深入了解Java SpringBoot自动装配原理

网友投稿 306 2022-08-21


深入了解Java SpringBoot自动装配原理

目录自动装配原理SpringBootApplicationEnableAutoConfigurationAutoConfigurationImportSelector总结

在使用springboot时,很多配置我们都没有做,都是springboot在帮我们完成,这很大一部分归功于springboot自动装配,那springboot的自动装配的原理是怎么实现的呢?

自动装配原理

springboot 版本:2.4.3

SpringBootApplication

springboot启动类必须要加@SpringBootApplication注解,那这个注解是什么意思呢?

@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 {

抛开元数据注解来说,SpringBootApplication注解主要由@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan组成。这三个又有不同的作用如下:

@SpringBootConfiguration:被@Configuration标记,表示这是个springboot配置,支持javaConfig的方式来进行配置。@EnableAutoConfiguration:表示开启自动装配(重点介绍)@ComponentScan:扫描注解,扫描basePackages包下的bean并将他们注入到IOC容器中,比如:@Service、@Controller、@Component等注解。

EnableAutoConfiguration

真正开启自动配置的还是@EnableAutoConfiguration注解,来看下EnableAutoConfiguration注解源码:

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@AutoConfigurationPackage

@Import(AutoConfigurationImportSelector.class)

public @interface EnableAutoConfiguration {

@EnableAutoConfiguration 又是由@AutoConfigurationPackage和@Import注解组成。

@AutoConfigurationPackage是一个复合注解的,它在内部使用@Import(AutoConfigurationPackages.Registrar.class)注解,Registrar是AutoConfigurationPackages的一个内部类,它的作用就是注册一个springboot启动类所在的包名,这个包名可以供列如JPA的使用。AutoConfigurationImportSelector通过selectImports方法将配置类导入,从而完成bean的装配

AutoConfigurationImportSelector

AutoConfigurationImportSelector实现了DeferredImportSelector接口,DeferredImportSelector是ImportSelector的变种,它是一个延迟选择器。实现了DeferredImportSelector接口的子类如果重新了getImportGroup方法并返回DeferredImportSelector内部接口Group的子类,DeferredImportSelector接口的子类的子类将不会调用selectImports而是调用Group的selectImports方法。

接下来看看AutoConfigurationImportSelector重写了getImportGroup方法并返回一个内部类AutoConfigurationGroup,AutoConfigurationGroup#selectImports方法只是对配置数组进行排序筛选,真正处理自动配置的流程的是process方法。process方法源码如下:

@Override

public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {

Assert.state(deferredImportSelector vLqmUinstanceof 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方法就是对AutoConfigurationGroup一些属性的填充,起作用的还是AutoConfigurationImportSelector.getAutoConfigurationEntry方法。

getAutoConfigurationEntry方法中经过各种判断过滤、去重等操作,最后返回AutoConfigurationEntry对象。源码如下

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {

if (!isEnabled(annotationMetadata)) {

return EMPTY_ENTRY;

}

//获取EnableAutoConfiguration注解的exclude和excludeName属性

AnnotationAttributes attributes = getAttributes(annotationMetadata);

//获取所有配置类

List configurations = getCandidateConfigurations(annotationMetadata, attributes);

//去除重复配置类

configurations = removeDuplicates(configurations);

Set exclusions = getExclusions(annotationMetadata, attributes);

checkExcludedClasses(configurations, exclusions);

//移除 exclude的配置类

configurations.removeAll(exclusions);

configurations = getConfigurationClassFilter().filter(configurations);

fireAutoConfigurationImportEvents(configurations, exclusions);

return new AutoConfigurationEntry(configurations, exclusions);

}

getAutoConfigurationEntry 筛选的自动配置类:

getAutoConfigurationEntry方法中要重点分析的是getCandidateConfigurations方法。getCandidateConfigurations的作用是获取所有自动装配的配置类的全限定名。

来看下getCandidateConfigurations方法源码:

protected List getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {

List 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;

}

这里面用到了SpringFactoriesLoader是spring提供的一种加载配置的方式,它会将类从配置文件中读取到,然后利用反射将bean加载到IOC容器中。

SpringFactoriesLoader.loadFactoryNames中会加载META-INF/spring.factories自动配置类。这些配置类在spring.factories文件中是以key=value的形式存储的,来看下部分自动配置类:

# 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.MessagvLqmUeSourceAutoConfiguration,\

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,\

来看下SpringFactoriesLoader.loadFactoryNames源码:

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

public static List 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> loadSpringFactories(ClassLoader classLoader) {

Map> result = cache.get(classLoader);

if (result != null) {

return result;

}

result = new HashMap<>();

try {

Enumeration 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方法只是传递一个org.springframework.boot.autoconfigure.EnableAutoConfiguration作为key,然后取到对应的自动配置类列表。

最终调用的是loadSpringFactories方法,loadSpringFactories会从jar包中找到spring.factories文件然后将其中的自动配置类存到一个map中,从下图可以看到map中存在很多bean,loadFactoryNames方法在加载自动配置类时只取了一个key。弱水三千,只取一瓢。

loadSpringFactories返回结果:

自动装配流程图大致如下:

总结

EnableAutoConfiguration注解开启自动装配,其上的标记的@Import(AutoConfigurationImportSelector.class)注解中导入配置类

AutoConfigurationImportSelector实现DeferredImportSelector接口,并重写了getImportGroup方法并返回AutoConfigurationImportSelector.AutoConfigurationGroup,AutoConfigurationImportSelector.AutoConfigurationGroup.process开始处理自动配置流程。

AutoConfigurationImportSelector.getCandidateConfigurations获取所有配置类getAutoConfigurationEntry方法筛选,去重、移除不符合条件的自动配置类。

SpringFactoriesLoader.loadSpringFactories从jar包中找到所有META-INF/spring.factories文件并读取自动配置类,存放到map中, loadFactoryNames方法通过全限定名org.springframework.boot.autoconfigure.EnableAutoConfiguration找到自动配置类。

最后经过层层筛选,去重、移除不符合条件的bean,由ConfigurationClassPostProcessor#processConfigBeanDefinitions注册所有的自动配置类。


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