SpringBoot之@EnableAutoConfiguration注解

首先Spring Boot项目中都会如下启动类：

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

  
  

  
  
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从上面代码可以看出，注解@SpringBootApplication和SpringApplication.run()方法是最为重要的部分。这里主要来看看@SpringBootApplication注解部分。

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
	...
}

  
  

  
  
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虽然定义使用了多个Annotation进行了原信息标注，但实际上重要的只有三个Annotation：

• @Configuration（@SpringBootConfiguration点开查看发现里面还是应用了@Configuration）
• @EnableAutoConfiguration
• @ComponentScan

如果在启动类使用这个三个注解，整个SpringBoot应用依然可以与之前的启动类功能一样。但每次写这3个比较啰嗦，所以写一个@SpringBootApplication方便点。

这三个注解中@Configuration和@ComponentScan对我们来说并不陌生，今天我们的主角是@EnableAutoConfiguration。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
Class<span class="token operator">&lt;</span><span class="token operator">?</span><span class="token operator">&gt;</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">exclude</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">default</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

String<span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">excludeName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">default</span> <span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">;</span>

}

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其中最关键的要属@Import(AutoConfigurationImportSelector.class)，借助AutoConfigurationImportSelector，@EnableAutoConfiguration可以帮助SpringBoot应用将所有符合条件的@Configuration配置都加载到当前SpringBoot创建并使用的IoC容器。

借助于Spring框架原有的一个工具类：SpringFactoriesLoader的支持，@EnableAutoConfiguration可以智能的自动配置功效才得以大功告成！

在AutoConfigurationImportSelector类中可以看到通过 SpringFactoriesLoader.loadFactoryNames()
把 spring-boot-autoconfigure.jar/META-INF/spring.factories中每一个xxxAutoConfiguration文件都加载到容器中，spring.factories文件里每一个xxxAutoConfiguration文件一般都会有下面的条件注解:

• @ConditionalOnClass ： classpath中存在该类时起效
• @ConditionalOnMissingClass ： classpath中不存在该类时起效
• @ConditionalOnBean ： DI容器中存在该类型Bean时起效
• @ConditionalOnMissingBean ： DI容器中不存在该类型Bean时起效
• @ConditionalOnSingleCandidate ： DI容器中该类型Bean只有一个或@Primary的只有一个时起效
• @ConditionalOnExpression ： SpEL表达式结果为true时
• @ConditionalOnProperty ： 参数设置或者值一致时起效
• @ConditionalOnResource ： 指定的文件存在时起效
• @ConditionalOnJndi ： 指定的JNDI存在时起效
• @ConditionalOnJava ： 指定的Java版本存在时起效
• @ConditionalOnWebApplication ： Web应用环境下起效
• @ConditionalOnNotWebApplication ： 非Web应用环境下起效

...
org.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.web.WebMvcAutoConfiguration,\
...

 
 

 
 
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SpringFactoriesLoader

SpringFactoriesLoader属于Spring框架私有的一种扩展方案(类似于Java的SPI方案java.util.ServiceLoader)，其主要功能就是从指定的配置文件META-INF/spring-factories加载配置，spring-factories是一个典型的java properties文件，只不过Key和Value都是Java类型的完整类名，比如：

example.MyService=example.MyServiceImpl1,example.MyServiceImpl2

 
 

 
 
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对于@EnableAutoConfiguration来说，SpringFactoriesLoader的用途稍微不同一些，其本意是为了提供SPI扩展的场景，而在@EnableAutoConfiguration场景中，它更多提供了一种配置查找的功能支持，即根据@EnableAutoConfiguration的完整类名org.springframework.boot.autoconfig.EnableAutoConfiguration作为查找的Key，获得对应的一组@Configuration类。

SpringFactoriesLoader是一个抽象类，类中定义的静态属性定义了其加载资源的路径public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories"，此外还有三个静态方法：

• loadFactories：加载指定的factoryClass并进行实例化。
• loadFactoryNames：加载指定的factoryClass的名称集合。
• instantiateFactory：对指定的factoryClass进行实例化。

在loadFactories方法中调用了loadFactoryNames以及instantiateFactory方法。

public static <T> List<T> loadFactories(Class<T> factoryClass, ClassLoader classLoader) {
   Assert.notNull(factoryClass, "'factoryClass' must not be null");
   ClassLoader classLoaderToUse = classLoader;
   if (classLoaderToUse == null) {
      classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
   }
   List<String> factoryNames = loadFactoryNames(factoryClass, classLoaderToUse);
   if (logger.isTraceEnabled()) {
      logger.trace("Loaded [" + factoryClass.getName() + "] names: " + factoryNames);
   }
   List<T> result = new ArrayList<T>(factoryNames.size());
   for (String factoryName : factoryNames) {
      result.add(instantiateFactory(factoryName, factoryClass, classLoaderToUse));
   }
   AnnotationAwareOrderComparator.sort(result);
   return result;
}

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loadFactories方法首先获取类加载器，然后调用loadFactoryNames方法获取所有的指定资源的名称集合、接着调用instantiateFactory方法实例化这些资源类并将其添加到result集合中。最后调用AnnotationAwareOrderComparator.sort方法进行集合的排序。

一个例子

上面介绍了很多原理的知识，下面结合一个例子来加深理解，例子展示的是当项目启动时如果某个类存在就自动配置这个Bean，并且这个属性可以在application.properties中配置

新建一个Maven项目,pom.xml文件如下：

<?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>

<span class="token generics function"><span class="token punctuation">&lt;</span>groupId<span class="token punctuation">&gt;</span></span>com<span class="token punctuation">.</span>chm<span class="token punctuation">.</span>test<span class="token operator">&lt;</span><span class="token operator">/</span>groupId<span class="token operator">&gt;</span>
<span class="token generics function"><span class="token punctuation">&lt;</span>artifactId<span class="token punctuation">&gt;</span></span>spring<span class="token operator">-</span>boot<span class="token operator">-</span>starter<span class="token operator">-</span>hello<span class="token operator">&lt;</span><span class="token operator">/</span>artifactId<span class="token operator">&gt;</span>
<span class="token generics function"><span class="token punctuation">&lt;</span>version<span class="token punctuation">&gt;</span></span><span class="token number">1.0</span><span class="token operator">-</span>SNAPSHOT<span class="token operator">&lt;</span><span class="token operator">/</span>version<span class="token operator">&gt;</span>
<span class="token generics function"><span class="token punctuation">&lt;</span>packaging<span class="token punctuation">&gt;</span></span>jar<span class="token operator">&lt;</span><span class="token operator">/</span>packaging<span class="token operator">&gt;</span>

<span class="token generics function"><span class="token punctuation">&lt;</span>name<span class="token punctuation">&gt;</span></span>spring<span class="token operator">-</span>boot<span class="token operator">-</span>starter<span class="token operator">-</span>hello<span class="token operator">&lt;</span><span class="token operator">/</span>name<span class="token operator">&gt;</span>
<span class="token operator">&lt;</span><span class="token operator">!</span><span class="token operator">--</span> FIXME change it to the project's website <span class="token operator">--</span><span class="token operator">&gt;</span>
<span class="token generics function"><span class="token punctuation">&lt;</span>url<span class="token punctuation">&gt;</span></span>http<span class="token operator">:</span><span class="token operator">/</span><span class="token operator">/</span>www<span class="token punctuation">.</span>example<span class="token punctuation">.</span>com<span class="token operator">&lt;</span><span class="token operator">/</span>url<span class="token operator">&gt;</span>

<span class="token generics function"><span class="token punctuation">&lt;</span>properties<span class="token punctuation">&gt;</span></span>
    <span class="token operator">&lt;</span>project<span class="token punctuation">.</span>build<span class="token punctuation">.</span>sourceEncoding<span class="token operator">&gt;</span>UTF<span class="token operator">-</span><span class="token number">8</span><span class="token operator">&lt;</span><span class="token operator">/</span>project<span class="token punctuation">.</span>build<span class="token punctuation">.</span>sourceEncoding<span class="token operator">&gt;</span>
    <span class="token operator">&lt;</span>maven<span class="token punctuation">.</span>compiler<span class="token punctuation">.</span>source<span class="token operator">&gt;</span><span class="token number">1.8</span><span class="token operator">&lt;</span><span class="token operator">/</span>maven<span class="token punctuation">.</span>compiler<span class="token punctuation">.</span>source<span class="token operator">&gt;</span>
    <span class="token operator">&lt;</span>maven<span class="token punctuation">.</span>compiler<span class="token punctuation">.</span>target<span class="token operator">&gt;</span><span class="token number">1.8</span><span class="token operator">&lt;</span><span class="token operator">/</span>maven<span class="token punctuation">.</span>compiler<span class="token punctuation">.</span>target<span class="token operator">&gt;</span>
<span class="token operator">&lt;</span><span class="token operator">/</span>properties<span class="token operator">&gt;</span>

<span class="token generics function"><span class="token punctuation">&lt;</span>dependencies<span class="token punctuation">&gt;</span></span>
    <span class="token generics function"><span class="token punctuation">&lt;</span>dependency<span class="token punctuation">&gt;</span></span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>groupId<span class="token punctuation">&gt;</span></span>junit<span class="token operator">&lt;</span><span class="token operator">/</span>groupId<span class="token operator">&gt;</span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>artifactId<span class="token punctuation">&gt;</span></span>junit<span class="token operator">&lt;</span><span class="token operator">/</span>artifactId<span class="token operator">&gt;</span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>version<span class="token punctuation">&gt;</span></span><span class="token number">4.11</span><span class="token operator">&lt;</span><span class="token operator">/</span>version<span class="token operator">&gt;</span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>scope<span class="token punctuation">&gt;</span></span>test<span class="token operator">&lt;</span><span class="token operator">/</span>scope<span class="token operator">&gt;</span>
    <span class="token operator">&lt;</span><span class="token operator">/</span>dependency<span class="token operator">&gt;</span>
    <span class="token generics function"><span class="token punctuation">&lt;</span>dependency<span class="token punctuation">&gt;</span></span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>groupId<span class="token punctuation">&gt;</span></span>org<span class="token punctuation">.</span>springframework<span class="token punctuation">.</span>boot<span class="token operator">&lt;</span><span class="token operator">/</span>groupId<span class="token operator">&gt;</span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>artifactId<span class="token punctuation">&gt;</span></span>spring<span class="token operator">-</span>boot<span class="token operator">-</span>autoconfigure<span class="token operator">&lt;</span><span class="token operator">/</span>artifactId<span class="token operator">&gt;</span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>version<span class="token punctuation">&gt;</span></span><span class="token number">2.0</span><span class="token number">.4</span><span class="token punctuation">.</span>RELEASE<span class="token operator">&lt;</span><span class="token operator">/</span>version<span class="token operator">&gt;</span>
    <span class="token operator">&lt;</span><span class="token operator">/</span>dependency<span class="token operator">&gt;</span>
    <span class="token generics function"><span class="token punctuation">&lt;</span>dependency<span class="token punctuation">&gt;</span></span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>groupId<span class="token punctuation">&gt;</span></span>org<span class="token punctuation">.</span>springframework<span class="token punctuation">.</span>boot<span class="token operator">&lt;</span><span class="token operator">/</span>groupId<span class="token operator">&gt;</span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>artifactId<span class="token punctuation">&gt;</span></span>spring<span class="token operator">-</span>boot<span class="token operator">-</span>configuration<span class="token operator">-</span>processor<span class="token operator">&lt;</span><span class="token operator">/</span>artifactId<span class="token operator">&gt;</span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>version<span class="token punctuation">&gt;</span></span><span class="token number">2.0</span><span class="token number">.4</span><span class="token punctuation">.</span>RELEASE<span class="token operator">&lt;</span><span class="token operator">/</span>version<span class="token operator">&gt;</span>
        <span class="token generics function"><span class="token punctuation">&lt;</span>optional<span class="token punctuation">&gt;</span></span><span class="token boolean">true</span><span class="token operator">&lt;</span><span class="token operator">/</span>optional<span class="token operator">&gt;</span>
    <span class="token operator">&lt;</span><span class="token operator">/</span>dependency<span class="token operator">&gt;</span>
<span class="token operator">&lt;</span><span class="token operator">/</span>dependencies<span class="token operator">&gt;</span>

</project>

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项目目录结构如下：

Hello.java

public class Hello {
<span class="token keyword">private</span> String msg<span class="token punctuation">;</span>

<span class="token keyword">public</span> String <span class="token function">sayHello</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token string">"hello "</span> <span class="token operator">+</span> msg<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> String <span class="token function">getMsg</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> msg<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setMsg</span><span class="token punctuation">(</span>String msg<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>msg <span class="token operator">=</span> msg<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

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HelloProperties.java

@ConfigurationProperties(prefix = "hello") //获取属性值
public class HelloProperties {
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> String MSG <span class="token operator">=</span> <span class="token string">"world"</span><span class="token punctuation">;</span>

<span class="token keyword">private</span> String msg <span class="token operator">=</span> MSG <span class="token punctuation">;</span>

<span class="token keyword">public</span> String <span class="token function">getMsg</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> msg<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setMsg</span><span class="token punctuation">(</span>String msg<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>msg <span class="token operator">=</span> msg<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

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HelloAutoConfiguration.java

@Configuration
//为带有@ConfigurationProperties注解的Bean提供有效的支持。
// 这个注解可以提供一种方便的方式来将带有@ConfigurationProperties注解的类注入为Spring容器的Bean。
@EnableConfigurationProperties(HelloProperties.class)//开启属性注入,通过@autowired注入
@ConditionalOnClass(Hello.class)//判断这个类是否在classpath中存在，如果存在，才会实例化一个Bean
// The Hello bean will be created if the hello.enable property exists and has a value other than false
// or the property doesn't exist at all.
@ConditionalOnProperty(prefix="hello", value="enabled", matchIfMissing = true)
public class HelloAutoConfiguration {
<span class="token annotation punctuation">@Autowired</span>
<span class="token keyword">private</span> HelloProperties helloProperties<span class="token punctuation">;</span>

<span class="token annotation punctuation">@Bean</span>
<span class="token annotation punctuation">@ConditionalOnMissingBean</span><span class="token punctuation">(</span>Hello<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token comment">//容器中如果没有Hello这个类,那么自动配置这个Hello</span>
<span class="token keyword">public</span> Hello <span class="token function">hello</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    Hello hello <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Hello</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    hello<span class="token punctuation">.</span><span class="token function">setMsg</span><span class="token punctuation">(</span>helloProperties<span class="token punctuation">.</span><span class="token function">getMsg</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> hello<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

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spring.factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.chm.test.HelloAutoConfiguration

 
 

 
 
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最后使用mvn package将上面项目打包，使用mvn install:install-file命令将打包文件上传到本地Maven仓库进行测试。下面再新建一个Maven项目用于测试。

pom.xml文件

<?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>

<groupId>com.chm.test</groupId>
<artifactId>test-starter</artifactId>
<version>1.0-SNAPSHOT</version>

<name>test-starter</name>
<!-- FIXME change it to the project’s website -->
<url>http://www.example.com</url>

<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.source>1.8</maven.compiler.source>
<maven.compiler.target>1.8</maven.compiler.target>
<boot.version>2.0.4.RELEASE</boot.version>
</properties>

<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.11</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>com.chm.test</groupId>
<artifactId>spring-boot-starter-hello</artifactId>
<version>0.0.1-SNAPSHOT</version>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<version>KaTeX parse error: Expected 'EOF', got '&' at position 95: …n punctuation">&̲lt;/</span>vers…{boot.version}</version>
<scope>test</scope>
</dependency>
</dependencies>
</project>

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App.java

@SpringBootApplication
@RestController
public class App {
<span class="token annotation punctuation">@Autowired</span>
<span class="token keyword">private</span> Hello hello<span class="token punctuation">;</span>

<span class="token annotation punctuation">@RequestMapping</span><span class="token punctuation">(</span><span class="token string">"/"</span><span class="token punctuation">)</span>
<span class="token keyword">public</span> String <span class="token function">index</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> hello<span class="token punctuation">.</span><span class="token function">sayHello</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    SpringApplication<span class="token punctuation">.</span><span class="token function">run</span><span class="token punctuation">(</span>App<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">,</span> args<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

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application.properties

#可以不配置
hello.enabled=true

hello.msg=charmingfst

#以debug模式运行
debug=true

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以debug模式运行，可以看到我们的配置：

启动项目，打开浏览器，运行结果如下：

参考文章

1. 《SpringBoot揭秘》
2. https://blog.youkuaiyun.com/a67474506/article/details/52013634

相关文章：JDK和Spring中SPI的实现原理和区别

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