Spring Boot 自动配置（auto-configurtion） 揭秘

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 ...@EnableAutoConfigurationpublic 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 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 hierarchy2017-01-03 10:17:38.155  INFO 10752 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startuppackages: [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 hierarchy2017-01-03 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 getCandidateConfigurations(AnnotationMetadata metadata,            AnnotationAttributes attributes) {        List 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：

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。

使用-debug参数

@EnableAutoConfigurationpublic 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.DebugModeExample2017-01-02 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 default2017-01-02 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 hierarchy2017-01-02 21:15:18.032  INFO 5704 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup2017-01-02 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:-----------    NoneUnconditional classes:----------------------    org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration    org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration    org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration    org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration2017-01-02 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 hierarchy2017-01-02 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.JmxAutoConfigurationUnconditional classes:----------------------    org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration    org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration    org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration    org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration