基于Java反射技术实现简单IOC容器

package cn.haidnor.bean;

public class Student {
  /** 学生姓名 */
  private String name;
  /** 学生性别 */
  private String gender;
  /** 学生年龄 */
  private int age;
  
  @Override
  public String toString() {
    return "Student{" +
        "name='" + name + '\'' +
        ", gender='" + gender + '\'' +
        ", age=" + age +
        '}';
  }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans>
  <bean id="stu1" class="cn.haidnor.bean.Student">
    <property name="name">Lucy</property>
    <property name="age">18</property>
    <property name="gender">female</property>
  </bean>

  <bean id="stu2" class="cn.haidnor.bean.Student">
    <property name="name">Tom</property>
    <property name="age">21</property>
    <property name="gender">male</property>
  </bean>

  <bean id="stu3" class="cn.haidnor.bean.Student">
    <property name="name">LiLi</property>
    <property name="age">23</property>
    <property name="gender">female</property>
  </bean>
</beans>

package cn.haidnor.core;

import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;


public class SpringIOC {

  /**
   * 配置文件地址
   */
  private static final String CONFIGURATION_PATH = "resources/applicationContext.xml";

  /**
   * ioc容器
   */
  private static Map<String, Object> ioc = new HashMap<>();

  static {
    initialization();
  }

  /**
   * 从 ioc 容器中获取指定 bean
   *
   * @param name 需要获取的 bean 的 id, 对应 XML 配置文件中的 bean id
   * @return bean
   */
  public static Object getBean(String name) {
    return ioc.get(name);
  }

  /**
   * 初始化容器
   */
  private static void initialization() {
    Document document = null;

    try {
      DocumentBuilderFactory bdf = DocumentBuilderFactory.newInstance();
      DocumentBuilder documentBuilder = bdf.newDocumentBuilder();
      document = documentBuilder.parse(CONFIGURATION_PATH);
    } catch (Exception e) {
      e.printStackTrace();
    }

    NodeList beanNodes = document.getElementsByTagName("bean");

    for (int i = 0; i < beanNodes.getLength(); i++) {
      Node node = beanNodes.item(i);
      reloadBean(node);
    }
  }

  /**
   * 装载 benn
   *
   * @param beanNode xml 文件 bean 根节点
   */
  private static void reloadBean(Node beanNode) {
    Element bean = (Element) beanNode;

    String id = bean.getAttribute("id");      // IOC 容器中 bean 的名字
    String beanClass = bean.getAttribute("class"); // 全类名

    // 每个 bean 节点下的全部 property 节点
    NodeList childNodes = beanNode.getChildNodes();
    Map<String, String> attributeMap = reloadAttribute(childNodes);

    // 使用反射构造 bean 对象
    Object instance = creatBean(beanClass, attributeMap);

    // 将所有的 bean 对象放入容器中
    ioc.put(id, instance);
  }

  /**
   * 加载 bean 的属性值
   *
   * @param attributeNodes 所有的属性 property 节点
   * @return Map 属性的名字和值集合
   */
  private static Map<String, String> reloadAttribute(NodeList attributeNodes) {
    Map<String, String> keyValue = new HashMap<>();
    for (int i = 0; i < attributeNodes.getLength(); i++) {
      Node filed = attributeNodes.item(i);
      if (filed.getNodeType() == Node.ELEMENT_NODE) {
        Element element = (Element) filed;
        String fileName = element.getAttribute("name");
        String value = element.getFirstChild().getNodeValue();
        keyValue.put(fileName, value);
      }
    }
    return keyValue;
  }

  /**
   * 构造bean对象
   *
   * @param className 全类名
   * @param attributes 每个对象的属性和
   * @return Object 构造完成的 bean 对象
   */
  private static Object creatBean(String className, Map<String, String> attributes) {
    Object instance = null;
    try {
      Class<?> clazz = Class.forName(className);
      instance = clazz.newInstance();
      Field[] fields = clazz.getDeclaredFields();

      for (Field field : fields) {
        setFiledValue(instance, field, attributes);
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
    return instance;
  }

  /**
   * 为实例对象的属性赋值
   *
   * @param instance  实例对象
   * @param field   属性字段对象
   * @param attributes 属性名与属性值的 Map 集合
   */
  private static void setFiledValue(Object instance, Field field, Map<String, String> attributes) {
    // 忽略 field 权限检查
    field.setAccessible(true);

    String type = field.getType().toString();
    String name = field.getName();

    try {
      switch (type) {
        case "char":
          field.setChar(instance, attributes.get(name).charAt(0));
          break;

        case "class java.lang.Boolean":
        case "boolean":
          field.setBoolean(instance, Boolean.parseBoolean(attributes.get(name)));
          break;

        case "class java.lang.Byte":
        case "byte":
          field.setByte(instance, Byte.parseByte(attributes.get(name)));
          break;

        case "class java.lang.Float":
        case "float":
          field.setFloat(instance, Float.parseFloat(attributes.get(name)));
          break;

        case "class java.lang.Integer":
        case "int":
          field.setInt(instance, Integer.parseInt(attributes.get(name)));
          break;

        case "class java.lang.Long":
        case "long":
          field.setLong(instance, Long.parseLong(attributes.get(name)));
          break;

        case "class java.lang.Short":
        case "short":
          field.setShort(instance, Short.parseShort(attributes.get(name)));
          break;

        default:
          field.set(instance, attributes.get(name));
          break;
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

package cn.haidnor.test;

import cn.haidnor.bean.Student;
import cn.haidnor.core.SpringIOC;

public class Test {
  public static void main(String[] args) {
    // 不使用 new 的方式创建对象, 从容器中获取
    Student stu1 = (Student) SpringIOC.getBean("stu3");
    // 调用学生类的方法,打印信息
    System.out.println(stu1.toString());
  }
}