本文主要探讨了SpringBoot的Bean自动化配置，重点在于@EnableAutoConfiguration注解的工作原理，它如何启用自动装配功能，以及@Import的三种用法，特别是ImportSelector方式的实现逻辑，包括从元数据和spring.factories文件中读取配置并进行装配。
Bean自动化配置
Springboot自动化配置是开箱即用的基础，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：
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)
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)
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 {};
}这里面包含了三个注解：
@SpringBootConfiguration @EnableAutoConfiguration @ComponentScan
@SpringBootConfiguration本质是一个@Configuration，等价于spring配置文件，我们比较熟悉，此处就不再赘述；@ComponentScan这个注解是组件扫描注解，我们也比较熟悉，就不多做探讨了；
接下来重点了解一下@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：
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.web.embedded.tomcat.TomcatServletWebServerFactory;
import org.springframework.boot.web.servlet.server.ServletWebServerFactory;
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 TomcatServletWebServerFactory} (unless you have defined your own
* {@link ServletWebServerFactory} 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
*/
@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 {};
}这个注解里面使用@Import注解，@Import通过快速导入的方式实现把实例加入spring的IOC容器中
@Import的三种用法
1、直接填class数组方式
2、ImportSelector方式
3、ImportBeanDefinitionRegistrar方式
此处使用的是第二种方式，这种方式的前提就是一个类要实现ImportSelector接口，我们看下AutoConfigurationImportSelector的源码：
/*
* Copyright 2012-2019 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
*
* https://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.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.TimeUnit;
import java.util.stream.Collectors;
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.context.properties.bind.Binder;
import org.springframework.context.EnvironmentAware;
import org.springframework.context.ResourceLoaderAware;
import org.springframework.context.annotation.Configuration;
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 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);
}
@Override
public Class<? extends Group> getImportGroup() {
return AutoConfigurationGroup.class;
}
protected boolean isEnabled(AnnotationMetadata metadata) {
if (getClass() == AutoConfigurationImportSelector.class) {
return getEnvironment().getProperty(EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class, true);
}
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<>(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<>();
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) {
Binder binder = Binder.get(getEnvironment());
return binder.bind(PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE, String[].class).map(Arrays::asList)
.orElse(Collections.emptyList());
}
String[] excludes = getEnvironment().getProperty(PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE, String[].class);
return (excludes != null) ? Arrays.asList(excludes) : Collections.emptyList();
}
private List<String> filter(List<String> configurations, AutoConfigurationMetadata autoConfigurationMetadata) {
long startTime = System.nanoTime();
String[] candidates = StringUtils.toStringArray(configurations);
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;
candidates[i] = null;
skipped = true;
}
}
}
if (!skipped) {
return configurations;
}
List<String> result = new ArrayList<>(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<>(result);
}
protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class, this.beanClassLoader);
}
protected final <T> List<T> removeDuplicates(List<T> list) {
return new ArrayList<>(new LinkedHashSet<>(list));
}
protected final List<String> asList(AnnotationAttributes attributes, String name) {
String[] value = attributes.getStringArray(name);
return Arrays.asList((value != null) ? value : new String[0]);
}
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;
}
private static class AutoConfigurationGroup
implements DeferredImportSelector.Group, BeanClassLoaderAware, BeanFactoryAware, ResourceLoaderAware {
private final Map<String, AnnotationMetadata> entries = new LinkedHashMap<>();
private final List<AutoConfigurationEntry> autoConfigurationEntries = new ArrayList<>();
private ClassLoader beanClassLoader;
private BeanFactory beanFactory;
private ResourceLoader resourceLoader;
private AutoConfigurationMetadata autoConfigurationMetadata;
@Override
public void setBeanClassLoader(ClassLoader classLoader) {
this.beanClassLoader = classLoader;
}
@Override
public void setBeanFactory(BeanFactory beanFactory) {
this.beanFactory = beanFactory;
}
@Override
public void setResourceLoader(ResourceLoader resourceLoader) {
this.resourceLoader = resourceLoader;
}
@Override
public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector,
() -> String.format("Only %s implementations are supported, got %s",
AutoConfigurationImportSelector.class.getSimpleName(),
deferredImportSelector.getClass().getName()));
AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
.getAutoConfigurationEntry(getAutoConfigurationMetadata(), annotationMetadata);
this.autoConfigurationEntries.add(autoConfigurationEntry);
for (String importClassName : autoConfigurationEntry.getConfigurations()) {
this.entries.putIfAbsent(importClassName, annotationMetadata);
}
}
@Override
public Iterable<Entry> selectImports() {
if (this.autoConfigurationEntries.isEmpty()) {
return Collections.emptyList();
}
Set<String> allExclusions = this.autoConfigurationEntries.stream()
.map(AutoConfigurationEntry::getExclusions).flatMap(Collection::stream).collect(Collectors.toSet());
Set<String> processedConfigurations = this.autoConfigurationEntries.stream()
.map(AutoConfigurationEntry::getConfigurations).flatMap(Collection::stream)
.collect(Collectors.toCollection(LinkedHashSet::new));
processedConfigurations.removeAll(allExclusions);
return sortAutoConfigurations(processedConfigurations, getAutoConfigurationMetadata()).stream()
.map((importClassName) -> new Entry(this.entries.get(importClassName), importClassName))
.collect(Collectors.toList());
}
private AutoConfigurationMetadata getAutoConfigurationMetadata() {
if (this.autoConfigurationMetadata == null) {
this.autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
}
return this.autoConfigurationMetadata;
}
private List<String> sortAutoConfigurations(Set<String> configurations,
AutoConfigurationMetadata autoConfigurationMetadata) {
return new AutoConfigurationSorter(getMetadataReaderFactory(), autoConfigurationMetadata)
.getInPriorityOrder(configurations);
}
private MetadataReaderFactory getMetadataReaderFactory() {
try {
return this.beanFactory.getBean(SharedMetadataReaderFactoryContextInitializer.BEAN_NAME,
MetadataReaderFactory.class);
}
catch (NoSuchBeanDefinitionException ex) {
return new CachingMetadataReaderFactory(this.resourceLoader);
}
}
}
protected static class AutoConfigurationEntry {
private final List<String> configurations;
private final Set<String> exclusions;
private AutoConfigurationEntry() {
this.configurations = Collections.emptyList();
this.exclusions = Collections.emptySet();
}
/**
* Create an entry with the configurations that were contributed and their
* exclusions.
* @param configurations the configurations that should be imported
* @param exclusions the exclusions that were applied to the original list
*/
AutoConfigurationEntry(Collection<String> configurations, Collection<String> exclusions) {
this.configurations = new ArrayList<>(configurations);
this.exclusions = new HashSet<>(exclusions);
}
public List<String> getConfigurations() {
return this.configurations;
}
public Set<String> getExclusions() {
return this.exclusions;
}
}
}
里面的public String[] selectImports(AnnotationMetadata annotationMetadata) 方法会返回组件的全类名的数组，返回的结果会注入到Spring的容器当中。接下来看下里面详细实现逻辑：
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());
}总共三步：
1.从META-INF/spring-autoconfigure-metadata.properties中读取元数据与元数据的相关属性；
2.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
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);
}
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;
}
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);
}
}
}3.返回符合条件的自动配置结果
由此可知，其实@EnableAutoConfiguration注解是利用了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，并自动装配到Spring中进行管理。
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