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

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

• 配置文件 spring.xml

<?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

<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

  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

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

  • parseXML()

  //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

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

• 测试 Test

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

D:\Java_JDK\JDK8\bin\java.exe ...
Book(id=1, name=《三体》, price=59.9)

Process finished with exit code 0