在使用springboot時(shí),很多配置我們都沒有做,都是springboot在幫我們完成,這很大一部分歸功于springboot自動(dòng)裝配,那springboot的自動(dòng)裝配的原理是怎么實(shí)現(xiàn)的呢?
自動(dòng)裝配原理
springboot 版本:2.4.3
SpringBootApplication
springboot啟動(dòng)類必須要加@SpringBootApplication注解,那這個(gè)注解是什么意思呢?
@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 {
拋開元數(shù)據(jù)注解來說,SpringBootApplication注解主要由@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan組成。這三個(gè)又有不同的作用如下:
- @SpringBootConfiguration:被@Configuration標(biāo)記,表示這是個(gè)springboot配置,支持JavaConfig的方式來進(jìn)行配置。
- @EnableAutoConfiguration:表示開啟自動(dòng)裝配(重點(diǎn)介紹)
- @ComponentScan:掃描注解,掃描basePackages包下的bean并將他們注入到IOC容器中,比如:@Service、@Controller、@Component等注解。
EnableAutoConfiguration
真正開啟自動(dòng)配置的還是@EnableAutoConfiguration注解,來看下EnableAutoConfiguration注解源碼:
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
@EnableAutoConfiguration 又是由@AutoConfigurationPackage和@Import注解組成。
- @AutoConfigurationPackage是一個(gè)復(fù)合注解的,它在內(nèi)部使用@Import(AutoConfigurationPackages.Registrar.class)注解,Registrar是AutoConfigurationPackages的一個(gè)內(nèi)部類,它的作用就是注冊(cè)一個(gè)springboot啟動(dòng)類所在的包名,這個(gè)包名可以供列如JPA的使用。
- AutoConfigurationImportSelector通過selectImports方法將配置類導(dǎo)入,從而完成bean的裝配
AutoConfigurationImportSelector
AutoConfigurationImportSelector實(shí)現(xiàn)了DeferredImportSelector接口,DeferredImportSelector是ImportSelector的變種,它是一個(gè)延遲選擇器。實(shí)現(xiàn)了DeferredImportSelector接口的子類如果重新了getImportGroup方法并返回DeferredImportSelector內(nèi)部接口Group的子類,DeferredImportSelector接口的子類的子類將不會(huì)調(diào)用selectImports而是調(diào)用Group的selectImports方法。
接下來看看AutoConfigurationImportSelector重寫了getImportGroup方法并返回一個(gè)內(nèi)部類AutoConfigurationGroup,AutoConfigurationGroup#selectImports方法只是對(duì)配置數(shù)組進(jìn)行排序篩選,真正處理自動(dòng)配置的流程的是process方法。 process方法源碼如下:
@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(annotationMetadata);
this.autoConfigurationEntries.add(autoConfigurationEntry);
for (String importClassName : autoConfigurationEntry.getConfigurations()) {
this.entries.putIfAbsent(importClassName, annotationMetadata);
}
}
process方法就是對(duì)AutoConfigurationGroup一些屬性的填充,起作用的還是AutoConfigurationImportSelector.getAutoConfigurationEntry方法。 getAutoConfigurationEntry方法中經(jīng)過各種判斷過濾、去重等操作,最后返回AutoConfigurationEntry對(duì)象。源碼如下
protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
}
//獲取EnableAutoConfiguration注解的exclude和excludeName屬性
AnnotationAttributes attributes = getAttributes(annotationMetadata);
//獲取所有配置類
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
//去除重復(fù)配置類
configurations = removeDuplicates(configurations);
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
//移除 exclude的配置類
configurations.removeAll(exclusions);
configurations = getConfigurationClassFilter().filter(configurations);
fireAutoConfigurationImportEvents(configurations, exclusions);
return new AutoConfigurationEntry(configurations, exclusions);
}
getAutoConfigurationEntry 篩選的自動(dòng)配置類:
getAutoConfigurationEntry方法中要重點(diǎn)分析的是getCandidateConfigurations方法。getCandidateConfigurations的作用是獲取所有自動(dòng)裝配的配置類的全限定名。
來看下getCandidateConfigurations方法源碼:
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;
}
protected Class<?> getSpringFactoriesLoaderFactoryClass() {
return EnableAutoConfiguration.class;
}
這里面用到了SpringFactoriesLoader是spring提供的一種加載配置的方式,它會(huì)將類從配置文件中讀取到,然后利用反射將bean加載到IOC容器中。
SpringFactoriesLoader.loadFactoryNames中會(huì)加載META-INF/spring.factories自動(dòng)配置類。這些配置類在spring.factories文件中是以key=value的形式存儲(chǔ)的,來看下部分自動(dòng)配置類:
# 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.CloudAutoConfiguration,\
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,\
來看下SpringFactoriesLoader.loadFactoryNames源碼:
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
ClassLoader classLoaderToUse = classLoader;
if (classLoaderToUse == null) {
classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
}
String factoryTypeName = factoryType.getName();
return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
}
private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
Map<String, List<String>> result = cache.get(classLoader);
if (result != null) {
return result;
}
result = new HashMap<>();
try {
Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
while (urls.hasMoreElements()) {
URL url = urls.nextElement();
UrlResource resource = new UrlResource(url);
Properties properties = PropertiesLoaderUtils.loadProperties(resource);
for (Map.Entry<?, ?> entry : properties.entrySet()) {
String factoryTypeName = ((String) entry.getKey()).trim();
String[] factoryImplementationNames =
StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
for (String factoryImplementationName : factoryImplementationNames) {
result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
.add(factoryImplementationName.trim());
}
}
}
// Replace all lists with unmodifiable lists containing unique elements
result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
.collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
cache.put(classLoader, result);
}
catch (IOException ex) {
throw new IllegalArgumentException("Unable to load factories from location [" +
FACTORIES_RESOURCE_LOCATION + "]", ex);
}
return result;
}
loadFactoryNames方法只是傳遞一個(gè)org.springframework.boot.autoconfigure.EnableAutoConfiguration作為key,然后取到對(duì)應(yīng)的自動(dòng)配置類列表。 最終調(diào)用的是loadSpringFactories方法,loadSpringFactories會(huì)從jar包中找到spring.factories文件然后將其中的自動(dòng)配置類存到一個(gè)map中,從下圖可以看到map中存在很多bean,loadFactoryNames方法在加載自動(dòng)配置類時(shí)只取了一個(gè)key。弱水三千,只取一瓢。
loadSpringFactories返回結(jié)果:自動(dòng)裝配流程圖大致如下:
總結(jié)
- EnableAutoConfiguration注解開啟自動(dòng)裝配,其上的標(biāo)記的@Import(AutoConfigurationImportSelector.class)注解中導(dǎo)入配置類
- AutoConfigurationImportSelector實(shí)現(xiàn)DeferredImportSelector接口,并重寫了getImportGroup方法并返回AutoConfigurationImportSelector.AutoConfigurationGroup,AutoConfigurationImportSelector.AutoConfigurationGroup.process開始處理自動(dòng)配置流程。
- AutoConfigurationImportSelector.getCandidateConfigurations獲取所有配置類getAutoConfigurationEntry方法篩選,去重、移除不符合條件的自動(dòng)配置類。
- SpringFactoriesLoader.loadSpringFactories從jar包中找到所有META-INF/spring.factories文件并讀取自動(dòng)配置類,存放到map中, loadFactoryNames方法通過全限定名org.springframework.boot.autoconfigure.EnableAutoConfiguration找到自動(dòng)配置類。
- 最后經(jīng)過層層篩選,去重、移除不符合條件的bean,由ConfigurationClassPostProcessor#processConfigBeanDefinitions注冊(cè)所有的自動(dòng)配置類。
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