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)@Documentedpublic @interface Import { /** * {@link Configuration}, {@link ImportSelector}, {@link ImportBeanDefinitionRegistrar} * or regular component classes to import. */ Class>[] value();}
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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 { }
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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); }
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可以看到这个接口唯一的方法是有两个参数的
- AnnotationMetadata：通过这个参数可以拿到类的元数据信息
- 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()); } }
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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 { }
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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：
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| 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 { }
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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：
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
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| public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) { List configCandidates = new ArrayList(); 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() { @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 candidates = new LinkedHashSet(configCandidates); Set alreadyParsed = new HashSet(configCandidates.size()); do {ConfigurationClassParser的parse方法进行解析，重点哈 parser.parse(candidates); parser.validate(); Set configClasses = new LinkedHashSet(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(); }}
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现在该进入ConfigurationClassParser类了
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| public void parse(Set configCandidates) { this.deferredImportSelectors = new LinkedList(); 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));}
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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
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| 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 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 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 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 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;}
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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
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| 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) { //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 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(); } }
}
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