关于java: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外围原理》

在之前的源码剖析中咱们曾经理解到，依赖注入（DI）的入口是getBean()办法，后面的IoC手写局部根本流程已通。先在GPApplicationContext中定义好IoC容器，而后将GPBeanWrapper对象保留到Map中。在GPApplicationContext中设计两个Map：factoryBeanObjectCache保留单例对象的缓存，factoryBeanInstanceCache保留GPBeanWrapper的缓存，变量命名也和原生Spring统一，这两个对象的设计其实就是注册式单例模式的经典利用。

public class GPApplicationContext extends GPDefaultListableBeanFactory implements GPBeanFactory {

    private String [] configLocations;

    private GPBeanDefinitionReader reader;

    //用来保障注册式单例的容器
    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()办法开始

上面咱们从欠缺getBean()办法开始：

    @Override
    public Object getBean(String beanName) {

        GPBeanDefinition beanDefinition = super.beanDefinitionMap.get(beanName);

        try{

            //生成告诉事件
            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);

            //在实例初始化当前调用一次
            beanPostProcessor.postProcessAfterInitialization(instance,beanName);

            populateBean(beanName,instance);

            //通过这样调用，相当于给咱们本人留有了可操作的空间
            return this.factoryBeanInstanceCache.get(beanName).getWrappedInstance();
        }catch (Exception e){
//            e.printStackTrace();
            return null;
        }
    }

2 instantiateBean()办法反射创立实例

    //传一个BeanDefinition，就返回一个实例Bean
    private Object instantiateBean(GPBeanDefinition beanDefinition){
        Object instance = null;
        String className = beanDefinition.getBeanClassName();
        try{

            //因为依据Class能力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()办法实现依赖注入

    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版本中只做阐明，不做具体实现，感兴趣的“小伙伴”能够持续深入研究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局部就手写实现了，也就是说实现了Spring的外围局部。“小伙伴们”是不是发现其实还是很简略的？