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浅谈SpringFramework的BeanFactory创建过程

博客前言介绍

提示：在前面章节的学习中，我们简单了解了Spring IoC容器启动初始化的主流程，不过并没有详细解释，因为代码比较复杂，没有做长篇大论，所以本文接着学习BenFactory的创建过程，学习源码建议带着疑问去学，一点点跟，时间积累之后就可以串起来
提示：以下是本篇文章正文内容，下面案例可供参考

获取BeanFactory主流程

在前面的学习中，#refresh是IoC容器创建很关键的主线，在代码里，可以找到obtainFreshBeanFactory这个比较关键的方法，这个方法就是BeanFactory的创建过程方法，本文ClassPathXmlApplicationContext为准，进行源码的学习，上篇博客是以AnnotationApplicationContext为准进行简单的分析，本文继续比较详细的讲述
实验环境：

• SpringFramework版本
• Springframework5.0.x

• 开发环境
• JAR管理：gradle 4.9/ Maven3.+
• 开发IDE：IntelliJ IDEA 2018.2.5
• JDK：jdk1.8.0_31
• Git Server：Git fro window 2.8.3
• Git Client：SmartGit18.1.5（可选）

maven项目，需要加上pom配置：

 <properties>
   <springframework.version>5.0.9.RELEASE</springframework.version>
 </properties>
 <dependencies>
     <dependency>
         <groupId>org.springframework</groupId>
         <artifactId>spring-context</artifactId>
         <version>${springframework.version}</version>
     </dependency>
 </dependencies>

如果是gradle环境，可以直接在Spring框架project里新增一个module，具体可以参考教程：加上对应的配置

SpringBean.java：

package com.example.bean;
import org.springframework.beans.factory.InitializingBean;
/**
 * 
<pre>
 *      SpringBean
 * </pre>
 *
 * 
<pre>
 * @author mazq
 * 修改记录
 *    修改后版本:     修改人：  修改日期: 2020/11/05 10:50  修改内容:
 * </pre>
 */
public class SpringBean implements InitializingBean {
    public SpringBean(){
        System.out.println("SpringBean构造函数");
    }
    @Override
    public void afterPropertiesSet() throws Exception {
        System.out.println("SpringBean afterPropertiesSet");
    }
}

applicationContext.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"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    <bean id="springBean" class="com.example.bean.SpringBean" ></bean>
</beans>

测试类，使用ClassPathXmlApplicationContext

package com.example;
import com.example.bean.SpringBean;
import org.springframework.context.support.ClassPathXmlApplicationContext;
/**
 * 
<pre>
 *      TestController
 * </pre>
 *
 * 
<pre>
 * @author mazq
 * 修改记录
 *    修改后版本:     修改人：  修改日期: 2020/11/05 10:22  修改内容:
 * </pre>
 */
public class TestApplication {
    public static void testClassPathXmlApplicationContext() {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath:applicationContext.xml");
        SpringBean  springBean = context.getBean(SpringBean.class);
        System.out.println(springBean);
    }
    public static void main(String[] args) {
        // 测试ClassPathXmlApplicationContext
        testClassPathXmlApplicationContext();
    }
}

在SpringBean 打断点，debug进行调试：找到#refresh方法

obtainFreshBeanFactory示例：

/**
 * Tell the subclass to refresh the internal bean factory.
 * @return the fresh BeanFactory instance
 * @see #refreshBeanFactory()
 * @see #getBeanFactory()
 */
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
    // 创建BeanFactory
    refreshBeanFactory();
    // 返回refreshBeanFactory方法创建好的BeanFactory
    ConfigurableListableBeanFactory beanFactory = getBeanFactory();
    if (logger.isDebugEnabled()) {
        logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
    }
    return beanFactory;
}

然后refreshBeanFactory是由哪个基类实现的？我们找来一张uml类图：

从图可以知道ClassPathXmlApplicationContext通过AbstractRefreshableApplicationContext实现AbstractApplicationContext，所以refreshBeanFactory是AbstractRefreshableApplicationContext#refreshBeanFactory
ok，所以主体的路线，就可以熟悉，用uml时序图表示：

refreshBeanFactory创建过程

refreshBeanFactory方法是BeanFactory的创建过程，接着跟
org.springframework.context.support.AbstractRefreshableApplicationContext#refreshBeanFactory代码如下（示例）：

/**
 * This implementation performs an actual refresh of this context's underlying
 * bean factory, shutting down the previous bean factory (if any) and
 * initializing a fresh bean factory for the next phase of the context's lifecycle.
 */
@Override
protected final void refreshBeanFactory() throws BeansException {
    // 先判断是否有BeanFactory
    if (hasBeanFactory()) {
        // destroy Beans
        destroyBeans();
        // 关闭BeanFactory
        closeBeanFactory();
    }
    try {
        // 实例化DefaultListableBeanFactory
        DefaultListableBeanFactory beanFactory = createBeanFactory();
        // 设置序列化id
        beanFactory.setSerializationId(getId());
        // 定义BeanFactory的一些属性（是否允许Bean覆盖，是否允许循环依赖）
        customizeBeanFactory(beanFactory);
        // 加载应用中的BeanDefinition
        loadBeanDefinitions(beanFactory);
        this.beanFactory = beanFactory;
    }
    catch (IOException ex) {
        throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
    }
}

{@link org.springframework.context.support.AbstractXmlApplicationContext#loadBeanDefinitions}

/**
 * Loads the bean definitions via an XmlBeanDefinitionReader.
 * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
 * @see #initBeanDefinitionReader
 * @see #loadBeanDefinitions
 */
@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
    // Create a new XmlBeanDefinitionReader for the given BeanFactory.
    // 给BeanFactory实例化一个XmlBeanDefinitionReader实例
    XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
    // Configure the bean definition reader with this context's
    // resource loading environment.
    beanDefinitionReader.setEnvironment(this.getEnvironment());
    beanDefinitionReader.setResourceLoader(this);
    beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
    // Allow a subclass to provide custom initialization of the reader,
    // then proceed with actually loading the bean definitions.
    // 给子类提供初始化XmlBeanDefinitionReader的模板方法
    initBeanDefinitionReader(beanDefinitionReader);
    // 重点在这，继续跟
    loadBeanDefinitions(beanDefinitionReader);
}
// org.springframework.context.support.AbstractXmlApplicationContext#loadBeanDefinitions(org.springframework.beans.factory.support.DefaultListableBeanFactory)
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
    Resource[] configResources = getConfigResources();
    if (configResources != null) {
        // 继续跟
        reader.loadBeanDefinitions(configResources);
    }
    String[] configLocations = getConfigLocations();
    if (configLocations != null) {
        reader.loadBeanDefinitions(configLocations);
    }
}

{@link org.springframework.beans.factory.support.AbstractBeanDefinitionReader#loadBeanDefinitions}

@Override
public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
    Assert.notNull(resources, "Resource array must not be null");
    int counter = 0;
    // 变量资源对象，加载BeanDefinition
    for (Resource resource : resources) {
        // 在这里，经过debug，调到XmlBeanDefinitionReader.loadBeanDefinitions(resource)
        counter += loadBeanDefinitions(resource);
    }
    // 返回统计数量，表示总共有多少个BeanDefinition
    return counter;
}

XmlBeanDefinitionReader类
{@link org.springframework.beans.factory.xml.XmlBeanDefinitionReader#loadBeanDefinitions}

@Override
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
    return loadBeanDefinitions(new EncodedResource(resource));
}
// 省略...
/**
 * Load bean definitions from the specified XML file.
 * @param encodedResource the resource descriptor for the XML file,
 * allowing to specify an encoding to use for parsing the file
 * @return the number of bean definitions found
 * @throws BeanDefinitionStoreException in case of loading or parsing errors
 */
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
    Assert.notNull(encodedResource, "EncodedResource must not be null");
    if (logger.isInfoEnabled()) {
        logger.info("Loading XML bean definitions from " + encodedResource);
    }
    // 用ThreadLocal存放资源对象，目的是保证线程安全
    Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
    if (currentResources == null) {
        currentResources = new HashSet<>(4);
        this.resourcesCurrentlyBeingLoaded.set(currentResources);
    }
    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);
            if (encodedResource.getEncoding() != null) {
                inputSource.setEncoding(encodedResource.getEncoding());
            }
            // 核心，往下跟
            return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
        }
        finally {
            inputStream.close();
        }
    }
    catch (IOException ex) {
        throw new BeanDefinitionStoreException(
                "IOException parsing XML document from " + encodedResource.getResource(), ex);
    }
    finally {
        currentResources.remove(encodedResource);
        if (currentResources.isEmpty()) {
            this.resourcesCurrentlyBeingLoaded.remove();
        }
    }
}
/**
 * Actually load bean definitions from the specified XML file.
 * @param inputSource the SAX InputSource to read from
 * @param resource the resource descriptor for the XML file
 * @return the number of bean definitions found
 * @throws BeanDefinitionStoreException in case of loading or parsing errors
 * @see #doLoadDocument
 * @see #registerBeanDefinitions
 */
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
        throws BeanDefinitionStoreException {
    try {
        // 读取xml信息，将xml信息保存在Document对象
        Document doc = doLoadDocument(inputSource, resource);
        // 封装BeanDefinitions并注册，继续解析Document对象，往下跟
        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);
    }
}
/**
 * Register the bean definitions contained in the given DOM document.
 * Called by {@code loadBeanDefinitions}.
 * <p>Creates a new instance of the parser class and invokes
 * {@code registerBeanDefinitions} on it.
 * @param doc the DOM document
 * @param resource the resource descriptor (for context information)
 * @return the number of bean definitions found
 * @throws BeanDefinitionStoreException in case of parsing errors
 * @see #loadBeanDefinitions
 * @see #setDocumentReaderClass
 * @see BeanDefinitionDocumentReader#registerBeanDefinitions
 */
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
    BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
    // 获取已有的BeanDefinition数量
    int countBefore = getRegistry().getBeanDefinitionCount();
    // 注册BeanDefinition，关注点：registerBeanDefinitions、createReaderContext
    documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
    // 返回新注册的BeanDefinition统计数量
    return getRegistry().getBeanDefinitionCount() - countBefore;
}

{@link org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader#registerBeanDefinitions}

/**
 * This implementation parses bean definitions according to the "spring-beans" XSD
 * (or DTD, historically).
 * <p>Opens a DOM Document; then initializes the default settings
 * specified at the {@code <beans/>} level; then parses the contained bean definitions.
 */
@Override
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
    this.readerContext = readerContext;
    logger.debug("Loading bean definitions");
    Element root = doc.getDocumentElement();
    // 往下跟，从xml根节点开始解析文件
    doRegisterBeanDefinitions(root);
}
/**
 * Register each bean definition within the given root {@code <beans/>} element.
 */
protected void doRegisterBeanDefinitions(Element root) {
    // Any nested <beans> elements will cause recursion in this method. In
    // order to propagate and preserve <beans> default-* attributes correctly,
    // keep track of the current (parent) delegate, which may be null. Create
    // the new (child) delegate with a reference to the parent for fallback purposes,
    // then ultimately reset this.delegate back to its original (parent) reference.
    // this behavior emulates a stack of delegates without actually necessitating one.
    BeanDefinitionParserDelegate parent = this.delegate;
    this.delegate = createDelegate(getReaderContext(), root, parent);
    if (this.delegate.isDefaultNamespace(root)) {
        // profile(环境校验)
        String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
        if (StringUtils.hasText(profileSpec)) {
            String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
                    profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            // 不是当前profile的，跳过
            if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
                if (logger.isInfoEnabled()) {
                    logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
                            "] not matching: " + getReaderContext().getResource());
                }
                return;
            }
        }
    }
    // preProcessXml模板方法
    preProcessXml(root);
    // 重点是parseBeanDefinitions
    parseBeanDefinitions(root, this.delegate);
    // postProcessXml 同样是模板方法 过
    postProcessXml(root);
    this.delegate = parent;
}
/**
 * Parse the elements at the root level in the document:
 * "import", "alias", "bean".
 * @param root the DOM root element of the document
 */
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
    if (delegate.isDefaultNamespace(root)) {
        NodeList nl = root.getChildNodes();
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            if (node instanceof Element) {
                Element ele = (Element) node;
                if (delegate.isDefaultNamespace(ele)) {
                    // 解析default namespace下面的元素
                    parseDefaultElement(ele, delegate);
                }
                else {
                    // 解析自定义标签元素
                    delegate.parseCustomElement(ele);
                }
            }
        }
    }
    else {
        delegate.parseCustomElement(root);
    }
}
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
    // import元素处理
    if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
        importBeanDefinitionResource(ele);
    }
    // alias元素处理
    else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
        processAliasRegistration(ele);
    }
    // bean元素处理
    else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
        processBeanDefinition(ele, delegate);
    }
    // 嵌套beans处理
    else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
        // recurse
        doRegisterBeanDefinitions(ele);
    }
}

bean元素处理，本文挑beana元素的处理进行讲述：

/**
 * Process the given bean element, parsing the bean definition
 * and registering it with the registry.
 */
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
    //解析Bean元素为BeanDefinition，再通过BeanDefinitionHolder封装成BeanDefinitionHolder对象
    BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
    if (bdHolder != null) {
        // 有自定义属性会进行相应的解析
        bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
        try {
            // Register the final decorated instance.
            // BeanDefinition的注册过程
            BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
        }
        catch (BeanDefinitionStoreException ex) {
            getReaderContext().error("Failed to register bean definition with name '" +
                    bdHolder.getBeanName() + "'", ele, ex);
        }
        // Send registration event.
        getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
    }
}

public static void registerBeanDefinition(
    BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
        throws BeanDefinitionStoreException {
    // Register bean definition under primary name.
    // 根据beanName注册BeanDefinition
    String beanName = definitionHolder.getBeanName();
    registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
    // Register aliases for bean name, if any.
    // 如果有别名，根据别名再注册一遍
    String[] aliases = definitionHolder.getAliases();
    if (aliases != null) {
        for (String alias : aliases) {
            registry.registerAlias(beanName, alias);
        }
    }
}