@override
    public void refresh() throws beansexception, illegalstateexception {
        synchronized (this.startupshutdownmonitor) {
            //1、 prepare this context for refreshing.
            preparerefresh();

               //创建defaultlistablebeanfactory（真正生产和管理bean的容器）
               //加载beandefition并注册到beandefitionregistry
               //通过namespacehandler解析自定义标签的功能（比如:context标签、aop标签、tx标签）
            //2、 tell the subclass to refresh the internal bean factory.
            configurablelistablebeanfactory beanfactory = obtainfreshbeanfactory();

            //3、 prepare the bean factory for use in this context.
            preparebeanfactory(beanfactory);

            try {
                //4、 allows post-processing of the bean factory in context subclasses.
                postprocessbeanfactory(beanfactory);

                     //实例化并调用实现了beanfactorypostprocessor接口的bean
                     //比如：propertyplaceholderconfigurer（context:property-placeholer）
                     //就是此处被调用的，作用是替换掉beandefinition中的占位符（${}）中的内容
                //5、 invoke factory processors registered as beans in the context.
                invokebeanfactorypostprocessors(beanfactory);

                     //创建并注册beanpostprocessor到beanfactory中（bean的后置处理器）
                     //比如：autowiredannotationbeanpostprocessor（实现@autowired注解功能）
                     //      requiredannotationbeanpostprocessor（实现@d注解功能）
                     //这些注册的beanpostprocessor
                //6、 register bean processors that intercept bean creation.
                registerbeanpostprocessors(beanfactory);

                //7、 initialize message source for this context.
                initmessagesource();

                //8、 initialize event multicaster for this context.
                initapplicationeventmulticaster();

                //9、 initialize other special beans in specific context subclasses.
                onrefresh();

                //10、 check for listener beans and register them.
                registerlisteners();

                     //创建非懒加载方式的单例bean实例（未设置属性）
                     //填充属性
                     //初始化实例（比如调用init-method方法）
                     //调用beanpostprocessor（后置处理器）对实例bean进行后置处理
                //11、 instantiate all remaining (non-lazy-init) singletons.
                finishbeanfactoryinitialization(beanfactory);

                //12、 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();
            }
        }
    }

    protected configurablelistablebeanfactory obtainfreshbeanfactory() {
        //使用的委派模式，调用2个抽象方法，定义了obtainfreshbeanfactory的算法骨架，实际的行为交给了子类abstractrefreshableapplicationcontext实现
        refreshbeanfactory();
        configurablelistablebeanfactory beanfactory = getbeanfactory();
        if (logger.isdebugenabled()) {
            logger.debug("bean factory for " + getdisplayname() + ": " + beanfactory);
        }
        return beanfactory;
    }

    /**
     * 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 {
         //若有容器，销毁容器中的bean，关闭容器，以此保证refresh()之后使用的是新建立起来的ioc容器
        if (hasbeanfactory()) {
            destroybeans();
            closebeanfactory();
        }
        try {
            //创建ioc容器
            defaultlistablebeanfactory beanfactory = createbeanfactory();
            beanfactory.setserializationid(getid());
            customizebeanfactory(beanfactory);
            //调用加载bean定义的方法，使用了委派模式，在当前类中定义了抽象的loadbeandefinitions方法，具体实现交给子类
            loadbeandefinitions(beanfactory);
            synchronized (this.beanfactorymonitor) {
                this.beanfactory = beanfactory;
            }
        }
        catch (ioexception ex) {
            throw new applicationcontextexception("i/o error parsing bean definition source for " + getdisplayname(), ex);
        }
    }st

    /**
     * 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 {
        // 为给定的bean工厂创建一个新的xmlbeanfinitionreader
        xmlbeandefinitionreader beandefinitionreader = new xmlbeandefinitionreader(beanfactory);
        //使用此上下文的bean定义读取器配置
        // 资源加载环境
        beandefinitionreader.setenvironment(this.getenvironment());
        beandefinitionreader.setresourceloader(this);
        beandefinitionreader.setentityresolver(new resourceentityresolver(this));

        // 允许子类提供读取器的自定义初始化，然后继续实际加载bean定义
        initbeandefinitionreader(beandefinitionreader);
        loadbeandefinitions(beandefinitionreader);
    }

    /**
     * create new xmlbeandefinitionreader for the given bean factory.
     * @param registry the beanfactory to load bean definitions into,
     * in the form of a beandefinitionregistry
     */
    public xmlbeandefinitionreader(beandefinitionregistry registry) {
        super(registry);
    }

    /**
     * create a new abstractbeandefinitionreader for the given bean factory.
     * <p>if the passed-in bean factory does not only implement the beandefinitionregistry
     * interface but also the resourceloader interface, it will be used as default
     * resourceloader as well. this will usually be the case for
     * {@link org.springframework.context.applicationcontext} implementations.
     * <p>if given a plain beandefinitionregistry, the default resourceloader will be a
     * {@link org.springframework.core.io.support.pathmatchingresourcepatternresolver}.
     * <p>if the passed-in bean factory also implements {@link environmentcapable} its
     * environment will be used by this reader.  otherwise, the reader will initialize and
     * use a {@link standardenvironment}. all applicationcontext implementations are
     * environmentcapable, while normal beanfactory implementations are not.
     * @param registry the beanfactory to load bean definitions into,
     * in the form of a beandefinitionregistry
     * @see #setresourceloader
     * @see #setenvironment
     */
    protected abstractbeandefinitionreader(beandefinitionregistry registry) {
        assert.notnull(registry, "beandefinitionregistry must not be null");
        this.registry = registry;

        // determine resourceloader to use.
        if (this.registry instanceof resourceloader) {
            this.resourceloader = (resourceloader) this.registry;
        }
        else {
            this.resourceloader = new pathmatchingresourcepatternresolver();
        }

        // inherit environment if possible
        if (this.registry instanceof environmentcapable) {
            this.environment = ((environmentcapable) this.registry).getenvironment();
        }
        else {
            this.environment = new standardenvironment();
        }
    }

    /**
     * load the bean definitions with the given xmlbeandefinitionreader.
     * <p>the lifecycle of the bean factory is handled by the {@link #refreshbeanfactory}
     * method; hence this method is just supposed to load and/or register bean definitions.
     * @param reader the xmlbeandefinitionreader to use
     * @throws beansexception in case of bean registration errors
     * @throws ioexception if the required xml document isn't found
     * @see #refreshbeanfactory
     * @see #getconfiglocations
     * @see #getresources
     * @see #getresourcepatternresolver
     */
    protected void loadbeandefinitions(xmlbeandefinitionreader reader) throws beansexception, ioexception {
        resource[] configresources = getconfigresources();
        if (configresources != null) {
            reader.loadbeandefinitions(configresources);
        }
        string[] configlocations = getconfiglocations();
        if (configlocations != null) {
            reader.loadbeandefinitions(configlocations);
        }
    }

    @override
    public int loadbeandefinitions(resource... resources) throws beandefinitionstoreexception {
        assert.notnull(resources, "resource array must not be null");
        int counter = 0;
        for (resource resource : resources) {
            counter += loadbeandefinitions(resource);
        }
        return counter;
    }

    /**
     * load bean definitions from the specified xml file.
     * @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
     */
    @override
    public int loadbeandefinitions(resource resource) throws beandefinitionstoreexception {
        return loadbeandefinitions(new encodedresource(resource));
    }

    /**
     * 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.getresource());
        }

        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());
                }
                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 {
            document doc = doloaddocument(inputsource, resource);
            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();
        int countbefore = getregistry().getbeandefinitioncount();
        documentreader.registerbeandefinitions(doc, createreadercontext(resource));
        return getregistry().getbeandefinitioncount() - countbefore;
    }

    /**
     * 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();
        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;
        //初始化bean默认的解析器beandefinitionparserdelegate
        this.delegate = createdelegate(getreadercontext(), root, parent);

        if (this.delegate.isdefaultnamespace(root)) {
            string profilespec = root.getattribute(profile_attribute);
            if (stringutils.hastext(profilespec)) {
                string[] specifiedprofiles = stringutils.tokenizetostringarray(
                        profilespec, beandefinitionparserdelegate.multi_value_attribute_delimiters);
                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(root);
        //解析dom
        parsebeandefinitions(root, this.delegate);
        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)) {
                        parsedefaultelement(ele, delegate);
                    }
                    else {
                        delegate.parsecustomelement(ele);
                    }
                }
            }
        }
        else {
            delegate.parsecustomelement(root);
        }
    }

    private void parsedefaultelement(element ele, beandefinitionparserdelegate delegate) {
        if (delegate.nodenameequals(ele, import_element)) { //import
            importbeandefinitionresource(ele);
        }
        else if (delegate.nodenameequals(ele, alias_element)) { //alias
            processaliasregistration(ele);
        }
        else if (delegate.nodenameequals(ele, bean_element)) { //bean
            processbeandefinition(ele, delegate);
        }
        else if (delegate.nodenameequals(ele, nested_beans_element)) { //beans
            // recurse
            doregisterbeandefinitions(ele);
        }
    }

configurablelistablebeanfactory beanfactory = obtainfreshbeanfactory();