本文的代码分析基于Spring Boot 2.1.5的版本。
Spring Boot相对于Spring的一大改变或者优势来说就是“约定大于配置”的思想，不像Spring一样所有的配置都需要我们自己去实现，Spring Boot集成了许多默认的配置。拿Spring MVC来举例，原来Spring时代是通过写两个XML配置文件来实现的，一个web.xml，另一个applicationContext.xml。这些文件内容复杂，且大部分情况下不需要改变，在各个项目中的迁移也只是复制粘贴里面的代码而已，这无疑增加了使用成本。而在Spring Boot中，只需要引入相关spring-boot-starter-web依赖即可，其他的配置都不需要。即使有需要其他配置的地方，统一在application.properties配置文件中进行配置即可，该文件写法是类似于json的键值对的格式，不像XML格式那样的重量级。
那么既然不需要相关配置，Spring Boot是如何实现自动装配类的呢？如何在项目启动的时候将需要加载的类都注入到Spring的IoC容器中？本文将探究这个问题。
通常在Spring Boot的启动类上会加上@SpringBootApplication的注解，如下所示：
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}其注解主要是由三个子注解构成的，分别是@SpringBootConfiguration、@EnableAutoConfiguration和@ComponentScan：
@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 {
//...
}这样我们可以直接使用@SpringBootApplication注解而不再需要使用以上三个注解来标识启动类了。其中@EnableAutoConfiguration是开启自动装配的功能，该注解的代码如下所示：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
//...
}由上所示，@EnableAutoConfiguration注解需要引入AutoConfigurationImportSelector这个类或者其子类，如果没有找到，则自动装配失败。
而在上述启动时会调用SpringApplication的run方法。该方法会最终调用如下所示的重载run方法：
/**
* Run the Spring application, creating and refreshing a new
* {@link ApplicationContext}.
* @param args the application arguments (usually passed from a Java main method)
* @return a running {@link ApplicationContext}
*/
public ConfigurableApplicationContext run(String... args) {
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
configureHeadlessProperty();
SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
configureIgnoreBeanInfo(environment);
Banner printedBanner = printBanner(environment);
context = createApplicationContext();
exceptionReporters = getSpringFactoriesInstances(
SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
prepareContext(context, environment, listeners, applicationArguments,
printedBanner);
refreshContext(context);
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass)
.logStarted(getApplicationLog(), stopWatch);
}
listeners.started(context);
callRunners(context, applicationArguments);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, listeners);
throw new IllegalStateException(ex);
}
try {
listeners.running(context);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, null);
throw new IllegalStateException(ex);
}
return context;
}以上方法会创建/刷新ApplicationContext、初始化Environment、listeners等一系列操作。在第23行代码中，其会调用getSpringFactoriesInstances方法，同时会将SpringBootExceptionReporter.class这个参数传入进去：
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = getClassLoader();
// Use names and ensure unique to protect against duplicates
Set<String> names = new LinkedHashSet<>(
SpringFactoriesLoader.loadFactoryNames(type, classLoader));
List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}在该方法中会调用SpringFactoriesLoader的loadFactoryNames方法。SpringFactoriesLoader类的作用是利用工厂的加载机制来读取装配资源的类，其部分源码如下所示：
public final class SpringFactoriesLoader {
/**
* The location to look for factories.
* <p>Can be present in multiple JAR files.
*/
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
//...
private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>();
//...
/**
* Load the fully qualified class names of factory implementations of the
* given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given
* class loader.
* @param factoryClass the interface or abstract class representing the factory
* @param classLoader the ClassLoader to use for loading resources; can be
* {@code null} to use the default
* @throws IllegalArgumentException if an error occurs while loading factory names
* @see #loadFactories
*/
public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
}
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
MultiValueMap<String, String> result = cache.get(classLoader);
if (result != null) {
return result;
}
try {
Enumeration<URL> urls = (classLoader != null ?
classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
result = new LinkedMultiValueMap<>();
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry<?, ?> entry : properties.entrySet()) {
String factoryClassName = ((String) entry.getKey()).trim();
for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
result.add(factoryClassName, factoryName.trim());
}
}
}
cache.put(classLoader, result);
return result;
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
}
//...
}
其中可以看到FACTORIES_RESOURCE_LOCATION这个常量，它就是读取配置资源的文件地址，也就是说会从spring.factories文件中读取所有需要注入的类出来，每一个jar包下基本上都会有一个spring.factories配置文件。而上面源码中第25行的loadFactoryNames方法会调用第30行的loadSpringFactories方法，该方法的作用是读取加载进来的所有jar包下的spring.factories配置文件中的内容，将其放到一个本地缓存cache中。缓存的意义在于第一次调用该方法时会读取spring.factories文件并将读取中的结果放到缓存中，之后再调用该方法时就不再读取文件，而直接返回缓存中的内容就行了。
在spring-boot-autoconfigure的jar包下META-INF目录中的spring.factories文件的部分内容如下：
# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer
# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener
# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
//...由上可以看到，其文件格式由键值对组成，每一行末尾的反斜杠表示换行，其他jar包下spring.factories文件的内容也都会有所不同。
而在上面说过的SpringFactoriesLoader类的loadFactoryNames方法中会通过ClassLoader来读取spring.factories文件中的内容。通过run方法中传入的SpringBootExceptionReporter.class这个参数，找到文件中键是SpringBootExceptionReporter所对应的值的集合，通过反射机制来实例化相关的类再返回即可。如前面所说，这里是第一次调用SpringFactoriesLoader类的工厂加载机制的地方，后续再调用的话会直接走缓存中的内容。
再回到最开始SpringApplication类中的run方法中的代码中，之前说的都是基于其中的getSpringFactoriesInstances方法的延展，在该方法执行完毕后，在第28行会调用refreshContext方法，在其中会调用AutoConfigurationImportSelector类的getAutoConfigurationEntry方法，该方法就是自动装配类的方法入口，而AutoConfigurationImportSelector类前面也说过，是@EnableAutoConfiguration这个注解必须要引入的类。在getAutoConfigurationEntry方法中会调用getCandidateConfigurations方法，这两个方法的源码如下：
/**
* 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);
}
/**
* 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;
}getCandidateConfigurations方法中也会像上面说过的getSpringFactoriesInstances方法一样，调用SpringFactoriesLoader的loadFactoryNames方法来获取配置文件中的内容，只不过这次传入的参数是getSpringFactoriesLoaderFactoryClass()，即EnableAutoConfiguration.class。也就是说会将需要自动装配的类都拿到，通过EnableAutoConfiguration所对应的值的类名的集合。但这次并不会读取spring.factories配置文件，而是会从缓存中读取，因为之前已经调用过loadSpringFactories方法了。读取出的内容依然会使用反射来实例化，将实例化的结果返回回去并最终完成自动装配的全过程。这些自动装配的类会通过一些@ConditionalOnClass、@ConditionalOnMissingClass、@Import之类的注解来加载它们需要的资源。
以上就是Spring Boot完成自动装配的大致核心流程，总结起来就是利用了SpringFactoriesLoader这个类来实现的工厂加载机制，读取jar包下的META-INF目录下的spring.factories配置文件中的内容，然后将需要的类名反射实例化即可。
需要自动装配的类在以前的Spring时代都是需要我们自己写的，但现在Spring Boot的自动装配机制帮我们实现了，只需要引入相关的依赖即可。例如Redis的依赖就是spring-boot-starter-data-redis，引用它就可以了，同时在application.properties配置文件中做些简单的配置即可，就可以直接用起来了，不再需要自己写连接客户端、连接池之类的代码。在上面的spring.factories配置文件的示例中也出现了像RedisAutoConfiguration这样的Redis的自动配置类。
