Spring Ioc底层实现

原理和步骤

1. Ioc容器的实现主要依赖的是xml解析和Java反射。
2. 步骤：读取配置文件 -> 将其逐层“剥开”，获取各项属性 -> 通过各属性配合反射生成对象 -> 将其放入容器中，以供调用

具体实现

• 实体类 Book

代码语言：javascript复制

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Book {
    private Integer id;
    private String name;
    private Double price;
}

• 配置文件 spring.xml

代码语言：javascript复制

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="threeBody" class="per.tan.ioc.Book">
        <property name="id" value="1"/>
        <property name="name" value="《三体》"/>
        <property name="price" value="59.9"/>
    </bean>
</beans>

• xml解析使用的是Dom4j，加入maven依赖

代码语言：javascript复制

<dependency>
    <groupId>dom4j</groupId>
    <artifactId>dom4j</artifactId>
    <version>1.1</version>
</dependency>

MyClassPathXmlApplicationContext。需要实现接口ApplicationContext，着重重写以下方法：

• 创建容器 Map

代码语言：javascript复制

private final Map<String, Object> iocMap;

• 重写构造方法 此处将解析xml的步骤写在自定义方法parseXML

代码语言：javascript复制

public MyClassPathXmlApplicationContext(String path) {
    iocMap = new HashMap<>();
    parseXML(path);
}

• parseXML()

代码语言：javascript复制

//使用Dom4j进行xml解析
public void parseXML(String path) {
    SAXReader saxReader = new SAXReader();
    try {
        //定义文档
        Document document = saxReader.read("src/main/resources/"   path);
        //获取根节点，即<beans>
        Element beans = document.getRootElement();
        //beans迭代器
        Iterator<Element> beansIterator = beans.elementIterator();
        while (beansIterator.hasNext()) {
            Element bean = beansIterator.next();
            String idStr = bean.attributeValue("id");
            String classStr = bean.attributeValue("class");

            Class<?> clazz = Class.forName(classStr);
            Constructor<?> constructor = clazz.getConstructor();
            //利用获取到的无参构造生成目标对象
            Object object = constructor.newInstance();
            //bean迭代器
            Iterator<Element> beanIterator = bean.elementIterator();
            while (beanIterator.hasNext()) {
                Element property = beanIterator.next();
                //获取属性
                String propertyName = property.attributeValue("name");
                String propertyValue = property.attributeValue("value");
                Field field = clazz.getDeclaredField(propertyName);
                //获取方法
                String methodStr = "set"   propertyName.substring(0, 1).toUpperCase()   propertyName.substring(1);
                Method method = clazz.getMethod(methodStr, field.getType());

                Object value = propertyValue;
                //判断属性类型并转换
                switch (field.getType().getName()) {
                    case "java.lang.Integer":
                        value = Integer.parseInt(propertyValue);
                        break;
                    case "java.lang.Double":
                        value = Double.parseDouble(propertyValue);
                        break;
                }
                method.invoke(object, value);
            }
            //装入容器
            iocMap.put(idStr, object);
        }
    } catch (DocumentException e) {
        e.printStackTrace();
    } catch (NoSuchMethodException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (InstantiationException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (InvocationTargetException e) {
        e.printStackTrace();
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    }
}

• 重写getBean，根据名字从容器中获取bean

代码语言：javascript复制

 @Override
 public Object getBean(String s) throws BeansException {
     return iocMap.get(s);
 } 

测试 Test

代码语言：javascript复制

public class Test {
public static void main(String[] args) {
   ApplicationContext context = new MyClassPathXmlApplicationContext("spring.xml");
   Book book = (Book) context.getBean("threeBody");
   System.out.println(book);
 }
}

• 成果

代码语言：javascript复制

D:Java_JDKJDK8binjava.exe ...
Book(id=1, name=《三体》, price=59.9)

Process finished with exit code 0

Q.E.D.

java xml maven ioc map

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