Spring Boot 自动配置（auto

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。

1package org.springframework.boot.autoconfigure;
2
3@Target(ElementType.TYPE)
4@Retention(RetentionPolicy.RUNTIME)
5@Documented
6@Inherited
7@AutoConfigurationPackage
8@Import(EnableAutoConfigurationImportSelector.class)
9public @interface EnableAutoConfiguration {
10
11   Class<?>[] exclude() default {};
12   String[] excludeName() default {};
13}

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

1package org.springframework.boot.autoconfigure;
2
3import ....
4
5@Target(ElementType.TYPE)
6@Retention(RetentionPolicy.RUNTIME)
7@Documented
8@Inherited
9@Import(AutoConfigurationPackages.Registrar.class)
10public @interface AutoConfigurationPackage {
11
12}

@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。

1package com.logicbig.example;
2
3import ...
4
5@EnableAutoConfiguration
6public class AutoConfigurationPackagesTest {
7
8   public static void main (String[] args) {
9
10       SpringApplication app =
11                     new SpringApplication(AutoConfigurationPackagesTest.class);
12       app.setBannerMode(Banner.Mode.OFF);
13       app.setLogStartupInfo(false);
14       ConfigurableApplicationContext c = app.run(args);
15       List<String> packages = AutoConfigurationPackages.get(c);
16       System.out.println("packages: "+packages);
17   }
18}

代码输出如下：

12017-01-03 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
22017-01-03 10:17:38.155  INFO 10752 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
3packages: [com.logicbig.example]
42017-01-03 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
52017-01-03 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。

1	protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
2			AnnotationAttributes attributes) {
3		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
4				getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
5		Assert.notEmpty(configurations,
6				"No auto configuration classes found in META-INF/spring.factories. If you "
7						+ "are using a custom packaging, make sure that file is correct.");
8		return configurations;
9	}

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

1 package org.springframework.boot.autoconfigure.jmx;
2
3   .......
4 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
5 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
6 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
7 import org.springframework.boot.autoconfigure.condition.SearchStrategy;
8  .....
9
10 @Configuration
11 @ConditionalOnClass({ MBeanExporter.class })
12 @ConditionalOnProperty(prefix = "spring.jmx", name = "enabled", havingValue = "true", matchIfMissing = true)
13 public class JmxAutoConfiguration implements
14                                    EnvironmentAware, BeanFactoryAware {
15    .....
16 }

@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：

1  @ConditionalOnProperty(prefix = "spring.jmx", name = "enabled",
2                         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

1@EnableAutoConfiguration
2public class DebugModeExample {
3
4  public static void main (String[] args) {
5      //just doing this programmatically for demo
6      String[] appArgs = {"--debug"};
7
8      SpringApplication app = new SpringApplication(DebugModeExample.class);
9      app.setBannerMode(Banner.Mode.OFF);
10      app.setLogStartupInfo(false);
11      app.run(appArgs);
12    }
13}

输出

12017-01-02 21:15:17.322 DEBUG 5704 --- [           main] o.s.boot.SpringApplication               : Loading source class com.logicbig.example.DebugModeExample
22017-01-02 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)
32017-01-02 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
42017-01-02 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
52017-01-02 21:15:18.032  INFO 5704 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
62017-01-02 21:15:18.047 DEBUG 5704 --- [           main] utoConfigurationReportLoggingInitializer :
7
8
9=========================
10AUTO-CONFIGURATION REPORT
11=========================
12
13
14Positive matches:
15-----------------
16
17   GenericCacheConfiguration matched:
18      - Cache org.springframework.boot.autoconfigure.cache.GenericCacheConfiguration automatic cache type (CacheCondition)
19
20   JmxAutoConfiguration matched:
21      - @ConditionalOnClass found required class 'org.springframework.jmx.export.MBeanExporter' (OnClassCondition)
22      - @ConditionalOnProperty (spring.jmx.enabled=true) matched (OnPropertyCondition)
23
24   JmxAutoConfiguration#mbeanExporter matched:
25      - @ConditionalOnMissingBean (types: org.springframework.jmx.export.MBeanExporter; SearchStrategy: current) did not find any beans (OnBeanCondition)
26
27   JmxAutoConfiguration#mbeanServer matched:
28      - @ConditionalOnMissingBean (types: javax.management.MBeanServer; SearchStrategy: all) did not find any beans (OnBeanCondition)
29
30   JmxAutoConfiguration#objectNamingStrategy matched:
31      - @ConditionalOnMissingBean (types: org.springframework.jmx.export.naming.ObjectNamingStrategy; SearchStrategy: current) did not find any beans (OnBeanCondition)
32
33   NoOpCacheConfiguration matched:
34      - Cache org.springframework.boot.autoconfigure.cache.NoOpCacheConfiguration automatic cache type (CacheCondition)
35
36   PropertyPlaceholderAutoConfiguration#propertySourcesPlaceholderConfigurer matched:
37      - @ConditionalOnMissingBean (types: org.springframework.context.support.PropertySourcesPlaceholderConfigurer; SearchStrategy: current) did not find any beans (OnBeanCondition)
38
39   RedisCacheConfiguration matched:
40      - Cache org.springframework.boot.autoconfigure.cache.RedisCacheConfiguration automatic cache type (CacheCondition)
41
42   SimpleCacheConfiguration matched:
43      - Cache org.springframework.boot.autoconfigure.cache.SimpleCacheConfiguration automatic cache type (CacheCondition)
44
45
46Negative matches:
47-----------------
48
49   ActiveMQAutoConfiguration:
50      Did not match:
51         - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.ActiveMQConnectionFactory' (OnClassCondition)
52
53   AopAutoConfiguration:
54      Did not match:
55         - @ConditionalOnClass did not find required classes 'org.aspectj.lang.annotation.Aspect', 'org.aspectj.lang.reflect.Advice' (OnClassCondition)
56
57   ArtemisAutoConfiguration:
58      Did not match:
59         - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.artemis.jms.client
60
61 ...............................
62 ....................
63
64
65Exclusions:
66-----------
67
68    None
69
70
71Unconditional classes:
72----------------------
73
74    org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration
75
76    org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration
77
78    org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration
79
80    org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration
81
82
83
842017-01-02 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
852017-01-02 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

1@EnableAutoConfiguration(exclude = {JmxAutoConfiguration.class})
2public class ExcludeConfigExample {
3
4    public static void main (String[] args) {
5         //just doing this programmatically for demo
6         String[] appArgs = {"--debug"};
7
8        SpringApplication app = new SpringApplication(ExcludeConfigExample.class);
9        app.setBannerMode(Banner.Mode.OFF);
10        app.setLogStartupInfo(false);
11        app.run(appArgs);
12    }
13}

输出

1  .............
2
3=========================
4AUTO-CONFIGURATION REPORT
5=========================
6
7
8Positive matches:
9-----------------
10
11   GenericCacheConfiguration matched:
12      - Cache org.springframework.boot.autoconfigure.cache.GenericCacheConfiguration automatic cache type (CacheCondition)
13
14   NoOpCacheConfiguration matched:
15      - Cache org.springframework.boot.autoconfigure.cache.NoOpCacheConfiguration automatic cache type (CacheCondition)
16
17   PropertyPlaceholderAutoConfiguration#propertySourcesPlaceholderConfigurer matched:
18      - @ConditionalOnMissingBean (types: org.springframework.context.support.PropertySourcesPlaceholderConfigurer; SearchStrategy: current) did not find any beans (OnBeanCondition)
19
20   RedisCacheConfiguration matched:
21      - Cache org.springframework.boot.autoconfigure.cache.RedisCacheConfiguration automatic cache type (CacheCondition)
22
23   SimpleCacheConfiguration matched:
24      - Cache org.springframework.boot.autoconfigure.cache.SimpleCacheConfiguration automatic cache type (CacheCondition)
25
26
27Negative matches:
28-----------------
29
30   ActiveMQAutoConfiguration:
31      Did not match:
32         - @ConditionalOnClass did not find required classes 'javax.jms.ConnectionFactory', 'org.apache.activemq.ActiveMQConnectionFactory' (OnClassCondition)
33
34   AopAutoConfiguration:
35      Did not match:
36         - @ConditionalOnClass did not find required classes 'org.aspectj.lang.annotation.Aspect', 'org.aspectj.lang.reflect.Advice' (OnClassCondition)
37
38
39  .................................
40
41Exclusions:
42-----------
43
44    org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration
45
46
47Unconditional classes:
48----------------------
49
50    org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration
51
52    org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration
53
54    org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration
55
56    org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration