Springboot @Import Springboot @Import 详解
爱笑的咖啡 人气:0SpringBoot 的 @Import 用于将指定的类实例注入之Spring IOC Container中。
今天抽空在仔细看了下Springboot 关于 @Import 的处理过程， 记下来以后看。
1. @Import
先看Spring对它的注释 (文档贴过来的）， 总结下来作用就是和xml配置的 <import />标签作用一样，允许通过它引入 @Configuration 注解的类 (java config)， 引入ImportSelector接口(这个比较重要， 因为要通过它去判定要引入哪些@Configuration) 和 ImportBeanDefinitionRegistrar 接口的实现， 也包括 @Component注解的普通类。
但是如果要引入另一个xml 文件形式配置的 bean, 则需要通过 @ImportResource 注解。
/**
* Indicates one or more {@link Configuration @Configuration} classes to import.
*
* <p>Provides functionality equivalent to the {@code <import/>} element in Spring XML.
* Allows for importing {@code @Configuration} classes, {@link ImportSelector} and
* {@link ImportBeanDefinitionRegistrar} implementations, as well as regular component
* classes (as of 4.2; analogous to {@link AnnotationConfigApplicationContext#register}).
*
* <p>{@code @Bean} definitions declared in imported {@code @Configuration} classes should be
* accessed by using {@link org.springframework.beans.factory.annotation.Autowired @Autowired}
* injection. Either the bean itself can be autowired, or the configuration class instance
* declaring the bean can be autowired. The latter approach allows for explicit, IDE-friendly
* navigation between {@code @Configuration} class methods.
*
* <p>May be declared at the class level or as a meta-annotation.
*
* <p>If XML or other non-{@code @Configuration} bean definition resources need to be
* imported, use the {@link ImportResource @ImportResource} annotation instead.
*
* @author Chris Beams
* @author Juergen Hoeller
* @since 3.0
* @see Configuration
* @see ImportSelector
* @see ImportResource
*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Import {
/**
* {@link Configuration}, {@link ImportSelector}, {@link ImportBeanDefinitionRegistrar}
* or regular component classes to import.
*/
Class<?>[] value();
}
2. ImportSelector
因为 @Import 的实现有很多时候需要借助 ImportSelector 接口， 所以我们再看下这个接口的描述, 总结下来就是需要通过这个接口的实现去决定要引入哪些 @Configuration。 它如果实现了以下四个Aware 接口， 那么spring保证会在调用它之前先调用Aware接口的方法。
至于为什么要保证调用Aware， 我个人觉得应该是你可以通过这些Aware去感知系统里边所有的环境变量， 从而决定你具体的选择逻辑。
/**
* Interface to be implemented by types that determine which @{@link Configuration}
* class(es) should be imported based on a given selection criteria, usually one or more
* annotation attributes.
*
* <p>An {@link ImportSelector} may implement any of the following
* {@link org.springframework.beans.factory.Aware Aware} interfaces, and their respective
* methods will be called prior to {@link #selectImports}:
* <ul>
* <li>{@link org.springframework.context.EnvironmentAware EnvironmentAware}</li>
* <li>{@link org.springframework.beans.factory.BeanFactoryAware BeanFactoryAware}</li>
* <li>{@link org.springframework.beans.factory.BeanClassLoaderAware BeanClassLoaderAware}</li>
* <li>{@link org.springframework.context.ResourceLoaderAware ResourceLoaderAware}</li>
* </ul>
*
* <p>ImportSelectors are usually processed in the same way as regular {@code @Import}
* annotations, however, it is also possible to defer selection of imports until all
* {@code @Configuration} classes have been processed (see {@link DeferredImportSelector}
* for details).
*
* @author Chris Beams
* @since 3.1
* @see DeferredImportSelector
* @see Import
* @see ImportBeanDefinitionRegistrar
* @see Configuration
*/
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);
}
3. Springboot 对@Import注解的处理过程
Springboot对注解的处理都发生在AbstractApplicationContext -> refresh() -> invokeBeanFactoryPostProcessors(beanFactory) -> ConfigurationClassPostProcessor -> postProcessBeanDefinitionRegistry()方法中。
(稍微说下也免得我自己忘了， springboot初始化的普通context(非web) 是AnnotationConfigApplicationContext, 在初始化的时候会初始化两个工具类， AnnotatedBeanDefinitionReader 和 ClassPathBeanDefinitionScanner 分别用来从 annotation driven 的配置和xml的配置中读取beanDefinition并向context注册, 那么在初始化 AnnotatedBeanDefinitionReader 的时候， 会向BeanFactory注册一个ConfigurationClassPostProcessor 用来处理所有的基于annotation的bean, 这个ConfigurationClassPostProcessor 是 BeanFactoryPostProcessor 的一个实现，springboot会保证在 invokeBeanFactoryPostProcessors(beanFactory) 方法中调用注册到它上边的所有的BeanFactoryPostProcessor)
如下代码显示是通过 ConfigurationClassParser 类来转换的
// Parse each @Configuration class
ConfigurationClassParser parser = new ConfigurationClassParser(
this.metadataReaderFactory, this.problemReporter, this.environment,
this.resourceLoader, this.componentScanBeanNameGenerator, registry);
那么在 ConfigurationClassParser -> processConfigurationClass() -> doProcessConfigurationClass() 方法中我们找到了 (这里边的流程还是很清楚的， 分别按次序处理了@PropertySource， @ComponentScan, @Import, @ImportResource, 在处理这些注解的时候是通过递归处理来保证所有的都被处理了)
// Process any @Import annotations
processImports(configClass, sourceClass, getImports(sourceClass), true);
那接下来就看它到底是怎么做的 . 流程依然清晰 ：
首先， 判断如果被import的是 ImportSelector.class 接口的实现， 那么初始化这个被Import的类， 然后调用它的selectImports方法去获得所需要的引入的configuration， 然后递归处理
其次， 判断如果被import的是 ImportBeanDefinitionRegistrar 接口的实现， 那么初始化后将对当前对象的处理委托给这个ImportBeanDefinitionRegistrar (不是特别明白， 只是我的猜测）
最后， 将import引入的类作为一个正常的类来处理 ( 调用最外层的doProcessConfigurationClass())
所以， 从这里我们知道， 如果你引入的是一个正常的component， 那么会作为@Compoent或者@Configuration来处理， 这样在BeanFactory里边可以通过getBean拿到， 但如果你是 ImportSelector 或者 ImportBeanDefinitionRegistrar 接口的实现， 那么spring并不会将他们注册到beanFactory中，而只是调用他们的方法。
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) {
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);
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);
}
}
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
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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