【Springboot】Springboot自动装配原理

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 {

    }

}