Spring Boot 自動配置（auto-configurtion） 揭秘

本章，我們為你揭秘Spring Boot自動配置（Auto Configuration）運行機制，談到auto-configuration，肯定離不開@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 {};
}

這里涉及了兩個元注解： @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 {

}

@AutoConfigurationPackage注解定義中使用了@Import元注解，注解屬性value取值為AutoConfigurationPackages.Registrar.class，AutoConfigurationPackages.Registrar類實現了接口ImportBeanDefinitionRegistrar

@Import注解可以接受以下幾種定義類型的Java類

• 使用@Configuration注解的類
• ImportSelector實現類：以代碼方式處理@Configuration注解類
• DeferredImportSelector實現類：與ImportSelector類似，區別在于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容器注冊Bean，Bean本身會存儲用戶自定義配置包列表。Spring Boot 本身會使用這個列表。例如：對于spring-boot-autoconfigure數據訪問配置類，可以通過靜態方法：AutoConfigurationPackages.get(BeanFactory)來獲取到這個配置列表，下面是示例代碼。

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);
   }
}

代碼輸出如下：

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

@Import(EnableAutoConfigurationImportSelector.class)注解是auto-configuration 機制的啟動入口。EnableAutoConfigurationImportSelector實現了接口DeferredImportSelector，其內部調用了SpringFactoriesLoader.loadFactoryNames()方法，方法會從META-INF/spring.factories中加載配置類。

    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;
    }

從spring.factories中查找鍵值org.springframework.boot.autoconfigure.EnableAutoConfiguration的值：

spring-boot-autoconfigure默認隱式包含在所有啟動程序中

下面其中的一個配置類JmxAutoConfiguration的代碼段

 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 {
    .....
 }

@ConditionalOnClass

@ConditionalOnClass是由元注解@Conditional(OnClassCondition.class定義的注解，我們知道，@Conditional是條件注解，只有條件為真時，@Conditional注解的類、方法才會被加載到Spring組件容器中。對于上面的實例代碼段，只有當MBeanExporter.class已經包含在classpath中（具體校驗類似于Class.forName的加載邏輯，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類型變量，是由元注解@Conditional(OnPropertyCondition.class)所定義的注解。只有當目標屬性包含了指定值，OnPropertyCondition#matches()才會返回真，還是上面的代碼段：

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

如果我們應用配置了spring.jmx.enabled=true,那么Spring容器將自動注冊JmxAutoConfiguration，matchIfMissing=true表示默認情況下（配置屬性未設置）為真。

其他一些條件注解

包‘org.springframework.boot.autoconfigure.condition，所有條件注解均遵循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，對于別的開發人員來說，最好也能了解基本的運行機制。

使用–debug參數

@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);
    }
}

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

在上面的輸出中

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

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);
    }
}

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