【Springboot】Springboot自动装配原理

提示：扫一扫查出行
【扫一扫了解最新限行尾号】
复制提示

1.核心注解就是 EnableAutoConfiguration 该注解会激活SpringBoot的自动装配功能:

代码如下:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration { String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration"; /**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
Class<?>[] exclude() default {}; /**
* Exclude specific auto-configuration class names such that they will never be
* applied.
* @return the class names to exclude
* @since 1.3.0
*/
String[] excludeName() default {};}

2.其中在 EnableAutoConfiguration 注解之上,还被 @Import(AutoConfigurationImportSelector.class)和 AutoConfigurationPackage修饰,

@Import的作用就是用来导入配置类或者一些需要前置加载的类:

@Import支持三种类导入方式:

第一种:

@import+@Configuration的的实现

第二种

@import+ImportSelector接口的实现

第三种

@import+ImportBeanDefinitionRegistrar 的实现

SpringBoot自动装配使用的是第二种方式:

3.其中核心的加载器 AutoConfigurationImportSelector.class 部分如下

public class AutoConfigurationImportSelector
implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
BeanFactoryAware, EnvironmentAware, Ordered { private static final AutoConfigurationEntry EMPTY_ENTRY = new AutoConfigurationEntry(); private static final String[] NO_IMPORTS = {}; private static final Log logger = LogFactory
.getLog(AutoConfigurationImportSelector.class); private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude"; private ConfigurableListableBeanFactory beanFactory; private Environment environment; private ClassLoader beanClassLoader; private ResourceLoader resourceLoader; @Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
autoConfigurationMetadata, annotationMetadata);
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
} /**
* Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}
* of the importing {@link Configuration @Configuration} class.
* @param autoConfigurationMetadata the auto-configuration metadata
* @param annotationMetadata the annotation metadata of the configuration class
* @return the auto-configurations that should be imported
*/
protected AutoConfigurationEntry getAutoConfigurationEntry(
AutoConfigurationMetadata autoConfigurationMetadata,
AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
}
AnnotationAttributes attributes = getAttributes(annotationMetadata);
List<String> configurations = getCandidateConfigurations(annotationMetadata,
attributes);
configurations = removeDuplicates(configurations);
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
configurations = filter(configurations, autoConfigurationMetadata);
fireAutoConfigurationImportEvents(configurations, exclusions);
return new AutoConfigurationEntry(configurations, exclusions);
}

在该导入类中,其中selectImpots是具体导入的方法,更深一层的方法 getCandidateConfigurations()便是用来获取需要被自动装配的类的全限定类名信息集合;这些集合信息也就是SpringBoot自动装配的一些组件,这些

组件会,等待实例化后被加入到Spring的IOC容器中(该类名信息集合存放在META-INF/spring.factories 中)

这些自动装配的类也不是完全会被实例化加入到IOC容器中,例如 AopAutoConfiguration 该类就依赖于条件注解 @ ConditionalOnClass 和 @ConditionalOnProperty

这也就解释了为什么在SpringBoot项目中如果添加了jdbc的starter后.如果没有在配置文件中配置数据库的信息程序启动会报错的原因;

@Configuration
@ConditionalOnClass({ EnableAspectJAutoProxy.class, Aspect.class, Advice.class,
AnnotatedElement.class })
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true",
matchIfMissing = true)
public class AopAutoConfiguration { @Configuration
@EnableAspectJAutoProxy(proxyTargetClass = false)
@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
havingValue = "false", matchIfMissing = false)
public static class JdkDynamicAutoProxyConfiguration { } @Configuration
@EnableAspectJAutoProxy(proxyTargetClass = true)
@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
havingValue = "true", matchIfMissing = true)
public static class CglibAutoProxyConfiguration { }}