浅谈基于动态代理的Spring AOP原理

浅谈基于动态代理的Spring AOP原理

1 什么时候会用到AOP？

• Bean的创建和初始化过程中
• 增加环绕、前置、后置方法
• 事务、日志
• 拦截器
• …

这篇文章就着重讲解下Bean创建和初始化的过程中使用到的AOP原理

2 回顾Bean的创建过程

在Bean的创建过程中比较重要的一步就是对Bean进行AOP处理，但是不是必要的，接下来我们从头开始手写一个自己的Spring进行实现。

3 扫描Bean

这里的流程主要是new一个ApplicationContext，传入xml文件或配置类，以此来扫描固定包下的类。

/**
 * 构造方法
 *
 * @param configClass
 */
public MyApplicationContext(Class configClass) {
    this.configClass = configClass;
    //扫描Bean
    scanBean();
    //根据BeanDefinition创建Bean
    for (Map.Entry<String, MyBeanDefinition> beanDefinitionEntry : beanDefinitionMap.entrySet()) {
        String key = beanDefinitionEntry.getKey();
        MyBeanDefinition value = beanDefinitionEntry.getValue();
        if (ScopeUtils.SINGLETON.equals(value.getScope())) {
            Object bean = createBean(key,value);
            singletonPool.put(key, bean);
        }
    }
}

/**
 * 扫描Bean
 */
private void scanBean() {
    //拿到配置类之后对其进行解析 扫描
    MyComponentScan scan = (MyComponentScan) configClass.getDeclaredAnnotation(MyComponentScan.class);
    String path = scan.value();
    //根据路径进行扫描
    ClassLoader classLoader = MyComponentScan.class.getClassLoader();
    String newPath = path.replace('.', '/');
    //获取文件路径
    URL url = classLoader.getResource(newPath);
    File file = new File(url.getFile());
    if (file.isDirectory()) {
        File[] files = file.listFiles();
        //获取到该包下的每个文件（Class）
        for (File f : files) {
            String absolutePath = f.getAbsolutePath();
            //判断是不是.class文件
            if (absolutePath.endsWith(".class")) {
                String className = absolutePath.substring(absolutePath.indexOf("org"), absolutePath.indexOf(".class"));
                String name = className.replace("\\", ".");
                try {
                    Class<?> clazz = classLoader.loadClass(name);
                    //判断是否有MyComponent注解
                    if (clazz.isAnnotationPresent(MyComponent.class)) {
                        MyComponent component = clazz.getDeclaredAnnotation(MyComponent.class);
                        String beanName = component.value();
                        if (beanName.getBytes().length == 0) {
                            beanName = name.substring(name.lastIndexOf(".") + 1);
                            beanName = beanName.substring(0, 1).toLowerCase(Locale.ROOT) + beanName.substring(1);
                        }
                        MyBeanDefinition beanDefinition = new MyBeanDefinition();
                        beanDefinition.setClazz(clazz);
                        //判断bean的作用域
                        if (clazz.isAnnotationPresent(MyScope.class)) {
                            MyScope scope = clazz.getDeclaredAnnotation(MyScope.class);
                            beanDefinition.setScope(scope.value());
                        } else {
                            beanDefinition.setScope("singleton");
                        }
                        //判断是否实现了BeanPostProcessor
                        if (MyBeanPostProcessor.class.isAssignableFrom(clazz)) {
                            //这一步简化，非spring源码
                            MyBeanPostProcessor instance = (MyBeanPostProcessor) clazz.getDeclaredConstructor().newInstance();
                            beanPostProcessorList.add(instance);
                        }
                        //将该bean的Definition放入map中
                        beanDefinitionMap.put(beanName, beanDefinition);
                    }
                } catch (ClassNotFoundException e) {
                    e.printStackTrace();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

4 创建Bean

/**
 * 创建bean
 *
 * @param beanDefinition
 * @return
 */
public Object createBean(String beanName,MyBeanDefinition beanDefinition) {
    Class clazz = beanDefinition.getClazz();
    Object newInstance = null;
    try {
        newInstance = clazz.getConstructor().newInstance();
        //处理依赖注入
        for (Field field : clazz.getDeclaredFields()) {
            if (field.isAnnotationPresent(MyAutowired.class)) {
                Object bean = getBean(field.getName());
                field.setAccessible(true);
                field.set(newInstance, bean);
            }
        }
        //BeanPostProcessor初始化前
        for (MyBeanPostProcessor processor : beanPostProcessorList) {
            //放入的和传出的不一定是同一个对象
            newInstance = processor.postProcessBeforeInitialization(newInstance, beanName);
        }
        //Bean的初始化方法
        if (newInstance instanceof MyInitializingBean) {
            ((MyInitializingBean) newInstance).afterPropertiesSet();
        }
        //BeanPostProcessor初始化后
        for (MyBeanPostProcessor processor : beanPostProcessorList) {
            //放入的和传出的不一定是同一个对象
            newInstance = processor.postProcessAfterInitialization(newInstance, beanName);
        }
    } catch (InstantiationException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (InvocationTargetException e) {
        e.printStackTrace();
    } catch (NoSuchMethodException e) {
        e.printStackTrace();
    }
    return newInstance;
}

5 获取Bean

/**
 * getBean方法
 *
 * @param beanName
 * @return
 */
public Object getBean(String beanName) {
    if (beanDefinitionMap.containsKey(beanName)) {
        MyBeanDefinition beanDefinition = beanDefinitionMap.get(beanName);
        if (ScopeUtils.SINGLETON.equals(beanDefinition.getScope())) {
            return singletonPool.get(beanName);
        } else {
            return createBean(beanName,beanDefinition);
        }
    } else {
        throw new MyNoSuchBeanDefinitionException("没有这个Bean");
    }
}

6 实现自己的BeanPostProcessor

Spring初始化Bean时对Bean的增强就是在BeanPostProcess方法中进行实现的，postProcessBeforeInitialization做前置处理，postProcessAfterInitialization做后置处理。可以看下Spring源码

/**
 * @desc: BeanPostProcessor抽象
 * @author: YanMingXin
 * @create: 2021/9/16-15:01
 **/
public interface MyBeanPostProcessor {

    Object postProcessBeforeInitialization(Object bean,String beanName);

    Object postProcessAfterInitialization(Object bean,String beanName);

}

实现该方法，使用JDK动态代理将Bean包装

/**
 * @desc: MyBeanPostProcessorHandler
 * @author: YanMingXin
 * @create: 2021/9/17-10:25
 **/
@MyComponent
public class MyBeanPostProcessorHandler implements MyBeanPostProcessor {

    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) {
        if ("studentServiceImpl".equals(beanName)) {
            System.out.println("初始化前");
        }
        return bean;
    }

    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) {
        if ("studentServiceImpl".equals(beanName)) {
            Object proxyInstance = Proxy.newProxyInstance(MyBeanPostProcessorHandler.class.getClassLoader(), bean.getClass().getInterfaces(), new InvocationHandler() {
                @Override
                public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                    //代理逻辑
                    System.out.println("代理逻辑");
                    //对象的方法
                    return method.invoke(bean, args);
                }
            });
            System.out.println("初始化后");
            return proxyInstance;
        }
        return bean;
    }
}

7 测试