SpringBoot 启动原理
SpringBoot干了什么？
没有SpringBoot的时候，使用Spring开发应用程序，需要自己去手动像IoC容器里注册Bean。无论是xml还是JavaConfig。比如说集成MyBatis，需要xml或者JavaConfig去注入MyBatis的工厂类到Spring容器。集成很多第三方程序的时候就需要很多配置。这些配置对于新建一个项目来说，很多都是重复且相同的，比如MyBatis工厂类的包位置，相同版本的谁来新建都是写的一样的东西。SpringBoot就采用约定大于配置的思想，把集成到spring的组件，拆分成starter，来自动配置。
核心原理
首先看看springboot程序的注解@SpringBootApplication
@SpringBootApplication
public class StudyProApplication {
public static void main(String[] args) {
SpringApplication.run(StudyProApplication.class, args);
}
}
@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 {
...
}
核心是这三个
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan
@SpringBootConfiguration就是@Configuration，表明这是一个Spring JavaConfig类。
@ComponentScan的功能其实就是自动扫描并加载符合条件的组件（比如@Component和@Repository等）或者bean定义，最终将这些bean定义加载到IoC容器中。不指定范围默认Spring框架实现会从声明@ComponentScan所在类的package进行扫描。
重点来看看@EnableAutoConfiguration
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
/**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
Class<?>[] exclude() default {};
/**
* Exclude specific auto-configuration class names such that they will never be
* applied.
* @return the class names to exclude
* @since 1.3.0
*/
String[] excludeName() default {};
}
@EnableAutoConfiguration是借助@Import的帮助，将所有符合自动配置条件的bean定义加载到IoC容器
@AutoConfigurationPackage：自动配置包
@Import: 导入自动配置的组件
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {
/**
* Base packages that should be registered with {@link AutoConfigurationPackages}.
* <p>
* Use {@link #basePackageClasses} for a type-safe alternative to String-based package
* names.
* @return the back package names
* @since 2.3.0
*/
String[] basePackages() default {};
/**
* Type-safe alternative to {@link #basePackages} for specifying the packages to be
* registered with {@link AutoConfigurationPackages}.
* <p>
* Consider creating a special no-op marker class or interface in each package that
* serves no purpose other than being referenced by this attribute.
* @return the base package classes
* @since 2.3.0
*/
Class<?>[] basePackageClasses() default {};
}
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
@Override
public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
register(registry, new PackageImports(metadata).getPackageNames().toArray(new String[0]));
}
@Override
public Set<Object> determineImports(AnnotationMetadata metadata) {
return Collections.singleton(new PackageImports(metadata));
}
}
new PackageImport(metadata).getPackageName()，它其实返回了当前主程序类的 同级以及子级 的包组件。
@Import(AutoConfigurationImportSelector.class)注解：
@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}
protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return EMPTY_ENTRY;
}
AnnotationAttributes attributes = getAttributes(annotationMetadata);
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
configurations = removeDuplicates(configurations);
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
configurations.removeAll(exclusions);
configurations = getConfigurationClassFilter().filter(configurations);
fireAutoConfigurationImportEvents(configurations, exclusions);
return new AutoConfigurationEntry(configurations, exclusions);
}
List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
它其实是去加载 public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
寻找classpath下所有的spring.factories文件
借助EnableAutoConfigurationImportSelector，@EnableAutoConfiguration可以帮助SpringBoot应用将所有符合条件的@Configuration配置都加载到当前SpringBoot创建并使用的IoC容器
