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- 在使用 @Import 注解來注冊 bean 的時候，Import 注解的值可以是 ImportSelector 或者 DeferredImportSelector 的實現(xiàn)類，spring 容器會實例化這個實現(xiàn)類，并執(zhí)行其 selectImports 方法，那么問題來了： ImportSelector 和 DeferredImportSelector 的區(qū)別在哪里，我們自定義 Imort 邏輯的時候該選擇哪個呢？ 本文通過分析相關的 spring 源碼來查找答案；
全文概覽
- 本文由以下幾部分組成：
看官方文檔
- 先看官方文檔看起，我選擇了 4.3.9 版本在線文檔（這是個 Release 版），地址：https://docs.spring.io/spring/docs/4.3.19.RELEASE/javadoc-api/
- 原文：A variation of ImportSelector that runs after all @Configuration beans have been processed. This type of selector can be particularly useful when the selected imports are @Conditional.Implementations can also extend the Ordered interface or use the Order annotation to indicate a precedence against other DeferredImportSelectors.
- 我的理解：
分析 spring 源碼中對這兩個接口的處理
- 接下來看看源碼：
- 在 spring-framework-4.1.8.RELEASE 工程中找到類 ConfigurationClassParser.java，這里面有處理配置類的主要邏輯；
- 找到方法 parse(Set configCandidates)：
public void parse(Set configCandidates) { this.deferredImportSelectors = new LinkedList(); //檢查每個bean的定義 for (BeanDefinitionHolder holder : configCandidates) { BeanDefinition bd = holder.getBeanDefinition(); try { if (bd instanceof AnnotatedBeanDefinition) { //對于每個有注解的類，都執(zhí)行方法parse(AnnotationMetadata metadata, String beanName) 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 (Exception ex) { throw new BeanDefinitionStoreException( “Failed to parse configuration class [” + bd.getBeanClassName() + “]”, ex); } } //最后再處理DeferredImportSelector的實現(xiàn)類 processDeferredImportSelectors(); }
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- 由以上代碼可以大致看出 DeferredImportSelector 的實現(xiàn)類被最后放在 processDeferredImportSelectors 方法中處理，那么前面的 parse(AnnotationMetadata metadata, String beanName)做了些什么呢？繼續(xù)看；
- 展開方法 parse(AnnotationMetadata metadata, String beanName)里面，是執(zhí)行 processConfigurationClass 方法;
- 再展開 processConfigurationClass 方法，看到核心邏輯是調(diào)用 doProcessConfigurationClass 方法，展開看看：
protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass) throws IOException { //為了聚焦Import相關處理，此處略去部分不相關代碼，不在這里展示了 … … // 處理@Import注解 processImports(configClass, sourceClass, getImports(sourceClass), true); // 處理@ImportResource注解 if (sourceClass.getMetadata().isAnnotated(ImportResource.class.getName())) { AnnotationAttributes importResource = AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class); String[] resources = importResource.getStringArray(“value”); Class readerClass = importResource.getClass(“reader”); for (String resource : resources) { String resolvedResource = this.environment.resolveRequiredPlaceholders(resource); configClass.addImportedResource(resolvedResource, readerClass); } } // 處理@Bean注解，注意是處理注解，不是執(zhí)行@Bean修飾的方法 Set beanMethods = sourceClass.getMetadata().getAnnotatedMethods(Bean.class.getName()); for (MethodMetadata methodMetadata : beanMethods) { configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass)); } // 處理Configuration類的父類，外面在調(diào)用doProcessConfigurationClass方法的時有迭代處理，確保所有父類的注解都會被處理 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(); } } // 再也沒有父類了，返回null表示當前Configuration處理完畢 return null; }
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- 根據(jù)上述代碼分析，可以梳理出下圖中的邏輯：
- 現(xiàn)在需要再看看 processImports 和 processDeferredImportSelectors 這兩個方法的具體代碼；
- 先看 processImports 方法：
private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass, Collection importCandidates, boolean checkForCircularImports) throws IOException { if (importCandidates.isEmpty()) { return; } if (checkForCircularImports && this.importStack.contains(configClass)) { this.problemReporter.error(new CircularImportProblem(configClass, this.importStack)); } else { this.importStack.push(configClass); try { for (SourceClass candidate : importCandidates) { //如果是ImportSelector接口的實現(xiàn)類，就在此處理 if (candidate.isAssignable(ImportSelector.class)) { // Candidate class is an ImportSelector -> delegate to it to determine imports Class candidateClass = candidate.loadClass(); //實例化這些ImportSelector的實現(xiàn)類 ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class); //如果這實現(xiàn)類還實現(xiàn)了BeanFactoryAware、EnvironmentAware這些接口，就要先執(zhí)行這些接口中聲明的方法 invokeAwareMethods(selector); //如果這個實現(xiàn)類也實現(xiàn)了DeferredImportSelector接口，就被加入到集合deferredImportSelectors中 if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) { this.deferredImportSelectors.add( new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector)); } else { //注意，這一行是關鍵代碼?。?！執(zhí)行實現(xiàn)類的selectImports方法 String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata()); Collection importSourceClasses = asSourceClasses(importClassNames); processImports(configClass, currentSourceClass, importSourceClasses, false); } } //此處略去的和ImportSelector不相關的邏輯代碼 … … … } } catch (BeanDefinitionStoreException ex) { throw ex; } catch (Exception ex) { throw new BeanDefinitionStoreException(“Failed to process import candidates for configuration class [” + configClass.getMetadata().getClassName() + “]”, ex); } finally { this.importStack.pop(); } } }
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- 以上代碼有兩個關鍵點：
- 第一、當前被處理的類，如果實現(xiàn)了 DeferredImportSelector 接口，就被加入到集合 deferredImportSelectors 中；
- 第二、當前被處理的類，如果沒有實現(xiàn) DeferredImportSelector 接口，但是實現(xiàn)了 ImportSelector 接口，就被執(zhí)行 selectImports 方法；
- 接下來看看 processDeferredImportSelectors 方法的源碼，提前推測應該是處理集合 deferredImportSelectors 中的所有類，這些類都實現(xiàn)了 DeferredImportSelector 接口：
private void processDeferredImportSelectors() { List deferredImports = this.deferredImportSelectors; this.deferredImportSelectors = null; //按照Order注解或者Ordered接口進行排序 Collections.sort(deferredImports, DEFERRED_IMPORT_COMPARATOR); for (DeferredImportSelectorHolder deferredImport : deferredImports) { ConfigurationClass configClass = deferredImport.getConfigurationClass(); try { //此處是關鍵代碼，執(zhí)行DeferredImportSelector實現(xiàn)類的selectImports方法 String[] imports = deferredImport.getImportSelector().selectImports(configClass.getMetadata()); processImports(configClass, asSourceClass(configClass), asSourceClasses(imports), false); } catch (BeanDefinitionStoreException ex) { throw ex; } catch (Exception ex) { throw new BeanDefinitionStoreException(“Failed to process import candidates for configuration class [” + configClass.getMetadata().getClassName() + “]”, ex); } } }
復制代碼
- 至此，源碼分析完畢了，從代碼可以很清晰的看出 ImportSelector 與 DeferredImportSelector 的區(qū)別，就是 selectImports 方法執(zhí)行時機有差別，這個差別期間，spring 容器對此 Configguration 類做了些其他的邏輯：包括對 @ImportResource、@Bean 這些注解的處理（注意，這里只是對 @Bean 修飾的方法的處理，并不是立即調(diào)用 @Bean 修飾的方法，這個區(qū)別很重要?。?；
實戰(zhàn)驗證
- 接下來到了實戰(zhàn)驗證的環(huán)節(jié)了，本次實戰(zhàn)的內(nèi)容是創(chuàng)建一個 springboot 工程，在里面自定義三個 ImportSelector 接口的實現(xiàn)類，如果您不想敲代碼，也可以去 github 下載源碼，地址和鏈接信息如下表所示：
- 這個 git 項目中有多個文件夾，本章源碼在文件夾 customizeimportselector 下，如下圖紅框所示：
- 開始編碼吧：
- 我們創(chuàng)建三個 ImportSelector 的實現(xiàn)類來檢查其先后順序，三個 Selector 類簡介如下表，有兩個是 DeferredImportSelector 的實現(xiàn)類，一個是 ImportSelector 的實現(xiàn)類，每個 Selector 負責向 spring 容器注冊一種實例：
- 基于 maven 創(chuàng)建 springboot 框架的 web 工程，pom.xml 內(nèi)容如下：
4.0.0 com.bolingcavalry customizeimportselector 0.0.1-SNAPSHOT jar customizeimportselector Demo project for Spring Boot org.springframework.boot spring-boot-starter-parent 1.5.9.RELEASE UTF-8 UTF-8 1.8 org.springframework.boot spring-boot-starter-web org.springframework.boot spring-boot-starter-test test org.springframework.boot spring-boot-maven-plugin
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- 創(chuàng)建三個接口 CustomizeService1、CustomizeService2、CustomizeService3，第一個源碼如下，另外兩個除了類名，其余部分一樣：
package com.bolingcavalry.customizeimportselector.service;public interface CustomizeService1 { void execute();}
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- 創(chuàng)建三個類，分別實現(xiàn)上面的三個接口，也是除了類名其余部分一樣：
package com.bolingcavalry.customizeimportselector.service.impl;import com.bolingcavalry.customizeimportselector.service.CustomizeService1;public class CustomizeServiceImpl1 implements CustomizeService1 { public CustomizeServiceImpl1() { System.out.println(“construct : ” + this.getClass().getSimpleName()); } @Override public void execute() { System.out.println(“execute : ” + this.getClass().getSimpleName()); }}
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- 創(chuàng)建 CustomizeImportSelector1：
package com.bolingcavalry.customizeimportselector.selector;import org.springframework.context.annotation.DeferredImportSelector;import org.springframework.context.annotation.ImportSelector;import org.springframework.core.annotation.Order;import org.springframework.core.type.AnnotationMetadata;@Order(102)public class CustomizeImportSelector1 implements DeferredImportSelector { @Override public String[] selectImports(AnnotationMetadata annotationMetadata) { System.out.println(“selectImports : ” + this.getClass().getSimpleName()); return new String[]{“com.bolingcavalry.customizeimportselector.service.impl.CustomizeServiceImpl1”}; }}
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- 創(chuàng)建 CustomizeImportSelector2：
package com.bolingcavalry.customizeimportselector.selector;import org.springframework.context.annotation.DeferredImportSelector;import org.springframework.core.annotation.Order;import org.springframework.core.type.AnnotationMetadata;@Order(101)public class CustomizeImportSelector2 implements DeferredImportSelector { @Override public String[] selectImports(AnnotationMetadata annotationMetadata) { System.out.println(“selectImports : ” + this.getClass().getSimpleName()); return new String[]{“com.bolingcavalry.customizeimportselector.service.impl.CustomizeServiceImpl2”}; }}
復制代碼
- 創(chuàng)建 CustomizeImportSelector3，實現(xiàn)的是 ImportSelector 接口：
package com.bolingcavalry.customizeimportselector.selector;import org.springframework.context.annotation.ImportSelector;import org.springframework.core.annotation.Order;import org.springframework.core.type.AnnotationMetadata;public class CustomizeImportSelector3 implements ImportSelector { @Override public String[] selectImports(AnnotationMetadata annotationMetadata) { System.out.println(“selectImports : ” + this.getClass().getSimpleName()); return new String[]{“com.bolingcavalry.customizeimportselector.service.impl.CustomizeServiceImpl3”}; }}
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- 創(chuàng)建配置類，將 CustomizeImportSelector1、CustomizeImportSelector2、CustomizeImportSelector3 全部用 Import 注解引入：
package com.bolingcavalry.customizeimportselector;import com.bolingcavalry.customizeimportselector.selector.CustomizeImportSelector1;import com.bolingcavalry.customizeimportselector.selector.CustomizeImportSelector2;import com.bolingcavalry.customizeimportselector.selector.CustomizeImportSelector3;import org.springframework.context.annotation.Configuration;import org.springframework.context.annotation.Import;@Configuration@Import({CustomizeImportSelector1.class, CustomizeImportSelector2.class, CustomizeImportSelector3.class})public class SysConfig {}
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- 創(chuàng)建啟動類 CustomizeimportselectorApplication.java：
package com.bolingcavalry.customizeimportselector;import org.springframework.boot.SpringApplication;import org.springframework.boot.autoconfigure.SpringBootApplication;@SpringBootApplicationpublic class CustomizeimportselectorApplication { public static void main(String[] args) { SpringApplication.run(CustomizeimportselectorApplication.class, args); }}
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- 啟動應用，可見輸入信息如下：
2018-09-09 15:43:45.790 INFO 15364 — [ main] c.b.c.CustomizeimportselectorApplication : Starting CustomizeimportselectorApplication on DESKTOP-82CCEBN with PID 15364 (D:githubblog_demoscustomizeimportselectorargetclasses started by 12167 in D:githubblog_demoscustomizeimportselector)2018-09-09 15:43:45.791 INFO 15364 — [ main] c.b.c.CustomizeimportselectorApplication : No active profile set, falling back to default profiles: default2018-09-09 15:43:45.825 INFO 15364 — [ main] ationConfigEmbeddedWebApplicationContext : Refreshing org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext@641147d0: startup date [Sun Sep 09 15:43:45 GMT+08:00 2018]; root of context hierarchyselectImports : CustomizeImportSelector3selectImports : CustomizeImportSelector2selectImports : CustomizeImportSelector12018-09-09 15:43:46.425 INFO 15364 — [ main] s.b.c.e.t.TomcatEmbeddedServletContainer : Tomcat initialized with port(s): 8080 (http)2018-09-09 15:43:46.430 INFO 15364 — [ main] o.apache.catalina.core.StandardService : Starting service [Tomcat]2018-09-09 15:43:46.431 INFO 15364 — [ main] org.apache.catalina.core.StandardEngine : Starting Servlet Engine: Apache Tomcat/8.5.232018-09-09 15:43:46.493 INFO 15364 — [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext2018-09-09 15:43:46.493 INFO 15364 — [ost-startStop-1] o.s.web.context.ContextLoader : Root WebApplicationContext: initialization completed in 670 ms2018-09-09 15:43:46.569 INFO 15364 — [ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean : Mapping servlet: ‘dispatcherServlet’ to [/]2018-09-09 15:43:46.572 INFO 15364 — [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: ‘characterEncodingFilter’ to: [/*]2018-09-09 15:43:46.572 INFO 15364 — [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: ‘hiddenHttpMethodFilter’ to: [/*]2018-09-09 15:43:46.572 INFO 15364 — [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: ‘httpPutFormContentFilter’ to: [/*]2018-09-09 15:43:46.572 INFO 15364 — [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: ‘requestContextFilter’ to: [/*]construct : CustomizeServiceImpl1construct : CustomizeServiceImpl2construct : CustomizeServiceImpl3
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- 從上述信息可以看出：
- 首先、三個 selector 實現(xiàn)類的 selectImports 方法執(zhí)行順序符合預期：先執(zhí)行 ImportSelector 實現(xiàn)類的，再執(zhí)行 DeferredImportSelector 實現(xiàn)類的，并且 DeferredImportSelector 實現(xiàn)類的執(zhí)行順序會按照 Order 的設置 從小到大 執(zhí)行；
- 其次、CustomizeServiceImpl1、CustomizeServiceImpl2、CustomizeServiceImpl3 的實例化順序并未受到影響；
- 至此，ImportSelector 與 DeferredImportSelector 的區(qū)別已經(jīng)分析和驗證完畢，隨著對 Configuration 初始化處理邏輯的深入了解，我們可以定制出更靈活強大的配置邏輯，以符合業(yè)務需求；
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