SpringBoot @SpringBootApplication 注解

2018-02-01

JDK: 1.8
SpringBoot: 1.5.10.RELEASE

@SpringBootApplication 注解示例

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

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

@SpringBootApplicaiton

SpringBoot 应用程序标注在某个类上说明这个类是 SpringBoot 的主配置类，也称为启动类。

SpringBoot 应用程序通过执行启动类的 main 方法来启动 SpringBoot 应用程序。

@SpringBootApplication 注解详解

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 {};
}

@SpringBootApplication 是一个组合注解：

@SpringBootConfiguration ：SpringBoot 应用程序的配置类注解，标注在某个类上，表示这是一个 SpringBoot 配置类

@EnableAutoConfiguration ：SpringBoot 应用程序的开启自动配置注解

@SpringBootConfiguration 注解详解

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Configuration;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}

@SpringBootConfiguration 是一个组合注解，实际表示了 @Configutation 注解，两者实际作用相同，

只不过一个是 Spring 提供的注解，一个是 SpringBoot 提供的注解。

@EnableAutoConfiguration 注解详解

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 {};
}

@EnableAutoConfiguration 是一个组合注解：

@AutoConfigurationPackage ：自动配置包

@Import({EnableAutoConfigurationImportSelector.class}) ：表示想容器中导入哪些组件的选择器

@AutoConfigurationPackage 注解详解

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 {
}

@AutoConfigurationPackage 注解商标注了 @Import({Registrar.class})

@Import 是 Spring 提供的注解，表示向容器中导入一个组件，这里就是表示向容器中导入 Registrar.class 类，

而 Registrar.class 类的作用就是得到注解标注类所在的包路径并注册

本质作用是将启动类所在包及子包里面的所有的组件扫描到 Spring 容器中

@Import({EnableAutoConfigurationImportSelector.class}) 详解

通过源码可以发现关键的如下方法：

public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    } else {
        try {
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);
            AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
            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);
        }
    }
}

将向容器中导入 SpringBoot 预定义的自动配置类，当触发自动配置时进行自动配置。