深入浅析SpringBoot中的自动装配

SpringBoot的自动装配是拆箱即用的基础，也是微服务化的前提。这次主要的议题是，来看看它是怎么样实现的，我们透过源代码来把握自动装配的来龙去脉。

一、自动装配过程分析

1.1、关于@SpringBootApplication

　　我们在编写SpringBoot项目时，@SpringBootApplication是最常见的注解了，我们可以看一下源代码：

/*
 * Copyright 2012-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.springframework.boot.autoconfigure;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.context.TypeExcludeFilter;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.ComponentScan.Filter;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.FilterType;
import org.springframework.core.annotation.AliasFor;
/**
 * Indicates a {@link Configuration configuration} class that declares one or more
 * {@link Bean @Bean} methods and also triggers {@link EnableAutoConfiguration
 * auto-configuration} and {@link ComponentScan component scanning}. This is a convenience
 * annotation that is equivalent to declaring {@code @Configuration},
 * {@code @EnableAutoConfiguration} and {@code @ComponentScan}.
 *
 * @author Phillip Webb
 * @author Stephane Nicoll
 * @since 1.2.0
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
 @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
 @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
 /**
 * Exclude specific auto-configuration classes such that they will never be applied.
 * @return the classes to exclude
 */
 @AliasFor(annotation = EnableAutoConfiguration.class, attribute = "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
 */
 @AliasFor(annotation = EnableAutoConfiguration.class, attribute = "excludeName")
 String[] excludeName() default {};
 /**
 * Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}
 * for a type-safe alternative to String-based package names.
 * @return base packages to scan
 * @since 1.3.0
 */
 @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
 String[] scanBasePackages() default {};
 /**
 * Type-safe alternative to {@link #scanBasePackages} for specifying the packages to
 * scan for annotated components. The package of each class specified will be scanned.
 * <p>
 * Consider creating a special no-op marker class or interface in each package that
 * serves no purpose other than being referenced by this attribute.
 * @return base packages to scan
 * @since 1.3.0
 */
 @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
 Class<?>[] scanBasePackageClasses() default {};
}

　　这里面包含了@SpringBootConfiguration，@EnableAutoConfiguration，@ComponentScan，此处@ComponentScan由于没有指定扫描包，因此它默认扫描的是与该类同级的类或者同级包下的所有类，另外@SpringBootConfiguration，通过源码得知它是一个@Configuration:

/*
 * Copyright 2012-2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.springframework.boot;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Configuration;
/**
 * Indicates that a class provides Spring Boot application
 * {@link Configuration @Configuration}. Can be used as an alternative to the Spring's
 * standard {@code @Configuration} annotation so that configuration can be found
 * automatically (for example in tests).
 * <p>
 * Application should only ever include <em>one</em> {@code @SpringBootConfiguration} and
 * most idiomatic Spring Boot applications will inherit it from
 * {@code @SpringBootApplication}.
 *
 * @author Phillip Webb
 * @since 1.4.0
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}

　　由此我们可以推断出@SpringBootApplication等同于@Configuration @ComponentScan @EnableAutoConfiguration

1.2、@EnableAutoConfiguration

　　一旦加上此注解，那么将会开启自动装配功能，简单点讲，Spring会试图在你的classpath下找到所有配置的Bean然后进行装配。当然装配Bean时，会根据若干个(Conditional)定制规则来进行初始化。我们看一下它的源码：

/*
 * Copyright 2012-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.springframework.boot.autoconfigure;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.context.embedded.EmbeddedServletContainerFactory;
import org.springframework.boot.context.embedded.tomcat.TomcatEmbeddedServletContainerFactory;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;
import org.springframework.core.io.support.SpringFactoriesLoader;
/**
 * Enable auto-configuration of the Spring Application Context, attempting to guess and
 * configure beans that you are likely to need. Auto-configuration classes are usually
 * applied based on your classpath and what beans you have defined. For example, If you
 * have {@code tomcat-embedded.jar} on your classpath you are likely to want a
 * {@link TomcatEmbeddedServletContainerFactory} (unless you have defined your own
 * {@link EmbeddedServletContainerFactory} bean).
 * <p>
 * When using {@link SpringBootApplication}, the auto-configuration of the context is
 * automatically enabled and adding this annotation has therefore no additional effect.
 * <p>
 * Auto-configuration tries to be as intelligent as possible and will back-away as you
 * define more of your own configuration. You can always manually {@link #exclude()} any
 * configuration that you never want to apply (use {@link #excludeName()} if you don't
 * have access to them). You can also exclude them via the
 * {@code spring.autoconfigure.exclude} property. Auto-configuration is always applied
 * after user-defined beans have been registered.
 * <p>
 * The package of the class that is annotated with {@code @EnableAutoConfiguration},
 * usually via {@code @SpringBootApplication}, has specific significance and is often used
 * as a 'default'. For example, it will be used when scanning for {@code @Entity} classes.
 * It is generally recommended that you place {@code @EnableAutoConfiguration} (if you're
 * not using {@code @SpringBootApplication}) in a root package so that all sub-packages
 * and classes can be searched.
 * <p>
 * Auto-configuration classes are regular Spring {@link Configuration} beans. They are
 * located using the {@link SpringFactoriesLoader} mechanism (keyed against this class).
 * Generally auto-configuration beans are {@link Conditional @Conditional} beans (most
 * often using {@link ConditionalOnClass @ConditionalOnClass} and
 * {@link ConditionalOnMissingBean @ConditionalOnMissingBean} annotations).
 *
 * @author Phillip Webb
 * @author Stephane Nicoll
 * @see ConditionalOnBean
 * @see ConditionalOnMissingBean
 * @see ConditionalOnClass
 * @see AutoConfigureAfter
 * @see SpringBootApplication
 */
@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.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 {};
}

　　虽然根据文档注释的说明它指点我们去看EnableAutoConfigurationImportSelector。但是该类在SpringBoot1.5.X版本已经过时了，因此我们看一下它的父类AutoConfigurationImportSelector：

/*
 * Copyright 2012-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.springframework.boot.autoconfigure;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.TimeUnit;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.Aware;
import org.springframework.beans.factory.BeanClassLoaderAware;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.BeanFactoryAware;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.boot.bind.RelaxedPropertyResolver;
import org.springframework.context.EnvironmentAware;
import org.springframework.context.ResourceLoaderAware;
import org.springframework.context.annotation.DeferredImportSelector;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.AnnotationAttributes;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.Environment;
import org.springframework.core.io.ResourceLoader;
import org.springframework.core.io.support.SpringFactoriesLoader;
import org.springframework.core.type.AnnotationMetadata;
import org.springframework.core.type.classreading.CachingMetadataReaderFactory;
import org.springframework.core.type.classreading.MetadataReaderFactory;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.StringUtils;
/**
 * {@link DeferredImportSelector} to handle {@link EnableAutoConfiguration
 * auto-configuration}. This class can also be subclassed if a custom variant of
 * {@link EnableAutoConfiguration @EnableAutoConfiguration}. is needed.
 *
 * @author Phillip Webb
 * @author Andy Wilkinson
 * @author Stephane Nicoll
 * @author Madhura Bhave
 * @since 1.3.0
 * @see EnableAutoConfiguration
 */
public class AutoConfigurationImportSelector
 implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
 BeanFactoryAware, EnvironmentAware, Ordered {
 private static final String[] NO_IMPORTS = {};
 private static final Log logger = LogFactory
 .getLog(AutoConfigurationImportSelector.class);
 private ConfigurableListableBeanFactory beanFactory;
 private Environment environment;
 private ClassLoader beanClassLoader;
 private ResourceLoader resourceLoader;
 @Override
 public String[] selectImports(AnnotationMetadata annotationMetadata) {
 if (!isEnabled(annotationMetadata)) {
 return NO_IMPORTS;
 }
 try {
 AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
  .loadMetadata(this.beanClassLoader);
 AnnotationAttributes attributes = getAttributes(annotationMetadata);
 List<String> configurations = getCandidateConfigurations(annotationMetadata,
  attributes);
 configurations = removeDuplicates(configurations);
 configurations = sort(configurations, autoConfigurationMetadata);
 Set<String> exclusions = getExclusions(annotationMetadata, attributes);
 checkExcludedClasses(configurations, exclusions);
 configurations.removeAll(exclusions);
 configurations = filter(configurations, autoConfigurationMetadata);
 fireAutoConfigurationImportEvents(configurations, exclusions);
 return configurations.toArray(new String[configurations.size()]);
 }
 catch (IOException ex) {
 throw new IllegalStateException(ex);
 }
 }
 protected boolean isEnabled(AnnotationMetadata metadata) {
 return true;
 }
 /**
 * Return the appropriate {@link AnnotationAttributes} from the
 * {@link AnnotationMetadata}. By default this method will return attributes for
 * {@link #getAnnotationClass()}.
 * @param metadata the annotation metadata
 * @return annotation attributes
 */
 protected AnnotationAttributes getAttributes(AnnotationMetadata metadata) {
 String name = getAnnotationClass().getName();
 AnnotationAttributes attributes = AnnotationAttributes
 .fromMap(metadata.getAnnotationAttributes(name, true));
 Assert.notNull(attributes,
 "No auto-configuration attributes found. Is " + metadata.getClassName()
  + " annotated with " + ClassUtils.getShortName(name) + "?");
 return attributes;
 }
 /**
 * Return the source annotation class used by the selector.
 * @return the annotation class
 */
 protected Class<?> getAnnotationClass() {
 return EnableAutoConfiguration.class;
 }
 /**
 * Return the auto-configuration class names that should be considered. By default
 * this method will load candidates using {@link SpringFactoriesLoader} with
 * {@link #getSpringFactoriesLoaderFactoryClass()}.
 * @param metadata the source metadata
 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
 * attributes}
 * @return a list of candidate configurations
 */
 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;
 }
 /**
 * Return the class used by {@link SpringFactoriesLoader} to load configuration
 * candidates.
 * @return the factory class
 */
 protected Class<?> getSpringFactoriesLoaderFactoryClass() {
 return EnableAutoConfiguration.class;
 }
 private void checkExcludedClasses(List<String> configurations,
 Set<String> exclusions) {
 List<String> invalidExcludes = new ArrayList<String>(exclusions.size());
 for (String exclusion : exclusions) {
 if (ClassUtils.isPresent(exclusion, getClass().getClassLoader())
  && !configurations.contains(exclusion)) {
 invalidExcludes.add(exclusion);
 }
 }
 if (!invalidExcludes.isEmpty()) {
 handleInvalidExcludes(invalidExcludes);
 }
 }
 /**
 * Handle any invalid excludes that have been specified.
 * @param invalidExcludes the list of invalid excludes (will always have at least one
 * element)
 */
 protected void handleInvalidExcludes(List<String> invalidExcludes) {
 StringBuilder message = new StringBuilder();
 for (String exclude : invalidExcludes) {
 message.append("\t- ").append(exclude).append(String.format("%n"));
 }
 throw new IllegalStateException(String
 .format("The following classes could not be excluded because they are"
  + " not auto-configuration classes:%n%s", message));
 }
 /**
 * Return any exclusions that limit the candidate configurations.
 * @param metadata the source metadata
 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
 * attributes}
 * @return exclusions or an empty set
 */
 protected Set<String> getExclusions(AnnotationMetadata metadata,
 AnnotationAttributes attributes) {
 Set<String> excluded = new LinkedHashSet<String>();
 excluded.addAll(asList(attributes, "exclude"));
 excluded.addAll(Arrays.asList(attributes.getStringArray("excludeName")));
 excluded.addAll(getExcludeAutoConfigurationsProperty());
 return excluded;
 }
 private List<String> getExcludeAutoConfigurationsProperty() {
 if (getEnvironment() instanceof ConfigurableEnvironment) {
 RelaxedPropertyResolver resolver = new RelaxedPropertyResolver(
  this.environment, "spring.autoconfigure.");
 Map<String, Object> properties = resolver.getSubProperties("exclude");
 if (properties.isEmpty()) {
 return Collections.emptyList();
 }
 List<String> excludes = new ArrayList<String>();
 for (Map.Entry<String, Object> entry : properties.entrySet()) {
 String name = entry.getKey();
 Object value = entry.getValue();
 if (name.isEmpty() || name.startsWith("[") && value != null) {
  excludes.addAll(new HashSet<String>(Arrays.asList(StringUtils
  .tokenizeToStringArray(String.valueOf(value), ","))));
 }
 }
 return excludes;
 }
 RelaxedPropertyResolver resolver = new RelaxedPropertyResolver(getEnvironment(),
 "spring.autoconfigure.");
 String[] exclude = resolver.getProperty("exclude", String[].class);
 return (Arrays.asList(exclude == null ? new String[0] : exclude));
 }
 private List<String> sort(List<String> configurations,
 AutoConfigurationMetadata autoConfigurationMetadata) throws IOException {
 configurations = new AutoConfigurationSorter(getMetadataReaderFactory(),
 autoConfigurationMetadata).getInPriorityOrder(configurations);
 return configurations;
 }
 private List<String> filter(List<String> configurations,
 AutoConfigurationMetadata autoConfigurationMetadata) {
 long startTime = System.nanoTime();
 String[] candidates = configurations.toArray(new String[configurations.size()]);
 boolean[] skip = new boolean[candidates.length];
 boolean skipped = false;
 for (AutoConfigurationImportFilter filter : getAutoConfigurationImportFilters()) {
 invokeAwareMethods(filter);
 boolean[] match = filter.match(candidates, autoConfigurationMetadata);
 for (int i = 0; i < match.length; i++) {
 if (!match[i]) {
  skip[i] = true;
  skipped = true;
 }
 }
 }
 if (!skipped) {
 return configurations;
 }
 List<String> result = new ArrayList<String>(candidates.length);
 for (int i = 0; i < candidates.length; i++) {
 if (!skip[i]) {
 result.add(candidates[i]);
 }
 }
 if (logger.isTraceEnabled()) {
 int numberFiltered = configurations.size() - result.size();
 logger.trace("Filtered " + numberFiltered + " auto configuration class in "
  + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime)
  + " ms");
 }
 return new ArrayList<String>(result);
 }
 protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
 return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class,
 this.beanClassLoader);
 }
 private MetadataReaderFactory getMetadataReaderFactory() {
 try {
 return getBeanFactory().getBean(
  SharedMetadataReaderFactoryContextInitializer.BEAN_NAME,
  MetadataReaderFactory.class);
 }
 catch (NoSuchBeanDefinitionException ex) {
 return new CachingMetadataReaderFactory(this.resourceLoader);
 }
 }
 protected final <T> List<T> removeDuplicates(List<T> list) {
 return new ArrayList<T>(new LinkedHashSet<T>(list));
 }
 protected final List<String> asList(AnnotationAttributes attributes, String name) {
 String[] value = attributes.getStringArray(name);
 return Arrays.asList(value == null ? new String[0] : value);
 }
 private void fireAutoConfigurationImportEvents(List<String> configurations,
 Set<String> exclusions) {
 List<AutoConfigurationImportListener> listeners = getAutoConfigurationImportListeners();
 if (!listeners.isEmpty()) {
 AutoConfigurationImportEvent event = new AutoConfigurationImportEvent(this,
  configurations, exclusions);
 for (AutoConfigurationImportListener listener : listeners) {
 invokeAwareMethods(listener);
 listener.onAutoConfigurationImportEvent(event);
 }
 }
 }
 protected List<AutoConfigurationImportListener> getAutoConfigurationImportListeners() {
 return SpringFactoriesLoader.loadFactories(AutoConfigurationImportListener.class,
 this.beanClassLoader);
 }
 private void invokeAwareMethods(Object instance) {
 if (instance instanceof Aware) {
 if (instance instanceof BeanClassLoaderAware) {
 ((BeanClassLoaderAware) instance)
  .setBeanClassLoader(this.beanClassLoader);
 }
 if (instance instanceof BeanFactoryAware) {
 ((BeanFactoryAware) instance).setBeanFactory(this.beanFactory);
 }
 if (instance instanceof EnvironmentAware) {
 ((EnvironmentAware) instance).setEnvironment(this.environment);
 }
 if (instance instanceof ResourceLoaderAware) {
 ((ResourceLoaderAware) instance).setResourceLoader(this.resourceLoader);
 }
 }
 }
 @Override
 public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
 Assert.isInstanceOf(ConfigurableListableBeanFactory.class, beanFactory);
 this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;
 }
 protected final ConfigurableListableBeanFactory getBeanFactory() {
 return this.beanFactory;
 }
 @Override
 public void setBeanClassLoader(ClassLoader classLoader) {
 this.beanClassLoader = classLoader;
 }
 protected ClassLoader getBeanClassLoader() {
 return this.beanClassLoader;
 }
 @Override
 public void setEnvironment(Environment environment) {
 this.environment = environment;
 }
 protected final Environment getEnvironment() {
 return this.environment;
 }
 @Override
 public void setResourceLoader(ResourceLoader resourceLoader) {
 this.resourceLoader = resourceLoader;
 }
 protected final ResourceLoader getResourceLoader() {
 return this.resourceLoader;
 }
 @Override
 public int getOrder() {
 return Ordered.LOWEST_PRECEDENCE - 1;
 }
}

　　首先该类实现了DeferredImportSelector接口，这个接口继承了ImportSelector:

/*
 * Copyright 2002-2013 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.springframework.context.annotation;
import org.springframework.core.type.AnnotationMetadata;
/**
 * Interface to be implemented by types that determine which @{@link Configuration}
 * class(es) should be imported based on a given selection criteria, usually one or more
 * annotation attributes.
 *
 * <p>An {@link ImportSelector} may implement any of the following
 * {@link org.springframework.beans.factory.Aware Aware} interfaces, and their respective
 * methods will be called prior to {@link #selectImports}:
 * <ul>
 * <li>{@link org.springframework.context.EnvironmentAware EnvironmentAware}</li>
 * <li>{@link org.springframework.beans.factory.BeanFactoryAware BeanFactoryAware}</li>
 * <li>{@link org.springframework.beans.factory.BeanClassLoaderAware BeanClassLoaderAware}</li>
 * <li>{@link org.springframework.context.ResourceLoaderAware ResourceLoaderAware}</li>
 * </ul>
 *
 * <p>ImportSelectors are usually processed in the same way as regular {@code @Import}
 * annotations, however, it is also possible to defer selection of imports until all
 * {@code @Configuration} classes have been processed (see {@link DeferredImportSelector}
 * for details).
 *
 * @author Chris Beams
 * @since 3.1
 * @see DeferredImportSelector
 * @see Import
 * @see ImportBeanDefinitionRegistrar
 * @see Configuration
 */
public interface ImportSelector {
 /**
 * Select and return the names of which class(es) should be imported based on
 * the {@link AnnotationMetadata} of the importing @{@link Configuration} class.
 */
 String[] selectImports(AnnotationMetadata importingClassMetadata);
}

　　该接口主要是为了导入@Configuration的配置项，而DeferredImportSelector是延期导入，当所有的@Configuration都处理过后才会执行。

　　回过头来我们看一下AutoConfigurationImportSelector的selectImport方法：

@Override
 public String[] selectImports(AnnotationMetadata annotationMetadata) {
 if (!isEnabled(annotationMetadata)) {
 return NO_IMPORTS;
 }
 try {
 AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
  .loadMetadata(this.beanClassLoader);
 AnnotationAttributes attributes = getAttributes(annotationMetadata);
 List<String> configurations = getCandidateConfigurations(annotationMetadata,
  attributes);
 configurations = removeDuplicates(configurations);
 configurations = sort(configurations, autoConfigurationMetadata);
 Set<String> exclusions = getExclusions(annotationMetadata, attributes);
 checkExcludedClasses(configurations, exclusions);
 configurations.removeAll(exclusions);
 configurations = filter(configurations, autoConfigurationMetadata);
 fireAutoConfigurationImportEvents(configurations, exclusions);
 return configurations.toArray(new String[configurations.size()]);
 }
 catch (IOException ex) {
 throw new IllegalStateException(ex);
 }
 }

　　该方法刚开始会先判断是否进行自动装配，而后会从META-INF/spring-autoconfigure-metadata.properties读取元数据与元数据的相关属性，紧接着会调用getCandidateConfigurations方法：

/**
 * Return the auto-configuration class names that should be considered. By default
 * this method will load candidates using {@link SpringFactoriesLoader} with
 * {@link #getSpringFactoriesLoaderFactoryClass()}.
 * @param metadata the source metadata
 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
 * attributes}
 * @return a list of candidate configurations
 */
 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;
 }
 /**
 * Return the class used by {@link SpringFactoriesLoader} to load configuration
 * candidates.
 * @return the factory class
 */
 protected Class<?> getSpringFactoriesLoaderFactoryClass() {
 return EnableAutoConfiguration.class;
 }

　　在这里又遇到我们的老熟人了--SpringFactoryiesLoader, 它会读取META-INF/spring.factories下的EnableAutoConfiguration的配置，紧接着在进行排除与过滤，进而得到需要装配的类。最后让所有配置在META-INF/spring.factories下的AutoConfigurationImportListener执行AutoConfigurationImportEvent事件，代码如下：

private void fireAutoConfigurationImportEvents(List<String> configurations,
 Set<String> exclusions) {
 List<AutoConfigurationImportListener> listeners = getAutoConfigurationImportListeners();
 if (!listeners.isEmpty()) {
 AutoConfigurationImportEvent event = new AutoConfigurationImportEvent(this,
  configurations, exclusions);
 for (AutoConfigurationImportListener listener : listeners) {
 invokeAwareMethods(listener);
 listener.onAutoConfigurationImportEvent(event);
 }
 }
 }
 protected List<AutoConfigurationImportListener> getAutoConfigurationImportListeners() {
 return SpringFactoriesLoader.loadFactories(AutoConfigurationImportListener.class,
 this.beanClassLoader);
 }

二、何时进行自动装配

　　在前面的环节里只是最终要确定哪些类需要被装配，在SpringBoot时何时处理这些自动装配的类呢？下面我们简要的分析一下：

2.1、AbstractApplicationContext的refresh方法：

　　这个方法老生常谈了其中请大家关注一下这个方法：

// Invoke factory processors registered as beans in the context.
 invokeBeanFactoryPostProcessors(beanFactory);

　　在这里是处理BeanFactoryPostProcessor的，那么我们在来看一下这个接口BeanDefinitionRegistryPostProcessor:

/*
 * Copyright 2002-2010 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.springframework.beans.factory.support;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
/**
 * Extension to the standard {@link BeanFactoryPostProcessor} SPI, allowing for
 * the registration of further bean definitions <i>before</i> regular
 * BeanFactoryPostProcessor detection kicks in. In particular,
 * BeanDefinitionRegistryPostProcessor may register further bean definitions
 * which in turn define BeanFactoryPostProcessor instances.
 *
 * @author Juergen Hoeller
 * @since 3.0.1
 * @see org.springframework.context.annotation.ConfigurationClassPostProcessor
 */
public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {
 /**
 * Modify the application context's internal bean definition registry after its
 * standard initialization. All regular bean definitions will have been loaded,
 * but no beans will have been instantiated yet. This allows for adding further
 * bean definitions before the next post-processing phase kicks in.
 * @param registry the bean definition registry used by the application context
 * @throws org.springframework.beans.BeansException in case of errors
 */
 void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException;
}

　　该接口继承了BeanFactoryPostProcessor。

2.2、ConfigurationClassPostProcessor 类

　　该类主要处理@Configuration注解的，它实现了BeanDefinitionRegistryPostProcessor,  那么也间接实现了BeanFactoryPostProcessor，关键代码如下：

@Override
 public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
 int factoryId = System.identityHashCode(beanFactory);
 if (this.factoriesPostProcessed.contains(factoryId)) {
  throw new IllegalStateException(
   "postProcessBeanFactory already called on this post-processor against " + beanFactory);
 }
 this.factoriesPostProcessed.add(factoryId);
 if (!this.registriesPostProcessed.contains(factoryId)) {
  // BeanDefinitionRegistryPostProcessor hook apparently not supported...
  // Simply call processConfigurationClasses lazily at this point then.
  processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
 }
 enhanceConfigurationClasses(beanFactory);
 beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
 }
/**
 * Build and validate a configuration model based on the registry of
 * {@link Configuration} classes.
 */
 public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
 //.....省略部分代码
 // Parse each @Configuration class
 ConfigurationClassParser parser = new ConfigurationClassParser(
  this.metadataReaderFactory, this.problemReporter, this.environment,
  this.resourceLoader, this.componentScanBeanNameGenerator, registry);
 Set<BeanDefinitionHolder> candidates = new LinkedHashSet<BeanDefinitionHolder>(configCandidates);
 Set<ConfigurationClass> alreadyParsed = new HashSet<ConfigurationClass>(configCandidates.size());
 do {
  parser.parse(candidates);
  parser.validate();
  Set<ConfigurationClass> configClasses = new LinkedHashSet<ConfigurationClass>(parser.getConfigurationClasses());
  configClasses.removeAll(alreadyParsed);
  // Read the model and create bean definitions based on its content
  if (this.reader == null) {
  this.reader = new ConfigurationClassBeanDefinitionReader(
   registry, this.sourceExtractor, this.resourceLoader, this.environment,
   this.importBeanNameGenerator, parser.getImportRegistry());
  }
  this.reader.loadBeanDefinitions(configClasses);
  alreadyParsed.addAll(configClasses);
  candidates.clear();
  if (registry.getBeanDefinitionCount() > candidateNames.length) {
  String[] newCandidateNames = registry.getBeanDefinitionNames();
  Set<String> oldCandidateNames = new HashSet<String>(Arrays.asList(candidateNames));
  Set<String> alreadyParsedClasses = new HashSet<String>();
  for (ConfigurationClass configurationClass : alreadyParsed) {
   alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
  }
  for (String candidateName : newCandidateNames) {
   if (!oldCandidateNames.contains(candidateName)) {
   BeanDefinition bd = registry.getBeanDefinition(candidateName);
   if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
    !alreadyParsedClasses.contains(bd.getBeanClassName())) {
    candidates.add(new BeanDefinitionHolder(bd, candidateName));
   }
   }
  }
  candidateNames = newCandidateNames;
  }
 }
 while (!candidates.isEmpty());
　　　　// ....省略部分代码
 } 

其实这里注释已经很清楚了，我们可以清楚的看到解析每一个@ConfigurationClass的关键类是：ConfigurationClassParser，那么我们继续看一看这个类的parse方法：

public void parse(Set<BeanDefinitionHolder> configCandidates) {
 this.deferredImportSelectors = new LinkedList<DeferredImportSelectorHolder>();
 for (BeanDefinitionHolder holder : configCandidates) {
  BeanDefinition bd = holder.getBeanDefinition();
  try {
  if (bd instanceof AnnotatedBeanDefinition) {
   parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
  }
  else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) {
   parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
  }
  else {
   parse(bd.getBeanClassName(), holder.getBeanName());
  }
  }
  catch (BeanDefinitionStoreException ex) {
  throw ex;
  }
  catch (Throwable ex) {
  throw new BeanDefinitionStoreException(
   "Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
  }
 }
 processDeferredImportSelectors();
 }

　　在这里大家留意一下最后一句processDeferredImportSelectors方法，在这里将会对DeferredImportSelector进行处理，这样我们就和AutoConfigurationSelectImporter结合到一起了：

private void processDeferredImportSelectors() {
 List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
 this.deferredImportSelectors = null;
 Collections.sort(deferredImports, DEFERRED_IMPORT_COMPARATOR);
 for (DeferredImportSelectorHolder deferredImport : deferredImports) {
  ConfigurationClass configClass = deferredImport.getConfigurationClass();
  try {
  String[] imports = deferredImport.getImportSelector().selectImports(configClass.getMetadata());
  processImports(configClass, asSourceClass(configClass), asSourceClasses(imports), false);
  }
  catch (BeanDefinitionStoreException ex) {
  throw ex;
  }
  catch (Throwable ex) {
  throw new BeanDefinitionStoreException(
   "Failed to process import candidates for configuration class [" +
   configClass.getMetadata().getClassName() + "]", ex);
  }
 }
 }

请大家关注这句代码：String[] imports = deferredImport.getImportSelector().selectImports(configClass.getMetadata());在这里deferredImport的类型为DeferredImportSelectorHolder：

private static class DeferredImportSelectorHolder {
 private final ConfigurationClass configurationClass;
 private final DeferredImportSelector importSelector;
 public DeferredImportSelectorHolder(ConfigurationClass configClass, DeferredImportSelector selector) {
  this.configurationClass = configClass;
  this.importSelector = selector;
 }
 public ConfigurationClass getConfigurationClass() {
  return this.configurationClass;
 }
 public DeferredImportSelector getImportSelector() {
  return this.importSelector;
 }
 }

　　在这个内部类里持有了一个DeferredImportSelector的引用，至此将会执行自动装配的所有操作

三、总结

　　1）自动装配还是利用了SpringFactoriesLoader来加载META-INF/spring.factoires文件里所有配置的EnableAutoConfgruation，它会经过exclude和filter等操作，最终确定要装配的类

　　2)  处理@Configuration的核心还是ConfigurationClassPostProcessor，这个类实现了BeanFactoryPostProcessor, 因此当AbstractApplicationContext执行refresh方法里的invokeBeanFactoryPostProcessors(beanFactory)方法时会执行自动装配

以上所述是小编给大家介绍的SpringBoot中的自动装配，希望对大家有所帮助，如果大家有任何疑问欢迎给我留言，小编会及时回复大家的！