手写SpringBoot（四）之bean动态加载

系列文章目录

手写SpringBoot（一）之简易版SpringBoot
手写SpringBoot（二）之动态切换Servlet容器
手写SpringBoot（三）之自动配置
手写SpringBoot（四）之bean动态加载

手写SpringBoot（四）之bean动态加载

本节巩固一下@ConditionalOnClass的相关逻辑，主要新增@ConditionalOnMissingBean注解

SpringBoot配置了很多类，如果用户不希望使用springboot自动配置的那个类怎么办？

如果用户强行配置了一个相同的类，SpringBoot会选择使用哪一个呢？

这就需要按需加载了，如果用户配置了相同的类，则SpringBoot就不加载自己配置的那个。这就是ConditionalOnMissingBean的由来，如果这个bean缺失了，则SpringBoot将这个Bean配置上，如果这个Bean由用户配置了，则自己配置的bean作弃。

前置工作，将MyConditionalOnClass，MyClassCondition 迁移到my-spring-boot-configuration模块下面

ConditionalOnMissingBean 逻辑实现

定义@MyConditionalOnMissingBean注解

package cn.axj.springboot.my.annotation;import cn.axj.springboot.my.condition.MyMissingBeanCondition;
import org.springframework.context.annotation.Conditional;import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE,ElementType.METHOD})
@Conditional(MyMissingBeanCondition.class)
public @interface MyConditionalOnMissingBean {String value();
}

实现MissBean的判断逻辑，新增MyMissingBeanCondition

package cn.axj.springboot.my.condition;import cn.axj.springboot.my.annotation.MyConditionalOnMissingBean;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;import java.util.Map;
import java.util.Objects;public class MyMissingBeanCondition implements Condition {@Overridepublic boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {Map<String, Object> annotationAttributes = metadata.getAnnotationAttributes(MyConditionalOnMissingBean.class.getName());/*** 获取{@link MyConditionalOnMissingBean}注解中的属性值* 例如：@MyConditionalOnMissingBean(value = "com.example.MyBean")* 则可以通过annotationAttributes.get("value")获取到"com.example.MyBean"*/String beanName = (String) annotationAttributes.get("value");ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();Object bean = null;try {//这里简单点，根据beanName获取bean，实际中需要更复杂的逻辑来判断是否缺失bean = beanFactory.getBean(beanName);return false;} catch (BeansException e) {BeanFactory parentBeanFactory = beanFactory.getParentBeanFactory();try {if(Objects.isNull(parentBeanFactory)){return true;}bean = parentBeanFactory.getBean(beanName);return false;} catch (BeansException ex) {return true;}}}
}

至此@ConditionalOnMissingBean逻辑开发完成

测试

在UserApplication中配置tomcatWebServer

	@Beanpublic TomcatWebServer tomcatWebServer(){System.out.println("userApplication tomcatWebServer init");return new TomcatWebServer();}

在my-spring-boot模块WebServerAutoConfiguration中，配置tomcatWebServer中加上注解@ConditionalOnMissingBean

@Bean@MyConditionalOnClass("org.apache.catalina.startup.Tomcat")@MyConditionalOnMissingBean("tomcatWebServer")public TomcatWebServer tomcatWebServer() {System.out.println("mySpringboot tomcatWebServer init");return new TomcatWebServer();}

启动user-application，

userApplication tomcatWebServer init
三月 29, 2024 5:20:31 下午 org.springframework.context.annotation.ConfigurationClassPostProcessor enhanceConfigurationClasses
信息: Cannot enhance @Configuration bean definition 'userApplication' since its singleton instance has been created too early. The typical cause is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor return type: Consider declaring such methods as 'static'.
启动TomcatWeb容器
三月 29, 2024 5:20:31 下午 org.apache.coyote.AbstractProtocol init
信息: Initializing ProtocolHandler ["http-nio-8080"]
三月 29, 2024 5:20:31 下午 org.apache.catalina.core.StandardService startInternal
信息: Starting service [Tomcat]
三月 29, 2024 5:20:31 下午 org.apache.catalina.core.StandardEngine startInternal
信息: Starting Servlet engine: [Apache Tomcat/9.0.75]
三月 29, 2024 5:20:32 下午 org.apache.catalina.util.SessionIdGeneratorBase createSecureRandom
警告: Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [320] milliseconds.
三月 29, 2024 5:20:32 下午 org.apache.coyote.AbstractProtocol start
信息: Starting ProtocolHandler ["http-nio-8080"]

可以看到只有userApplication tomcatWebServer init打印，没有mySpringboot tomcatWebServer init打印，证明spring-boot只加载了用户定义的那个tomcatWebServer