springboot中自動(dòng)配置原理解析

SpringBoot作為當(dāng)前Java開發(fā)的熱門框架，有絞手架之稱。“約定大于配置”也一直是SpringBoot的標(biāo)簽，那么，SpringBoot要實(shí)現(xiàn)自身優(yōu)勢，自動(dòng)配置功不可沒。

一、SpringBoot自動(dòng)配置是什么

SpringBoot自動(dòng)配置是指在應(yīng)用程序啟動(dòng)時(shí)，SpringBoot根據(jù)classpath路徑下的jar包自動(dòng)配置應(yīng)用程序所需的一系列bean和組件，從而減少開發(fā)者的配置工作，提高開發(fā)效率。文章源自四五設(shè)計(jì)網(wǎng)-http://m.wasochina.com/45484.html

二、@Import注解

在剖析SpringBoot自動(dòng)配置原理之前，我們先了解一下@Import注解的使用文章源自四五設(shè)計(jì)網(wǎng)-http://m.wasochina.com/45484.html

1. 方式一： .class方式

定義兩個(gè)類A、B，并將其加入到Spring IOC容器中：文章源自四五設(shè)計(jì)網(wǎng)-http://m.wasochina.com/45484.html

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@Data
public class A{
????private Integer id=0;
????private String name="classA";
????public void print(){
????????System.out.println(this.name);
????}
}
@Data
public class B{
????private Integer id=1;
????private String name="classB";
????public void print(){
????????System.out.println(this.name);
????}
}

創(chuàng)建一個(gè)配置類，并使用@Import注解將類A、B添加到 IOC 容器中：文章源自四五設(shè)計(jì)網(wǎng)-http://m.wasochina.com/45484.html

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@Import({A.class,B.class})
@Configuration
public class ClassConfig{
???????}

2.方式二：ImportSelector方式

該方法需要定義類來實(shí)現(xiàn)ImportSelector接口，并重寫其中的selectImports( )方法，該方法的返回值是需要添加到IOC容器中的類的全限定類名數(shù)組：文章源自四五設(shè)計(jì)網(wǎng)-http://m.wasochina.com/45484.html

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@Data
public class C{
????private Integer id=2;
????private String name="classC";
????public void print(){
????????System.out.println(this.name);
????}
}

編寫類來實(shí)現(xiàn)ImportSelector接口：文章源自四五設(shè)計(jì)網(wǎng)-http://m.wasochina.com/45484.html

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public class ImportSelectorTest implements ImportSelector{
????@Override
????public String[] selectImports(AnnotationMetadata annotationMetada){
????????return new String[]{"C.class"};
????}
}

使用該類:文章源自四五設(shè)計(jì)網(wǎng)-http://m.wasochina.com/45484.html

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@Import({ImportSelectorTest.class})
@Configuration
public class ImportSelectorConfig{
???????}

3.方式三：ImportBeanDefinitionRegistrar方式

定義一個(gè)類并實(shí)現(xiàn)ImportBeanDefinitionRegistrar接口，重寫其中的registerBeanDefinitions( )方法，此方式可以自定義Bean在容器中的名稱：文章源自四五設(shè)計(jì)網(wǎng)-http://m.wasochina.com/45484.html

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@Data
public class D{
????private Integer id=3;
????private String name="ClassD";
????public void print(){
????????System.out.println(this.name);
????}
}
//定義類實(shí)現(xiàn)ImPortBeanDefinitionRegistrar接口，重寫其中的registerBeanDefinitions()方法
public class ImportBeanDefinitionRegistrarTest{
????@Override
????public void registerBeanDefninitions(AnnotationMetadata annotationMetadata,BeanDefinitionRegistry registry{
????????RootBeanDefinition rootBeanDefinition=new RootBeanDefinition(D.class);
????????registry.registerBeanDefinition("自定義名稱",rootBeanDefinition);
????}
}
//使用上面的類進(jìn)行導(dǎo)入
@Import({ImportBeanDefinitionRegistrarTest})
@Configuration
public class ImportBeanDefinitionRegistrarConfig{
}

三、SpringBoot自動(dòng)配置原理解析

為了容易分析和理解，我們在IDEA中創(chuàng)建一個(gè)SpringBoot項(xiàng)目，創(chuàng)建過程省略，直接跳到該項(xiàng)目的主配置類進(jìn)行分析：文章源自四五設(shè)計(jì)網(wǎng)-http://m.wasochina.com/45484.html

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@SpringBootApplication
public class SpringBootTestApplication {
????public static void main(String[] args) {
????????SpringApplication.run(SpringBootTestApplication.class, args);
????}
}

其中@SpringBootApplication注解是SpringBoot項(xiàng)目的重點(diǎn)，按住ctrl鍵進(jìn)入其中，看到它由以下部分組成:文章源自四五設(shè)計(jì)網(wǎng)-http://m.wasochina.com/45484.html

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@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
????excludeFilters = {@Filter(
????type = FilterType.CUSTOM,
????classes = {TypeExcludeFilter.class}
), @Filter(
????type = FilterType.CUSTOM,
????classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
????//內(nèi)容省略
}

我們主要關(guān)注以下幾個(gè)注解:

  • @SpringBootConfiguration：標(biāo)記當(dāng)前類為配置類
  • @EnableAutoConfiuration：開啟自動(dòng)配置
  • @ComponentScan：掃描主類所在包及其子包、同級(jí)包中的Bean

1、@SpringBootConfiguration注解：標(biāo)記當(dāng)前類為配置類

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@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
@Indexed
public @interface SpringBootConfiguration {
????@AliasFor(
????????annotation = Configuration.class
????)
????boolean proxyBeanMethods() default true;
}

根據(jù)其源碼可以知道，@SpringBootConfiguration注解包含@Configuration，所以其擁有@Configuration注解相似的功能，而@Configuration注解又包含@Companent注解，所以配置類也存在于IOC容器中。

2、@EnableAutoConfiguration注解：開啟自動(dòng)配置

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@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
????String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
????Class<?>[] exclude() default {};
????String[] excludeName() default {};
}

根據(jù)其源碼得出其主要由@AutoConfigurationPackages注解和@Import注解組成

@AutoConfigurationPackages注解

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@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import({Registrar.class})
public @interface AutoConfigurationPackage {
????String[] basePackages() default {};
????Class<?>[] basePackageClasses() default {};
}
//其中的@Import注解導(dǎo)入了Registrar類，該類是AutoConfigurationPackages的靜態(tài)類部類
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
????????Registrar() {
????????}
????????/**
????????????根據(jù)傳入的元注解信息獲取所在的包，將包中組件類封裝為數(shù)組進(jìn)行注冊
????????*/
????????public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
????????????AutoConfigurationPackages.register(registry, (String[])(new AutoConfigurationPackages.PackageImports(metadata)).getPackageNames().toArray(new String[0]));
????????}
????????public Set<Object> determineImports(AnnotationMetadata metadata) {
????????????return Collections.singleton(new AutoConfigurationPackages.PackageImports(metadata));
????????}
????}

@Import({AutoConfigurationImportSelector.class})注解

這一步就用到了@Import注解使用方式中的第二中：實(shí)現(xiàn)ImportSelector接口

那么AutoConfigurationImportSelector接口何時(shí)如何被執(zhí)行呢?

SpringBoot 啟動(dòng)時(shí)會(huì)使用 ConfigurationClassParser 來解析被 @Configuration 標(biāo)識(shí)的配置類, 然后再處理這個(gè)類內(nèi)部被其他注解修飾的情況, 比如 @Import 注解, @ComponentScan 注解，@Bean 注解等

若發(fā)現(xiàn)注解中存在 @Import(ImportSelector) 的情況下，就會(huì)創(chuàng)建一個(gè)相應(yīng)的 ImportSelector 對(duì)象，并調(diào)用其 process 方法

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public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
????????????Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector, () -> {
????????????????return String.format("Only %s implementations are supported, got %s", AutoConfigurationImportSelector.class.getSimpleName(), deferredImportSelector.getClass().getName());
????????????});
????????????AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector)deferredImportSelector).getAutoConfigurationEntry(annotationMetadata);
????????????this.autoConfigurationEntries.add(autoConfigurationEntry);
????????????Iterator var4 = autoConfigurationEntry.getConfigurations().iterator();
????????????while(var4.hasNext()) {
????????????????String importClassName = (String)var4.next();
????????????????this.entries.putIfAbsent(importClassName, annotationMetadata);
????????????}
????????}

process方法又調(diào)用了getAutoConfigurationEntry方法:

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protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
????????if (!this.isEnabled(annotationMetadata)) {
????????????return EMPTY_ENTRY;
????????} else {
????????????AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
????????????List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
????????????configurations = this.removeDuplicates(configurations);
????????????Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
????????????this.checkExcludedClasses(configurations, exclusions);
????????????configurations.removeAll(exclusions);
????????????configurations = this.getConfigurationClassFilter().filter(configurations);
????????????this.fireAutoConfigurationImportEvents(configurations, exclusions);
????????????return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
????????}
????}

getAutoConfigurationEntry方法調(diào)用了getCandidateConfigurations方法：

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protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
????????List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());
????????Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct.");
????????return configurations;
????}

getCandidateConfigurations方法中使用了Spring Cor中的類SpringFactoriesLoader，該類的loadFactoryNames方法可以根據(jù)接口獲取接口類的名稱，這個(gè)方法返回的是類名的列表，loadFactoryNames方法會(huì)遍歷整個(gè)springboot項(xiàng)目的classpath下的ClassLoader中所有jar包下的spring.factories文件。至此自動(dòng)配置結(jié)束

總結(jié)

SpringBoot自動(dòng)配置是SpringBoot的核心，所以了解SpringBoot的自動(dòng)配置是非常有必要的，大家可以自行查找資料解釋以下為什么不使用@ComponentScan注解替換@Import注解來進(jìn)行類的導(dǎo)入

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