一、问题剖析

1.spring容器的启动过程，启动期间都做了什么？什么时候创立单实例bean？
2.ioc是如何创立这些单实例bean，是如何治理的？保留在哪里？

二、Debug剖析

ClassPathXmlApplicationContext的继承树

1.创立spring容器，读取配置文件，类门路下加载配置文件

    ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");

2.最先加载的应该是动态初始化块，它会随着类的加载而加载

static {
//在WebLogic 8.1中，在应用程序敞开时，紧急加载ContextCLosedEvent类以防止奇怪的类加载器问题。
    ContextClosedEvent.class.getName();
}

3.调用ClassPathXmlApplicationContext的构造方法

/**
 * 创立一个新的classPathXmlApplicationcontext，从给定的XML文件加载定义，并主动刷新该ontext。
 * @param applicationContext.xml
 * 如果上下文创立失败，@抛出BeansException
 */
public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
//调用类中其余结构
    this(new String[] {configLocation}, true, null);
}

4.参数解析

/**
 * 创立一个新的classPathXmlApplicationContextwith给定的父类，从给定的XML文件加载定义。
 * @param configLocations 资源地位数组
 * @param refresh 是否主动刷新上下文
 * 加载所有bean定义并创立所有单例。或者，在进一步的configuri之后手动调用刷新康泰克斯的
 * @param 父上下文后手动调用刷新
 * 如果上下文创立失败，@抛出BeansException
 * @see #refresh()
 */

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent) throws BeansException {
    // 1.初始化父类
    super(parent);
    // 2.设置本地的配置信息
    setConfigLocations(configLocations);
    // 3.实现Spring容器的初始化
    if (refresh) { //true 进入refresh办法
        refresh(); //所有单实例创立实现
    }
}

5.refresh()解析

synchronized (this.startupShutdownMonitor) {
    // 筹备此上下文以进行刷新
    prepareRefresh();

    // spring解析xml配置文件，将要创立的所有bean 的配置信息保存起来
    ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

5.1 ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory()解析

5.1.1 obtainFreshBeanFactory();

    protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        refreshBeanFactory(); //刷新beanFactory
        return getBeanFactory();
    }

5.1.2 refreshBeanFactory();

//对上下文的底层bean工厂执行一次理论刷新，敞开上一个bean工厂(如果有的话)和为上下文生命周期的下一阶段初始化afreshbean工厂。
if (hasBeanFactory()) { //判断是否曾经创立beanFactory 如果存在beanFactory则销毁并敞开
    destroyBeans();
    closeBeanFactory();
}
try {
    DefaultListableBeanFactory beanFactory = createBeanFactory();//创立一个新的beanFactory
    beanFactory.setSerializationId(getId());
    customizeBeanFactory(beanFactory);
    loadBeanDefinitions(beanFactory);//通过beanFactory加载beanDefinition
    synchronized (this.beanFactoryMonitor) {
        this.beanFactory = beanFactory;//对beanFactory进行从新赋值。
    }
}
catch (IOException ex) {
    throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}

5.1.3 createBeanFactory()

protected DefaultListableBeanFactory createBeanFactory() {//传入这个context的父context外部的beanFactory
   
return new DefaultListableBeanFactory(getInternalParentBeanFactory());//由此可见这个context外部的beanFacotry

5.2 解析完xml后

    // 筹备在此上下文中应用的bean工厂
    prepareBeanFactory(beanFactory);

    try {
        // 容许在上下文子类中对bean工厂进行后处理
        postProcessBeanFactory(beanFactory);

        // 调用上下文中注册为bean的工厂处理器
        invokeBeanFactoryPostProcessors(beanFactory);

        // 注册拦挡bean创立的bean处理器
        registerBeanPostProcessors(beanFactory);

        // 初始化消息来源,用来反对国际化性能
        initMessageSource();

        // 初始化多事件派发器
        initApplicationEventMulticaster();

        // 初始化特定上下文子类中的其余专门化bean，专门留给子类的办法
        onRefresh();

        // 查看侦听器bean并注册它们注册监听器
        registerListeners();
        
        // 实例化所有残余的(非提早-init)单例
        finishBeanFactoryInitialization(beanFactory);

5.3 finishBeanFactoryInitialization(beanFactory)实现单实例bean创立;

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // 初始化类型转换的组件服务 （次要用于自定义类型转化）
    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));
    }
    //如果没有bean前期解决
    //(例如PropertyPlaceholderConfigurer bean)之前注册任何:
    //此时，次要用于解析正文属性值。
    if (!beanFactory.hasEmbeddedValueResolver()) {
        beanFactory.addEmbeddedValueResolver(strVal ->   getEnvironment().resolvePlaceholders(strVal));
    }

    // 初始化加载时的Autowire
    String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    for (String weaverAwareName : weaverAwareNames) {
        getBean(weaverAwareName);
    }

    //停止使用长期类加载器进行类型匹配
    beanFactory.setTempClassLoader(null);
    
    //容许缓存所有bean定义元数据，不冀望进一步更改
    beanFactory.freezeConfiguration();

    //实例化所有残余的(非提早-init)单单实例bean
    beanFactory.preInstantiateSingletons();
}

beanFactory.preInstantiateSingletons()解析

//日志记录
    if (logger.isTraceEnabled()) {
        logger.trace("Pre-instantiating singletons in " + this);
    }

    //迭代一个正本以容许init办法注册新的bean定义。
    //尽管这可能不是惯例工厂疏导程序的一部分，但它在其余方面工作得很好。
    //beanDefinitionNames 所有定义bean的名字
    List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

    // 按程序初始化所有非懒加载的单实例bean
    for (String beanName : beanNames) {
        //依据bean的id获取到bean的具体定义信息
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        //判断bean不是一个形象的、非懒加载的单实例bean，所有这是ioc中bean的默认个性
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            //如果该bean是工厂bean，实现了FactoryBean接口的bean，就进入判断
            if (isFactoryBean(beanName)) { 
                Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                if (bean instanceof FactoryBean) {
                    final FactoryBean<?> factory = (FactoryBean<?>) bean;
                    boolean isEagerInit;
                    if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                        isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
                                        ((SmartFactoryBean<?>) factory)::isEagerInit,
                                getAccessControlContext());
                    }
                    else {
                        isEagerInit = (factory instanceof SmartFactoryBean &&
                                ((SmartFactoryBean<?>) factory).isEagerInit());
                    }
                    if (isEagerInit) {
                        getBean(beanName);
                    }
                }
            }
            else {
                getBean(beanName); //因为我并没有应用工厂创立bean，所以来到这行
            }
        }
    }

getBean(beanName):bean创立的细节

    @Override
    public Object getBean(String name) throws BeansException {
        //重写BeanFactory的办法
        return doGetBean(name, null, null, false); //调用了doGetBean接口
    }

doGetBean(name, null, null, false)：bean创立的细节（重点）

参数

    protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
            @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
        //先拿到要创立的bean名称
        final String beanName = transformedBeanName(name);
        Object bean;

        // 从曾经注册的单实例bean外面查看有没有这个bean
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) { //缓存里有则走上面的代码
            if (logger.isTraceEnabled()) {
                if (isSingletonCurrentlyInCreation(beanName)) {
                    logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
                            "' that is not fully initialized yet - a consequence of a circular reference");
                }
                else {
                    logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
                }
            }
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else { //第一次创立bean是没有的，所有走到这
            //如果咱们曾经创立了这个bean实例，则会失败:咱们假如在循环援用中。
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            //查看父工厂中是否存在bean定义。
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // Not found -> check parent.
                String nameToLookup = originalBeanName(name);
                if (parentBeanFactory instanceof AbstractBeanFactory) {
                    return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                            nameToLookup, requiredType, args, typeCheckOnly);
                }
                else if (args != null) {
                    // Delegation to parent with explicit args.
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else if (requiredType != null) {
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
                else {
                    return (T) parentBeanFactory.getBean(nameToLookup);
                }
            }

            if (!typeCheckOnly) { //typeCheckOnly false
                //标记bean的状态为曾经被创立，为了避免多线程并发创立bean
                markBeanAsCreated(beanName); 
            }

            try {
                //获取合并后的bean的定义信息
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                //拿到以后bean创立之前所须要提前创立创立的bean，depends-on属性，调用依赖关系设置，如果有此属性就循环创立deponds-on属性依赖的bean
                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    for (String dep : dependsOn) {
                        if (isDependent(beanName, dep)) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                        }
                        registerDependentBean(dep, beanName);
                        try {
                            getBean(dep);
                        }
                        catch (NoSuchBeanDefinitionException ex) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                        }
                    }
                }

                // 如果是单例，创立bean
                if (mbd.isSingleton()) {
                // getSington办法里创立bean
                    sharedInstance = getSingleton(beanName, () -> {
                        try {
                        //bean的创立次要办法
                            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);
                }

getSingleton(beanName, () -> {} 细节

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "Bean name must not be null");
    synchronized (this.singletonObjects) {
        //先从单实例对象中将这些beanget进去
        Object singletonObject = this.singletonObjects.get(beanName);
        

singletonObjects是一个hashMap

//依照对象的名字和对象实例缓存所有单实例bean
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

        if (singletonObject == null) { //第一次创立为null。进入上面的办法
            if (this.singletonsCurrentlyInDestruction) {
                throw new BeanCreationNotAllowedException(beanName,
                        "Singleton bean creation not allowed while 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 newSingleton = false;
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet<>();
            }
            try {
                //在这里利用了反射创立了单实例bean
                singletonObject = singletonFactory.getObject();             
                newSingleton = true;
            }
            catch (IllegalStateException ex) {
                //让单例对象同时隐式地呈现
                //如果是，持续，因为异样表明状态。
>               singletonObject = this.singletonObjects.get(beanName);
                if (singletonObject == null) {
                    throw ex;
                }
            }
            catch (BeanCreationException ex) {
                if (recordSuppressedExceptions) {
                    for (Exception suppressedException : this.suppressedExceptions) {
                        ex.addRelatedCause(suppressedException);
                    }
                }
                throw ex;
            }
            finally {
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = null;
                }
                afterSingletonCreation(beanName);
            }
            if (newSingleton) { 
                //创立完单实例bean后调用此办法
                addSingleton(beanName, singletonObject);
                

addSingleton(beanName, singletonObject)细节

    protected void addSingleton(String beanName, Object singletonObject) {
        synchronized (this.singletonObjects) { 
        //singletonObjects 之前创立的map汇合，将bean对象退出到map中
        //所以，创立好的对象最终会保留在这个map中
            this.singletonObjects.put(beanName, singletonObject);
            this.singletonFactories.remove(beanName);
            this.earlySingletonObjects.remove(beanName);
            this.registeredSingletons.add(beanName);
        }
    }

            }
        }
        return singletonObject;
    }
}

                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 name '" + scopeName + "'");
                    }
                    try {
                        Object scopedInstance = scope.get(beanName, () -> {
                            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 && !requiredType.isInstance(bean)) {
            try {
                T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
                if (convertedBean == null) {
                    throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
                }
                return convertedBean;
            }
            catch (TypeMismatchException ex) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Failed to convert bean '" + name + "' to required type '" +
                            ClassUtils.getQualifiedName(requiredType) + "'", ex);
                }
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
        }
        return (T) bean;
    }

    // Trigger post-initialization callback for all applicable beans...
    for (String beanName : beanNames) {
        Object singletonInstance = getSingleton(beanName);
        if (singletonInstance instanceof SmartInitializingSingleton) {
            final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                    smartSingleton.afterSingletonsInstantiated();
                    return null;
                }, getAccessControlContext());
            }
            else {
                smartSingleton.afterSingletonsInstantiated();
            }
        }
    }
}

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

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

        // Destroy already created singletons to avoid dangling resources.
        destroyBeans();

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

        // Propagate exception to caller.
        throw ex;
    }

    finally {
        // Reset common introspection caches in Spring's core, since we
        // might not ever need metadata for singleton beans anymore...
        resetCommonCaches();
    }
}

总结

最终单实例bean对象保留DefaultSingletonBeanRegistry类中的singletonObjects属性
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);