大家好,我是丁甲,很久没有写博客了,最近由于工作原因,需要重新面试,所以最近会写几篇关于面试的博客。
Spring Boot的运行机制是什么?
这个问题通常会被问到,之前丁甲也只是背书应付面试,但这样始终是好的,于是我决定看看Spring Boot注解的源码,并从源码层面上来看看Spring Boot的运行机制究竟是什么。
首先,我们随便打开一个Spring Boot项目,我们能够发现他的启动类是都是有一个非常重要的注解:
@SpringBootApplication
public class Demo123Application {
public static void main(String[] args) {
SpringApplication.run(Demo123Application.class, args);
}
}
这个非常重要的注解就是@SpringBootApplicaiton。
其次,任意打开一个SpringBoot项目,不难发现他的启动类(使用@SpringBootApplication注解的类)目录层级非常高,是顶级目录,这是为什么呢?今天我们从源码的角度出发,进一步的解析SpringBoot项目的运行机制。
@SpringBootApplication注解是一个复合注解
@SpringBootApplication是一个复合注解,并非一个单一的注解,我们点开一个@SpringBootApplication就不难发现:
@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;
}
从这段源码中不难看出,@SpringBootApplication注解是由多个注解组合而成的,而在这几个组合注解中其中最重要的核心注解当属@EnableAutoConfiguration、@ComponentScan、@Configuration。
我们从这三个核心注解来分下SpringBoot项目的运行机制。
@Configuration注解
源码:
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
@AliasFor(
annotation = Component.class
)
String value() default "";
boolean proxyBeanMethods() default true;
}
@coofiguration注解是一个十分常用的注解,经常都会用到。它的作用通过java类将写好的类在Spring容器初始化的时候将其加载到Spring容器中去,摒弃了xml配置文件来初始化Bean,十分的方便,和他一起使用的注解为@Bean
@ComponentScan
源码:
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Documented
@Repeatable(ComponentScans.class)
public @interface ComponentScan {
@AliasFor("basePackages")
String[] value() default {};
@AliasFor("value")
String[] basePackages() default {};
Class<?>[] basePackageClasses() default {};
Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;
Class<? extends ScopeMetadataResolver> scopeResolver() default AnnotationScopeMetadataResolver.class;
ScopedProxyMode scopedProxy() default ScopedProxyMode.DEFAULT;
String resourcePattern() default "**/*.class";
boolean useDefaultFilters() default true;
ComponentScan.Filter[] includeFilters() default {};
ComponentScan.Filter[] excludeFilters() default {};
boolean lazyInit() default false;
@Retention(RetentionPolicy.RUNTIME)
@Target({})
public @interface Filter {
FilterType type() default FilterType.ANNOTATION;
@AliasFor("classes")
Class<?>[] value() default {};
@AliasFor("value")
Class<?>[] classes() default {};
String[] pattern() default {};
}
}
@componentScan注解,是一个扫描包的注解。在Spring容器初始化的时候,invokeBeanFactoryPostProcessor()方法中会将所有的@componentScan下面的包扫描并将类通过一系列的操作(java -> class ->BeanDefinitonMap ->Singleton)加载入Spring容器中。
@EnableAutoConfiguration
源码:
@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 {};
}
@EnableAotuConfiguration类中的核心注解为@import以及AutoConfigurationImportSelector类。@Import注解也是我们平时用得非常多的一个注解,同一模块下的引包,以及JDK自带包的引用都是需要用@Import注解使用的,这个注解是非常好理解的,然后我们再来说一说@Import的AutoConfigurationimportSelector类。
首先我们来看看autoSonfigurationImportSelector类的源码:
public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
private static final AutoConfigurationImportSelector.AutoConfigurationEntry EMPTY_ENTRY = new AutoConfigurationImportSelector.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 ConfigurableListableBeanFactory beanFactory;
private Environment environment;
private ClassLoader beanClassLoader;
private ResourceLoader resourceLoader;
private AutoConfigurationImportSelector.ConfigurationClassFilter configurationClassFilter;
public AutoConfigurationImportSelector() {
}
首先这个类是实现了DeferredImportSelector接口,然后在看来他的核心方法 :
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());
Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you are using a custom packaging, make sure that file is correct.");
return configurations;
}
他的核心方法就是这个SpringFactoriesLoader.loadFactoryNames(),作用就是将META-INF/spring.factories的文件信息读取出来。然后作为Bean加载到Spring容器中。
总结:
1、SpringBoot的运行机制要从他的核心注解@SpringBootAppliccation说起
2、@SpringBootApplication的核心注解为:@ComponentScan、@Configuration、@EnabledAutoConfiguration
3、@ComponentScan发现扫描包并将包下面的类加载进Spring容器作为Spring Bean
4、@Configuration + @Bean将自定义的配置类也加载入Spring容器中作为SpringBean
5、@EnabledAutoConfiguration、@Import(AutoConfigurationIpmortSelector)、@AutoConfigurationPackage三个注解将Spring容器里面的Bean实现发现与加载