Spring Boot 自動配置（auto-configurtion） 揭祕

王下七匹狼發表於2019-07-18
Spring Boot

本章，我們為你揭祕Spring Boot自動配置（Auto Configuration）執行機制，談到auto-configuration，肯定離不開@EnableAutoConfiguration註解。

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 {};
}
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這裡涉及了兩個元註解： @AutoConfigurationPackage, @Import(EnableAutoConfigurationImportSelector.class)，其中@AutoConfigurationPackage定義如下：

package org.springframework.boot.autoconfigure;

import ....

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

}
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@AutoConfigurationPackage註解定義中使用了@Import元註解，註解屬性value取值為AutoConfigurationPackages.Registrar.class，AutoConfigurationPackages.Registrar類實現了介面ImportBeanDefinitionRegistrar

@Import註解可以接受以下幾種定義型別的Java類

  • 使用@Configuration註解的類
  • ImportSelector實現類：以程式碼方式處理@Configuration註解類
  • DeferredImportSelector實現類：與ImportSelector類似，區別在於處理操作被延遲到所有其他配置項都處理完畢再進行。
  • ImportBeanDefinitionRegistrar實現類

AutoConfigurationPackages.Registrar會向Spring容器註冊Bean，Bean本身會儲存使用者自定義配置包列表。Spring Boot 本身會使用這個列表。例如：對於spring-boot-autoconfigure資料訪問配置類，可以通過靜態方法：**AutoConfigurationPackages.get(BeanFactory)**來獲取到這個配置列表，下面是示例程式碼。

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);
   }
}
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程式碼輸出如下：

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
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**@Import(EnableAutoConfigurationImportSelector.class)註解是auto-configuration 機制的啟動入口。EnableAutoConfigurationImportSelector實現了介面DeferredImportSelector，其內部呼叫了SpringFactoriesLoader.loadFactoryNames()**方法，方法會從META-INF/spring.factories中載入配置類。

	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;
	}
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從spring.factories中查詢鍵值org.springframework.boot.autoconfigure.EnableAutoConfiguration的值：

Spring Boot 自動配置（auto-configurtion） 揭祕

spring-boot-autoconfigure預設隱式包含在所有啟動程式中

下面其中的一個配置類JmxAutoConfiguration的程式碼段

 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 {
    .....
 }
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@ConditionalOnClass

@ConditionalOnClass是由元註解**@Conditional(OnClassCondition.class**定義的註解，我們知道，@Conditional是條件註解，只有條件為真時，@Conditional註解的類、方法才會被載入到Spring元件容器中。對於上面的例項程式碼段，只有當MBeanExporter.class已經包含在classpath中（具體校驗類似於Class.forName的載入邏輯，當目標類包含在classpath中，方法返回為true，否則返回false），OnClassCondition#matches()才會返回為true。

@ConditionalOnProperty

與**@ConditionalOnClass類似，@ConditionalOnProperty是另一個@Conditional型別變數，是由元註解@Conditional(OnPropertyCondition.class)**所定義的註解。只有當目標屬性包含了指定值，**OnPropertyCondition#matches()**才會返回真，還是上面的程式碼段：

  @ConditionalOnProperty(prefix = "spring.jmx", name = "enabled",
                         havingValue = "true", matchIfMissing = true)
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如果我們應用配置了spring.jmx.enabled=true,那麼Spring容器將自動註冊JmxAutoConfiguration，matchIfMissing=true表示預設情況下（配置屬性未設定）為真。

其他一些條件註解

包‘org.springframework.boot.autoconfigure.condition，所有條件註解均遵循ConditionalOnXyz`命名約定。如果想要開發自定義啟動包，你需要了解這些API，對於別的開發人員來說，最好也能瞭解基本的執行機制。

Spring Boot 自動配置（auto-configurtion） 揭祕

使用–debug引數

@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);
    }
}
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輸出

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
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在上面的輸出中

  • Positive matches：@Conditional條件為真，配置類被Spring容器載入。
  • Negative matches: @Conditional條件為假，配置類未被Spring容器載入。
  • Exclusions： 應用端明確排除載入配置
  • Unconditional classes： 自動配置類不包含任何類級別的條件，也就是說，類始終會被自動載入。

禁止特定類的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);
    }
}
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輸出


  .............

=========================
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
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