springboot自动配置原理（高低版本比较）spring.factories文件的作用

@SpringBootApplication
public class SpringSecurityApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringSecurityApplication.class, args);
    }

}

注解@SpringBootApplication
@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
)
Class<?>[] exclude() default {};

@AliasFor(
    annotation = EnableAutoConfiguration.class
)
String[] excludeName() default {};

@AliasFor(
    annotation = ComponentScan.class,
    attribute = "basePackages"
)
String[] scanBasePackages() default {};

@AliasFor(
    annotation = ComponentScan.class,
    attribute = "basePackageClasses"
)
Class<?>[] scanBasePackageClasses() default {};

@AliasFor(
    annotation = ComponentScan.class,
    attribute = "nameGenerator"
)
Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;

@AliasFor(
    annotation = Configuration.class
)
boolean proxyBeanMethods() default true;

}

核心3个注解：
@SpringBootConfiguration     改注解和@Configuration类似，表面这个类是配置类
@EnableAutoConfiguration    springboot实现自动化配置的核心注解
@ComponentScan    默认扫描启动类当前包以及子包

要了解springboot自动配置原理当然我们需要看@EnableAutoConfiguration

在这2.4版本以前
@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 {};

}

3.4.1版本
@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 {};

}

开启自动配置注解我们需要看这个注解做了什么事？
看上面的代码我们可以发现一个关键的@import注解，
低版本：开启导入自动配置选择器：
源码：
@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;
    }
}
``

3.4.1版本
public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
    static final int ORDER = 2147483646;
    private static final AutoConfigurationEntry EMPTY_ENTRY = new AutoConfigurationEntry();
    private static final String[] NO_IMPORTS = new String[0];
    private static final Log logger = LogFactory.getLog(AutoConfigurationImportSelector.class);
    private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";
    private final Class<?> autoConfigurationAnnotation;
    private ConfigurableListableBeanFactory beanFactory;
    private Environment environment;
    private ClassLoader beanClassLoader;
    private ResourceLoader resourceLoader;
    private volatile ConfigurationClassFilter configurationClassFilter;
    private volatile AutoConfigurationReplacements autoConfigurationReplacements;

    public AutoConfigurationImportSelector() {
        this((Class)null);
    }

    AutoConfigurationImportSelector(Class<?> autoConfigurationAnnotation) {
        this.autoConfigurationAnnotation = autoConfigurationAnnotation != null ? autoConfigurationAnnotation : AutoConfiguration.class;
    }

    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!this.isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        } else {
            AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);
            return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
        }
    }

    public Predicate<String> getExclusionFilter() {
        return this::shouldExclude;
    }

    private boolean shouldExclude(String configurationClassName) {
        return this.getConfigurationClassFilter().filter(Collections.singletonList(configurationClassName)).isEmpty();
    }

    protected 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 AutoConfigurationEntry(configurations, exclusions);
        }
    }

    public Class<? extends DeferredImportSelector.Group> getImportGroup() {
        return AutoConfigurationGroup.class;
    }

    protected boolean isEnabled