[Re] Spring-4(SourceCode)

1. 引入

• IOC
  • IOC 是一个容器
  • 容器启动的时候创建所有单实例对象
  • 我们可以直接从容器中获取到这个对象
• Spring IOC
  • IOC 容器的启动过程？启动期间都做了什么（什么时候创建所有单实例bean）
  • IOC 是如何创建这些单实例 bean，并如何管理的，到底保存在了那里？

2. ClassPathXMLApplicationContext

public ClassPathXmlApplicationContext(String[] configLocations,
        boolean refresh, ApplicationContext parent) throws BeansException {
    super(parent);
    setConfigLocations(configLocations);
    if (refresh) {
        refresh(); // 所有单实例 bean 创建完成
    }
}

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

3. AbstractApplicationContext

@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();

        // Tell the subclass to refresh the internal bean factory.
        // Spring 解析 XML 配置文件，将要创建的所有 bean 的配置信息保存起来
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Prepare the bean factory for use in this context.
        prepareBeanFactory(beanFactory);

        try {
            // Allows post-processing of the bean factory in context subclasses.
            postProcessBeanFactory(beanFactory);

            // Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);

            // Register bean processors that intercept bean creation.
            // 注册Spring内置的bean后置处理器
            registerBeanPostProcessors(beanFactory);

            // Initialize message source for this context.
            // 用来支持国际化功能的
            initMessageSource();

            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses.
            // 留给子类的
            onRefresh();

            // Check for listener beans and register them.
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons.
            // 初始化所有单实例 bean ====> Step Into
            finishBeanFactoryInitialization(beanFactory);

            // Last step: publish corresponding event.
            finishRefresh();

        } catch (BeansException ex) {
            // Destroy already created singletons to avoid dangling resources.
            destroyBeans();

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

            // Propagate exception to caller.
            throw ex;
        }
    }
}

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // Initialize conversion service for this context.
    if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
        beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
        beanFactory.setConversionService(
        beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
    }

    // Initialize LoadTimeWeaverAware beans early
    // to allow for registering their transformers early.
    String[] weaverAwareNames = beanFactory
            .getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    for (String weaverAwareName : weaverAwareNames) {
        getBean(weaverAwareName);
    }

    // Stop using the temporary ClassLoader for type matching.
    beanFactory.setTempClassLoader(null);

    // Allow for caching all bean definition metadata, not expecting further changes.
    beanFactory.freezeConfiguration();

    // Instantiate all remaining (non-lazy-init) singletons.
    // 初始化所有单实例 bean ====> Step Into
    beanFactory.preInstantiateSingletons();
}

4. DefaultListableBeanFactory

@Override
public void preInstantiateSingletons() throws BeansException {
    if (this.logger.isDebugEnabled()) {
        this.logger.debug("Pre-instantiating singletons in " + this);
    }
    // 所有要创建的 bean 的 id
    List<String> beanNames;
    synchronized (this.beanDefinitionMap) {
        // Iterate over a copy to allow for init methods which in turn register new bean definitions.
        // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
        beanNames = new ArrayList<String>(this.beanDefinitionNames);
    }
    // 按顺序创建 bean
    for (String beanName : beanNames) {
        // 根据 beanId 获取到 bean 的定义信息
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        // 如果 bean (不是抽象的 && bean 是单例的 && 不是懒加载的)
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            // 如果是工厂 bean(实现了 FactoryBean 接口的 bean)
            if (isFactoryBean(beanName)) {
                final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
                boolean isEagerInit;
                if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                    isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
                            @Override
                            public Boolean run() {
                                return ((SmartFactoryBean<?>) factory).isEagerInit();
                            }
                        }, getAccessControlContext());
                } else {
                    isEagerInit = (factory instanceof SmartFactoryBean &&
                            ((SmartFactoryBean<?>) factory).isEagerInit());
                }
                if (isEagerInit) {
                    getBean(beanName);
                }
            } else {
                getBean(beanName); // core！！！
            }
        }
    }
}

5. AbstractBeanFactory

@Override
public Object getBean(String name) throws BeansException {
    return doGetBean(name, null, null, false);
}

protected <T> T doGetBean(final String name, final Class<T> requiredType
        , final Object[] args, boolean typeCheckOnly) throws BeansException {

    final String beanName = transformedBeanName(name);
    Object bean;

    // 先从已经注册的所有单实例 bean 中看有没有这个 bean (第一次肯定没有)
    // Eagerly check singleton cache for manually registered singletons.
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        if (logger.isDebugEnabled()) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                logger.debug("Returning eagerly cached instance of singleton bean '" + beanName
                    + "' that is not fully initialized yet - a consequence of a circular reference");
            } else {
                logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
            }
        }
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else {
        // Fail if we're already creating this bean instance:
        // We're assumably within a circular reference.
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }

        // Check if bean definition exists in this factory.
        BeanFactory parentBeanFactory = getParentBeanFactory();
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            // Not found -> check parent.
            String nameToLookup = originalBeanName(name);
            if (args != null) {
                // Delegation to parent with explicit args.
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            }
            else {
                // No args -> delegate to standard getBean method.
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
        }

        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);
        }

        try {
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);

            // Guarantee initialization of beans that the current bean depends on.
            // 拿到创建当前 bean 之前需要提前创建的 bean → depends-on 属性
            String[] dependsOn = mbd.getDependsOn();
            // 如果有，就循环创建
            if (dependsOn != null) {
                for (String dependsOnBean : dependsOn) {
                    if (isDependent(beanName, dependsOnBean)) {
                        throw new BeanCreationException("Circular depends-on relationship between '" +
                        beanName + "' and '" + dependsOnBean + "'");
                    }
                    registerDependentBean(dependsOnBean, beanName);
                    getBean(dependsOnBean); // 创建
                }
            }

            // Create bean instance.
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                    @Override
                    public Object getObject() throws BeansException {
                        try {
                            return createBean(beanName, mbd, args);
                        }
                        catch (BeansException ex) {
                            // Explicitly remove instance from singleton cache: It might have been put there
                            // eagerly by the creation process, to allow for circular reference resolution.
                            // Also remove any beans that received a temporary reference to the bean.
                            destroySingleton(beanName);
                            throw ex;
                        }
                    }
                });
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }

            else if (mbd.isPrototype()) {
                // It's a prototype -> create a new instance.
                Object prototypeInstance = null;
                try {
                    beforePrototypeCreation(beanName);
                    prototypeInstance = createBean(beanName, mbd, args);
                }
                finally {
                    afterPrototypeCreation(beanName);
                }
                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
            }

            else {
                String scopeName = mbd.getScope();
                final Scope scope = this.scopes.get(scopeName);
                if (scope == null) {
                    throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
                }
                try {
                    Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
                        @Override
                        public Object getObject() throws BeansException {
                            beforePrototypeCreation(beanName);
                            try {
                                return createBean(beanName, mbd, args);
                            }
                            finally {
                                afterPrototypeCreation(beanName);
                            }
                        }
                    });
                    bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                } catch (IllegalStateException ex) {
                    throw new BeanCreationException(beanName, "Scope '" + scopeName
                        + "' is not active for the current thread; consider defining a scoped "
                        + "proxy for this bean if you intend to refer to it from a singleton", ex);
                }
            }
        } 
        catch (BeansException ex) {
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
    }

    // Check if required type matches the type of the actual bean instance.
    if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
        try {
            return getTypeConverter().convertIfNecessary(bean, requiredType);
        }
        catch (TypeMismatchException ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Failed to convert bean '" + name + "' to required type [" +
                ClassUtils.getQualifiedName(requiredType) + "]", ex);
            }
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
    }
    return (T) bean;
}

6. DefaultSingletonBeanRegistry

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "'beanName' must not be null");
    synchronized (this.singletonObjects) {
        // 先从一个地方把这个 bean 拿出来
        /** Cache of singleton objects: bean name --> bean instance */
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            if (this.singletonsCurrentlyInDestruction) {
                throw new BeanCreationNotAllowedException(beanName,
                "Singleton bean creation not allowed while the singletons of this factory are in destruction " +
                "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
            }
            beforeSingletonCreation(beanName);
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet<Exception>();
            }
            try {
                // bean 创建出来了！！！
                singletonObject = singletonFactory.getObject();
            } catch (BeanCreationException ex) {
                if (recordSuppressedExceptions) {
                    for (Exception suppressedException : this.suppressedExceptions) {
                        ex.addRelatedCause(suppressedException);
                    }
                }
                throw ex;
            } finally {
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = null;
                }
                afterSingletonCreation(beanName);
            }
            // 把 bean 对象加入 singletonObjects 这个 Map 中
            addSingleton(beanName, singletonObject);
        }
        return (singletonObject != NULL_OBJECT ? singletonObject : null);
    }
}

• 创建好的对象最终会保存在一个 Map 中。IOC 容器之一：保存单实例 bean 的 ConcurrentHashMap，名为 singletonObjects
  
• getBean 的时候，如果是单实例 bean，直接找 singletonObjects 这个 Map 要
  

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7. BeanFactory 和 ApplicationContext 区别

• BeanFactory 是最底层的接口，ApplicationContext 是留给程序员使用的 IOC 容器接口；ApplicationContext 是 BeanFactory 的子接口。
• BeanFactory（对象创建）
  • 是 bean 工厂接口，负责创建 bean 实例
  • 容器中保存所有单例 bean 的其实是一个 Map
• ApplicationContext（对象之间的动态装配、管理）
  • 是 IOC 容器接口，更多的负责容器功能的实现
  • 可以基于 BeanFactory 创建好的 bean 之上，完成强大的容器功能
  • 容器从 Map 中获取这个 bean，继而进行 DI，AOP 等操作
• Spring 里面最大的模式就是【工厂模式】。BeanFactory 如字面意思，就是一个 bean 工厂，根据用户提供的配置信息，创建 bean。

8. IOC 容器创建和销毁的时机

• 项目启动：IOC 容器创建
• 项目关闭：IOC 容器销毁

ContextLoaderListener

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    public ContextLoaderListener() {}

    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }

    @Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
    }

    @Override
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }
}

ContextLoader

/**
 * The root WebApplicationContext instance that this loader manages.
 * WebApplicationContext extends ApplicationContext
 */
private WebApplicationContext context;

public static WebApplicationContext getCurrentWebApplicationContext() {
    ClassLoader ccl = Thread.currentThread().getContextClassLoader();
    if (ccl != null) {
        WebApplicationContext ccpt = currentContextPerThread.get(ccl);
        if (ccpt != null) {
            return ccpt;
        }
    }
    return currentContext;
}

/**
 * Initialize Spring's web application context for the given servlet context,
 * using the application context provided at construction time, or creating a new one
 */
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
    if (servletContext.getAttribute(WebApplicationContext
            .ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
        throw new IllegalStateException(...);
    }

    Log logger = LogFactory.getLog(ContextLoader.class);
    servletContext.log("Initializing Spring root WebApplicationContext");
    if (logger.isInfoEnabled()) {
        logger.info("Root WebApplicationContext: initialization started");
    }
    long startTime = System.currentTimeMillis();

    // 包裹如下代码的 try...catch 已被省略
    // Store context in local instance variable, to guarantee that
    // it is available on ServletContext shutdown.
    if (this.context == null) {
        this.context = createWebApplicationContext(servletContext);
    }
    if (this.context instanceof ConfigurableWebApplicationContext) {
        ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
        if (!cwac.isActive()) {
            // The context has not yet been refreshed -> provide services such as
            // setting the parent context, setting the application context id, etc
            if (cwac.getParent() == null) {
                // The context instance was injected without an explicit parent ->
                // determine parent for root web application context, if any.
                ApplicationContext parent = loadParentContext(servletContext);
                cwac.setParent(parent);
            }
            configureAndRefreshWebApplicationContext(cwac, servletContext);
        }
    }
    // 把最先加载的容器设置为根容器（也就是 SpringIOC 容器）放置到servletContext中
    servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

    ClassLoader ccl = Thread.currentThread().getContextClassLoader();
    if (ccl == ContextLoader.class.getClassLoader()) {
        currentContext = this.context;
    } else if (ccl != null) {
        currentContextPerThread.put(ccl, this.context);
    }

    if (logger.isDebugEnabled()) {
        logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" +
        WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
    }
    if (logger.isInfoEnabled()) {
        long elapsedTime = System.currentTimeMillis() - startTime;
        logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
    }

    return this.context;
}