Spring使用xml启动源码解析

本文章出处Spring使用xml启动源码解析
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工程准备

• 引入Spring最小依赖

    
        UTF-8
        5.1.8.RELEASE

    

    
        
            junit
            junit
            4.12
            test
        

        
            org.springframework
            spring-context
            ${spring.version}
        

    

• applicationContext.xml配置

• Spring 启动代码

package org.ting.spring.study;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class App 
{
    public static void main( String[] args ){
        ApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");
        App myBean = (App) app.getBean("myBean");
        System.out.println(myBean);
    }
}

了解ClassPathXmlApplicationContext

ClassPathXmlApplicationContext结构图

这个类基本上都是构造函数

ClassPathXmlApplicationContext方法

可以看出，这个没有任何业务逻辑代码，都是通过继承抽象类扩展功能，主要业务都在父类中。下面我们看下构造方法是怎么初始化Spring容器的

public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
        this(new String[] {configLocation}, true, null);
    }

进入this构造方法

public ClassPathXmlApplicationContext(
            String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
            throws BeansException {
         //设置父容器
        super(parent);
        setConfigLocations(configLocations);
        if (refresh) {
            refresh();
        }
    }

可以看出，这个才是ClassPathXmlApplicatonContext实例化方法，设置父容器对象，添加配置文件路径到容器中，开始启动容器工作。
super(parent)方法依次调用服务父类AbstractXmlApplicationContext、AbstractRefreshableConfigApplicationContext、AbstractRefreshableApplicationContext、AbstractApplicationContext的构造方法。

    public AbstractApplicationContext(@Nullable ApplicationContext parent) {
        this();
        setParent(parent);
    }
@Override
    public void setParent(@Nullable ApplicationContext parent) {
        this.parent = parent;
        if (parent != null) {
            Environment parentEnvironment = parent.getEnvironment();
            if (parentEnvironment instanceof ConfigurableEnvironment) {
                getEnvironment().merge((ConfigurableEnvironment) parentEnvironment);
            }
        }
    }

这个方法主要是向上调用父类构造方法，一直到AbstractApplicationContext对象。参数不为空的话将父上下文对像Environment设置合并到当前容器中，将两个容器配置文件合并起来，一般在web环境有使用到。

setConfigLocations

    public void setConfigLocations(@Nullable String... locations) {
        if (locations != null) {
            Assert.noNullElements(locations, "Config locations must not be null");
            this.configLocations = new String[locations.length];
            for (int i = 0; i < locations.length; i++) {
          //处理路径，从环境变量中替换占位符 ${x}
                this.configLocations[i] = resolvePath(locations[i]).trim(); 
            }
        }
        else {
            this.configLocations = null;
        }
    }

将路径添加到AbstractRefreshableConfigApplicationContextconfigLocations 属性中，并且出来掉路径中环境变量占位符，保证configLocaiton得到是可用地址。

refresh解读

@Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            //准备刷新上下文对象
            prepareRefresh();

            // 创建Spring Factory对象，清空缓存，解析xml文件转换成BeanDefinition 保存到缓存中
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            //初始化BeanFactory 内部属性值，添加类型转换 ,前置处理器
            prepareBeanFactory(beanFactory);

            try {
                // 空方法，让字类实现查看或者修改beanFacory内部属性值
                postProcessBeanFactory(beanFactory);

                // 注册实例化BeanFactoryPostProcessor，并且执行所有类接口方法
                invokeBeanFactoryPostProcessors(beanFactory);

                //注册所有后置处理器
                registerBeanPostProcessors(beanFactory);

                //初始化messageSource bean
                initMessageSource();

                //初始化组播器
                initApplicationEventMulticaster();

                // 空方法，让字类自行初始化特别bean
                onRefresh();

                // 初始化所有事件监听器，加入组播器中，广播事件
                registerListeners();

                //启动所有非延迟单例bean 
                finishBeanFactoryInitialization(beanFactory);

                // 完成applicationContext初始化工作
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // 销毁已经创建成功bean
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                //清空所有类型注解扫描，反射等元数据集合
                resetCommonCaches();
            }
        }
    }

下面我们一个个方法解析

prepareRefresh

protected void prepareRefresh() {
        // Switch to active.
        this.startupDate = System.currentTimeMillis();
        this.closed.set(false);
        this.active.set(true);

        if (logger.isDebugEnabled()) {
            if (logger.isTraceEnabled()) {
                logger.trace("Refreshing " + this);
            }
            else {
                logger.debug("Refreshing " + getDisplayName());
            }
        }

        // 初始化PropertySource ，默认不做任何处理，让子类实现
        initPropertySources();

        // Validate that all properties marked as required are resolvable:
        // see ConfigurablePropertyResolver#setRequiredProperties
        getEnvironment().validateRequiredProperties();

        // Store pre-refresh ApplicationListeners...
        if (this.earlyApplicationListeners == null) {
            this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);
        }
        else {
            // Reset local application listeners to pre-refresh state.
            this.applicationListeners.clear();
            this.applicationListeners.addAll(this.earlyApplicationListeners);
        }

        // Allow for the collection of early ApplicationEvents,
        // to be published once the multicaster is available...
        this.earlyApplicationEvents = new LinkedHashSet<>();
    }

设置启动时间，将关闭标记设置false，活动开关设置true，initPropertySources(),本身是一个空方法，由子类自行去实现，让子类在容器创建之前修改ConfigurableEnvironment的PropertySources对象属性。PropertySourcesname/value键值对的封装接口，主要用来装配配置变量。虽然ClassPathXmlApplicatonContext和他的父类都没有实现这个方法，在Spring MVC中， GenericWebApplicationContext实现initPropertySources方法，将servletContext配置变量合并到ProertySources中。接着看getEnvironment().validateRequiredProperties()这个方法实现在

validateRequiredProperties

    @Override
    public void validateRequiredProperties() {
        MissingRequiredPropertiesException ex = new MissingRequiredPropertiesException();
        for (String key : this.requiredProperties) {
            if (this.getProperty(key) == null) {
                ex.addMissingRequiredProperty(key);
            }
        }
        if (!ex.getMissingRequiredProperties().isEmpty()) {
            throw ex;
        }
    }

主要校验ProertySources中key value，是否有不对应的情况。其实这个主要校验上一个initPropertySources()方法安全措施来的。比如老爸放手让自己儿子去做一件事，自己完全不干预，但是他会偷偷去查看事件做得怎么样？如果做不不行，狠狠打儿子一顿。

ConfigurableListableBeanFactory初始化

这部分应该是整个代码解析最核心的功能了，这个类就是我们经常说的Spring容器,Spring工厂的实现DefaultListableBeanFactory。主要负责bean注册实例化，bean查找，bean别名，bean销毁。这里实例化这个对象，也开始说明Spring生命周期正式开始。下面是 DefaultListableBeanFactory的继承关系

DefaultListableBeanFactory

查看obtainFreshBeanFactory方法具体内容

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        refreshBeanFactory();
        return getBeanFactory();
    }

refreshBeanFactory主要是清除ConfigurableListableBeanFactory bean缓存，重新实例化。

refreshBeanFactory

这个方法在AbstractRefreshableApplicationContext中

protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) { //判断ConfigurableListableBeanFactory 是否已经创建了 
            destroyBeans();
            closeBeanFactory();
        }
        try {
                        //使用构造函数实例化new DefaultListableBeanFactory（parent）
            DefaultListableBeanFactory beanFactory = createBeanFactory(); 
            beanFactory.setSerializationId(getId());
                //Spring 工厂自定义设置，是否运行已经注册bean被相同beanName覆盖，默认是true，设置是否允许bean之间的循环引用 - 并尝试自动解决它默认也是true
            customizeBeanFactory(beanFactory);
            //开始解析xml配置文件，注册BeanDefinition,下面具体分析
            loadBeanDefinitions(beanFactory);
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

先判断ConfigurableListableBeanFactory对象是否已经创建成功了，如果已经存在了，执行销毁Spring容器内已经注册bean销毁，重新实例化ConfigurableListableBeanFactory对象，设置id，设置beanFactory bean重名，依赖解决方式。开始加载xml配置文件，注册BeanDefinition，将已经实例化BeanFactory指引再指向this.beanFactory。

主要看下destroyBeans()

protected void destroyBeans() {
        getBeanFactory().destroySingletons();
    }

调用了ConfigurableListableBeanFactory的destroySingletons方法，接口的实现类是DefaultListableBeanFactory。

@Override
    public void destroySingletons() {
        super.destroySingletons();
        updateManualSingletonNames(Set::clear, set -> !set.isEmpty());
        clearByTypeCache();
    }

destroySingletons

super.destroySingletons()调用了DefaultSingletonBeanRegistry的destroySingletons方法

public void destroySingletons() {
        if (logger.isTraceEnabled()) {
            logger.trace("Destroying singletons in " + this);
        }
        synchronized (this.singletonObjects) {
            this.singletonsCurrentlyInDestruction = true;
        }

        String[] disposableBeanNames;
        synchronized (this.disposableBeans) { //将map key 转化成string 数组
            disposableBeanNames = StringUtils.toStringArray(this.disposableBeans.keySet());
        }
        for (int i = disposableBeanNames.length - 1; i >= 0; i--) {
            destroySingleton(disposableBeanNames[i]);
        }

        this.containedBeanMap.clear();
        this.dependentBeanMap.clear();
        this.dependenciesForBeanMap.clear();

        clearSingletonCache();
    }

destroySingleton

public void destroySingleton(String beanName) {
        // Remove a registered singleton of the given name, if any.
        removeSingleton(beanName);

        // Destroy the corresponding DisposableBean instance.
        DisposableBean disposableBean;
        synchronized (this.disposableBeans) {
            disposableBean = (DisposableBean) this.disposableBeans.remove(beanName);
        }
        destroyBean(beanName, disposableBean);
    }

查看removeSingleton方法

protected void removeSingleton(String beanName) {
        synchronized (this.singletonObjects) {
            this.singletonObjects.remove(beanName);
            this.singletonFactories.remove(beanName);
            this.earlySingletonObjects.remove(beanName);
            this.registeredSingletons.remove(beanName);
        }
    }

这几个分别就是内部容器来的，看下主要用途

    /**缓存单例bean对象 ： beanName 对应 bean对象 */
    private final Map singletonObjects = new ConcurrentHashMap<>(256);

    /**缓存单例工厂: beanName 对应 bean工厂实体. */
    private final Map> singletonFactories = new HashMap<>(16);

    /** 缓存bean早期对象，主要是bean注册的实体依赖对象: beanName 对应 实体对象 */
    private final Map earlySingletonObjects = new HashMap<>(16);

    /**已经成功注册beanName,按住注册顺序保存 */
    private final Set registeredSingletons = new LinkedHashSet<>(256);

destroyBean

    protected void destroyBean(String beanName, @Nullable DisposableBean bean) {
        // Trigger destruction of dependent beans first...
        Set dependencies;
        synchronized (this.dependentBeanMap) {
            // Within full synchronization in order to guarantee a disconnected Set
            dependencies = this.dependentBeanMap.remove(beanName);
        }
        if (dependencies != null) {
            if (logger.isTraceEnabled()) {
                logger.trace("Retrieved dependent beans for bean '" + beanName + "': " + dependencies);
            }
            for (String dependentBeanName : dependencies) {
                destroySingleton(dependentBeanName);
            }
        }

        // Actually destroy the bean now...
        if (bean != null) {
            try {
                bean.destroy();
            }
            catch (Throwable ex) {
                if (logger.isInfoEnabled()) {
                    logger.info("Destroy method on bean with name '" + beanName + "' threw an exception", ex);
                }
            }
        }

        // Trigger destruction of contained beans...
        Set containedBeans;
        synchronized (this.containedBeanMap) {
            // Within full synchronization in order to guarantee a disconnected Set
            containedBeans = this.containedBeanMap.remove(beanName);
        }
        if (containedBeans != null) {
            for (String containedBeanName : containedBeans) {
                destroySingleton(containedBeanName);
            }
        }

在删除DisposableBean实例的时候，获取bean下依赖实例，逐个移除。最后将Spring容器依赖实体逐个移除。接着看closeBeanFactory()办法

loadBeanDefinitions 解析

    @Override
    protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        // Create a new XmlBeanDefinitionReader for the given BeanFactory.
        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

        // Configure the bean definition reader with this context's
        // resource loading environment.
        beanDefinitionReader.setEnvironment(this.getEnvironment());
        beanDefinitionReader.setResourceLoader(this);
         //xml  文件约束校验
        beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

        // Allow a subclass to provide custom initialization of the reader,
        // 设置xml校验方式
        initBeanDefinitionReader(beanDefinitionReader);
                //开始解析xml转化BeanDefinition
        loadBeanDefinitions(beanDefinitionReader);
    }

实例化XmlBeanDefinitionReader将资源文件xml解析成BeanDefinition，将Spring上下文对象赋值给对象属性。loadBeanDefinitions会将xml文件转化成Document对象，由于这部分太过繁琐，需要根据文件名获取ResouceLoader，再获取到文件流对象，解析xml成Document，觉得省略这些代码解析，主要放在Document如何转化成BeanDefinition，在DefaultBeanDefinitionDocumentReader实现。

doRegisterBeanDefinitions

    protected void doRegisterBeanDefinitions(Element root) {
        // Any nested  elements will cause recursion in this method. In
        // order to propagate and preserve  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)) {
            String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
                    //判断文件profile属性，转化成array
            if (StringUtils.hasText(profileSpec)) {
                String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
                        profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
                // We cannot use Profiles.of(...) since profile expressions are not supported
                // in XML config. See SPR-12458 for details.
                if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
                    if (logger.isDebugEnabled()) {
                        logger.debug("Skipped XML bean definition file due to specified profiles [" + profileSpec +
                                "] not matching: " + getReaderContext().getResource());
                    }
                    return;
                }
            }
        }
        //运行自定义实现类 预处理xml文件，空方法，让子类实现。
        preProcessXml(root);
        parseBeanDefinitions(root, this.delegate);
       //运行自定义自主实心方法，修改xml属性，空方法。  
        postProcessXml(root);

        this.delegate = parent;
    }

BeanDefinitionParserDelegate主要是解析Document命名空间，标签元素，属性，将xml标签转化成对象。

prseBeanDefinitions

    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)) {
                                              //注册BeanDefinition
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }

循环遍历beans的子标签，符合"import", "alias", "bean"命名空间，进入标签解析环节

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
            importBeanDefinitionResource(ele);
        }
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
            processAliasRegistration(ele);
        }
        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
            processBeanDefinition(ele, delegate);
        }
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }

主要看下bean标签解析

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if (bdHolder != null) {
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
            try {
                // Register the final decorated instance.
                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));
        }
    }

这个方法将xml解析返回 BeanDefinition对象，如果对象不为空，继续解析元素的自定义属性，并将元素自定义属性设置给刚刚创建BeanDefiniton对象，最后广播注册BeanDefinition对象。关于如何生成BeanDefiniton对象，这个方法我不具体深入了解了，毕竟里面篇幅很多的，以后会专门出一个章节去说明。

prepareBeanFactory 方法解析

prepareBeanFactory方法主要是配置BeanFactory 类加载器，Spring el表达式实现，bean前置处理器等等的属性设置，主要逻辑详看下面的代码

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        // Tell the internal bean factory to use the context's class loader etc.
    //设置工厂类加载器  beanFactory.setBeanClassLoader(getClassLoader());
        //设置Spring el表达式实现 
        beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
       //添加属性转化器
        beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));
        /*
         * 添加后置处理器实现, 如果实现了相应的 Aware 接口，则注入对应的资源
         * 1. EnvironmentAware
         * 2. EmbeddedValueResolverAware
         * 3. ResourceLoaderAware
         * 4. ApplicationEventPublisherAware
         * 5. MessageSourceAware
         * 6. ApplicationContextAware
         */
        beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
          //忽略 这些类自动注入，与上面后置处理器对应
        beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
        beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
        beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
        beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
        beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
        beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

              //将Spring 内部对象缓存起来，注册到容器内部。
        beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
        beanFactory.registerResolvableDependency(ResourceLoader.class, this);
        beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
        beanFactory.registerResolvableDependency(ApplicationContext.class, this);

        // 注册 事件监听器前置处理
        beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

        // 判断存在AOP bean name 则注册aop前置处理器
        if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
            beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
            // Set a temporary ClassLoader for type matching.
            beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
        }

        // 注册 默认environment beans.
        if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
                //向Spring容器注册bean
            beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
        }
        if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) { //注册系统环境变量
            beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
        }
        if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
            beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());0

        }
    }

BeanPostProcessor

addBeanPostProcessor方法主要添加BeanPostProcessor接口实现类，Bean 的后置处理器，主要是在 bean 初始化前后进行一些处理工作,spring bean 创建委派给个大后置处理器创建。

public interface BeanPostProcessor {

    /**
     * bean 在创建之前，对bean进行处理，将处理过实体返回给BeanFactory容器
     */
    @Nullable
    default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

    /**
     * bean创建之后，对bean进行处理，相当于给已经创建的进行功能加强
     */
    @Nullable
    default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

}

ApplicationContextAwareProcessor

    @Override
    @Nullable
    public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
        AccessControlContext acc = null;

        if (System.getSecurityManager() != null &&
                (bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
                        bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
                        bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {
            acc = this.applicationContext.getBeanFactory().getAccessControlContext();
        }

        if (acc != null) {
            AccessController.doPrivileged((PrivilegedAction