Spring處理@Async導致的循環依賴失敗問題的方案詳解

package com.knife;
 
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.scheduling.annotation.EnableAsync;
 
@EnableAsync
@SpringBootApplication
public class DemoApplication {
 
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
 
}

package com.knife.service;
 
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;
 
@Component
public class A {
    @Autowired
    private B b;
 
    @Async
    public void print() {
        System.out.println("Hello World");
    }
}

package com.knife.service;
 
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
 
@Component
public class B {
    @Autowired
    private A a;
}

package com.knife.controller;
 
import com.knife.service.A;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
 
@RestController
public class HelloController {
    @Autowired
    private A a;
 
    @GetMapping("/test")
    public String test() {
        a.print();
        return "test success";
    }
}

    // 這個方法主要是為有@Async 註解的 bean 生成代理對象
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) {
        if (this.advisor == null || bean instanceof AopInfrastructureBean) {
            // Ignore AOP infrastructure such as scoped proxies.
            return bean;
        }
 
        // 如果此Bean已經被代理瞭（比如已經被事務那邊給代理瞭~~）
        if (bean instanceof Advised) {
            Advised advised = (Advised) bean;
        
            if (!advised.isFrozen() && isEligible(AopUtils.getTargetClass(bean))) {
                // Add our local Advisor to the existing proxy's Advisor chain...
                // beforeExistingAdvisors決定這該advisor最先執行還是最後執行
                // 此處的advisor為：AsyncAnnotationAdvisor  它切入Class和Method標註有@Aysnc註解的地方~~~
                if (this.beforeExistingAdvisors) {
                    advised.addAdvisor(0, this.advisor);
                } else {
                    advised.addAdvisor(this.advisor);
                }
                return bean;
            }
        }
        // 若不是代理對象，則進行處理
        if (isEligible(bean, beanName)) {
            //copy屬性 proxyFactory.copyFrom(this); 工廠模式生成一個新的 ProxyFactory
            ProxyFactory proxyFactory = prepareProxyFactory(bean, beanName);
            // 如果沒有采用CGLIB，就去探測它的接口
            if (!proxyFactory.isProxyTargetClass()) {
                evaluateProxyInterfaces(bean.getClass(), proxyFactory);
            }
            // 切入切面並創建一個getProxy 代理對象
            proxyFactory.addAdvisor(this.advisor);
            customizeProxyFactory(proxyFactory);
            return proxyFactory.getProxy(getProxyClassLoader());
        }
 
        // No proxy needed.
        return bean;
    }
 
    protected boolean isEligible(Object bean, String beanName) {
        return isEligible(bean.getClass());
    }
 
    protected boolean isEligible(Class<?> targetClass) {
        //首次從 eligibleBeans 這個 map 中獲取值肯定為 null
        Boolean eligible = this.eligibleBeans.get(targetClass);
        if (eligible != null) {
            return eligible;
        }
        //如果沒有配置 advisor（即：切面），返回 false
        if (this.advisor == null) {
            return false;
        }
        
        // 若類或方法有 @Aysnc 註解，AopUtils.canApply 會判斷為 true
        eligible = AopUtils.canApply(this.advisor, targetClass);
        this.eligibleBeans.put(targetClass, eligible);
        return eligible;
    }

package com.knife.service;
 
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Lazy;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;
 
@Component
public class A {
    @Lazy
    @Autowired
    private B b;
 
    @Async
    public void print() {
        System.out.println("Hello World");
    }
}

package com.knife.service;
 
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Lazy;
import org.springframework.stereotype.Component;
 
@Component
public class B {
    @Lazy
    @Autowired
    private A a;
}

package com.knife.service;
 
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Lazy;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;
 
@Component
public class A {
    @Lazy
    @Autowired
    private B b;
 
    @Async
    public void print() {
        System.out.println("Hello World");
    }
}

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory;
import org.springframework.stereotype.Component;
 
@Component
public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        ((AbstractAutowireCapableBeanFactory) beanFactory).setAllowRawInjectionDespiteWrapping(true);
    }
}

public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
        implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {
        
    // 其他代碼
    
    @Override
    public Object getEarlyBeanReference(Object bean, String beanName) {
        Object cacheKey = getCacheKey(bean.getClass(), beanName);
        this.earlyProxyReferences.put(cacheKey, bean);
        return wrapIfNecessary(bean, beanName, cacheKey);
    }
    
}

public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
        implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware {
        
    // 其他代碼
    
    @Override
    public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
        if (bean != null) {
            Object cacheKey = getCacheKey(bean.getClass(), beanName);
            if (this.earlyProxyReferences.remove(cacheKey) != bean) {
                return wrapIfNecessary(bean, beanName, cacheKey);
            }
        }
        return bean;
    }
    
}