Spring Framework: Bean definition and creation

Fields in org.springframework.beans.factory.config.BeanDefinition Fields Meaning name Name of bean parentName beanClassName String of bean class scope singleton/prototype/request/session/refresh... lazyInit true if actual bean will be instantiated when being called dependsOn beans which is essential for this bean autowireCandidate to specify if this can be injected into other bean in autowire operation primary true is the first candidate in autowire operation factoryBeanName with factoryMethodName to specify class method to create a bean factoryMethodName initMethodName initialization method after bean is instantiated destroyMethodName last method called before bean destroyed role ROLE_APPLICATION/ROLE_SUPPORT/ROLE_INFRASTRUCTURE which described in org.springframework.beans.factory.config.BeanDefinition org.springframework.context.annotation.ComponentScan annotation The base package of the class annotated with @ComponentScan will become the base package start to scanning. And the classes will following annotations will be picked up. @Component All annotated with @Component @Configuration @Repository @Service @Controller Please note that org.springframework.boot.autoconfigure.SpringBootApplication is annotated with @ComponentScan annotation. Function of org.springframework.context.annotation.Configuration annotation This annotation is for annotates class (configuration class) which provide methods of creating bean (will annotated with @bean) Configuration classes behave like normal beans, and will be created at the beginning of bean creation of starting ApplicationContext. proxyBeanMethods element: default is true, it is to avoid double bean instance creation when a method in the configuration class refers to another method of the class. ConditionalOnXXX annotation Following are commonly used annotations. Annotation Function ConditionalOnBean Defined bean can be found in BeanFactory ConditionalOnClass Defined class can be found in Classpath ConditionalOnMissingBean Defined bean cannot be found in BeanFactory ConditionalOnMissingClass Defined class cannot be found in Classpath ConditionalOnProperty To check particular value of a property ConditionalOnResource Defined resource can be found ConditionalOnSingleCandidate Only one bean can be found in BeanFactory Control code of ConditionalOnXXX annotation Below is the definition of ConditionalOnClass annotation package org.springframework.boot.autoconfigure.condition; 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.Conditional; @Target({ ElementType.TYPE, ElementType.METHOD }) @Retention(RetentionPolicy.RUNTIME) @Documented @Conditional(OnClassCondition.class) public @interface ConditionalOnClass { Class[] value() default {}; String[] name() default {}; } It is annotated with Conditional annotation, the class OnClassCondition is the actual implementation to define the behavior of ConditionalOnClass annotation. Multiple annotation If more than one ConditionalOnXXX annotation are annotated on a configuration class. This will be instantiated only if all annotations hold (i.e. AND condition). On following example @Configuration(proxyBeanMethods = false) @ConditionalOnMissingBean(ReactiveWebServerFactory.class) @ConditionalOnClass({ org.apache.catalina.startup.Tomcat.class }) static class EmbeddedTomcat { @Bean TomcatReactiveWebServerFactory tomcatReactiveWebServerFactory( ObjectProvider connectorCustomizers, ObjectProvider contextCustomizers, ObjectProvider

package org.springframework.boot.autoconfigure.condition;

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.Conditional;

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnClassCondition.class)
public @interface ConditionalOnClass {
  Class[] value() default {};
  String[] name() default {};
}

@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingBean(ReactiveWebServerFactory.class)
@ConditionalOnClass({ org.apache.catalina.startup.Tomcat.class })
static class EmbeddedTomcat {

  @Bean
  TomcatReactiveWebServerFactory tomcatReactiveWebServerFactory(
      ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
      ObjectProvider<TomcatContextCustomizer> contextCustomizers,
      ObjectProvider<TomcatProtocolHandlerCustomizer> protocolHandlerCustomizers) {
    //...
  }

}

@AutoConfiguration
@ConditionalOnProperty(prefix = "spring.aop", name = "auto", havingValue = "true", matchIfMissing = true)
public class AopAutoConfiguration {
  //...
}

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