Java依赖注入容器超详细全面讲解

Java依赖注入容器 Java依赖注入 2023-01-12 09:01:23 574人浏览泡泡鱼

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摘要

目录一、依赖注入Dependency Injection二、解析2.1 典型的配置文件2.2 配置文件所对应的Java类2.3 定义解析器三、bean工厂（根据bean定义创建bea

一、依赖注入Dependency Injection

DI容器底层最基本的设计思路就是基于工厂模式。

DI容器的核心功能：配置解析、对象创建、对象声明周期。

完整的代码：Dependency Injection。

二、解析

通过配置，让DI容器知道要创建哪些对象。

DI容器读取文件，根据配置文件来创建对象。

2.1 典型的配置文件

下面是一个典型的配置文件：

<?xml version="1.0" encoding="UTF-8"?>
<beans>
    <bean id="productInfo" class="com.hef.review.designpatterns.creational.di.beans.ProductInfo">
        <constructor-arg type="String" value="P01"/>
        <constructor-arg type="int" value="200"/>
    </bean>
    <bean id="productSell" class="com.hef.review.designpatterns.creational.di.beans.ProductSell">
        <constructor-arg ref="productInfo"/>
    </bean>
</beans>

2.2 配置文件所对应的Java类

public class ProductInfo {
    private String productName;
    private int productVersion;
    public ProductInfo(String productName, int productVersion) {
        this.productName = productName;
        this.productVersion = productVersion;
    }
  // 省略 getter 和 setter
}
public class ProductSell {
    private ProductInfo productInfo;
    public ProductSell(ProductInfo productInfo) {
        this.productInfo = productInfo;
    }
    public void sell() {
        System.out.println("销售：" + productInfo);
    }
  // 省略 getter 和 setter
}

2.3 定义解析器

Bean定义：


public class BeanDefinition {
    private String id;
    private String className;
    private List<ConstructorArg> constructorArgs = new ArrayList<>();
    private Scope scope = Scope.SINGLETON;
    private boolean lazyInit = false;
    public BeanDefinition(){}
    public BeanDefinition(String id, String className) {
        this.id = id;
        this.className = className;
    }
  // 省略getter 和 setter方法
}

配置解析接口：


public interface BeanConfigParser {
    
    List<BeanDefinition> parse(InputStream in);
}

XML解析实现（使用Java自带的DOM解析类库）：


public class BeanXmlConfigParser implements BeanConfigParser {
    
    @Override
    public List<BeanDefinition> parse(InputStream in) {
        try {
            List<BeanDefinition> result = new ArrayList<>();
            DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();
            DocumentBuilder documentBuilder = documentBuilderFactory.newDocumentBuilder();
            Document doc = documentBuilder.parse(in);
            doc.getDocumentElement().nORMalize();
            nodeList beanList = doc.getElementsByTagName("bean");
            for (int i = 0; i < beanList.getLength(); i++) {
                Node node = beanList.item(i);
                if (!Objects.equals(node.getNodeType(), Node.ELEMENT_NODE)) continue;
                Element element = (Element) node;
                BeanDefinition beanDefinition = new BeanDefinition(element.getAttribute("id"), element.getAttribute("class"));
                if (element.hasAttribute("scope")
                        && StringUtils.equals(element.getAttribute("scope"), BeanDefinition.Scope.PROTOTYPE.name())) {
                    beanDefinition.setScope(BeanDefinition.Scope.PROTOTYPE);
                }
                if (element.hasAttribute("lazy-init")
                        && Boolean.valueOf(element.getAttribute("lazy-init"))) {
                    beanDefinition.setLazyInit(true);
                }
                List<BeanDefinition.ConstructorArg> constructorArgs = createConstructorArgs(element);
                if (CollectionUtils.isNotEmpty(constructorArgs)) {
                    beanDefinition.setConstructorArgs(constructorArgs);
                }
                result.add(beanDefinition);
            }
            return result;
        }catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
    
    private List<BeanDefinition.ConstructorArg> createConstructorArgs(Element element) {
        List<BeanDefinition.ConstructorArg> result = new ArrayList<>();
        NodeList nodeList = element.getElementsByTagName("constructor-arg");
        if (nodeList.getLength()==0) return result;
        for (int i = 0; i < nodeList.getLength(); i++) {
            Node node = nodeList.item(i);
            if (!Objects.equals(node.getNodeType(), Node.ELEMENT_NODE)) continue;
            Element ele = (Element) node;
            BeanDefinition.ConstructorArg arg = new BeanDefinition.ConstructorArg();
            if (ele.hasAttribute("type") && StringUtils.isNoneBlank(ele.getAttribute("type"))) {
                String type = ele.getAttribute("type");
                String value = ele.getAttribute("value");
                arg.setType(fetchClassType(type));
                arg.setArg(fetchArgValue(type, value));
                arg.setRef(false);
            }else if (ele.hasAttribute("ref")) {
                arg.setRef(true);
                arg.setArg(ele.getAttribute("ref"));
            }
            result.add(arg);
        }
        return result;
    }
    
    private Object fetchArgValue(String typeValue, String value) {
        if (StringUtils.equals(typeValue, "int") || StringUtils.contains(typeValue, "Integer")) {
            return Integer.parseInt(value);
        }else if (StringUtils.contains(typeValue, "String")) {
            return value;
        } else {
            throw new RuntimeException("未知类型");
        }
    }
    
    private Class<?> fetchClassType(String typeValue) {
        if (StringUtils.equals(typeValue, "int")){
            return Integer.TYPE;
        } else if(StringUtils.contains(typeValue, "Integer")) {
            return Integer.class;
        }else if (StringUtils.contains(typeValue, "String")) {
            return String.class;
        } else {
            throw new RuntimeException("未知类型");
        }
    }
}

三、bean工厂（根据bean定义创建bean对象）

根据bean工厂创建bean的对象：


public class BeansFactory {
    private ConcurrentHashMap<String, Object> singletonObjects = new ConcurrentHashMap<>();
    private ConcurrentHashMap<String, BeanDefinition> beanDefinitions = new ConcurrentHashMap<>();
    
    public void addBeanDefinitions(List<BeanDefinition> beanDefinitionList) {
        for (BeanDefinition beanDefinition : beanDefinitionList) {
            this.beanDefinitions.putIfAbsent(beanDefinition.getId(), beanDefinition);
        }
        for (BeanDefinition beanDefinition : beanDefinitionList) {
            if (!beanDefinition.isLazyInit() && beanDefinition.isSingleton()) {
                singletonObjects.put(beanDefinition.getId(), createBean(beanDefinition));
            }
        }
    }
    
    public Object getBean(String beanId) {
        BeanDefinition beanDefinition = beanDefinitions.get(beanId);
        checkState(Objects.nonNull(beanDefinition), "Bean is not defined:" + beanId);
        return createBean(beanDefinition);
    }
    
    private Object createBean(BeanDefinition beanDefinition) {
        if (beanDefinition.isSingleton() && singletonObjects.containsKey(beanDefinition.getId())) {
            return singletonObjects.get(beanDefinition.getId());
        }
        Object result = null;
        try {
            Class<?> beanClass = Class.forName(beanDefinition.getClassName());
            List<BeanDefinition.ConstructorArg> constructorArgs = beanDefinition.getConstructorArgs();
            if (CollectionUtils.isEmpty(constructorArgs)) {
                result =  beanClass.newInstance();
            } else {
                Class[] arGClasses = new Class[constructorArgs.size()];
                Object[] arGobjects = new Object[constructorArgs.size()];
                for (int k = 0; k < constructorArgs.size(); k++) {
                    BeanDefinition.ConstructorArg arg = constructorArgs.get(k);
                    if (!arg.isRef()) {
                        argClasses[k] = arg.getType();
                        argObjects[k] = arg.getArg();
                    } else {
                        BeanDefinition refBeanDefinition = beanDefinitions.get(arg.getArg());
                        checkState(Objects.nonNull(refBeanDefinition), "Bean is not defined: " + arg.getArg());
                        argClasses[k] = Class.forName(refBeanDefinition.getClassName());
                        argObjects[k] = createBean(refBeanDefinition);
                    }
                }
                result = beanClass.getConstructor(argClasses).newInstance(argObjects);
            }
            if (Objects.nonNull(result) && beanDefinition.isSingleton()) {
                singletonObjects.putIfAbsent(beanDefinition.getId(), result);
                return singletonObjects.get(beanDefinition.getId());
            }
            return result;
        }catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

四、DI容器（上下文）

4.1 容器接口


public interface ApplicationContext {
    Object getBean(String beanId);
    void loadBeanDefinitions(String configLocation);
}

4.2 XML容器


public class ClassPathXmlApplicationContext implements ApplicationContext {
    private BeansFactory beansFactory;
    private BeanConfigParser beanConfigParser;
    public ClassPathXmlApplicationContext(String configLocation) {
        this.beansFactory = new BeansFactory();
        this.beanConfigParser = new BeanXmlConfigParser();
        loadBeanDefinitions(configLocation);
    }
    
    public void loadBeanDefinitions(String configLocation) {
        try (InputStream in = this.getClass().getClassLoader().getResourceAsStream(configLocation)) {
            if (in==null) {
                throw new RuntimeException("未发现配置文件:" + configLocation);
            }
            List<BeanDefinition> beanDefinitionList = beanConfigParser.parse(in);
            beansFactory.addBeanDefinitions(beanDefinitionList);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
    
    @Override
    public Object getBean(String beanId) {
        return beansFactory.getBean(beanId);
    }
}

五、使用DI容器


public class Demo {
    public static void main(String[] args) {
//        testReadResourceXML();
//        testParseXML();
        ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");
        Object productInfo = context.getBean("productInfo");
        System.out.println(productInfo);
        ProductSell productSell = (ProductSell)context.getBean("productSell");
        productSell.sell();
    }
  // 省略 testReadResourceXML() 和 testParseXML()
}

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