30个类手写Spring核心原理之依赖注入功能（3）

本文节选自《Spring 5核心原理》

在之前的源码分析中我们已经了解到，依赖注入（DI）的入口是getBean()方法，前面的IoC手写部分基本流程已通。先在GPApplicationContext中定义好IoC容器，然后将GPBeanWrapper对象保存到Map中。在GPApplicationContext中设计两个Map：factoryBeanObjectCache保存单例对象的缓存，factoryBeanInstanceCache保存GPBeanWrapper的缓存，变量命名也和原生Spring一致，这两个对象的设计其实就是注册式单例模式的经典应用。

public class GPApplicationContext extends GPDefaultListableBeanFactory implements GPBeanFactory {private String [] configLocations;private GPBeanDefinitionReader reader;//用来保证注册式单例的容器private Map<String,Object> factoryBeanObjectCache = new HashMap<String, Object>();//用来存储所有的被代理过的对象private Map<String,GPBeanWrapper> factoryBeanInstanceCache = new ConcurrentHashMap<String, GPBeanWrapper>();...}

1 从getBean()方法开始

下面我们从完善getBean()方法开始：

@Overridepublic Object getBean(String beanName) {GPBeanDefinition beanDefinition = super.beanDefinitionMap.get(beanName);try{//生成通知事件GPBeanPostProcessor beanPostProcessor = new GPBeanPostProcessor();Object instance = instantiateBean(beanDefinition);if(null == instance){ return  null;}//在实例初始化以前调用一次beanPostProcessor.postProcessBeforeInitialization(instance,beanName);GPBeanWrapper beanWrapper = new GPBeanWrapper(instance);this.factoryBeanInstanceCache.put(beanName,beanWrapper);//在实例初始化以后调用一次beanPostProcessor.postProcessAfterInitialization(instance,beanName);populateBean(beanName,instance);//通过这样调用，相当于给我们自己留有了可操作的空间return this.factoryBeanInstanceCache.get(beanName).getWrappedInstance();}catch (Exception e){//            e.printStackTrace();return null;}}

2 instantiateBean()方法反射创建实例

//传一个BeanDefinition，就返回一个实例Beanprivate Object instantiateBean(GPBeanDefinition beanDefinition){Object instance = null;String className = beanDefinition.getBeanClassName();try{//因为根据Class才能确定一个类是否有实例if(this.factoryBeanObjectCache.containsKey(className)){instance = this.factoryBeanObjectCache.get(className);}else{Class<?> clazz = Class.forName(className);instance = clazz.newInstance();this.factoryBeanObjectCache.put(beanDefinition.getFactoryBeanName(),instance);}return instance;}catch (Exception e){e.printStackTrace();}return null;}

3 populateBean()方法完成依赖注入

private void populateBean(String beanName,Object instance){Class clazz = instance.getClass();if(!(clazz.isAnnotationPresent(GPController.class) ||clazz.isAnnotationPresent(GPService.class))){return;}Field [] fields = clazz.getDeclaredFields();for (Field field : fields) {if (!field.isAnnotationPresent(GPAutowired.class)){ continue; }GPAutowired autowired = field.getAnnotation(GPAutowired.class);String autowiredBeanName = autowired.value().trim();if("".equals(autowiredBeanName)){autowiredBeanName = field.getType().getName();}field.setAccessible(true);try {field.set(instance,this.factoryBeanInstanceCache.get(autowiredBeanName). getWrappedInstance());} catch (IllegalAccessException e) {//                e.printStackTrace();}}}

4 GPBeanPostProcessor后置处理器

原生Spring中的BeanPostProcessor是为对象初始化事件设置的一种回调机制。这个Mini版本中只做说明，不做具体实现，感兴趣的“小伙伴”可以继续深入研究Spring源码。

package com.tom.spring.formework.beans.config;public class GPBeanPostProcessor {//为在Bean的初始化之前提供回调入口public Object postProcessBeforeInitialization(Object bean, String beanName) throws Exception {return bean;}//为在Bean的初始化之后提供回调入口public Object postProcessAfterInitialization(Object bean, String beanName) throws Exception {return bean;}}

至此，DI部分就手写完成了，也就是说完成了Spring的核心部分。“小伙伴们”是不是发现其实还是很简单的？