[置頂] Spring的DI依賴實現分析

DI(依賴注入)是Spring最底層的核心容器要實現的功能之一，利用DI能夠實現程序功能的控制反轉(控制反轉即程序之間之間的依賴關係再也不由程序員來負責，而是由Spring容器來負責)

一個簡單的例子(DI例子）

 

一個接口的源代碼(表示一我的說話)

package com.pp;
public interface SaySentence {
public void say();
}

一個類實現了上面的接口，表示要說的一句具體話

package com.pp;
public class Person {
	private Sentence sce;
	private String name;
	public Person(String name,Sentence sce){
		this.name=name;
		this.sce=sce;
	}
        public Person(){}
	public Sentence getSce() {
		return sce;
	}
	public void setSce(Sentence sce) {
		this.sce = sce;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public void say(){
		/*若是沒有DI機制，可能要說一句話就得以下
		 *Sentenct sce=new SaySentence();
		 *sce.say();
		 *或者由程序員顯示調用setSentence()方法設置sce
		 *或者由程序員經過構造函數的方式對sce顯示賦值
		 * */
		sce.say();
	}
}

Spring的配置文件(person.xml)

<?xml version="1.0" encoding="UTF-8"?>
<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"
	xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
	<bean id="sentence" class="com.pp.Sentence"/>
	<bean id="person" class="com.pp.Person">
		<constructor-arg value="pp"/>
		<constructor-arg ref="sentence"/>
	</bean>
</beans>

測試的源代碼

 

package com.pp;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.xml.XmlBeanFactory;
import org.springframework.core.io.ClassPathResource;
public class MainTest {
public static void main(String[] args) {
	BeanFactory factory=new XmlBeanFactory(new ClassPathResource("person.xml"));
	Person person=(Person)factory.getBean("person");
	person.say();
}
}

看到上面的，你應該也瞭解了什麼是IOC,什麼是DI,但是我想分析的不是這，這真的很簡單，我想分析就是這個DI是怎麼實現的:)下面代碼，只是摘錄，太多了，只貼出幾個關鍵函數，有興趣的，能夠共同分析

 

 

	//加載的Bean定義
	//參數是一個資源文件，這個資源文件指定了編碼方式
	//返回值是配置文件中的bean定義個數
	public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
		//斷言要解析的XML文件配置存在，不能爲空
		Assert.notNull(encodedResource, "EncodedResource must not be null");
		//向日志系統輸出日誌系統，輸出XML bean的加載源
		if (logger.isInfoEnabled()) {
			logger.info("Loading XML bean definitions from " + encodedResource.getResource());
		}
    //獲取當前線程裏的ThreadLocal裏的變量集合
		Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
		if (currentResources == null) {
		//若是爲空的狀況下，從新申請一下HashSet集合
			currentResources = new HashSet<EncodedResource>(4);
			this.resourcesCurrentlyBeingLoaded.set(currentResources);
		}
		//將encodeResource填加到當前線程的局部變量集合中
		if (!currentResources.add(encodedResource)) {
			throw new BeanDefinitionStoreException(
					"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
		}
		try {
			InputStream inputStream = encodedResource.getResource().getInputStream();
			try {
				InputSource inputSource = new InputSource(inputStream);
				//若是設置了編譯方式，對輸入流進行編碼的設置
				if (encodedResource.getEncoding() != null) {
					inputSource.setEncoding(encodedResource.getEncoding());
				}
				return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
			}
			finally {
				inputStream.close();
			}
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException(
					"IOException parsing XML document from " + encodedResource.getResource(), ex);
		}
		finally {
		  //釋放內存空間
			currentResources.remove(encodedResource);
			if (currentResources.isEmpty()) {
				this.resourcesCurrentlyBeingLoaded.remove();
			}
		}
	}

上面這個函數最關鍵的部分是doLoadBeanDefinitions這個函數，看看它的源代碼

 

 

//真正的從指定的XML文件中加載Bean的定義
	protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
			throws BeanDefinitionStoreException {
		try {
		  //獲取資源的校驗模式
			int validationMode = getValidationModeForResource(resource);
			//文檔定義
			Document doc = this.documentLoader.loadDocument(
					inputSource, getEntityResolver(), this.errorHandler, validationMode, isNamespaceAware());
			return registerBeanDefinitions(doc, resource);
		}
		catch (BeanDefinitionStoreException ex) {
			throw ex;
		}
		catch (SAXParseException ex) {
			throw new XmlBeanDefinitionStoreException(resource.getDescription(),
					"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
		}
		catch (SAXException ex) {
			throw new XmlBeanDefinitionStoreException(resource.getDescription(),
					"XML document from " + resource + " is invalid", ex);
		}
		catch (ParserConfigurationException ex) {
			throw new BeanDefinitionStoreException(resource.getDescription(),
					"Parser configuration exception parsing XML from " + resource, ex);
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException(resource.getDescription(),
					"IOException parsing XML document from " + resource, ex);
		}
		catch (Throwable ex) {
			throw new BeanDefinitionStoreException(resource.getDescription(),
					"Unexpected exception parsing XML document from " + resource, ex);
		}
	}

 

這個函數最關鍵部分是這個registerBeanDefinitions，跟蹤到裏面，略去源代碼，直接分析其最核心的代碼registerBeanDefinitions

核心源碼以下

 

		public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
		this.readerContext = readerContext;
    //向日志文件輸出日誌信息
		logger.debug("Loading bean definitions");
		//獲取根元素
		Element root = doc.getDocumentElement();
    //如下是對Xml文件進行解析(各類解析，各類提取)
		BeanDefinitionParserDelegate delegate = createHelper(readerContext, root);
		preProcessXml(root);
		parseBeanDefinitions(root, delegate);
		postProcessXml(root);
	}

 

未完，待續