1 How does SpringBoot automatically assemble and how to start the built-in Tomcat to run.
1.1 SpringBootApplication annotation and composition
@SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}
The above code calls the static run method of SpringApplication, runs a SpringApplication instance with the default configuration, and returns a running Spring context. Among them, the default configuration is loaded through the @SpringBootApplication annotation, which means that the automatic assembly of SpringBoot is completed through this annotation.
@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 {
@SpringBootApplication is a composite annotation, used to quickly configure the startup class. There are 7 annotations on the annotations, of which the first 4 are meta annotations, which are used to modify the current annotations and have no actual function. There are three important notes.
-
@ComponentScan: Tell Spring to automatically scan the classes that use @Service, @Component, @Repository, and @Controller in the package, and register them as beans.
-
@SpringBootConfiguration: Identified by @Configuration, declares that the current class is a configuration class, which is equivalent to the xml file of Spring configuration.
-
@EnableAutoConfiguration: The function is to enable automatic assembly, so that Spring Boot will automatically set the current project according to the jar package dependencies in the classpath. For example, if Spring-boot-starter-web dependency is added, Tomcat and SpringMVC dependencies will be automatically added, and both will be automatically configured at startup.
SpringBoot only scans the same-level packages and subpackages of the class where the @SpringBootApplication annotation is located.
1.2 @EnableAutoConfiguration automatic assembly
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration
@EnableAutoConfiguration will import the default configuration of the AutoConfigurationImportSelector selector
// AutoConfigurationImportSelector
@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
// 核心加载语句
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);
....
}
// 返回自动配置的类名
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
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;
}
Through code tracing, we can learn that @EnableAutoConfiguration annotation mainly loads the automatic configuration classes in ETA-INF/spring.factories. In spring.factories, we can find classes related to Tomcat and SpringMVC.
// 内置servlet/Reactive服务器的相关配置
// 如果使用到Tomcat，则会加载内联Tomcat配置（Jetty/Netty同理）
org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration
// 内部会加载Tomcat
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration