（一）实现一个简易版IoC容器【手撸Spring】

一、前言

相信大家在看本篇文章的时候，对IoC应该有一个比较清晰的理解，我在这里再重新描述下：它的作用就是实现一个容器将一个个的Bean（这里的Bean可以是一个Java的业务对象，也可以是一个配置对象）统一管理起来。在Java中，我们创建一个对象最简单的方法是使用new关键字。Spring框架的IoC容器则是将创建Bean的动作与使用Bean解耦，将应用层程序员无需关注底层对象的构建以及其生命周期，以便更好的专注于业务开发。

本节我们则开始进入手写Spring框架的第一步：实现一个最简易的IoC容器。

二、一个最简易的IoC容器

1、简易流程

IoC简易流程

我们在面向Spring框架开发时，想要使用一个Bean时，通常会将bean的一些元信息配置在xml文件中（也可以通过注解），Spring IoC容器会加载指定路径的xml文件，将其进一步解析成BeanDefinition并存储到IoC容器中，当我们（应用层）去获取Bean实例（通过getBean）时，如果该Bean没有被初始化，则会触发Bean实例创建的动作，创建实例由反射实现。

2、简易功能下UML图

简易IoC-UML图

3、相关代码

BeanDefinition

package com.tiny.spring.beans.factory.config;/*** @author: markus* @date: 2023/10/7 8:14 PM* @Description: Bean配置元信息* @Blog: https://markuszhang.com* It's my honor to share what I've learned with you!*/
public class BeanDefinition {private String id;private String className;public BeanDefinition() {}public BeanDefinition(String id, String className) {this.id = id;this.className = className;}public String getId() {return id;}public void setId(String id) {this.id = id;}public String getClassName() {return className;}public void setClassName(String className) {this.className = className;}
}

ClassPathXmlApplicationContext

package com.tiny.spring.context.support;import com.tiny.spring.beans.factory.config.BeanDefinition;
import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;/*** @author: markus* @date: 2023/10/7 8:16 PM* @Description: 基于xml的Spring应用上下文* @Blog: https://markuszhang.com* It's my honor to share what I've learned with you!*/
public class ClassPathXmlApplicationContext {private List<BeanDefinition> beanDefinitions = new ArrayList<>();private Map<String, Object> singletons = new HashMap<>();public ClassPathXmlApplicationContext(String pathname) {this.readXml(pathname);this.instanceBeans();}private void readXml(String pathname) {SAXReader saxReader = new SAXReader();try {URL xmlPath = this.getClass().getClassLoader().getResource(pathname);Document document = saxReader.read(xmlPath);Element rootElement = document.getRootElement();// 对配置文件的每一个<bean>标签进行处理for (Element element : rootElement.elements()) {// 获取Bean的基本信息String beanId = element.attributeValue("id");String beanClassName = element.attributeValue("class");BeanDefinition beanDefinition = new BeanDefinition(beanId, beanClassName);// 将Bean的定义存放到BeanDefinitionbeanDefinitions.add(beanDefinition);}} catch (DocumentException e) {e.printStackTrace();}}/*** 利用反射创建Bean实例，并存储在singletons中*/private void instanceBeans() {for (BeanDefinition beanDefinition : beanDefinitions) {try {singletons.put(beanDefinition.getId(), Class.forName(beanDefinition.getClassName()).newInstance());} catch (InstantiationException | IllegalAccessException | ClassNotFoundException e) {e.printStackTrace();}}}/*** 对外提供的方法，让外部程序获取Bean实例* @param beanName* @return*/public Object getBean(String beanName) {return singletons.get(beanName);}
}

测试类

package com.tiny.spring.test;import com.tiny.spring.beans.BeansException;
import com.tiny.spring.context.support.ClassPathXmlApplicationContext;
import com.tiny.spring.test.service.AService;/*** @author: markus* @date: 2023/10/7 8:37 PM* @Description: 最原始的IoC容器功能测试* @Blog: https://markuszhang.com* It's my honor to share what I've learned with you!*/
public class OriginalIoCContainerTest {public static void main(String[] args) throws BeansException {ClassPathXmlApplicationContext classPathXmlApplicationContext = new ClassPathXmlApplicationContext("beans.xml");AService aService = (AService) classPathXmlApplicationContext.getBean("aService");aService.sayHello();}
}

控制台

三、单一职责原则

我们可以看到，上面的ClassPathXmlApplicationContext承担了太多的功能，这不符合对象单一职责原则。

原本ClassPathXmlApplicationContext既承担了对外提供Bean实例访问，对内进行配置文件的加载并解析成BeanDefinition存储起来以及进行Bean实例的创建操作。

我们需要对此进行优化，将IoC容器的核心功能（Bean实例访问+BeanDefinition注册）和外部信息的访问剥离出来。

1、优化后的UML类图

单一职责优化后的UML图

2、相关代码

Resource

package com.tiny.spring.core.io;import java.util.Iterator;/*** @author: markus* @date: 2023/10/7 8:45 PM* @Description: 外部的配置信息抽象* @Blog: https://markuszhang.com* It's my honor to share what I've learned with you!*/
public interface Resource extends Iterator<Object> {
}

ClassPathXmlResource

package com.tiny.spring.core.io;import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;import java.net.URL;
import java.util.Iterator;/*** @author: markus* @date: 2023/10/7 8:47 PM* @Description:* @Blog: https://markuszhang.com* It's my honor to share what I've learned with you!*/
public class ClassPathXmlResource implements Resource {Document document;Element rootElement;Iterator<Element> elementIterator;public ClassPathXmlResource(String pathname) {SAXReader saxReader = new SAXReader();URL xmlPath = this.getClass().getClassLoader().getResource(pathname);try {this.document = saxReader.read(xmlPath);this.rootElement = document.getRootElement();this.elementIterator = this.rootElement.elementIterator();} catch (DocumentException e) {e.printStackTrace();}}@Overridepublic boolean hasNext() {return this.elementIterator.hasNext();}@Overridepublic Object next() {return this.elementIterator.next();}
}

XmlBeanDefinitionReader

package com.tiny.spring.beans.factory.xml;import com.tiny.spring.beans.factory.BeanFactory;
import com.tiny.spring.beans.factory.config.BeanDefinition;
import com.tiny.spring.core.io.Resource;
import org.dom4j.Element;/*** @author: markus* @date: 2023/10/7 8:50 PM* @Description:* @Blog: https://markuszhang.com* It's my honor to share what I've learned with you!*/
public class XmlBeanDefinitionReader {BeanFactory beanFactory;public XmlBeanDefinitionReader(BeanFactory beanFactory) {this.beanFactory = beanFactory;}public void loadBeanDefinitions(Resource resource) {while (resource.hasNext()) {Element element = (Element) resource.next();String beanId = element.attributeValue("id");String className = element.attributeValue("class");BeanDefinition beanDefinition = new BeanDefinition(beanId, className);this.beanFactory.registerBeanDefinition(beanDefinition);}}
}

BeanFactory

package com.tiny.spring.beans.factory;import com.tiny.spring.beans.BeansException;
import com.tiny.spring.beans.factory.config.BeanDefinition;/*** @author: markus* @date: 2023/10/7 8:43 PM* @Description: IoC底层容器的根类* @Blog: https://markuszhang.com* It's my honor to share what I've learned with you!*/
public interface BeanFactory {/*** 根据beanName获取Bean实例* @param beanName* @return* @throws BeansException*/Object getBean(String beanName) throws BeansException;/*** 注册Bean配置元信息* @param beanDefinition*/void registerBeanDefinition(BeanDefinition beanDefinition);
}

ClassPathXmlApplicationContext

package com.tiny.spring.context.support;import com.tiny.spring.beans.BeansException;
import com.tiny.spring.beans.factory.BeanFactory;
import com.tiny.spring.beans.factory.config.BeanDefinition;
import com.tiny.spring.beans.factory.support.SimpleBeanFactory;
import com.tiny.spring.beans.factory.xml.XmlBeanDefinitionReader;
import com.tiny.spring.core.io.ClassPathXmlResource;
import com.tiny.spring.core.io.Resource;
import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;/*** @author: markus* @date: 2023/10/7 8:16 PM* @Description: 基于xml的Spring应用上下文* @Blog: https://markuszhang.com* It's my honor to share what I've learned with you!*/
public class ClassPathXmlApplicationContext implements BeanFactory {BeanFactory beanFactory;public ClassPathXmlApplicationContext(String pathname) {Resource resource = new ClassPathXmlResource(pathname);BeanFactory beanFactory = new SimpleBeanFactory();XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanFactory);reader.loadBeanDefinitions(resource);this.beanFactory = beanFactory;}/*** 对外提供的方法，让外部程序获取Bean实例** @param beanName* @return*/public Object getBean(String beanName) throws BeansException {return this.beanFactory.getBean(beanName);}@Overridepublic void registerBeanDefinition(BeanDefinition beanDefinition) {this.beanFactory.registerBeanDefinition(beanDefinition);}
}