Spring AOP 代理分析

1. Spring AOP 代理

1.1 代理时机

• Spring 使用模板模式，在 Bean 的创建过程中安插了许多锚点，用户寻找对应的锚点，通过重写方法介入到 Bean 的创建过程当中
• 介入 Spring Ioc 创建 Bean 的过程，最好的方式就是实现 BeanPostProcessor

1.2 代理创建

Bean后置处理器 BeanPostProcessor
postProcessBeforeInstantiation() 可在 Spring 实例化 Bean 对象之前，创建一个 Bean
postProcessBeforeInitialization() 可在 Bean 的初始化方法被执行前，修改 Bean
postProcessAfterInitialization() 可在 Bean 的初始化方法被执行前，修改 Bean

实现类 AbstractAutoProxyCreator

方法postProcessAfterInitialization() 在 Bean 初始化后调用

@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;
}

wrapIfNecessary() 创建 ProxyFactory 工厂

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
    // ...
    Object proxy = createProxy(bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
    // ...
}

protected Object createProxy(Class<?> beanClass, @Nullable String beanName,
            @Nullable Object[] specificInterceptors, TargetSource targetSource) {
    // ...
    ProxyFactory proxyFactory = new ProxyFactory();
    proxyFactory.copyFrom(this);
    // ...
}

ProxyFactory 的无参构造方法里没有逻辑

public class ProxyFactory extends ProxyCreatorSupport {
    public ProxyFactory() {
    }
}

父类 ProxyCreatorSupport 的无参构造如下：

public class ProxyCreatorSupport extends AdvisedSupport {
    private AopProxyFactory aopProxyFactory;

    public ProxyCreatorSupport() {
        this.aopProxyFactory = new DefaultAopProxyFactory();
    }
}

继续顺着继承关系往上找，父类 AdvisedSupport 的无参构造如下：

public class AdvisedSupport extends ProxyConfig implements Advised {
    public AdvisedSupport() {
        this.methodCache = new ConcurrentHashMap<>(32);
    }
}

2. Spring Boot 自动配置类 AopAutoConfiguration

Spring 需要手动添加 @EnableAspectJAutoProxy 注解进行集成
SpringBoot 可自动装配 spring-boot-autoconfigure.jar/META-INF/spring.factories

package org.springframework.boot.autoconfigure.aop;

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

    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass(Advice.class)
    static class AspectJAutoProxyingConfiguration {

        @Configuration(proxyBeanMethods = false)
        @EnableAspectJAutoProxy(proxyTargetClass = false)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false")
        static class JdkDynamicAutoProxyConfiguration {

        }

        @Configuration(proxyBeanMethods = false)
        @EnableAspectJAutoProxy(proxyTargetClass = true)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true",
                matchIfMissing = true)
        static class CglibAutoProxyConfiguration {

        }

    }
}

参考

• Spring 源码阅读 48：用于创建 AOP 代理的后处理器分析
• Spring 源码阅读 56：创建 ProxyFactory