花了點時間摸了一個IoC容器出來，沒實現元件掃描和變數的註解注入，就單純實現了使用xml進行的配置。

applicationContext.xml

配置了一下，將UserDao注入到UserService當中。

<?xml version="1.0" encoding="UTF-8"?>

<beans>
    <bean id="userDao" class="indi.zhuyst.dao.UserDao"/>
    <bean id="userService" class="indi.zhuyst.service.UserService"
 
>
        <property name="userDao" ref="userDao"/>
    </bean>
</beans>

ApplicationContext

首先定義好XML的位置以及一個存放Bean的Map

    private static final String XML_PATH = "/applicationContext.xml";

    private Map<String,Bean> beans = new HashMap<>();

    public ApplicationContext
 
(){
        this.init();
    }

之後順序就很明確了

1. 讀取XML
2. 初始化所有Bean
3. 設定Bean中的屬性

/**
     * 讀取XML到{@link #beans}中
     * @see #XML_PATH
     */
    private void readXml(){
        InputStream inputStream = this.getClass().getResourceAsStream(XML_PATH);
        SAXParserFactory factory = SAXParserFactory.newInstance();
        try
 
 {
            SAXParser parser = factory.newSAXParser();
            ApplicationContextHandler handler = new ApplicationContextHandler();

            parser.parse(inputStream,handler);
            initBean(handler.getBeans());

        } catch (ParserConfigurationException | SAXException | IOException e) {
            throw new RuntimeException("XML解析錯誤");

        }
    }

    /**
     * 通過反射，初始化所有的Bean
     * @param beanList Bean列表
     */
    private void initBean(List<Bean> beanList){
        try {
            for(Bean bean : beanList){
                Object object = Class.forName(bean.getClassPath()).newInstance();
                bean.setObject(object);
                beans.put(bean.getId(),bean);

                System.out.println("初始化 —— " + bean.getClassPath());
            }

        } catch (ClassNotFoundException e) {
            throw new RuntimeException("沒有找到類：" + e.getMessage());
        } catch (IllegalAccessException | InstantiationException e) {
            throw new RuntimeException("類初始化失敗");
        }
    }

    /**
     * 核心：通過setter設定Bean中需要的類
     */
    private void setBeanProperties(){
        for(Map.Entry<String,Bean> entry : beans.entrySet()){
            Bean bean = entry.getValue();

            List<Property> properties = bean.getProperties();
            if(properties != null){

                for(Property property : properties){
                    String ref = property.getRef();
                    if(beans.containsKey(ref)){
                        Object object = beans.get(ref).getObject();
                        setter(bean.getObject(),property.getName(),object);

                        System.out.println("向 " + bean.getClassPath() + " 設定 " + property.getName());
                    }

                    else {
                        throw new RuntimeException("無法找到" + property.getRef());
                    }
                }

            }
        }
    }

一個簡單的Ioc容器就完成了！