Spring Boot 自动配置（auto-configurtion）揭秘

野性酷女 2022-05-09 03:48 741阅读 0赞

本章，我们为你揭秘Spring Boot自动配置（Auto Configuration）运行机制，谈到auto-configuration，肯定离不开@EnableAutoConfiguration注解。

package org.springframework.boot.autoconfigure;
    
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    @Import(EnableAutoConfigurationImportSelector.class)
    public @interface EnableAutoConfiguration {
    
       Class<?>[] exclude() default {};
       String[] excludeName() default {};
    }

这里涉及了两个元注解： `@AutoConfigurationPackage`, `@Import(EnableAutoConfigurationImportSelector.class)`，其中`@AutoConfigurationPackage`定义如下：

package org.springframework.boot.autoconfigure;
    
    import ....
    
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @Import(AutoConfigurationPackages.Registrar.class)
    public @interface AutoConfigurationPackage {
    
    }

`@AutoConfigurationPackage`注解定义中使用了@Import元注解，注解属性value取值为`AutoConfigurationPackages.Registrar.class`，`AutoConfigurationPackages.Registrar`类实现了接口`ImportBeanDefinitionRegistrar`

> @Import注解可以接受以下几种定义类型的Java类
> 
>  *  使用@Configuration注解的类
>  *  ImportSelector实现类：以代码方式处理@Configuration注解类
>  *  DeferredImportSelector实现类：与ImportSelector类似，区别在于处理操作被延迟到所有其他配置项都处理完毕再进行。
>  *  ImportBeanDefinitionRegistrar实现类

`AutoConfigurationPackages.Registrar`会向Spring容器注册Bean，Bean本身会存储用户自定义配置包列表。Spring Boot 本身会使用这个列表。例如：对于spring-boot-autoconfigure数据访问配置类，可以通过静态方法：`AutoConfigurationPackages.get(BeanFactory)`来获取到这个配置列表，下面是示例代码。

package com.logicbig.example;
    
    import ...
    
    @EnableAutoConfiguration
    public class AutoConfigurationPackagesTest {
    
       public static void main (String[] args) {
    
           SpringApplication app =
                         new SpringApplication(AutoConfigurationPackagesTest.class);
           app.setBannerMode(Banner.Mode.OFF);
           app.setLogStartupInfo(false);
           ConfigurableApplicationContext c = app.run(args);
           List<String> packages = AutoConfigurationPackages.get(c);
           System.out.println("packages: "+packages);
       }
    }

代码输出如下：

2017-01-03 10:17:37.372  INFO 10752 --- [           main] s.c.a.AnnotationConfigApplicationContext : Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@67b467e9: startup date [Tue Jan 03 10:17:37 CST 2017]; root of context hierarchy
    2017-01-03 10:17:38.155  INFO 10752 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
    packages: [com.logicbig.example]
    2017-01-03 10:17:38.170  INFO 10752 --- [       Thread-1] s.c.a.AnnotationConfigApplicationContext : Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@67b467e9: startup date [Tue Jan 03 10:17:37 CST 2017]; root of context hierarchy
    2017-01-03 10:17:38.171  INFO 10752 --- [       Thread-1] o.s.j.e.a.AnnotationMBeanExporter        : Unregistering JMX-exposed beans on shutdown

`@Import(EnableAutoConfigurationImportSelector.class)`注解是auto-configuration 机制的启动入口。`EnableAutoConfigurationImportSelector`实现了接口DeferredImportSelector，其内部调用了`SpringFactoriesLoader.loadFactoryNames()`方法，方法会从META-INF/spring.factories中加载配置类。

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
    			AnnotationAttributes attributes) {
    		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
    				getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
    		Assert.notEmpty(configurations,
    				"No auto configuration classes found in META-INF/spring.factories. If you "
    						+ "are using a custom packaging, make sure that file is correct.");
    		return configurations;
    	}

从spring.factories中查找键值org.springframework.boot.autoconfigure.EnableAutoConfiguration的值：  
![在这里插入图片描述][70]

`spring-boot-autoconfigure`默认隐式包含在所有启动程序中

下面其中的一个配置类JmxAutoConfiguration的代码段

package org.springframework.boot.autoconfigure.jmx;
    
       .......
     import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
     import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
     import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
     import org.springframework.boot.autoconfigure.condition.SearchStrategy;
      .....
    
     @Configuration
     @ConditionalOnClass({ MBeanExporter.class })
     @ConditionalOnProperty(prefix = "spring.jmx", name = "enabled", havingValue = "true", matchIfMissing = true)
     public class JmxAutoConfiguration implements
                                        EnvironmentAware, BeanFactoryAware {
        .....
     }

**@ConditionalOnClass**

`@ConditionalOnClass`是由元注解`@Conditional(OnClassCondition.class`定义的注解，我们知道，@Conditional是条件注解，只有条件为真时，@Conditional注解的类、方法才会被加载到Spring组件容器中。对于上面的实例代码段，只有当MBeanExporter.class已经包含在classpath中（具体校验类似于Class.forName的加载逻辑，当目标类包含在classpath中，方法返回为true，否则返回false），OnClassCondition\#matches()才会返回为true。

**@ConditionalOnProperty**

与`@ConditionalOnClass`类似，`@ConditionalOnProperty`是另一个@Conditional类型变量，是由元注解`@Conditional(OnPropertyCondition.class)`所定义的注解。只有当目标属性包含了指定值，`OnPropertyCondition#matches()`才会返回真，还是上面的代码段：

@ConditionalOnProperty(prefix = "spring.jmx", name = "enabled",
                             havingValue = "true", matchIfMissing = true)

如果我们应用配置了spring.jmx.enabled=true,那么Spring容器将自动注册JmxAutoConfiguration，`matchIfMissing=true`表示默认情况下（配置属性未设置）为真。

**其他一些条件注解**

包‘org.springframework.boot.autoconfigure.condition`，所有条件注解均遵循`ConditionalOnXyz\`命名约定。如果想要开发自定义启动包，你需要了解这些API，对于别的开发人员来说，最好也能了解基本的运行机制。  
![在这里插入图片描述][70 1]

**使用–debug参数**

@EnableAutoConfiguration
    public class DebugModeExample {
    
      public static void main (String[] args) {
          //just doing this programmatically for demo
          String[] appArgs = {"--debug"};
    
          SpringApplication app = new SpringApplication(DebugModeExample.class);
          app.setBannerMode(Banner.Mode.OFF);
          app.setLogStartupInfo(false);
          app.run(appArgs);
        }
    }

输出

2017-01-02 21:15:17.322 DEBUG 5704 --- [           main] o.s.boot.SpringApplication               : Loading source class com.logicbig.example.DebugModeExample
    2017-01-02 21:15:17.379 DEBUG 5704 --- [           main] o.s.b.c.c.ConfigFileApplicationListener  : Skipped (empty) config file 'file:/D:/LogicBig/example-projects/spring-boot/boot-customizing-autoconfig/target/classes/application.properties' (classpath:/application.properties)
    2017-01-02 21:15:17.379 DEBUG 5704 --- [           main] o.s.b.c.c.ConfigFileApplicationListener  : Skipped (empty) config file 'file:/D:/LogicBig/example-projects/spring-boot/boot-customizing-autoconfig/target/classes/application.properties' (classpath:/application.properties) for profile default
    2017-01-02 21:15:17.384  INFO 5704 --- [           main] s.c.a.AnnotationConfigApplicationContext : Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@2f0a87b3: startup date [Mon Jan 02 21:15:17 CST 2017]; root of context hierarchy
    2017-01-02 21:15:18.032  INFO 5704 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
    2017-01-02 21:15:18.047 DEBUG 5704 --- [           main] utoConfigurationReportLoggingInitializer :
    
    
    =========================
    AUTO-CONFIGURATION REPORT
    =========================
    
    
    Positive matches:
    -----------------
    
       GenericCacheConfiguration matched:
          - Cache org.springframework.boot.autoconfigure.cache.GenericCacheConfiguration automatic cache type (CacheCondition)
    
       JmxAutoConfiguration matched:
          - @ConditionalOnClass found required class 'org.springframework.jmx.export.MBeanExporter' (OnClassCondition)
          - @ConditionalOnProperty (spring.jmx.enabled=true) matched (OnPropertyCondition)
    
       JmxAutoConfiguration#mbeanExporter matched:
          - @ConditionalOnMissingBean (types: org.springframework.jmx.export.MBeanExporter; SearchStrategy: current) did not find any beans (OnBeanCondition)
    
       JmxAutoConfiguration#mbeanServer matched:
          - @ConditionalOnMissingBean (types: javax.management.MBeanServer; SearchStrategy: all) did not find any beans (OnBeanCondition)
    
       JmxAutoConfiguration#objectNamingStrategy matched:
          - @ConditionalOnMissingBean (types: org.springframework.jmx.export.naming.ObjectNamingStrategy; SearchStrategy: current) did not find any beans (OnBeanCondition)
    
       NoOpCacheConfiguration matched:
          - Cache org.springframework.boot.autoconfigure.cache.NoOpCacheConfiguration automatic cache type (CacheCondition)
    
       PropertyPlaceholderAutoConfiguration#propertySourcesPlaceholderConfigurer matched:
          - @ConditionalOnMissingBean (types: org.springframework.context.support.PropertySourcesPlaceholderConfigurer; SearchStrategy: current) did not find any beans (OnBeanCondition)
    
       RedisCacheConfiguration matched:
          - Cache org.springframework.boot.autoconfigure.cache.RedisCacheConfiguration automatic cache type (CacheCondition)
    
       SimpleCacheConfiguration matched:
          - Cache org.springframework.boot.autoconfigure.cache.SimpleCacheConfiguration automatic cache type (CacheCondition)
    
    
    Negative matches:
    -----------------
    
       ActiveMQAutoConfiguration:
          Did not match:
             - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.ActiveMQConnectionFactory' (OnClassCondition)
    
       AopAutoConfiguration:
          Did not match:
             - @ConditionalOnClass did not find required classes 'org.aspectj.lang.annotation.Aspect', 'org.aspectj.lang.reflect.Advice' (OnClassCondition)
    
       ArtemisAutoConfiguration:
          Did not match:
             - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.artemis.jms.client
    
     ...............................
     ....................
    
    
    Exclusions:
    -----------
    
        None
    
    
    Unconditional classes:
    ----------------------
    
        org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration
    
        org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration
    
        org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration
    
        org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration
    
    
    
    2017-01-02 21:15:18.058  INFO 5704 --- [       Thread-1] s.c.a.AnnotationConfigApplicationContext : Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@2f0a87b3: startup date [Mon Jan 02 21:15:17 CST 2017]; root of context hierarchy
    2017-01-02 21:15:18.059  INFO 5704 --- [       Thread-1] o.s.j.e.a.AnnotationMBeanExporter        : Unregistering JMX-exposed beans on shutdown

在上面的输出中

*  **Positive matches：**@Conditional条件为真，配置类被Spring容器加载。
 *  **Negative matches:** @Conditional条件为假，配置类未被Spring容器加载。
 *  **Exclusions：** 应用端明确排除加载配置
 *  **Unconditional classes：** 自动配置类不包含任何类级别的条件，也就是说，类始终会被自动加载。

**禁止特定类的auto-configuration**

@EnableAutoConfiguration(exclude = {JmxAutoConfiguration.class})
    public class ExcludeConfigExample {
    
        public static void main (String[] args) {
             //just doing this programmatically for demo
             String[] appArgs = {"--debug"};
    
            SpringApplication app = new SpringApplication(ExcludeConfigExample.class);
            app.setBannerMode(Banner.Mode.OFF);
            app.setLogStartupInfo(false);
            app.run(appArgs);
        }
    }

输出

.............
    
    =========================
    AUTO-CONFIGURATION REPORT
    =========================
    
    
    Positive matches:
    -----------------
    
       GenericCacheConfiguration matched:
          - Cache org.springframework.boot.autoconfigure.cache.GenericCacheConfiguration automatic cache type (CacheCondition)
    
       NoOpCacheConfiguration matched:
          - Cache org.springframework.boot.autoconfigure.cache.NoOpCacheConfiguration automatic cache type (CacheCondition)
    
       PropertyPlaceholderAutoConfiguration#propertySourcesPlaceholderConfigurer matched:
          - @ConditionalOnMissingBean (types: org.springframework.context.support.PropertySourcesPlaceholderConfigurer; SearchStrategy: current) did not find any beans (OnBeanCondition)
    
       RedisCacheConfiguration matched:
          - Cache org.springframework.boot.autoconfigure.cache.RedisCacheConfiguration automatic cache type (CacheCondition)
    
       SimpleCacheConfiguration matched:
          - Cache org.springframework.boot.autoconfigure.cache.SimpleCacheConfiguration automatic cache type (CacheCondition)
    
    
    Negative matches:
    -----------------
    
       ActiveMQAutoConfiguration:
          Did not match:
             - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.ActiveMQConnectionFactory' (OnClassCondition)
    
       AopAutoConfiguration:
          Did not match:
             - @ConditionalOnClass did not find required classes 'org.aspectj.lang.annotation.Aspect', 'org.aspectj.lang.reflect.Advice' (OnClassCondition)
    
    
      .................................
    
    Exclusions:
    -----------
    
        org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration
    
    
    Unconditional classes:
    ----------------------
    
        org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration
    
        org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration
    
        org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration
    
        org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration

原文链接：  
[https://www.logicbig.com/tutorials/spring-framework/spring-boot/auto-config-mechanism.html][https_www.logicbig.com_tutorials_spring-framework_spring-boot_auto-config-mechanism.html]

[70]: /images/20220509/104f67475c354318ab83c298d91a80c5.png
[70 1]: /images/20220509/e91c1066f6934203b62fc7522bca7cbd.png
[https_www.logicbig.com_tutorials_spring-framework_spring-boot_auto-config-mechanism.html]: https://www.logicbig.com/tutorials/spring-framework/spring-boot/auto-config-mechanism.html