Spring @Import注解源码解析

Posted 2022-09-26 zhixiang-org-cn

简介

Spring 3.0之前，创建Bean可以通过xml配置文件与扫描特定包下面的类来将类注入到Spring IOC容器内。而在Spring 3.0之后提供了JavaConfig的方式，也就是将IOC容器里Bean的元信息以java代码的方式进行描述。我们可以通过@Configuration与@Bean这两个注解配合使用来将原来配置在xml文件里的bean通过java代码的方式进行描述

@Import注解提供了@Bean注解的功能，同时还有xml配置文件里标签组织多个分散的xml文件的功能，当然在这里是组织多个分散的@Configuration

先看一下@Import注解的源码：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Import 
??? /**
????? * @link Configuration, @link ImportSelector, @link ImportBeanDefinitionRegistrar
????? * or regular component classes to import.
????? */
    ??????? Class<?>[] value();

从源码里可以看出@Import可以配合 Configuration ,ImportSelector, ImportBeanDefinitionRegistrar 来使用，下面的or表示也可以把Import当成普通的Bean使用
@Import只允许放到类上面，不能放到方法上。下面我们来看具体的使用方式
??? ?

普通使用方法

这种方式可以直接把类加入到Spring IOC容器

@Configuration
@Import(value=UserServiceImpl.class)
public class Config 
?

但是这种方式有一些问题，那就是只能使用类的无参构造方法来创建bean，对于有参数的构造方法就无能为力了

结合ImportBeanDefinitionRegistrar接口

ImportBeanDefinitionRegistrar接口的源码如下：

public interface ImportBeanDefinitionRegistrar 
??? public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry);
??? ?

可以看到这个接口唯一的方法是有两个参数的

1. AnnotationMetadata：通过这个参数可以拿到类的元数据信息
2. BeanDefinitionRegistry：通过这个参数可以操作IOC容器

我们可以使用一个类来实现这个接口

public class UserServiceBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar 
?
??? public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,BeanDefinitionRegistry registry) 
??????? BeanDefinitionBuilder userService = BeanDefinitionBuilder.rootBeanDefinition(UserServiceImpl.class);
??????? registry.registerBeanDefinition("userService", userService.getBeanDefinition());
??? 
?

可以看到我们在这个方法里面做一些特殊操作什么的都是可以的，相比较于普通的方式可是灵活了很多

接着我们在@Import注解引入的地方只需要修改为引入UserServiceBeanDefinitionRegistrar就可以了

@Configuration
@Import(value=UserServiceBeanDefinitionRegistrar.class)
public class Config 
?

??? ?

结合ImportSelector接口

相比较与实现ImportBeanDefinitionRegistrar接口之后直接操作Bean容器来说，使用ImportSelector会更加优雅一些，只需要返回需要注入类的全限定名就可以了

ImportSelector接口的源码如下：

public interface ImportSelector 
??? String[] selectImports(AnnotationMetadata importingClassMetadata);



public class UserServiceImportSelect implements ImportSelector
?
??? public String[] selectImports(AnnotationMetadata importingClassMetadata) 
?????? ?
?????? return new String[]UserServiceImpl.class.getName();
??? 
?


@Configuration()
@Import(value=UserServiceImportSelect.class)
public class Config 
?

相比较三种方式来说可以看到最后这种才是最优雅的方式

源码解析

首先我们就以第三种比较优雅的方式出发，使用Call Hierarchy看一下ImportSelector接口的selectImports方法调用链关系：

1

看过之前Spring源码解析文章的同学都知道，refresh方法是用来初始化容器上下文的。跟着这个调用链走下来到中间有一个类是ConfigurationClassPostProcessor，根据类名我们就可以猜到这个类应该是处理配置类（也就是标注@Configuration）的。那么从这开始看吧

??? ?
??? ?

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) 
??? List<BeanDefinitionHolder> configCandidates = new ArrayList<BeanDefinitionHolder>();
??? String[] candidateNames = registry.getBeanDefinitionNames();

??? for (String beanName : candidateNames) 
??????? BeanDefinition beanDef = registry.getBeanDefinition(beanName);
     
??????? if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
??????????????? ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) 
??????????? if (logger.isDebugEnabled()) 
??????????????? logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
??????????? 
??????? 
       //查看是否是配置类
??????? else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) 

??????????? configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
??????? 
??? 
    //如果没有配置类就直接返回
??? if (configCandidates.isEmpty()) 
??????? return;
??? 

    //对这些配置类根据Order排序
??? Collections.sort(configCandidates, new Comparator<BeanDefinitionHolder>() 
??????? @Override
??????? public int compare(BeanDefinitionHolder bd1, BeanDefinitionHolder bd2) 
??????????? int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
??????????? int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
??????????? return (i1 < i2) ? -1 : (i1 > i2) ? 1 : 0;
??????? 
??? );

??? SingletonBeanRegistry sbr = null;
??? if (registry instanceof SingletonBeanRegistry) 
??????? sbr = (SingletonBeanRegistry) registry;
??????? if (!this.localBeanNameGeneratorSet && sbr.containsSingleton(CONFIGURATION_BEAN_NAME_GENERATOR)) 
??????????? BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(CONFIGURATION_BEAN_NAME_GENERATOR);
??????????? this.componentScanBeanNameGenerator = generator;
??????????? this.importBeanNameGenerator = generator;
??????? 
??? 
???????????? //创建配置类的解析类
??? ConfigurationClassParser parser = new ConfigurationClassParser(
??????????? this.metadataReaderFactory, this.problemReporter, this.environment,
??????????? this.resourceLoader, this.componentScanBeanNameGenerator, registry);

??? Set<BeanDefinitionHolder> candidates = new LinkedHashSet<BeanDefinitionHolder>(configCandidates);
??? Set<ConfigurationClass> alreadyParsed = new HashSet<ConfigurationClass>(configCandidates.size());
??? do 
ConfigurationClassParser的parse方法进行解析，重点哈
??????? parser.parse(candidates);
??????? parser.validate();

??????? Set<ConfigurationClass> configClasses = new LinkedHashSet<ConfigurationClass>(parser.getConfigurationClasses());
??????? configClasses.removeAll(alreadyParsed);

??????? // Read the model and create bean definitions based on its content
??????? if (this.reader == null) 
??????????? this.reader = new ConfigurationClassBeanDefinitionReader(
??????????????????? registry, this.sourceExtractor, this.resourceLoader, this.environment,
??????????????????? this.importBeanNameGenerator, parser.getImportRegistry());
??????? 
??????? this.reader.loadBeanDefinitions(configClasses);
??????? alreadyParsed.addAll(configClasses);

??????? candidates.clear();
??????? if (registry.getBeanDefinitionCount() > candidateNames.length) 
??????????? String[] newCandidateNames = registry.getBeanDefinitionNames();
??????????? Set<String> oldCandidateNames = new HashSet<String>(Arrays.asList(candidateNames));
??????????? Set<String> alreadyParsedClasses = new HashSet<String>();
??????????? for (ConfigurationClass configurationClass : alreadyParsed) 
??????????????? alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
??????????? 
??????????? for (String candidateName : newCandidateNames) 
??????????????? if (!oldCandidateNames.contains(candidateName)) 
??????????????????? BeanDefinition bd = registry.getBeanDefinition(candidateName);
??????????????????? if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
??????????????????????????? !alreadyParsedClasses.contains(bd.getBeanClassName())) 
??????????????????????? candidates.add(new BeanDefinitionHolder(bd, candidateName));
??????????????????? 
??????????????? 
??????????? 
??????????? candidateNames = newCandidateNames;
??????? 
??? 
??? while (!candidates.isEmpty());

??? if (sbr != null) 
??????? if (!sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) 
??????????? sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
??????? 
??? 

??? if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory) 
??????? ((CachingMetadataReaderFactory) this.metadataReaderFactory).clearCache();
??? 

??? ?

现在该进入ConfigurationClassParser类了

public void parse(Set<BeanDefinitionHolder> configCandidates) 
    this.deferredImportSelectors = new LinkedList<DeferredImportSelectorHolder>();
    
    for (BeanDefinitionHolder holder : configCandidates) 
    ??? BeanDefinition bd = holder.getBeanDefinition();
    ??? try 
    ??????? if (bd instanceof AnnotatedBeanDefinition) 
    ??????????? parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
    ??????? 
    ??????? else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) 
    ??????????? parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
    ??????? 
    ??????? else 
    ??????????? parse(bd.getBeanClassName(), holder.getBeanName());
    ??????? 
    ??? 
    ??? catch (BeanDefinitionStoreException ex) 
    ??????? throw ex;
    ??? 
    ??? catch (Throwable ex) 
    ??????? throw new BeanDefinitionStoreException(
    ??????????????? "Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
    ??? 
    
    
    processDeferredImportSelectors();

//下面三种方法用于处理不同的BeanDefinition 类型，但最终都是使用的processConfigurationClass方法
protected final void parse(String className, String beanName) throws IOException 
??? MetadataReader reader = this.metadataReaderFactory.getMetadataReader(className);
??? processConfigurationClass(new ConfigurationClass(reader, beanName));


protected final void parse(Class<?> clazz, String beanName) throws IOException 
??? processConfigurationClass(new ConfigurationClass(clazz, beanName));


protected final void parse(AnnotationMetadata metadata, String beanName) throws IOException 
??? processConfigurationClass(new ConfigurationClass(metadata, beanName));

可以看到配置类可能会是三种形式的存在，这三种形式的Bean在操作上有着部分不一样，但是大部分又是一样，所以Spring用这种模式来处理。不得不感叹人家设计的真好

接着往下看

protected void processConfigurationClass(ConfigurationClass configClass) throws IOException 
??? if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) 
??????? return;
??? 

??? ConfigurationClass existingClass = this.configurationClasses.get(configClass);
????//在这里处理Configuration重复import
    //如果同一个配置类被处理两次，两次都属于被import的则合并导入类，返回。如果配置类不是被导入的，则移除旧使用新的配置类
??? if (existingClass != null) 
??????? if (configClass.isImported()) 
??????????? if (existingClass.isImported()) 
??????????????? existingClass.mergeImportedBy(configClass);
??????????? 
??????????? return;
??????? 
??????? else 
??????????? this.configurationClasses.remove(configClass);
??????????? for (Iterator<ConfigurationClass> it = this.knownSuperclasses.values().iterator(); it.hasNext();) 
??????????????? if (configClass.equals(it.next())) 
??????????????????? it.remove();
??????????????? 
??????????? 
??????? 
??? 

??? SourceClass sourceClass = asSourceClass(configClass);
??? do 
      //接着往下看吧
??????? sourceClass = doProcessConfigurationClass(configClass, sourceClass);
??? 
??? while (sourceClass != null);

??? this.configurationClasses.put(configClass, configClass);




protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
??????? throws IOException 

??? // 处理递归类
??? processMemberClasses(configClass, sourceClass);

??? // 处理@PropertySource注解
??? for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
??????????? sourceClass.getMetadata(), PropertySources.class,
??????????? org.springframework.context.annotation.PropertySource.class)) 
??????? if (this.environment instanceof ConfigurableEnvironment) 
??????????? processPropertySource(propertySource);
??????? 
??????? else 
??????????? logger.warn("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
??????????????????? "]. Reason: Environment must implement ConfigurableEnvironment");
??????? 
??? 

??? // 处理 @ComponentScan 注解
??? Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
??????????? sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
??? if (!componentScans.isEmpty() &&
??????????? !this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) 
??????? for (AnnotationAttributes componentScan : componentScans) 
??????????? // The config class is annotated with @ComponentScan -> perform the scan immediately
??????????? Set<BeanDefinitionHolder> scannedBeanDefinitions =
??????????????????? this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
??????????? // Check the set of scanned definitions for any further config classes and parse recursively if needed
??????????? for (BeanDefinitionHolder holder : scannedBeanDefinitions) 
??????????????? BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
??????????????? if (bdCand == null) 
??????????????????? bdCand = holder.getBeanDefinition();
??????????????? 
??????????????? if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) 
??????????????????? parse(bdCand.getBeanClassName(), holder.getBeanName());
??????????????? 
??????????? 
??????? 
??? 
????//处理Import注解，这个是咱们的菜
??? processImports(configClass, sourceClass, getImports(sourceClass), true);

??? // 处理@ImportResource 注解
??? if (sourceClass.getMetadata().isAnnotated(ImportResource.class.getName())) 
??????? AnnotationAttributes importResource =
??????????????? AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
??????? String[] resources = importResource.getStringArray("locations");
??????? Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
??????? for (String resource : resources) 
??????????? String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
??????????? configClass.addImportedResource(resolvedResource, readerClass);
??????? 
??? 

??? //处理包含@Bean注解的方法
??? Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
??? for (MethodMetadata methodMetadata : beanMethods) 
??????? configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
??? 

??? // 处理普通方法
??? processInterfaces(configClass, sourceClass);

???
??? if (sourceClass.getMetadata().hasSuperClass()) 
??????? String superclass = sourceClass.getMetadata().getSuperClassName();
??????? if (!superclass.startsWith("java") && !this.knownSuperclasses.containsKey(superclass)) 
??????????? this.knownSuperclasses.put(superclass, configClass);
??????????? // Superclass found, return its annotation metadata and recurse
??????????? return sourceClass.getSuperClass();
??????? 
??? 

??? return null;

??? ?

看到这里好像突然发现了新大陆呀，原来我们经常见的@Bean、@ImportResource 、@Import、@ComponentScan、@PropertySource都是在这里处理的呀

咱们的重点还是放在@Import上，对其他几个注解感兴趣的同学可以自行研究一下

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
??????? Collection<SourceClass> 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) 
                //如果实现了ImportSelector接口
??????????????? if (candidate.isAssignable(ImportSelector.class)) 
??????????????????? Class<?> candidateClass = candidate.loadClass();
??????????????????? ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
??????????????????? ParserStrategyUtils.invokeAwareMethods(
??????????????????????????? selector, this.environment, this.resourceLoader, this.registry);
??????????????????? if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) 
??????????????????????? this.deferredImportSelectors.add(
??????????????????????????????? new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector));
??????????????????? 
??????????????????? else 
??????????????????????? String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
??????????????????????? Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
??????????????????????? processImports(configClass, currentSourceClass, importSourceClasses, false);
??????????????????? 
??????????????? 
                //如果实现了ImportBeanDefinitionRegistrar接口
??????????????? else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) 
??????????????????? 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 
                     //将import当成Configuration来使用就是我们的第一种应用的方式
??????????????????? 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();
??????? 
???