Spring Boot自动装配原理和源码分析

该篇的主要内容

1. Spring Boot的自动装配原理【有案例模拟】
2. Spring Boot自动装配源码分析

SpringBoot中的自动装载原理

SpringBoot具备开箱即用的特点，它默默的帮我们做了很多事情【自动创建和装配很多对象】

自动装配的实现

主要是通过ImportSelector接口完成的自动装配

ImportSelector

ImportSelector接口是Spring导入外部配置的核心接口，在SpringBoot的自动化配置和@EnableXXX(功能性注解)中起到了决定性的作用。当在@Configuration标注的Class上使用@Import引入了一个 ImportSelector实现类后，会把实现类中返回的Class名称都定义为bean。

package org.springframework.context.annotation;

import org.springframework.core.type.AnnotationMetadata;

public interface ImportSelector {

	/**
	 * Select and return the names of which class(es) should be imported based on
	 * the {@link AnnotationMetadata} of the importing @{@link Configuration} class.
	 */
	String[] selectImports(AnnotationMetadata importingClassMetadata);

} 

该接口的继承关系图

​DeferredImportSelector

由上图可知DeferredImportSelector接口继承自ImportSelector，它和ImportSelector的区别在于装载bean的时机上，DeferredImportSelector需要等所有的@Configuration都执行完毕后才会进行装载。

Spring Boot自动装配案例

项目案例传送门：autoConfig

案例主要代码

1. 创建一个生产Bean的配置类，但是我们并不使用@Configuration注解来声明它。

public class MyConfig {
    @Bean(value = "chenfu", name = "chenfu")
    public Map getMap() {
        HashMap map = new HashMap<>();
        map.put("code", 200);
        map.put("msg", "success");
        String nowDate = LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE_TIME);
        map.put("data", nowDate);
        return map;
    }
} 

2.实现ImportSelector接口，返回我们上边的配置类名

public class MyConfigImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
//        返回配置名称
        return new String[]{MyConfig.class.getName()};
    }
} 

3.运行测试

@SpringBootApplication
@Import(value = MyConfigImportSelector.class)
public class AutoConfigApp {
    public static void main(String[] args) {
        ConfigurableApplicationContext app = SpringApplication.run(AutoConfigApp.class, args);
        Object chenfu = app.getBean("chenfu");
        System.out.println(chenfu);
    }
} 

案例大体内容如上，通过案例可知经过Spring Boot自动装配的对象并没有使用Spring的对象创建注解声明 （@Controller，@Service，@Repostiroty），而是使用编程的方式动态的载入bean。在Spring Boor中对这些对象的解析步骤主要是在ConfigurationClassParser类的processImports方法内进行的

ImportSelector的解析过程

ConfigurationClassParser

ConfigurationClassParser类的源码

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
		Collection importCandidates, boolean checkForCircularImports) {
	if (importCandidates.isEmpty()) {
		return;
	}
	if (checkForCircularImports && isChainedImportOnStack(configClass)) {
		this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
	}
	else {
		this.importStack.push(configClass);
		try {
			for (SourceClass candidate : importCandidates) {
				//对ImportSelector类型的类进行处理
				if (candidate.isAssignable(ImportSelector.class)) {
					// Candidate class is an ImportSelector -> delegate to it to determine imports
					Class candidateClass = candidate.loadClass();
					ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
					ParserStrategyUtils.invokeAwareMethods(
							selector, this.environment, this.resourceLoader, this.registry);
 					//上边我们说过的另外一种Selector类型，可以理解为延迟加载
					if (selector instanceof DeferredImportSelector) {
						//该方法内部会将该Selector保存到一个集合【deferredImportSelectors】中
						this.deferredImportSelectorHandler.handle(
								configClass, (DeferredImportSelector) selector);
					} else {
						String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
						Collection importSourceClasses = asSourceClasses(importClassNames);
						processImports(configClass, currentSourceClass, importSourceClasses, false);
					}
				} else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
					// Candidate class is an ImportBeanDefinitionRegistrar ->
					// delegate to it to register additional bean definitions
					Class candidateClass = candidate.loadClass();
					ImportBeanDefinitionRegistrar registrar =
							BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);
					ParserStrategyUtils.invokeAwareMethods(
							registrar, this.environment, this.resourceLoader, this.registry);
					configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
				} else {
					// Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
					// process it as an @Configuration class
					// 直接把上边的官方英文注释硬翻译了，当前类不是ImportSelector或ImportBeanDefinitionRegistrar类型，直接让其走@Configuration类的处理流程
					this.importStack.registerImport(
							currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
					processConfigurationClass(candidate.asConfigClass(configClass));
				}
			}
		} catch (BeanDefinitionStoreException ex) {
			throw ex;
		} catch (Throwable ex) {
			throw new BeanDefinitionStoreException(
					"Failed to process import candidates for configuration class [" +
					configClass.getMetadata().getClassName() + "]", ex);
		} finally {
			this.importStack.pop();
		}
	}
} 

综上，大致流程就是ImportSelector接口的返回值会递归进行解析，然后把解析到的类全名最终按照 @Configuration进行处理。

ImportSelector总结

SpringBoot开箱即用的特点，很大程度上归功于ImportSelector。

Spring Boot源码分析

Spring Boot在Spring的基础上做了一些扩展。

1. 在SpringBoot的SpringBootApplication注解中声明了一个 @EnableAutoConfiguration注解

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication { 

2.@EnableAutoConfiguration中通过Import引入了SpringBoot定义的@Import({
AutoConfigurationImportSelector.class})

@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration { 

接下来开始对
AutoConfigurationImportSelector进行源码分析

AutoConfigurationImportSelector

AutoConfigurationImportSelector是selectImports的实现类，我们来看selectImports方法

selectImports

public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    } else {
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
        AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }
} 

该方法的主要逻辑都在getAutoConfigurationEntry方法内

getAutoConfigurationEntry

protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return EMPTY_ENTRY;
    } else {
        AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
        // 2.1通过getCandidateConfigurations方法获取所有需要加载的bean
        List configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
        // 去重
        configurations = this.removeDuplicates(configurations);
        // 获取不需要加载的bean，我们可以通过spring.autoconfigure.exclude配置
        Set exclusions = this.getExclusions(annotationMetadata, attributes);
        this.checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = this.filter(configurations, autoConfigurationMetadata);
        // 发送事件，通知所有的AutoConfigurationImportListener进行监听
        this.fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
    }
} 

接下来再看一下上边源码中调用的
getCandidateConfigurations方法

getCandidateConfigurations

protected List getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
	// 这里的getSpringFactoriesLoaderFactoryClass()最终返回的是EnableAutoConfiguration.class
    List configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
} 

从上面的逻辑可以看出,终获取bean的渠道在
SpringFactoriesLoader.loadFactoryNames内SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());

SpringFactoriesLoader

loadFactoryNames

public static List loadFactoryNames(Class factoryClass, @Nullable ClassLoader classLoader) {
	// 通过factoryClassName获取相应的bean全称
    String factoryClassName = factoryClass.getName();
    return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
} 

loadSpringFactories

private static Map> loadSpringFactories(@Nullable ClassLoader classLoader) {
    MultiValueMap result = (MultiValueMap)cache.get(classLoader);
    if (result != null) {
        return result;
    } else {
        try {
        	// 获取项目中所有META-INF/spring.factories文件，将其组装成Map
            Enumeration urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
            LinkedMultiValueMap result = new LinkedMultiValueMap();

            while(urls.hasMoreElements()) {
                URL url = (URL)urls.nextElement();
                UrlResource resource = new UrlResource(url);
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                Iterator var6 = properties.entrySet().iterator();

                while(var6.hasNext()) {
                    Entry entry = (Entry)var6.next();
                    String factoryClassName = ((String)entry.getKey()).trim();
                    String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                    int var10 = var9.length;

                    for(int var11 = 0; var11 < var10; ++var11) {
                        String factoryName = var9[var11];
                        result.add(factoryClassName, factoryName.trim());
                    }
                }
            }

            cache.put(classLoader, result);
            return result;
        } catch (IOException var13) {
            throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
        }
    }
} 

每个jar包都可以定义自己的META-INF/spring.factories，在该jar被加载的同时 spring.factories里面定义的 bean就会被自动加载。我们可以来看一下Spring Boot的该配置文件内容【只拿出部分内容】

# 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,\ 

故，我们可以对上边的案例进行改造，在resouces目录下创建该文件，然后添加如下内容

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
top.chenfu.auto.config.MyConfig   

去掉启动类中的@import或者自定义@EnableXXX注解