@SpringBootApplication原理

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1.@SpringBootApplication是一个组合注解，我们来看一下它的源码：

//// 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@ComponentScan(    excludeFilters = {@Filter(    type = FilterType.CUSTOM,    classes = {TypeExcludeFilter.class}), @Filter(    type = FilterType.CUSTOM,    classes = {AutoConfigurationExcludeFilter.class})})public @interface SpringBootApplication {    @AliasFor(        annotation = EnableAutoConfiguration.class,        attribute = "exclude"    )    Class<?>[] exclude() default {};    @AliasFor(        annotation = EnableAutoConfiguration.class,        attribute = "excludeName"    )    String[] excludeName() default {};    @AliasFor(        annotation = ComponentScan.class,        attribute = "basePackages"    )    String[] scanBasePackages() default {};    @AliasFor(        annotation = ComponentScan.class,        attribute = "basePackageClasses"    )    Class<?>[] scanBasePackageClasses() default {};}

2.而@SpringBootApplication的核心功能是由@EnableAutoConfiguration提供的：

//// 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.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 {};}

3.在@EnableAutoConfiguration中个，关键的注解是@Import这个注解，它导入了“EnableAutoConfigurationImportSelector”这个类；

4.而“EnableAutoConfigurationImportSelector”这个类使用了“SpringFactoriesLoader。loadFactoryName”这个方法来扫描具有META-INF/spring,factories文件的jar包；

5.而在我们导入的“spring-boot-autoconfigure-1.3.0.x.jar”包中就有一个spring.factories文件，次文件中声明了有哪些自动配置

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