手写基于XML的IOC容器 -- 理解反射和XML解析

IOC = XML解析文件配置 + 使用反射通过解析到Bean的class属性初始化类实例

引入Pom文件

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.cy</groupId>
    <artifactId>my-ioc-xml</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <!-- https://mvnrepository.com/artifact/dom4j/dom4j -->
        <dependency>
            <groupId>dom4j</groupId>
            <artifactId>dom4j</artifactId>
            <version>1.6.1</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/commons-lang/commons-lang -->
        <dependency>
            <groupId>commons-lang</groupId>
            <artifactId>commons-lang</artifactId>
            <version>2.6</version>
        </dependency>
    </dependencies>
</project>

编写XML配置文件

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p"
	xmlns:context="http://www.springframework.org/schema/context"
	xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
    	 http://www.springframework.org/schema/beans/spring-beans.xsd
     	 http://www.springframework.org/schema/context
         http://www.springframework.org/schema/context/spring-context.xsd
         http://www.springframework.org/schema/aop
         http://www.springframework.org/schema/aop/spring-aop.xsd
         http://www.springframework.org/schema/tx
     	 http://www.springframework.org/schema/tx/spring-tx.xsd">

	<bean id="user" class="com.cy.spring.entity.User"></bean>
</beans>

编写对应的实体类

public class User {

	private Integer id;
	private String userName;

	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	public String getUserName() {
		return userName;
	}

	public void setUserName(String userName) {
		this.userName = userName;
	}

}

XML的解析工具类

public class XmlUtils {

	public static void main(String[] args) throws SAXException, DocumentException {
		XmlUtils xmlUtils = new XmlUtils();
		xmlUtils.test001();

	}

	public void test001() throws DocumentException {
		SAXReader saxReader = new SAXReader();
		Document read = saxReader.read(getClassPath("student.xml"));
		// 获取根节点
		Element rootElement = read.getRootElement();
		getNodes(rootElement);
	}

	public InputStream getClassPath(String xmlPath) {
		InputStream resourceAsStream = getClass().getClassLoader().getResourceAsStream(xmlPath);
		return resourceAsStream;
	}

	public static void getNodes(Element rootElement) {
		System.out.println("获取当前名称:" + rootElement.getName());
		// 获取属性信息
		List<Attribute> attributes = rootElement.attributes();
		for (Attribute attribute : attributes) {
			System.out.println("属性:" + attribute.getName() + "---" + attribute.getText());
		}
		// 获取属性value
		String value = rootElement.getTextTrim();
		if (!StringUtils.isEmpty(value)) {
			System.out.println("value:" + value);
		}
		// 使用迭代器遍历,继续遍历子节点
		Iterator<Element> elementIterator = rootElement.elementIterator();
		while (elementIterator.hasNext()) {
			Element next = elementIterator.next();
			getNodes(next);
		}
	}

}

模拟实现IOC容器

public class ClassPathXmlApplicationContext {
	// xml路径地址
	private String xmlPath;

	public ClassPathXmlApplicationContext(String xmlPath) {
		this.xmlPath = xmlPath;
	}

	public Object getBean(String beanId) throws Exception {
		// 1. 读取配置文件
		List<Element> elements = readerXml();
		if (elements == null) {
			throw new Exception("该配置文件没有子元素");
		}
		// 2. 使用beanId查找对应的class地址
		String beanClass = findXmlByIDClass(elements, beanId);
		if (StringUtils.isEmpty(beanClass)) {
			throw new Exception("未找到对应的class地址");
		}
		// 3. 使用反射机制初始化，对象
		Class<?> forName = Class.forName(beanClass);
		return forName.newInstance();
	}

	// 读取配置文件信息
	public List<Element> readerXml() throws DocumentException {
		SAXReader saxReader = new SAXReader();
		if (StringUtils.isEmpty(xmlPath)) {
			new Exception("xml路径为空...");
		}
		Document read = saxReader.read(getClassXmlInputStream(xmlPath));
		// 获取根节点信息
		Element rootElement = read.getRootElement();
		// 获取子节点
		List<Element> elements = rootElement.elements();
		if (elements == null || elements.isEmpty()) {
			return null;
		}
		return elements;
	}

	// 使用beanid查找该Class地址
	public String findXmlByIDClass(List<Element> elements, String beanId) throws Exception {
		for (Element element : elements) {
			// 读取节点上是否有value
			String beanIdValue = element.attributeValue("id");
			if (beanIdValue == null) {
				throw new Exception("使用该beanId为查找到元素");
			}
			if (!beanIdValue.equals(beanId)) {
				continue;
			}
			// 获取Class地址属性
			String classPath = element.attributeValue("class");
			if (!StringUtils.isEmpty(classPath)) {
				return classPath;
			}
		}
		return null;
	}

	// 读取xml配置文件
	public InputStream getClassXmlInputStream(String xmlPath) {
		InputStream resourceAsStream = getClass().getClassLoader().getResourceAsStream(xmlPath);
		return resourceAsStream;
	}
}

测试基于XML的IOC是否完成

public class Test002 {

	public static void main(String[] args) throws Exception {
		ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring.xml");
		User user = (User) applicationContext.getBean("user");
		System.out.println(user);
	}
}

sout:com.itmayiedu.spring.entity.User@7291c18f
说明完成了IOC容器对实例的初始化