SpringIoc底层实现原理

• 读取bean 的XML配置文件
• 使用beanId查找bean配置，并获取配置文件class地址
• 使用java反射技术实例化对象
• 获取属性配置，使用反射技术进行赋值

步骤

1.利用传入的参数获取xml文件的流,并且利用dom4j解析成Document对象
2.对于Document对象获取根元素对象<beans>后对下面的<bean>标签进行遍历,判断是否有符合的id.
3.如果找到对应的id,相当于找到了一个Element元素,开始创建对象,先获取class属性,根据属性值利用反射建立对象.
4.遍历<bean>标签下的property标签,并对属性赋值.注意,需要单独处理int,float类型的属性.因为在xml配置中这些属性都是以字符串的形式来配置的,因此需要额外处理.
5.如果属性property标签有ref属性,说明某个属性的值是一个对象,那么根据id(ref属性的值)去获取ref对应的对象,再给属性赋值.
6.返回建立的对象,如果没有对应的id,或者<beans>下没有子标签都会返回null

实现

新建maven工程
添加依赖

 <dependencies>
        <!--工具包-->
        <dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-lang3</artifactId>
            <version>3.4</version>
        </dependency>
        <!--解析dom-->
        <dependency>
            <groupId>org.dom4j</groupId>
            <artifactId>dom4j</artifactId>
            <version>2.0.0</version>
        </dependency>
    </dependencies>

目录结构.png

public class UserEntity {
    private String userId;
    private String userName;

    public UserEntity() {
    }

    public String getUserId() {

        return userId;
    }

    public void setUserId(String userId) {

        this.userId = userId;
    }

    public String getUserName() {

        return userName;
    }

    public void setUserName(String userName) {

        this.userName = userName;
    }

    @Override
    public String toString() {
        return "UserEntity [userId=" + userId + ", userName=" + userName + "]";
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans>
    <bean id="user1" class="com.springioc.test.entity.UserEntity">
        <property name="userId" value="a1"></property>
        <property name="userName" value="小明"></property>
    </bean>
    <bean id="user2" class="com.springioc.test.entity.UserEntity">
        <property name="userId" value="a2"></property>
        <property name="userName" value="小军"></property>
    </bean>
</beans>   

public class ClassPathXmlApplicationContext {
    private static String PATH;
    private static String ID;
    private static String CLASS;
    private static String NAME;
    private static String VALUE;

    public void init() {
        ID = "id";
        CLASS = "class";
        NAME = "name";
        VALUE = "value";
    }

    public ClassPathXmlApplicationContext(String path) {
        PATH = path;
        init();
    }

    public Object getBean(String beanId) throws DocumentException, ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchFieldException {
        //解析xml
        if (StringUtils.isEmpty(beanId)) {
            return null;
        }
        SAXReader saxReader = new SAXReader();
        //获取xml
        Document applicationContext = saxReader.read(this.getClass().getClassLoader().getResource(PATH));
        //获取最外层根元素beans
        Element rootElement = applicationContext.getRootElement();
        //所有的二级节点
        List<Element> elements = rootElement.elements();
        for (Element element : elements) {
            String id = element.attributeValue(ID);
            if (!beanId.equals(id)) {
                continue;
            }
            //获取class节点属性
            String clazz = element.attributeValue(CLASS);
            //使用java反射机制初始化类
            Class<?> forName = Class.forName(clazz);
            //获取类的对象
            Object newInstance = forName.newInstance();
            //获取属性
            List<Element> childElemets = element.elements();
            for (Element childElemet : childElemets) {
                String name = childElemet.attributeValue(NAME);
                String value = childElemet.attributeValue(VALUE);
                Field declaredField = forName.getDeclaredField(name);
                //设置私有客可访问权限
                declaredField.setAccessible(true);
                declaredField.set(newInstance, value);
            }
            return newInstance;
        }
        return null;
    }

    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        try {
            UserEntity user1 = (UserEntity) context.getBean("user1");
            if (null != user1) {
                System.out.println(user1.toString());
            } else {
                System.out.println("获取失败");
            }

        } catch (DocumentException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
    }
}

运行结果

image.png