Spring Boot和外部包实现直接存在紧耦合关系。

Spring Factories机制解法： 在外部包的META-INF/spring.factories文件中添加配置文件，Spring Boot项目会自动扫描这个配置文件，获取外部包的Bean的详细信息。基本思想如下图所示，从图中可以看到，Spring Boot项目中已经不包含外部包的相关逻辑，实现与外部包之间的解耦关系。

Spring Factories机制原理

核心类SpringFactoriesLoader

从上文可知，Spring Factories机制通过META-INF/spring.factories文件获取相关的实现类的配置信息，而SpringFactoriesLoader的功能就是读取META-INF/spring.factories，并加载配置中的类。SpringFactoriesLoader主要有两个方法：loadFactories和loadFactoryNames。

SpringFactoriesLoader官方说明 : General purpose factory loading mechanism for internal use within the framework.

META-INF/spring.factories中定义了实现类和对应的接口之间的关系，其定义格式如下：

example.MyService=example.MyServiceImpl1,example.MyServiceImpl2 其中的example.MyService为接口全称，example.MyServiceImpl1,example.MyServiceImpl2为两个实现类的全称。

官方说明： SpringFactoriesLoader loads and instantiates factories of a given type from META-INF/spring.factories files which may be present in multiple JAR files in the classpath. The file must be in {@link Properties} format, where the key is the fully qualified name of the interface or abstract class, and the value is a comma-separated list of implementation class names. For example: example.MyService=example.MyServiceImpl1,example.MyServiceImpl2 where {@code example.MyService} is the name of the interface, and {@code MyServiceImpl1} and {@code MyServiceImpl2} are two implementations.

loadFactoryNames

用于按接口获取Spring Factories文件中的实现类的全称，其方法定义如下所示，其中参数factoryType指定了需要获取哪个接口的实现类，classLoader用于读取配置文件资源。 public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader)

loadFactories

用于按接口获取Spring Factories文件中的实现类的实例，其方法定义如下所示，其中参数factoryType指定了需要获取哪个接口的实现类，classLoader用于读取配置文件资源和加载类。 public static <T> List<T> loadFactories(Class<T> factoryType, @Nullable ClassLoader classLoader)

Spring Boot Factories

Spring boot中默认使用了很多factories机制，主要包含：

1. ApplicationContextInitializer：用于在spring容器刷新之前初始化Spring ConfigurableApplicationContext的回调接口。
   
2. ApplicationListener：用于处理容器初始化各个阶段的事件。
   
3. AutoConfigurationImportListener：导入配置类的时候，获取类的详细信息（Listener that can be registered with spring.factories to receive details of imported auto-configurations.）。
   
4. AutoConfigurationImportFilter：用于按条件过滤导入的配置类（Filter that can be registered in spring.factories to limit the auto-configuration classes considered. This interface is designed to allow fast removal of auto-configuration classes before their bytecode is even read.）
   
5. EnableAutoConfiguration：指定自动加载的配置类列表（Enable auto-configuration of the Spring Application Context, attempting to guess and configure beans that you are likely to need. Auto-configuration classes are usually applied based on your classpath and what beans you have defined. For example, if you have tomcat-embedded.jar on your classpath you are likely to want a TomcatServletWebServerFactory (unless you have defined your own ServletWebServerFactory bean).
   
6. FailureAnalyzer：在启动时拦截异常并将其转换为易读的消息，并将其包含在FailureAnalysis中。 Spring Boot为应用程序上下文相关异常、JSR-303验证等提供了此类分析器（A FailureAnalyzer is used to analyze a failure and provide diagnostic information that can be displayed to the user.）
   
7. TemplateAvailabilityProvider：模版引擎配置。（Collection of TemplateAvailabilityProvider beans that can be used to check which (if any) templating engine supports a given view. Caches responses unless the spring.template.provider.cache property is set to false.）
   

自定义Spring Factories机制

本节将使用一个例子来展示Spring Factories机制，案例分为两个项目spring-factories-provider和spring-factories-consume，其中spring-factories-consume可以类比于上文中的Spring Boot项目，包含Bean的接口和Bean的调用，spring-factories-provider可以类比于外部包，包含了bean的实现。项目目录结构如下：

以下为各个文件的内容：

1. DemoService
   

public interface DemoService{    void printName();}

2. PrintDemoService
   

@Servicepublic class PrintDemoService{    @PostConstruct    public void printService(){        List<String> serviceNames = SpringFactoriesLoader.loadFactoryNames(DemoService.class,null);        for (String serviceName:serviceNames){            System.out.println(serviceName);        }

        List<DemoService> services = SpringFactoriesLoader.loadFactories(DemoService.class,null);        for (DemoService demoService:services){            demoService.printName();        }    }}

3. DemoServiceImpl1
   

public class DemoServiceImpl1 implements DemoService{    @Override    public void printName(){        System.out.println("This is in demo 1.");    }}

4. DemoServiceImpl2
   

public class DemoServiceImpl2 implements DemoService{    @Override    public void printName(){        System.out.println("This is in demo 2.");    }}

5. FactoriesApplication
   

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

6. spring.factories
   

com.wangzemin.learning.factories.interfac.DemoService=\  com.wangzemin.learning.factories.impl.DemoServiceImpl1,\  com.wangzemin.learning.factories.impl.DemoServiceImpl2

程序的运行结果如下图所示，可以看到，Factories机制已经成功发挥作用。

附录：spring boot 自带factories文件示例

# Initializersorg.springframework.context.ApplicationContextInitializer=\org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

# Application Listenersorg.springframework.context.ApplicationListener=\org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listenersorg.springframework.boot.autoconfigure.AutoConfigurationImportListener=\org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filtersorg.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\org.springframework.boot.autoconfigure.condition.OnBeanCondition,\org.springframework.boot.autoconfigure.condition.OnClassCondition,\org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

# Auto Configureorg.springframework.boot.autoconfigure.EnableAutoConfiguration=\org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\org.springframework.boot.autoconfigure.context.LifecycleAutoConfiguration,\org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.elasticsearch.ReactiveElasticsearchRestClientAutoConfiguration,\org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.r2dbc.R2dbcDataAutoConfiguration,\org.springframework.boot.autoconfigure.data.r2dbc.R2dbcRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.r2dbc.R2dbcTransactionManagerAutoConfiguration,\org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\org.springframework.boot.autoconfigure.elasticsearch.ElasticsearchRestClientAutoConfiguration,\org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,\org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\org.springframework.boot.autoconfigure.availability.ApplicationAvailabilityAutoConfiguration,\org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\org.springframework.boot.autoconfigure.r2dbc.R2dbcAutoConfiguration,\org.springframework.boot.autoconfigure.rsocket.RSocketMessagingAutoConfiguration,\org.springframework.boot.autoconfigure.rsocket.RSocketRequesterAutoConfiguration,\org.springframework.boot.autoconfigure.rsocket.RSocketServerAutoConfiguration,\org.springframework.boot.autoconfigure.rsocket.RSocketStrategiesAutoConfiguration,\org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\org.springframework.boot.autoconfigure.security.rsocket.RSocketSecurityAutoConfiguration,\org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyAutoConfiguration,\org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

# Failure analyzersorg.springframework.boot.diagnostics.FailureAnalyzer=\org.springframework.boot.autoconfigure.data.redis.RedisUrlSyntaxFailureAnalyzer,\org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\org.springframework.boot.autoconfigure.flyway.FlywayMigrationScriptMissingFailureAnalyzer,\org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,\org.springframework.boot.autoconfigure.r2dbc.ConnectionFactoryBeanCreationFailureAnalyzer,\org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer

# Template availability providersorg.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\org.springframework.boot.autoconfigure.web.servlet.JspTemplateAvailabilityProvider

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