spring容器

一、簡介

　　spring容器是整個spring框架的核心,通常我們說的spring容器就是bean工廠,bean工廠負責創建和初始化bean、裝配bean並且管理應用程序中的bean.spring中提供了兩個核心接口：BeanFactory和ApplicationContext,ApplicationContext是BeanFactory子接口,它提供了比BeanFactory更完善的功能.

二、ApplicationContext的工作原理

　　先建立一個新的java項目,搭建好spring的開發環境.然后啟動spring的容器,如下面的代碼:

public class Demo {
    public static void main(String[] args) {
　　　　
        ApplicationContext cxt = new ClassPathXmlApplicationContext("beans.xml");
        
    }
}

　spring容器啟動時,會完成兩個重要的工作:加載bean的定義信息（BeanDefinition）以及初始化所有單例bean,在初始化bean的過程中注入bean的依賴.bean的定義信息是指：bean的基本屬性,例如完整類名,是否單例等等,其實就是<bean id="" class="" scope="">元素的那些屬性.在創建bean時需要用到這些屬性,所以必須要先加載bean以及它的定義信息.

  先說spring容器加載bean的定義信息的實現原理,spring中有兩種配置bean的方法:

• 使用配置文件配置bean,需要在<bean>元素中聲明bean的信息;spring容器啟動時,會讀取配置文件並進行解析,這種情況下,只要解析bean元素就可以獲取bean的beanName和它的定義信息.

• 使用注解配置bean,需要在配置文件中配置bean的路徑,例如:<context:component-scan base-package="cn.spring"/>,這樣容器啟動時就會掃描cn.spring包以及子包下面的所有類,如果類上有@Controller 或者 @Service 或者@Repository或者@Component注解,spring就會加載這些類的定義信息;這里就會有幾個問題,第一個問題是如何獲取base-package的子包以及包下的所有類?spring的做法是將包名轉化成文件系統中的路徑,然后traverse獲取該目錄下的所有.class文件,非常巧妙的一個解決方案!接下來的問題是如何從.class文件中獲取bean的定義信息呢?有兩種方式,第一種就是把通過.class文件的路徑獲取該類的包名,然后通過類加載器加載該類獲取它的定義信息,第二種方式是用asm框架從class文件中直接讀取類的定義信息。spring用的是第二種方式,個人覺得spring選擇第二種方式是有以下幾個原因,其一,可能需要對class文件進行增強處理,也就是在class文件中增加一些新的指令,在生成代理時可能會需要這樣做;其二,反射無法獲取類完完全全的信息(例如:方法的參數名稱),其三,反射的性能問題;

　接下來,就是容器初始化單例bean的過程:

　　spring容器在加載完所有bean的定義信息以后,會有一個refresh()操作,在refresh容器過程中完成兩個重要的操作,第一個就是創建所有單例bean,第二個就是裝配這些創建bean（注入它們所需要的依賴）;

　　因為前面的操作已經加載了所有bean的定義信息,並且維護了一個<beanName,BeanDefinition>對應關系的Map,遍歷Map,就可以取得每個bean的定義信息,從bean的定義信息可以知道bean是否是單例,如果是單例的,下一步就會根據bean的定義信息來決定bean實例的創建策略,如果配置了bean的factory-method,就調用factory-method創建bean實例,如果沒有配置factory-method,默認會調用bean的無參構造函數創建bean實例.

　　創建bean實例之后的工作就是裝配bean,現在已經拿到了bean實例,如果bean是在配置文件中配置的,此時就會先把配置文件中配置的屬性賦值給bean實例上對應的屬性;而后由bean的后處理器(BeanPostProcessor)完成bean實例其他屬性(通過注解配置的)的注入.如果bean是通過注解進行配置,這時直接就會由bean的后處理器完成bean的裝配.完成bean裝配的后處理器的工作原理:遍歷bean對象的字段和方法,根據字段和方法上應的注解完成相對應的注入操作.

　　在裝配bean的過程中會出現一個問題:A依賴B,裝配A的時候B的實例還沒有創建,spring解決這個問題的辦法是:先創建B對象,裝配好bean,然后把B注入A,繼續完成A的裝配.

三、容器初始化過程的源碼分析

　　我們從ApplicationContext的構造函數開始,如下代碼:　

　　/**
     * Create a new ClassPathXmlApplicationContext with the given parent,
     * loading the definitions from the given XML files.
     * @param configLocations array of resource locations
     * @param refresh whether to automatically refresh the context,
     * loading all bean definitions and creating all singletons. --->加載所有bean的定義信息,創建所有單例bean
     * Alternatively, call refresh manually after further configuring the context.
     * @param parent the parent context
     * @throws BeansException if context creation failed
     * @see #refresh()
     */
    public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
            throws BeansException {

        super(parent);
　　　　　// 解析給定的配置文件,完成加載所有bean的定義信息的操作
        setConfigLocations(configLocations);
        if (refresh) {
　　　　　　　// refresh容器,完成創建單例bean的操作
            refresh();
        }
    }

　　構造方法的注釋上寫的so nice.接下來,看加載bean的定義信息的過程,setConfigLocations()是在父類中實現的,接收到配置文件以后,容器開始解析配置文件.經過一系列的調用,會調用org.springframework.beans.factory.xml.XmlBeanDefinitionReader的doLoadBeanDefinitions(),到這里終於看到Document,下面是該方法的源碼:

　　/**
     * 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
     */
    protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {
        try {
            int validationMode = getValidationModeForResource(resource);
  // 取得Document對象  
            Document doc = this.documentLoader.loadDocument(
                    inputSource, getEntityResolver(), this.errorHandler, validationMode, isNamespaceAware());
 // 從Document對象中解析bean的定義信息   
            return registerBeanDefinitions(doc, resource);
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
　　　　 // ...各種異常的處理
    }      

　　registerBeanDefinitions()又會經過一系列的檢查和處理,然后調用

org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader的doRegisterBeanDefinitions(root)方法進行處理,這部分會從bean元素中解析出bean的定義信息,源代碼如下:

　　

/**
     * Register each bean definition within the given root {@code <beans/>} element.
     * @throws IllegalStateException if {@code <beans profile="..."} attribute is present
     * and Environment property has not been set
     * @see #setEnvironment
     */
    protected void doRegisterBeanDefinitions(Element root) {
        String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
        if (StringUtils.hasText(profileSpec)) {
            Assert.state(this.environment != null, "environment property must not be null");
            String[] specifiedProfiles = StringUtils.tokenizeToStringArray(profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            if (!this.environment.acceptsProfiles(specifiedProfiles)) {
                return;
            }
        }

        // 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 = createHelper(readerContext, root, parent);

        preProcessXml(root);
               // 重點部分,解析bean的定義信息
        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); // 重點:解析bean元素
                    }
                    else {
                        delegate.parseCustomElement(ele); // 重點:解析其他元素,例如:<context<context:component-scan> or <annotation:config/>
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }

　　到這里,終於到了關鍵的地方,如果bean是在配置文件中配置的,由parseDefaultElement(ele, delegate)處理bean元素的解析,如果是注解配置,parseCustomElement(ele)會掃描包下的class文件,並完成解析.我們先看配置文件中bean元素的解析方式。

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)) {
　　　　　　// 重點
            processBeanDefinition(ele, delegate);
        }
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }

　　/**
     * Process the given bean element, parsing the bean definition
     * and registering it with the registry.
     */
    protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
　　　　 // 獲取bean的定義信息,用BeanDefinitionHodler對象封裝
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if (bdHolder != null) {
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
            try {
                // Register the final decorated instance.---》關鍵,將bean的定義信息保存到容器
                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));
        }
    }

　　接下來就是調用org.springframework.beans.factory.support.BeanDefinitionReaderUtils的registerBeanDefinition()保存bean定義信息到容器的方法了.

/**
     * Register the given bean definition with the given bean factory.
     * @param definitionHolder the bean definition including name and aliases
     * @param registry the bean factory to register with
     * @throws BeanDefinitionStoreException if registration failed
     */
    public static void registerBeanDefinition(
            BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
            throws BeanDefinitionStoreException {

        // Register bean definition under primary name.---》重點
        String beanName = definitionHolder.getBeanName();
        registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

        // Register aliases for bean name, if any.
        String[] aliases = definitionHolder.getAliases();
        if (aliases != null) {
            for (String aliase : aliases) {
                registry.registerAlias(beanName, aliase);
            }
        }
    }

　　ok,來看最終的保存代碼:

org.springframework.beans.factory.support.DefaultListableBeanFactory中的registerBeanDefinition();先看DefaultListableBeanFactory保存bean定義信息的Map的聲明：

　　

    /** Map of bean definition objects, keyed by bean name */
    private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>();

　　　　/** List of bean definition names, in registration order */
　　　　private final List<String> beanDefinitionNames = new ArrayList<String>();

　　保存bean定義信息的方法:

　　//---------------------------------------------------------------------
    // Implementation of BeanDefinitionRegistry interface
    //---------------------------------------------------------------------

    public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
            throws BeanDefinitionStoreException {

        Assert.hasText(beanName, "Bean name must not be empty");
        Assert.notNull(beanDefinition, "BeanDefinition must not be null");

        if (beanDefinition instanceof AbstractBeanDefinition) {
            try {
                ((AbstractBeanDefinition) beanDefinition).validate();
            }
            catch (BeanDefinitionValidationException ex) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Validation of bean definition failed", ex);
            }
        }
　　　　// 保存bean定義信息,線程同步
        synchronized (this.beanDefinitionMap) {
　　　　　　// 判斷當前bean的定義信息是否已經保存
            Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);
            if (oldBeanDefinition != null) {
                if (!this.allowBeanDefinitionOverriding) {
                    throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                            "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                            "': There is already [" + oldBeanDefinition + "] bound.");
                }
                else {
                    if (this.logger.isInfoEnabled()) {
                        this.logger.info("Overriding bean definition for bean '" + beanName +
                                "': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
                    }
                }
            }
            else {
　　　　　　　　　// 保存beanName
                this.beanDefinitionNames.add(beanName);
                this.frozenBeanDefinitionNames = null;
            }
　　　　　　 // 保存beanName和bean的定義信息到Map
            this.beanDefinitionMap.put(beanName, beanDefinition);
　　　　　

            resetBeanDefinition(beanName);
        }
    }

 上面就是spring解析配置文件中的bean定義信息,然后保存beanName和bean定義信息到Map中.這個過程主要就是xml的解析.接下來我們看spring是如何解析注解方式配置的bean.回到parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate)方法,現在重點關注:delegate.parseCustomElement(ele)方法.如果我們在配置文件用<context:component-scan  base-package="">方式來指定自動掃描的包,之后就會調用org.springframework.context.annotation.ComponentScanBeanDefinitionParser的parse().下面是parse()方法的源代碼:　　

public BeanDefinition parse(Element element, ParserContext parserContext) {
　　　　 // 解析<context:component-scan元素,獲取base-package
        String[] basePackages = StringUtils.tokenizeToStringArray(element.getAttribute(BASE_PACKAGE_ATTRIBUTE),
                ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);

        // Actually scan for bean definitions and register them.
        ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);
　　　　 //重點: 掃描basePackage下所有的class文件,讀取bean的定義信息
        Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages);
        registerComponents(parserContext.getReaderContext(), beanDefinitions, element);

        return null;
    }

重點關注scanner.doScan(basePackges)方法,該方法完成整個核心操作--->根據包名獲取包下所有的class的定義信息.直接看org.springframework.context.annotation.ClassPathBeanDefinitionScanner的scan():注意,看源碼時一定要多關注注釋,例如下面方法上的注釋就非常有意義.

　　/**
     * Perform a scan within the specified base packages, 
     * returning the registered bean definitions.
     * <p>This method does <i>not</i> register an annotation config processor
     * but rather leaves this up to the caller.
     * @param basePackages the packages to check for annotated classes
     * @return set of beans registered if any for tooling registration purposes (never {@code null})
     */
    protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
        Assert.notEmpty(basePackages, "At least one base package must be specified");
        Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>();
        for (String basePackage : basePackages) { // 遍歷每一個basepackages
　　　　　　 // 1.獲取basePackage下bean的定義信息
            Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
            for (BeanDefinition candidate : candidates) {
　　　　　　　　  //2.根據掃描的信息,解析bean的一些定義信息
                ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
                candidate.setScope(scopeMetadata.getScopeName());
                String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
                if (candidate instanceof AbstractBeanDefinition) {
                    postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
                }
                if (candidate instanceof AnnotatedBeanDefinition) {
                    AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
                }
                if (checkCandidate(beanName, candidate)) {
                    BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
                    definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
                    beanDefinitions.add(definitionHolder);
　　　　　　　　　　　　// 3.將bean的定義信息添加到容器中
                    registerBeanDefinition(definitionHolder, this.registry);
                }
            }                        
        }
        return beanDefinitions;
    }

　　第1個步驟我們核心關注點,它完成從文件系統中讀取class文件的操作,第3個步驟在之前已經說了,就是保存bean的定義信息到容器的DefaultListableBeanFactory的beanDefinitionMap 中.重點關注第1個步驟,看findCandidateComponents()的源代碼:

    /**
     * Scan the class path for candidate components.
     * @param basePackage the package to check for annotated classes
     * @return a corresponding Set of autodetected bean definitions
     */
    public Set<BeanDefinition> findCandidateComponents(String basePackage) {
        Set<BeanDefinition> candidates = new LinkedHashSet<BeanDefinition>();
        try {
            String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
                    resolveBasePackage(basePackage) + "/" + this.resourcePattern;
　　　　　　　// 1.獲取包下的class文件路徑,例如E:\Program Files (x86)\MyEclipse10\workplace2\spr\bin\cn\jack\domain\User.class,
　　　　　　　// 每一個class文件的路徑封裝成Resource對象.
            Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);
            boolean traceEnabled = logger.isTraceEnabled();
            boolean debugEnabled = logger.isDebugEnabled();
            for (Resource resource : resources) {
                if (traceEnabled) {
                    logger.trace("Scanning " + resource);
                }
                if (resource.isReadable()) {
                    try {
　　　　　　　　　　　　　　// 2.使用asm框架讀取class文件,獲取類的定義信息
                        MetadataReader metadataReader = this.metadataReaderFactory.getMetadataReader(resource);
                        if (isCandidateComponent(metadataReader)) {
                            ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
                            sbd.setResource(resource);
                            sbd.setSource(resource);
                            if (isCandidateComponent(sbd)) {
                                if (debugEnabled) {
                                    logger.debug("Identified candidate component class: " + resource);
                                }
                                candidates.add(sbd);
                            }
                            else {
                                if (debugEnabled) {
                                    logger.debug("Ignored because not a concrete top-level class: " + resource);
                                }
                            }
                        }
                        else {
                            if (traceEnabled) {
                                logger.trace("Ignored because not matching any filter: " + resource);
                            }
                        }
                    }
                    catch (Throwable ex) {
                        throw new BeanDefinitionStoreException(
                                "Failed to read candidate component class: " + resource, ex);
                    }
                }
                else {
                    if (traceEnabled) {
                        logger.trace("Ignored because not readable: " + resource);
                    }
                }
            }
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
        }
　　　　　//3.返回benadefinition集合
        return candidates;
    }

 第1個步驟主要是解析文件路徑,然后遍歷文件夾獲取每個class文件的地址;第2個步驟用asm框架來讀取class文件獲取類的信息封裝成BeanDefinition對象.

第2個步驟最后調用的是org.springframework.core.type.classreading.SimpleMetadataReader的構造函數,下面是該類的部分源代碼：

final class SimpleMetadataReader implements MetadataReader {

    private final Resource resource;
    private final ClassMetadata classMetadata;
    private final AnnotationMetadata annotationMetadata;

    SimpleMetadataReader(Resource resource, ClassLoader classLoader) throws IOException {
        InputStream is = resource.getInputStream();
        ClassReader classReader = null;
        try {
　　　　　　　// asm框架讀取class文件
            classReader = new ClassReader(is);
        } finally {
            is.close();
        }
　　　　// 采用訪問者模式來獲取class類信息
        AnnotationMetadataReadingVisitor visitor = new AnnotationMetadataReadingVisitor(classLoader);
        classReader.accept(visitor, true);
        
        this.annotationMetadata = visitor;
        // (since AnnotationMetadataReader extends ClassMetadataReadingVisitor)
        this.classMetadata = visitor;
        this.resource = resource;
    }

ClassReader是asm框架中核心類,具體用法可以參考asm的官網.

上面說的過程就是spring容器加載bean定義信息的過程.過程很長,但實現原理卻並不復雜.

2. 初始化單例bean的過程

　　上面分析了spring容器加載bean定義信息的過程,接下來分析bean的初始化以及創建bean的過程.回到ApplicationContext中的構造函數,入口為refresh().refresh方法在父類中實現的。下面是AbstractApplicationContext類refresh()方法的源代碼:

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

            // Tell the subclass to refresh the internal bean factory.
            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);// 容器的后處理器

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);//調用容器的后處理器

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);//注冊bean的后處理器

                // Initialize message source for this context.
                initMessageSource();

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

                // Initialize other special beans in specific context subclasses.
                onRefresh();

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

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);// 重點,注釋寫的so nice，初始化所有單例bean

                // 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;
            }
        }
    }

后處理器是一種特殊bean,用於完成一些自身操作.

容器后處理器：對容器本身進行處理，在容器實例化其他任何Bean之前讀取配置文件的元數據並可能修改這些數據.PropertyPlaceholderConfigurer就是一個容器后處理器,用於完成beans.xml中引入其他配置文件中內容操作.

Bean后處理器：即當Spring容器創建完Bean實例之后對bean進行一些處理，例如:完成bean的裝配等操作。

回到refresh()方法,重點關注:finishBeanFactoryInitialization(beanFactory);這個方法會調用DefaultListableBeanFactory.preInstantiateSingletons方法.

public void preInstantiateSingletons() throws BeansException {
        if (this.logger.isInfoEnabled()) {
            this.logger.info("Pre-instantiating singletons in " + this);
        }
        synchronized (this.beanDefinitionMap) { // 線程同步
            // 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.
            List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames);
            for (String beanName : beanNames) {// 遍歷beanNames
                RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
                if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {//單例非延遲實例的bean
                    if (isFactoryBean(beanName)) { // 工廠bean.FactoryBean接口的子類
                        final FactoryBean factory = (FactoryBean) getBean(FACTORY_BEAN_PREFIX + beanName);
                        boolean isEagerInit;
                        if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                            isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
                                public Boolean run() {
                                    return ((SmartFactoryBean) factory).isEagerInit();
                                }
                            }, getAccessControlContext());
                        }
                        else {//普通bean
                            isEagerInit = (factory instanceof SmartFactoryBean &&
                                    ((SmartFactoryBean) factory).isEagerInit());
                        }
                        if (isEagerInit) {
                            getBean(beanName);
                        }
                    }
                    else {
                        getBean(beanName); // 從容器中獲取bean,如果沒有創建,並完成裝配
                    }
                }
            }
        }
    }

getBean(beanName)方法會調用doGetBean方法.這是個很關鍵的地方,切記注釋很重要

    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * @param name the name of the bean to retrieve
     * @param requiredType the required type of the bean to retrieve
     * @param args arguments to use if creating a prototype using explicit arguments to a
     * static factory method. It is invalid to use a non-null args value in any other case.
     * @param typeCheckOnly whether the instance is obtained for a type check,
     * not for actual use
     * @return an instance of the bean
     * @throws BeansException if the bean could not be created
     */
    @SuppressWarnings("unchecked")
    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;

        // Eagerly check singleton cache for manually registered singletons.
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) { //bean已創建,調用方法返回該bean
            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 + "'");
                }
            }
　　　　　　 //如果是工廠bean,則返回beanFactory.getObject(),普通bean直接返回sharedInstance
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else { //bean未創建
            // 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();
　　　　　　 // 檢查父容器是否已經創建該bean,有則從父容器獲取bean返回
            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);
            }

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

            // Guarantee initialization of beans that the current bean depends on.
            String[] dependsOn = mbd.getDependsOn();
            if (dependsOn != null) {
                for (String dependsOnBean : dependsOn) {
                    getBean(dependsOnBean);
                    registerDependentBean(dependsOnBean, beanName);
                }
            }

            // Create bean instance.---》創建單例bean
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, new ObjectFactory() {
                    public Object getObject() throws BeansException {
                        try {
                            return createBean(beanName, mbd, args);// ---> 創建bean的方法
                        }
                        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;
                        }
                    }
                });
　　　　　　　　　// 返回創建的單例bean
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }

            else if (mbd.isPrototype()) { // 創建原型bean,scope="prototype"
                // 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() {
                        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);
                }
            }
        }

        // 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());
            }
        }
　　　　// 返回bean
        return (T) bean;
    }

createBean(beanName, mbd, args)方法會調用doCreateBean()完成bean的創建工作,源代碼如下：

/**
     * Actually create the specified bean. Pre-creation processing has already happened
     * at this point, e.g. checking <code>postProcessBeforeInstantiation</code> callbacks.
     * <p>Differentiates between default bean instantiation, use of a
     * factory method, and autowiring a constructor.
     * @param beanName the name of the bean
     * @param mbd the merged bean definition for the bean
     * @param args arguments to use if creating a prototype using explicit arguments to a
     * static factory method. This parameter must be <code>null</code> except in this case.
     * @return a new instance of the bean
     * @throws BeanCreationException if the bean could not be created
     * @see #instantiateBean
     * @see #instantiateUsingFactoryMethod
     * @see #autowireConstructor
     */
    protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
        // Instantiate the bean.
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
　　　　　　  // 1.創建bean的包裝類,裝飾設計模式
            instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
        Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);

        // Allow post-processors to modify the merged bean definition.
        synchronized (mbd.postProcessingLock) {
            if (!mbd.postProcessed) {
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
                mbd.postProcessed = true;
            }
        }

        // Eagerly cache singletons to be able to resolve circular references
        // even when triggered by lifecycle interfaces like BeanFactoryAware.
        boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                isSingletonCurrentlyInCreation(beanName));
        if (earlySingletonExposure) {
            if (logger.isDebugEnabled()) {
                logger.debug("Eagerly caching bean '" + beanName +
                        "' to allow for resolving potential circular references");
            }
            addSingletonFactory(beanName, new ObjectFactory() {
                public Object getObject() throws BeansException {
                    return getEarlyBeanReference(beanName, mbd, bean);
                }
            });
        }

        // Initialize the bean instance.
        Object exposedObject = bean;
        try {
　　　　　　 //2.裝配bean
            populateBean(beanName, mbd, instanceWrapper);
            if (exposedObject != null) {
                exposedObject = initializeBean(beanName, exposedObject, mbd);
            }
        }
        catch (Throwable ex) {
            if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
                throw (BeanCreationException) ex;
            }
            else {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
            }
        }

        if (earlySingletonExposure) {
            Object earlySingletonReference = getSingleton(beanName, false);
            if (earlySingletonReference != null) {
                if (exposedObject == bean) {
                    exposedObject = earlySingletonReference;
                }
                else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                    String[] dependentBeans = getDependentBeans(beanName);
                    Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
                    for (String dependentBean : dependentBeans) {
                        if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                            actualDependentBeans.add(dependentBean);
                        }
                    }
                    if (!actualDependentBeans.isEmpty()) {
                        throw new BeanCurrentlyInCreationException(beanName,
                                "Bean with name '" + beanName + "' has been injected into other beans [" +
                                StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                                "] in its raw version as part of a circular reference, but has eventually been " +
                                "wrapped. This means that said other beans do not use the final version of the " +
                                "bean. This is often the result of over-eager type matching - consider using " +
                                "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
                    }
                }
            }
        }

        // Register bean as disposable.
        try {
            registerDisposableBeanIfNecessary(beanName, bean, mbd);
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
        }
　　　　 //3.返回
        return exposedObject;
    }

首先看第1個步驟,這個步驟中會創建bean實例和bean的包裝類,這里使用了裝飾設計模式.創建bean的實例過程比較簡單,如果配置bean時指定了bean的創建方法 factory-method,就用factory-method創建bean實例,默認會使用無參構造函數創建bean實例.這部分重點關注裝配bean的過程.

/**
     * Create a new instance for the specified bean, using an appropriate instantiation strategy:
     * factory method, constructor autowiring, or simple instantiation.
     * @param beanName the name of the bean
     * @param mbd the bean definition for the bean
     * @param args arguments to use if creating a prototype using explicit arguments to a
     * static factory method. It is invalid to use a non-null args value in any other case.
     * @return BeanWrapper for the new instance
     * @see #instantiateUsingFactoryMethod
     * @see #autowireConstructor
     * @see #instantiateBean
     */
    protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
        // Make sure bean class is actually resolved at this point.
        Class beanClass = resolveBeanClass(mbd, beanName);

        if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                    "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
        }

        if (mbd.getFactoryMethodName() != null)  {// 使用工廠方法創建bean,<bean factory-method="">
            return instantiateUsingFactoryMethod(beanName, mbd, args);
        }

        // Shortcut when re-creating the same bean...
        boolean resolved = false;
        boolean autowireNecessary = false;
        if (args == null) {
            synchronized (mbd.constructorArgumentLock) {
                if (mbd.resolvedConstructorOrFactoryMethod != null) {
                    resolved = true;
                    autowireNecessary = mbd.constructorArgumentsResolved;
                }
            }
        }
        if (resolved) {
            if (autowireNecessary) {
                return autowireConstructor(beanName, mbd, null, null);
            }
            else {
                return instantiateBean(beanName, mbd);
            }
        }

        // Need to determine the constructor...
        Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
        if (ctors != null ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
            return autowireConstructor(beanName, mbd, ctors, args);
        }
　　　　// 用默認的構造函數創建bean,反射獲取構造函數,constructor.newInstance()創建bean.
        // No special handling: simply use no-arg constructor. 
        return instantiateBean(beanName, mbd);
    }

　　現在來看裝配bean的過程,這個過程完成注入bean的依賴對象,如果bean是在配置文件配置的,則把從xml中解析出來的屬性注入給bean實例,如果是用注解配置的依賴(@Resource 或者@AutoWired),則會解析bean的字段或者方法上的注解,根據這些注解找到對應的依賴,如果依賴對象已經創建,就直接注入依賴,否則,先創建依賴對象,在完成注入操作.

/**
     * Populate the bean instance in the given BeanWrapper with the property values
     * from the bean definition.
     * @param beanName the name of the bean
     * @param mbd the bean definition for the bean
     * @param bw BeanWrapper with bean instance
     */
    protected void populateBean(String beanName, AbstractBeanDefinition mbd, BeanWrapper bw) {
        PropertyValues pvs = mbd.getPropertyValues();

        if (bw == null) {
            if (!pvs.isEmpty()) {
                throw new BeanCreationException(
                        mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
            }
            else {
                // Skip property population phase for null instance.
                return;
            }
        }

        // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
        // state of the bean before properties are set. This can be used, for example,
        // to support styles of field injection.
        boolean continueWithPropertyPopulation = true;

        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            for (BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                        continueWithPropertyPopulation = false;
                        break;
                    }
                }
            }
        }

        if (!continueWithPropertyPopulation) {
            return;
        }
　　　　 // 根據beanName或者type完成自動裝配
        if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
            MutablePropertyValues newPvs = new MutablePropertyValues(pvs);

            // Add property values based on autowire by name if applicable.
            if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
                autowireByName(beanName, mbd, bw, newPvs);
            }

            // Add property values based on autowire by type if applicable.
            if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
                autowireByType(beanName, mbd, bw, newPvs);
            }

            pvs = newPvs;
        }

        boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
        boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
　　　　　// 調用bean后處理器
        if (hasInstAwareBpps || needsDepCheck) {
            PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw);
            if (hasInstAwareBpps) {
                for (BeanPostProcessor bp : getBeanPostProcessors()) {
                    if (bp instanceof InstantiationAwareBeanPostProcessor) {
                        InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
　　　　　　　　　　　　　　// 重點:獲取bean要裝配的屬性和屬性值
                        pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                        if (pvs == null) {
                            return;
                        }
                    }
                }
            }
            if (needsDepCheck) {
                checkDependencies(beanName, mbd, filteredPds, pvs);
            }
        }
　　　　 // 給bean的屬性賦值
        applyPropertyValues(beanName, mbd, bw, pvs);
    }

 　　InstantiationAwareBeanPostProcessor是BeanPostProcessor的子接口,能在bean初始化前后對bean進行處理.InstantiationAwareBeanPostProcessor有以下幾個子類：

　　

　　@Resource注解注入依賴的工作就是由CommonAnnotationBeanPostProcessor完成的.下面是該類postProcessPropertyValues()的源碼:

public PropertyValues postProcessPropertyValues(
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
　　　　 //1.根據bean的字節碼,遍歷所有的字段和方法,獲取需要注入的字段或者方法
        InjectionMetadata metadata = findResourceMetadata(bean.getClass());
        try {
　　　　　　　//2.從容器中查找依賴對象,並賦值給相應的字段,完成bean的裝配
            metadata.inject(bean, beanName, pvs);
        }
        catch (Throwable ex) {
            throw new BeanCreationException(beanName, "Injection of resource dependencies failed", ex);
        }
        return pvs;
    }

　　第1個步驟主要就是根據字節碼獲取所有字段和方法，然后遍歷查找有@Resource注解的字段或方法以及依賴bean的beanName,第2個步驟從容器中依賴對象的beanName(@Resource的name屬性值),如果容器中沒有該依賴對象就創建,有的話就直接獲取,並賦值給bean的屬性.這樣,就通過bean的后處理器完成了bean的裝配過程.

　　到這里,容器的啟動過程就完成了,此時就可以對外提供服務了.上面就是本人對spring容器部分源碼學習的一些總結.日后,了解更多會不定時更新上來!