简介
- 简介
- 启动过程
- starter 依赖扩展ApplicationContext的入口 ApplicationContextInitializer
- tomcat 是如何内置的——ServletWebServerApplicationContext
- @EnableAutoConfiguration 如何工作
几个问题
- 如何自动加载配置，将依赖bean 注入到spring ioc
- 如何自动规范依赖jar？继承 spring-boot-starter-parent
- tomcat 是如何内置的
建议先阅读下 回头看Spring IOC 对IOC 和 ApplicationContext 等概念有所了解。
启动过程
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
this.webApplicationType = WebApplicationType.deduceFromClasspath();
setInitializers((Collection) getSpringFactoriesInstances(
ApplicationContextInitializer.class));
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
this.mainApplicationClass = deduceMainApplicationClass();
}
getSpringFactoriesInstances 会从META-INF/spring.factories读取key为org.springframework.context.ApplicationContextInitializer的value 并创建ApplicationContextInitializer 实例，ApplicationListener类似。
SpringApplication.run 创建并刷新ApplicationContext，算是开始进入正题了。
public ConfigurableApplicationContext run(String... args) {
ConfigurableApplicationContext context = null;
...
SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
args);
ConfigurableEnvironment environment = prepareEnvironment(listeners,
applicationArguments);
...
// 创建AnnotationConfigServletWebServerApplicationContext
context = createApplicationContext();
...
prepareContext(context,environment,listeners,applicationArguments,printedBanner);
refreshContext(context);
listeners.started(context);
callRunners(context, applicationArguments);
listeners.running(context);
return context;
}
- 所谓的 SpringApplicationRunListeners 就是在SpringApplication.run 方法执行的不同阶段去执行一些操作， SpringApplicationRunListener 也可在
META-INF/spring.factories配置 - Environment代表着程序的运行环境，主要包含了两种信息，一种是profiles，用来描述哪些bean definitions 是可用的；一种是properties，用来描述系统的配置，其来源可能是配置文件、jvm属性文件、操作系统环境变量等。
- AnnotationConfigServletWebServerApplicationContext 的默认构造方法中初始化了两个成员变量，类型分别为AnnotatedBeanDefinitionReader 和 ClassPathBeanDefinitionScanner 用来加载Bean 定义。
其中在 prepareContext 中
private void prepareConte(ConfigurableApplicationContext context,ConfigurableEnvironment environment,SpringApplicationRunListeners listeners,ApplicationArguments applicationArguments, Banner printedBanner) {
context.setEnvironment(environment);
postProcessApplicationContext(context);
applyInitializers(context);
listeners.contextPrepared(context);
// Add boot specific singleton beans
ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
...
// Load the sources
Set<Object> sources = getAllSources();
load(context, sources.toArray(new Object[0]));
listeners.contextLoaded(context);
}
applyInitializers 会执行initializer.initialize(context)
starter 依赖扩展ApplicationContext的入口 ApplicationContextInitializer
public class XXApplicationContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext{
@Override
public void initialize(ConfigurableApplicationContext applicationContext) {
...
applicationContext.getBeanFactory().addBeanPostProcessor(xx);
applicationContext.getBeanFactory().registerSingleton("xx", xx);
...
}
}
通过BeanFactory 对象，便可以将自定义的业务对象 注入到ioc 容器中，为spring 所用。
tomcat 是如何内置的——ServletWebServerApplicationContext
@EnableAutoConfiguration 如何工作
@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 {
...
}
如果使用了@SpringBootApplication注解，那么自动就启用了EnableAutoConfiguration
@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 {};
}
@Import 的作用类似xml 中的<import> Provides functionality equivalent to the <import/> element in Spring XML.Allows for importing @Configuration classes,ImportSelector and
ImportBeanDefinitionRegistrar implementations
public class AutoConfigurationImportSelector implements DeferredImportSelector{
protected AutoConfigurationEntry getAutoConfigurationEntry(
AutoConfigurationMetadata autoConfigurationMetadata,
AnnotationMetadata annotationMetadata) {
...
List<String> configurations = getCandidateConfigurations(annotationMetadata,
attributes);
...
}
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
AnnotationAttributes attributes) {
List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
EnableAutoConfiguration.class, getBeanClassLoader());
return configurations;
}
}
自动配置类就是普通的Spring @Configuration类，通过SpringFactoriesLoader机制完成加载，实现上通常使用@Conditional(比如@ConditionalOnClass或者@ConditionalOnMissingBean)
spring-boot-autoconfigure META-INF/spring.factories内容示例
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
...
总结一下就是，@EnableAutoConfiguration 会push springboot 加载各个依赖jar META-INF/spring.factories 中key=org.springframework.boot.autoconfigure.EnableAutoConfiguration 指定的@Configuration 类