Spring-Import注解源码解析

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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 <import> 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();
        }
    }
}

正文完
spring
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2019-09-03
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