源码学习系列之SpringBoot自动配置（篇一）
- 2019 年 11 月 2 日
- 筆記
源码学习系列之SpringBoot自动配置源码学习（篇一）
ok，本博客尝试跟一下Springboot的自动配置源码，做一下笔记记录，自动配置是Springboot的一个很关键的特性，也容易被忽略的属性，因为这个属性被包括在@SpringBootApplication注解里，所以不去跟一下源码都不知道还有这个属性，ps：本博客源码基于SpringBoot1.5.7版本
@SpringBootApplication
ok，跟一下@SpringBootApplication，发现@SpringBootApplication其实是一个复合的注解，由很多注解构成，@EnableAutoConfiguration其实只是其一部分，@EnableAutoConfiguration就是开启自动配置的注解
// // Source code recreated from a .class file by IntelliJ IDEA // (powered by Fernflower decompiler) // package org.springframework.boot.autoconfigure; import java.lang.annotation.Documented; import java.lang.annotation.ElementType; import java.lang.annotation.Inherited; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; import org.springframework.boot.SpringBootConfiguration; import org.springframework.boot.context.TypeExcludeFilter; import org.springframework.context.annotation.ComponentScan; import org.springframework.context.annotation.FilterType; import org.springframework.context.annotation.ComponentScan.Filter; import org.springframework.core.annotation.AliasFor; @Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @SpringBootConfiguration @EnableAutoConfiguration //自动配置注解 @ComponentScan( excludeFilters = {@Filter( type = FilterType.CUSTOM, classes = {TypeExcludeFilter.class} ), @Filter( type = FilterType.CUSTOM, classes = {AutoConfigurationExcludeFilter.class} )} ) public @interface SpringBootApplication { .... Class<?>[] scanBasePackageClasses() default {}; } @EnableAutoConfiguration
点进@EnableAutoConfiguration，比较重要的列出来：
- @AutoConfigurationPackage
- @Import({EnableAutoConfigurationImportSelector.class})
package org.springframework.boot.autoconfigure; import java.lang.annotation.Documented; import java.lang.annotation.ElementType; import java.lang.annotation.Inherited; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; import org.springframework.context.annotation.Import; @Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @AutoConfigurationPackage @Import({EnableAutoConfigurationImportSelector.class}) public @interface EnableAutoConfiguration { String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration"; Class<?>[] exclude() default {}; String[] excludeName() default {}; }@AutoConfigurationPackage
先看@AutoConfigurationPackage源码，这个注解是开启自动配置包的，关注点在@Import({Registrar.class})，核心在Registrar类
备注：@import注解是Spring的底层注解，作用是导入一个组件到容器里
package org.springframework.boot.autoconfigure; import java.lang.annotation.Documented; import java.lang.annotation.ElementType; import java.lang.annotation.Inherited; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; import org.springframework.boot.autoconfigure.AutoConfigurationPackages.Registrar; import org.springframework.context.annotation.Import; @Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @Import({Registrar.class}) public @interface AutoConfigurationPackage { }Registrar 类是AutoConfigurationPackages类的静态内部类，
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports { Registrar() { } public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) { AutoConfigurationPackages.register(registry, (new AutoConfigurationPackages.PackageImport(metadata)).getPackageName()); } public Set<Object> determineImports(AnnotationMetadata metadata) { return Collections.singleton(new AutoConfigurationPackages.PackageImport(metadata)); } }看一下(new AutoConfigurationPackages.PackageImport(metadata)).getPackageName()获取的是什么，用idea的工具，计算表达式，可以看到其实获取的是SpringBoot启动类上面的包名
AnnotationMetadata：SpringBoot注解元数据
所以，@AutoConfigurationPackage开启后，就可以将主配置类(@SpringBootApplication)所在包及其子包里面的所有组件都扫描到Spring容器里
public static void register(BeanDefinitionRegistry registry, String... packageNames) { if (registry.containsBeanDefinition(BEAN)) { BeanDefinition beanDefinition = registry.getBeanDefinition(BEAN); ConstructorArgumentValues constructorArguments = beanDefinition.getConstructorArgumentValues(); constructorArguments.addIndexedArgumentValue(0, addBasePackages(constructorArguments, packageNames)); } else {// Spring容器里没有找到对应组件 /* 将组件注册到Spring容器里 */ GenericBeanDefinition beanDefinition = new GenericBeanDefinition(); beanDefinition.setBeanClass(AutoConfigurationPackages.BasePackages.class); beanDefinition.getConstructorArgumentValues().addIndexedArgumentValue(0, packageNames); beanDefinition.setRole(2); registry.registerBeanDefinition(BEAN, beanDefinition); } }ok，跟了源码，当然只是简单跟一下，没有特别细的跟，这个过程可以看出这里的组件扫描只是扫描主配置类(@SpringBootApplication)所在包及其子包里面的所有组件，所以，写了例子验证一下：
在Application类包外写个Controller测试类
@RestController public class TestController { @GetMapping("/hello") public String hello(){ return "hello world!"; } }启动项目，进行访问，发现都是404找不到这个接口，再将这个Controller放在包里面，才可以扫描到
@Import({EnableAutoConfigurationImportSelector.class})
package org.springframework.boot.autoconfigure; import org.springframework.core.type.AnnotationMetadata; /** @deprecated */ @Deprecated public class EnableAutoConfigurationImportSelector extends AutoConfigurationImportSelector { public EnableAutoConfigurationImportSelector() { } protected boolean isEnabled(AnnotationMetadata metadata) { return this.getClass().equals(EnableAutoConfigurationImportSelector.class) ? (Boolean)this.getEnvironment().getProperty("spring.boot.enableautoconfiguration", Boolean.class, true) : true; } } 主要看一下其基类AutoConfigurationImportSelector代码，看一下selectImport方法：
/**装载很多自动配置类，以全类名的方式返回一个字符数组**/ public String[] selectImports(AnnotationMetadata annotationMetadata) { if (!this.isEnabled(annotationMetadata)) { return NO_IMPORTS; } else { try { //获取自动配置的元数据 AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader); //装载配置属性 AnnotationAttributes attributes = this.getAttributes(annotationMetadata); //通过类加载器读取所有的配置类全类名 List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes); configurations = this.removeDuplicates(configurations); configurations = this.sort(configurations, autoConfigurationMetadata); Set<String> exclusions = this.getExclusions(annotationMetadata, attributes); this.checkExcludedClasses(configurations, exclusions); configurations.removeAll(exclusions); configurations = this.filter(configurations, autoConfigurationMetadata); this.fireAutoConfigurationImportEvents(configurations, exclusions); return (String[])configurations.toArray(new String[configurations.size()]); } catch (IOException var6) { throw new IllegalStateException(var6); } } }- AutoConfigurationMetadata信息，获取整个JavaEE体系的一些配置类，当然是Springboot集成的，比如有WebMvcAutoConfiguration自动配置类
跟一下getCandidateConfigurations方法，SpringFactoriesLoader是Spring-code工程的工厂加载类，使用SpringFactoriesLoader，需要在模块的META-INF/spring.factories文件自己配置属性，这个Properties格式的文件中的key是接口、注解、或抽象类的全名，value是以逗号 “ , “ 分隔的实现类，使用SpringFactoriesLoader可以实现将相应的实现类注入Spirng容器
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; }loadFactoryNames方法代码，
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories"; /* 将spring.factories的类都装载到Spring容器*/ public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) { String factoryClassName = factoryClass.getName(); try { //将META-INF/spring.factories文件里配置的属性都装载到Enumeration数据结构里 Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories"); ArrayList result = new ArrayList(); //遍历获取属性，然后再获取对应的配置类全类名 while(urls.hasMoreElements()) { URL url = (URL)urls.nextElement(); Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url)); String factoryClassNames = properties.getProperty(factoryClassName); result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames))); } return result; } catch (IOException var8) { throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + "META-INF/spring.factories" + "]", var8); } }然后META-INF/spring.factories文件放在哪？很显然是放在Springboot的自动配置模块里，如图：
所以，@Import({EnableAutoConfigurationImportSelector.class})开启之后，主要是EnableAutoConfigurationImportSelector这个类的作用就是在SpringBoot启动时候将从SpringBoot自动配置工程的META-INF/spring.factories文件中获取指定的值，经过SpringFactoriesLoader加载之后将很多自动配置类加载到Spring容器，所以我们不需要配置，mvc等等默认配置就已经随着SpringBoot启动而自动生效
ok，Springboot的自动配置类都在这个包里，源码很多，所以本博客只是简单跟一下源码
