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深入了解Java SpringBoot自动装配原理

作者:码猿笔记

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。本文将详细为大家讲解SpringBoot的自动装配原理,需要的可以参考一下

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:

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。

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 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<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
    //去除重复配置类
        configurations = removeDuplicates(configurations);
        Set<String> 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<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;
    }

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.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,\

来看下SpringFactoriesLoader.loadFactoryNames源码:

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。

到此这篇关于深入了解Java SpringBoot自动装配原理的文章就介绍到这了,更多相关SpringBoot自动装配内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家!

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