前面介紹了Spring 框架簡介。今天我們來實(shí)現(xiàn)一個(gè)簡單的Spring IOC容器。

根據(jù)對Spring IOC框架的了解，我們可以將IOC容器簡單抽象為如下四個(gè)步驟：
（1）加載 xml 配置文件，遍歷其中的 <bean> 標(biāo)簽
（2）獲取<bean>標(biāo)簽中的 id 和 class 屬性，加載 class 屬性對應(yīng)的類，并創(chuàng)建 bean
（3）遍歷 <bean> 標(biāo)簽中的 <property> 標(biāo)簽，獲取屬性值，并將屬性值填充到 bean 中
（4）將 bean 注冊到 bean 容器中

只需要如下的幾個(gè)類即可:

• SimpleIOC ：IOC 的實(shí)現(xiàn)類，作用為加載XML文件注冊到容器之中
• SimpleIOCTest : 測試類
• Car ：bena 類
• Wheel ： bean 類
• ioc.xml ： XML配置文件

項(xiàng)目代碼

1. SimpleIOC.java

public class SimpleIOC {
    private Map<String, Object> beanMap = new HashMap<>();

    public SimpleIOC(String location) throws Exception {
        loadBeans(location);
    }

    public Object getBean(String name) {
        Object bean = beanMap.get(name);
        if (bean == null) {
            throw new IllegalArgumentException("there is no bean with name " + name);
        }

        return bean;
    }

    private void loadBeans(String location) throws Exception {
        // 加載 xml 配置文件
        InputStream inputStream = new FileInputStream(location);
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder docBuilder = factory.newDocumentBuilder();
        Document doc = docBuilder.parse(inputStream);
        Element root = doc.getDocumentElement();
        NodeList nodes = root.getChildNodes();

        // 遍歷 <bean></bean> 標(biāo)簽
        for (int i = 0; i < nodes.getLength(); i++) {
            Node node = nodes.item(i);
            if (node instanceof Element) {
                Element ele = (Element) node;
                String id = ele.getAttribute("id");
                String className = ele.getAttribute("class");
                Class beanClass = null;
                try {
                    beanClass = Class.forName(className);
                } catch (ClassNotFoundException e) {
                    e.printStackTrace();
                    return;
                }

                Object bean = beanClass.newInstance();

                // 處理 bean property 屬性
                NodeList propertyNodes = ele.getElementsByTagName("property");
                for (int j = 0; j < propertyNodes.getLength(); j++) {
                    Node propertyNode = propertyNodes.item(j);
                    if (propertyNode instanceof Element) {
                        Element propertyElement = (Element) propertyNode;
                        String name = propertyElement.getAttribute("name");
                        String value = propertyElement.getAttribute("value");

                        Field declaredField = bean.getClass().getDeclaredField(name);
                        declaredField.setAccessible(true);

                        if (value != null && value.length() > 0) {
                            declaredField.set(bean, value);
                        } else {
                            String ref = propertyElement.getAttribute("ref");
                            if (ref == null || ref.length() == 0) {
                                throw new IllegalArgumentException("ref config error");
                            }

                            declaredField.set(bean, getBean(ref));
                        }

                        // 將 bean 注冊到 bean 容器中
                        registerBean(id, bean);
                    }
                }
            }
        }
    }

    private void registerBean(String id, Object bean) {
        beanMap.put(id, bean);
    }
}

2. SimpleIOCTest.java

public class SimpleIOCTest {

    @Test
    public void getBean() throws Exception {
        String location = SimpleIOC.class.getClassLoader().getResource("toy-spring-ioc.xml").getFile();
        SimpleIOC bf = new SimpleIOC(location);
        Wheel wheel = (Wheel) bf.getBean("wheel");
        System.out.println(wheel);
        Car car = (Car) bf.getBean("car");
        System.out.println(car);
    }
}

3. Car.java

public class Car {

    private String name;
    private String length;
    private String width;
    private String height;
    private Wheel wheel;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getLength() {
        return length;
    }

    public void setLength(String length) {
        this.length = length;
    }

    public String getWidth() {
        return width;
    }

    public void setWidth(String width) {
        this.width = width;
    }

    public String getHeight() {
        return height;
    }

    public void setHeight(String height) {
        this.height = height;
    }

    public Wheel getWheel() {
        return wheel;
    }

    public void setWheel(Wheel wheel) {
        this.wheel = wheel;
    }

    @Override
    public String toString() {
        return "Car{" +
                "name='" + name + '\'' +
                ", length='" + length + '\'' +
                ", width='" + width + '\'' +
                ", height='" + height + '\'' +
                ", \nwheel=" + wheel +
                '}';
    }
}

4. Wheel.java

public class Wheel {

    private String brand;
    private String specification ;

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public String getSpecification() {
        return specification;
    }

    public void setSpecification(String specification) {
        this.specification = specification;
    }

    @Override
    public String toString() {
        return "Wheel{" +
                "brand='" + brand + '\'' +
                ", specification='" + specification + '\'' +
                '}';
    }
}

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