手撕spring框架（3）

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手撕spring框架（1）
手撕spring框架（2）

InitializingBean 接口详解

什么是 InitializingBean 接口？

InitializingBean 接口是 Spring 框架中的一个接口，用于在 Bean 初始化完成后执行特定的操作。当一个 Bean 实现了 InitializingBean 接口，Spring 容器在实例化 Bean 后会调用其 afterPropertiesSet() 方法，从而实现在 Bean 初始化阶段执行一些定制化的逻辑。

InitializingBean 接口的作用

InitializingBean 接口的主要作用是在 Bean 初始化完成后执行一些必要的操作，例如初始化一些资源、建立一些连接等。通过实现 InitializingBean 接口，开发人员可以在 Bean 初始化阶段进行一些自定义的操作，从而更好地控制 Bean 的生命周期。

InitializingBean 接口的使用方法

要实现 InitializingBean 接口，只需要在 Bean 类中实现 afterPropertiesSet() 方法即可。在 afterPropertiesSet() 方法中编写需要在 Bean 初始化阶段执行的逻辑代码。下面是一个简单的示例：

package com.spring;public interface InitializingBean {void afterPropertiesSet();
}

调整DzendApplicationContext中的createBean方法

具体看最后文件

BeanPostProcessor类详解

什么是BeanPostProcessor？

在Spring框架中，BeanPostProcessor是一个接口，它允许在Spring容器实例化bean之后和调用bean的初始化方法之前对bean进行自定义处理。BeanPostProcessor接口定义了两个方法：postProcessBeforeInitialization和postProcessAfterInitialization。通过实现这两个方法，我们可以在bean初始化前后做一些额外的操作，比如修改bean的属性值、添加额外的逻辑等。

BeanPostProcessor的使用场景

BeanPostProcessor可以用于很多场景，比如：

AOP（面向切面编程）：通过BeanPostProcessor可以在bean初始化前后动态代理bean，实现AOP功能。
属性注入：可以在bean初始化前后修改bean的属性值，实现自定义的属性注入逻辑。
Bean初始化：可以在bean初始化前后执行一些额外的逻辑，比如打印日志、记录性能指标等。

BeanPostProcessor的实现方式

要实现一个BeanPostProcessor，我们需要创建一个实现了BeanPostProcessor接口的类，并实现postProcessBeforeInitialization和postProcessAfterInitialization方法。下面是一个简单的示例：

package com.spring;public interface BeanPostProcessor {default Object postProcessBeforeInitialization(Object bean, String beanName) {// 在bean初始化前执行的逻辑return bean;}default Object postProcessAfterInitialization(Object bean, String beanName) {// 在bean初始化后执行的逻辑return bean;}
}

调整DzendApplicationContext的代码

主要流程是添加一个BeanPostProcessorList，在扫描路径的时候，把符合条件的类添加到beanPostProcessorList队列中

添加一个beanPostProcessorList列表

private List<BeanPostProcessor> beanPostProcessorList = new ArrayList<>();

scan方法中加入

在

if (clazz.isAnnotationPresent(Component.class)) {

代码下添加以下代码

if (BeanPostProcessor.class.isAssignableFrom(clazz)) {BeanPostProcessor instance = (BeanPostProcessor) clazz.getConstructor().newInstance();beanPostProcessorList.add(instance);}

在createBean方法中调整代码

for (BeanPostProcessor beanPostProcessor : beanPostProcessorList) {instance = beanPostProcessor.postProcessBeforeInitialization(instance,beanName);}
for (BeanPostProcessor beanPostProcessor : beanPostProcessorList) {instance = beanPostProcessor.postProcessAfterInitialization(instance,beanName);}

最后附修改后的代码

DzendApplicationContext.java

package com.spring;import java.beans.Introspector;
import java.io.File;
import java.io.UnsupportedEncodingException;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.net.URL;
import java.net.URLDecoder;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;public class DzendApplicationContext {private Class<?> configClass;private Map<String,BeanDefinition> beanDefinitionMap= new HashMap<>();private Map<String, Object> singletonObjects=new HashMap<>();private List<BeanPostProcessor> beanPostProcessorList = new ArrayList<>();public DzendApplicationContext(Class configClass)   {this.configClass=configClass;scan(configClass);}private void scan(Class configClass) {if(configClass.isAnnotationPresent(ComponentScan.class)){ComponentScan componentScan = (ComponentScan) configClass.getAnnotation(ComponentScan.class);String path = componentScan.value();path = path.replace(".","/");ClassLoader classLoader = DzendApplicationContext.class.getClassLoader();URL resource = classLoader.getResource(path);try {path = URLDecoder.decode(path, "UTF-8");} catch (UnsupportedEncodingException e) {throw new RuntimeException(e);}File file = null;try {file = new File(  URLDecoder.decode(resource.getFile(), "UTF-8"));} catch (UnsupportedEncodingException e) {throw new RuntimeException(e);}if(file.isDirectory()){for (File f : file.listFiles()) {String absolutePath = f.getAbsolutePath();absolutePath = absolutePath.substring(absolutePath.indexOf("com"),absolutePath.indexOf(".class"));absolutePath=absolutePath.replace("\\",".");try {Class<?>  clazz = classLoader.loadClass(absolutePath);if (clazz.isAnnotationPresent(Component.class)) {if (BeanPostProcessor.class.isAssignableFrom(clazz)) {BeanPostProcessor instance = (BeanPostProcessor) clazz.getConstructor().newInstance();beanPostProcessorList.add(instance);}Component componentAnnotaion = clazz.getAnnotation(Component.class);String beanName= componentAnnotaion.value();if("".equals(beanName)){beanName = Introspector.decapitalize(clazz.getSimpleName());}BeanDefinition beanDefinition = new BeanDefinition();beanDefinition.setType(clazz);if (clazz.isAnnotationPresent(Scope.class)) {Scope scopeAnnotation = clazz.getAnnotation(Scope.class);String value = scopeAnnotation.value();beanDefinition.setScope(value);}else{beanDefinition.setScope("singleton");}beanDefinitionMap.put(beanName,beanDefinition);}} catch (ClassNotFoundException e) {e.printStackTrace();} catch (InvocationTargetException e) {throw new RuntimeException(e);} catch (InstantiationException e) {throw new RuntimeException(e);} catch (IllegalAccessException e) {throw new RuntimeException(e);} catch (NoSuchMethodException e) {throw new RuntimeException(e);}}}}}public Object getBean(String beanName){if(!beanDefinitionMap.containsKey(beanName)){throw new NullPointerException();}BeanDefinition beanDefinition = beanDefinitionMap.get(beanName);if(beanDefinition.getScope().equals("singleton")){Object singletonObject = singletonObjects.get(beanName);if(singletonObject == null){singletonObject = createBean(beanName,beanDefinition);singletonObjects.put(beanName,singletonObject);}return singletonObject;}else{//原型Object prototypeBean = createBean(beanName, beanDefinition);return prototypeBean;}}private Object createBean(String beanName, BeanDefinition beanDefinition) {Class clazz = beanDefinition.getType();Object instance = null;try {instance = clazz.getConstructor().newInstance();for (Field field : clazz.getDeclaredFields()) {if(field.isAnnotationPresent(Autowired.class)){field.setAccessible(true);field.set(instance,getBean(field.getName()));}}for (BeanPostProcessor beanPostProcessor : beanPostProcessorList) {instance = beanPostProcessor.postProcessBeforeInitialization(instance,beanName);}if(instance instanceof BeanNameAware){((BeanNameAware) instance).setBeanName(beanName);}if(instance instanceof InitializingBean){((InitializingBean) instance).afterPropertiesSet();}for (BeanPostProcessor beanPostProcessor : beanPostProcessorList) {instance = beanPostProcessor.postProcessAfterInitialization(instance,beanName);}} catch (InstantiationException e) {throw new RuntimeException(e);} catch (IllegalAccessException e) {throw new RuntimeException(e);} catch (InvocationTargetException e) {throw new RuntimeException(e);} catch (NoSuchMethodException e) {throw new RuntimeException(e);}return instance;}
}