Spring解密 - Bean的加載流程

Spring是一個開源的設計層面框架,解決了業務邏輯層和其餘各層的鬆耦合問題,將面向接口的編程思想貫穿整個系統應用,同時它也是 Java工做中必備技能之一...

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前言

Spring解密 - XML解析 與 Bean註冊 中,講了 Bean的解析,本章將詳細講解Spring中Bean的加載過程,相比解析而言,加載稍微複雜一點.git

入口

public class Application {

    public static void main(String[] args) {

        ApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");
        Person person = context.getBean("person", Person.class);
        System.out.println(person.toString());
    }
}

重點分析 context.getBean();spring

解密

在分析 Bean 的加載過程前,咱們看看 BeanFactory 的由來,以及 getBean 所屬的類是誰。磨刀不誤砍柴工編程

BeanFactory 的由來

1.查看getBean()的源碼,此處的 getBeanFactory() 是怎麼來的?緩存

public class AbstractApplicationContext{

    @Override
    public <T> T getBean(String name, @Nullable Class<T> requiredType) throws BeansException {
        assertBeanFactoryActive();
        // getBeanFactory() 是怎麼來的?咱們接着看
        return getBeanFactory().getBean(name, requiredType);
    }
}

2.應用程序的上下文微信

new ClassPathXmlApplicationContext("bean.xml"); 中,跳轉到重載的構造方法中,咱們會發現以下代碼session

public class ClassPathXmlApplicationContext {

    public ClassPathXmlApplicationContext(
            String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
            throws BeansException {

        super(parent);
        // 解析 bean.xml 文件
        setConfigLocations(configLocations);
        if (refresh) {
            refresh();
        }
    }

}

它的目的是將普通路徑解析爲 類(classpath) 路徑資源名稱。多個配置文件的狀況下,後加載的 Bean 會覆蓋先前定義好的 Bean,這樣作的目的是爲了經過額外的XML文件來特地重寫某個 Bean這裏咱們能夠看到調用了一個 refresh(),它的做用是什麼呢?app

3.初始化框架

刷新容器
public class AbstractApplicationContext {

    @Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // 準備刷新上下文
            prepareRefresh();
            // 通知子類,刷新內部 BeanFactory(建立 BeanFactory 的入口)
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
        }
    }

    protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        // 跟蹤該方法
        refreshBeanFactory();
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (logger.isDebugEnabled()) {
            logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
        }
        return beanFactory;
    }

}

初次加載完 bean.xml 後,確定會進入到 refresh() 方法中,這個時候會作容器初始化的工做,也就是 標籤解析、Bean加載 等等工做,因爲本章重點介紹 Bean加載,因此只貼了少許代碼(看官莫急)ide

建立 DefaultListableBeanFactory
public class AbstractRefreshableApplicationContext {

    @Override
    protected final void refreshBeanFactory() throws BeansException {
        // 判斷是否存在
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            // 建立 DefaultListableBeanFactory
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            customizeBeanFactory(beanFactory);
            loadBeanDefinitions(beanFactory);
            synchronized (this.beanFactoryMonitor) {
                // 而後賦值給 beanFactory 變量
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

    // 提取 BeanFactory
    @Override
    public final ConfigurableListableBeanFactory getBeanFactory() {
        synchronized (this.beanFactoryMonitor) {
            if (this.beanFactory == null) {
                throw new IllegalStateException("BeanFactory not initialized or already closed - " +
                        "call 'refresh' before accessing beans via the ApplicationContext");
            }
            return this.beanFactory;
        }
    }
}

從上面的代碼塊中,咱們能夠看到 若是已經存在 beanFactory,將銷燬舊的實例,而後在建立新的 DefaultListableBeanFactory,最後 getBeanFactory() 就能夠正常使用了。

Bean 加載

整個 Bean 的裝載過程當中,重點圍繞 AbstractBeanFactory,只要把它搞定剩下的就輕鬆了

public abstract class AbstractBeanFactory {

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

    protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
            @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
        // 別名轉換
        final String beanName = transformedBeanName(name);
        Object bean;

        // 檢查緩存中是否存在 該 Bean 的單例(Bean默認的Scope = singleton)
        // 好比容器初始化的時候或者其餘地方調用過getBean,已經完成了初始化
        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 + "'");
                }
            }
            // 返回實例,FactoryBean 的狀況下,並非直接返回實例自己而是返回指定方法返回的實例
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // 存在循環依賴則報錯
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // 判斷工廠中是否含有當前 Bean 的定義
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // 若是沒有,查詢父工廠
                String nameToLookup = originalBeanName(name);
                if (parentBeanFactory instanceof AbstractBeanFactory) {
                    return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                            nameToLookup, requiredType, args, typeCheckOnly);
                }
                else if (args != null) {
                    // 執行帶有 args 參數的 getBean 方法
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else {
                    // 若是沒有參數,執行標準的 getBean 方法 
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }

            if (!typeCheckOnly) { // 若是不是作類型檢查,則須要標記此 Bean 正在建立之中
                markBeanAsCreated(beanName);
            }

            try {
                // 將存儲XML配置文件的GernericBeanDefinition轉換成RootBeanDefinition,若是BeanName是子Bean的話會合並父類的相關屬性
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // 獲取依賴的 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);
                        getBean(dep);
                    }
                }

                // 終於開始建立 Bean 實例了,若是是單例的,那麼會建立一個單例的匿名工廠,
                // 若是是原型模式的,則不須要建立單例的工廠的,其餘的如 request、session 做用域的,則根據自身的須要
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, () -> {
                        try {
                            return createBean(beanName, mbd, args);// 調用默認構造函數,有興趣本身跟進下代碼
                        }
                        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 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;
            }
        }

        // 類型檢查,若是不能進行類型轉換,則拋出異常
        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.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;
    }

}

整個代碼塊的內容仍是很多的,接下來咱們一步一步的分析。

轉換 BeanName

final String beanName = transformedBeanName(name);

protected String transformedBeanName(String name) {
    return canonicalName(BeanFactoryUtils.transformedBeanName(name));
}

去除 FactoryBean 的修飾符,最終取指定 alias 所表示的 beanName。由於有可能獲取到以 & 開頭的 FactoryBean,因此要進行轉化(關於 BeanFactoryFactoryBean 後面會進行區分)

加載單例

檢查緩存中是否存在 該 Bean 的單例(Bean默認的Scope = singleton),如容器初始化的時候或者其餘地方已經調用過 getBean() 完成了初始化

Object sharedInstance = getSingleton(beanName);
緩存加載 Bean

默認在裝載 Bean 的時候會先去檢查 singletonObjects 是否存在,若是存在直接提取緩存的。

public class DefaultSingletonBeanRegistry {

    /** 保存 BeanName 和建立 bean 實例之間的關係 bean name --> bean instance */
    private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

    /** 保存 BeanName 和建立 bean 實例的工廠之間的關係 bean name --> ObjectFactory */
    private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

    /** 保存 BeanName 和建立 bean 實例之間的關係 bean name --> bean instance */
    /** 與 singletonObjects 不一樣的是當一個單例 bean 被放到裏面後,那麼在 bean 在建立過程當中,就能夠經過 getBean 方法獲取到,能夠用來檢測循環引用。 **/
    private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);

    /** 保存當前全部已註冊的 bean */
    private final Set<String> registeredSingletons = new LinkedHashSet<>(256);

    @Nullable
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        // 嘗試從緩存獲取實例
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
            synchronized (this.singletonObjects) {
                // 若該 bean 正在加載則不處理
                singletonObject = this.earlySingletonObjects.get(beanName);
                if (singletonObject == null && allowEarlyReference) {
                    ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        singletonObject = singletonFactory.getObject();
                        // 存入到緩存中
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return singletonObject;
    }

}
構建實例(無緩存狀況)

緩存中沒有,那就當場構建一個 bean 出來,能夠看到 getSingleton(String beanName, ObjectFactory<?> singletonFactory) 有兩個參數,其中的 ObjectFactory 是怎麼來的呢? 接着往下看

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    synchronized (this.singletonObjects) {
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            // 記錄加載狀態,,以便對循環依賴進行檢測
            beforeSingletonCreation(beanName);
            singletonObject = singletonFactory.getObject();
            // 移除加載狀態
            afterSingletonCreation(beanName);
            addSingleton(beanName, singletonObject);
        }
        return (singletonObject != NULL_OBJECT ? singletonObject : null);
    }
}
構建 ObjectFactory

此處與其說是構建 ObjectFactory ,不如說是在建立一個單例 Bean

if (mbd.isSingleton()) {
    sharedInstance = getSingleton(beanName, () -> {
        try {
            // 對 JDK8 lambda 表達式熟悉的小夥伴就不會陌生了
            return createBean(beanName, mbd, args);
        }
        catch (BeansException ex) {
            destroySingleton(beanName);
            throw ex;
        }
    });
    // 劃重點了
    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}

建立 Bean

public class AbstractAutowireCapableBeanFactory {

    @Override
    protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
            throws BeanCreationException {

        if (logger.isDebugEnabled()) {
            logger.debug("Creating instance of bean '" + beanName + "'");
        }
        RootBeanDefinition mbdToUse = mbd;

        // 首先判斷須要建立的bean是否能夠被實例化,這個類是否能夠經過類裝載器來載入。
        Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
        if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            mbdToUse = new RootBeanDefinition(mbd);
            mbdToUse.setBeanClass(resolvedClass);
        }

        // 準備方法覆蓋
        try {
            mbdToUse.prepareMethodOverrides();
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                    beanName, "Validation of method overrides failed", ex);
        }

        try {
            // 用 BeanPostProcessors 返回代理來替代真正的實例(若是 Bean 配置了 PostProcessor,那麼這裏返回的是一個代理)
            Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
            if (bean != null) {
                return bean;
            }
        }
        catch (Throwable ex) {
            throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
                    "BeanPostProcessor before instantiation of bean failed", ex);
        }

        try {
            // 重點來了
            Object beanInstance = doCreateBean(beanName, mbdToUse, args);
            if (logger.isDebugEnabled()) {
                logger.debug("Finished creating instance of bean '" + beanName + "'");
            }
            return beanInstance;
        }
        catch (BeanCreationException ex) {
            // A previously detected exception with proper bean creation context already...
            throw ex;
        }
        catch (ImplicitlyAppearedSingletonException ex) {
            // An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...
            throw ex;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(
                    mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
        }
    }


}

在建立 Bean 以前,Spring 還作了很多工做。

  • 判斷建立的 bean 是否能夠被實例化,這個類是否能夠經過 ClassLoader 來載入,根據設置的 class 屬性或根據 className 來解析 class
  • 對覆蓋進行標記並驗證,在 Spring 配置中存在 lookup-mothodreplace-method 的,這兩個配置的加載時將配置統一存放在 BeanDefinition 中的 methodOverrides 屬性裏,這個方法的操做也是針對於這兩個配置的;
  • 應用初始化前的後處理器,最後建立 bean。在 createBean() 方法裏執行完 resolveBeforeInstantiation 方法後,若是建立了代理且不爲空的話就直接返回,不然須要進行常規 bean 的建立,這個建立過程是在 doCreateBean 中完成的

初始化 Bean

上面 createBean 交給了 doCreateBean 來建立 bean (上面還有一個重要的方法getObjectForBeanInstance,在後面分析)

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
        throws BeanCreationException {

    // 實例化 Bean
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
        instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        // 根據指定 bean 使用相應策略建立實例(正確狀況會調用無參構造函數)
        instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    // 獲取實例化好的 Bean(Person person = new Person()),此處還未進行賦值
    final Object bean = instanceWrapper.getWrappedInstance();
    // 得到實例化好的 class
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
        mbd.resolvedTargetType = beanType;
    }

    // 記錄建立 Bean 的 ObjectFactory,初始化前調用 post-processors,可讓咱們在 bean 實例化以前作一些定製操做
    synchronized (mbd.postProcessingLock) {
        if (!mbd.postProcessed) {
            try {
                applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
            }
            catch (Throwable ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                        "Post-processing of merged bean definition failed", ex);
            }
            mbd.postProcessed = true;
        }
    }

    // 檢測循環依賴,是否須要提前初始化(只能解決單例Bean)
    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, () -> getEarlyBeanReference(beanName, mbd, bean));
    }

    // 初始化bean實例。
    Object exposedObject = bean;
    try {
        // 在這個方法裏面初始化對象,配置 xml 中的各類屬性
        populateBean(beanName, mbd, instanceWrapper);
        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<>(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.");
                }
            }
        }
    }

    // 用於銷燬方法
    try {
        registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
}

上面源碼完成的操做能夠歸納爲如下幾點

  • RootBeanDefinition 是否是單例,若是是單例先移除緩存
  • 實例化 bean,將 RootBeanDefinition 轉換爲 BeanWrapper
  • 使用 MergedBeanDefinitionPostProcessorAutowired註解 就是經過此方法實現類型的預解析;
  • 解決循環依賴問題
  • populateBean() 中填充屬性,配置在 XML 中的各類屬性
  • 註冊到 DisposableBean
  • 完成建立並返回 Bean 的實例

接下來咱們看下 Spring 是如何建立 bean 實例的。跟蹤 createBeanInstance() 方法

建立Bean實例
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
    // 獲取 beanClass , 要先確保 Bean 是正確的,已經解析到當前節點了
    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());
    }

    Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
    if (instanceSupplier != null) {
        return obtainFromSupplier(instanceSupplier, beanName);
    }

    // 若工廠方法不爲空則使用工廠方法初始化
    if (mbd.getFactoryMethodName() != null)  {
        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;
            }
        }
    }
    // 若是已經解析過則使用解析好的構造方法不須要再次鎖定
    if (resolved) {
        if (autowireNecessary) {
            // 構造方法自動注入
            return autowireConstructor(beanName, mbd, null, null);
        }
        else {
            // 使用默認構造方法
            return instantiateBean(beanName, mbd);
        }
    }

    // 根據參數解析構造方法
    Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    if (ctors != null ||
            mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
            mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
        return autowireConstructor(beanName, mbd, ctors, args);
    }

    // 沒有特殊處理的狀況下,只需使用無參數的構造函數。
    return instantiateBean(beanName, mbd);
}

能夠看出若是在 RootBeanDefinition 中存在 factoryMethodName屬性,或者說配置文件中配置了 factory-method,那麼 Spring 會嘗試使用 instantiateUsingFactoryMethod(beanName, mbd, args) 方法根據 RootBeanDefinition 中的配置生成bean實例。而後再解析構造方法並進行實例化,Spring 會根據參數及類型判斷使用哪一個構造方法進行實例化。判斷調用哪一個構造方法的過程會採用緩存機制,若是已經解析過則不須要重複解析而是從 RootBeanDefinition 中的屬性 resolvedConstructorOrFactoryMethod 緩存的值去取,不然需再次解析。

populateBean 屬性注入

這塊代碼比較多,有興趣請自行跟進,就不貼出來了

populateBean 方法的中的主要處理流程:

  • InstantiationAwareBeanPostProcessor 處理器的 postProcessAfterInstantiation 方法控制程序是否繼續填充屬性;
  • 根據注入類型提取依賴的 bean,並存入 PropertyValues 中;
  • InstantiationAwareBeanPostProcessor 處理器的 postProcessPropertyValues 方法對屬性在填充前再次處理(主要仍是驗證屬性);
  • 將全部 PropertyValues 中的屬性填充到 BeanWrapper 中;
initializeBean 初始化Bean

學過 Spring 的都知道 bean 配置時有一個 init-method 屬性,這個屬性的做用是在 bean 實例化前調用 init-method 指定的方法進行須要的操做,如今就進入這個方法了;Spring 執行過 bean 的實例化,而且進行屬性填充後,就會調用用戶設定的初始化方法。

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
    if (System.getSecurityManager() != null) {
        AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
            invokeAwareMethods(beanName, bean);
            return null;
        }, getAccessControlContext());
    }
    else {
        // 特殊bean處理
        invokeAwareMethods(beanName, bean);
    }

    Object wrappedBean = bean;
    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
    }

    try {
        // 調用配置的 init-method
        invokeInitMethods(beanName, wrappedBean, mbd);
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
                (mbd != null ? mbd.getResourceDescription() : null),
                beanName, "Invocation of init method failed", ex);
    }
    if (mbd == null || !mbd.isSynthetic()) {
        wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
    }

    return wrappedBean;
}

最後加載完 Bean 並執行完初始化操做後,一個 bean 的加載基本就結束了。

加強的 Bean

經過上面的描述,咱們已經知道了一個的 Bean 是如何初始化的,已經具有普通 Bean 的功能。可是 Spring 還提供了一種加強的 Bean(FactoryBean),具有 factory 能力的 Bean,這個能力主要在 getObjectForBeanInstance 獲得。

public class AbstractBeanFactory {

    protected Object getObjectForBeanInstance(
            Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
    
        // 若是想要獲取 FactoryBean 自己,那麼 beanInstance 必須是 FactoryBean 的實例
        if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
            throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
        }
    
        // 若是 instance 不是 FactoryBean 實例,或者想要獲取的就是 FactoryBean 實例,那麼直接返回就好
        if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
            return beanInstance;
        }
    
        Object object = null;
        if (mbd == null) {
            // 獲取緩存的實例
            object = getCachedObjectForFactoryBean(beanName);
        }
        if (object == null) {
            // 緩存中沒有對象,那麼從頭準備 bean defition 實例化一個
            FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
            // Caches object obtained from FactoryBean if it is a singleton.
            if (mbd == null && containsBeanDefinition(beanName)) {
                mbd = getMergedLocalBeanDefinition(beanName);
            }
            boolean synthetic = (mbd != null && mbd.isSynthetic());
            // 在這裏面得到最終的FactoryBean
            object = getObjectFromFactoryBean(factory, beanName, !synthetic);
        }
        return object;
    }

}

關於 BeanFactoryFactoryBean 的區別:

  • BeanFactory: 容器的基本接口,是一個工廠,用來生產 Bean
  • FactoryBean: 是一個特殊的 Bean,能夠看成工廠使用的 Bean

循環依賴

上面有簡單提到過 循環依賴(只有在單例狀況下才會嘗試解決循環依賴)。 沒法解決就只能拋出 BeanCurrentlyInCreationException 異常

構造器循環依賴

沒法解決,只能拋出 BeanCurrentlyInCreationException 異常

<bean id="A" class="com.battcn.A">
  <constructor-arg index="0" ref="B" />
</bean>
<bean id="B" class="com.battcn.B">
  <constructor-arg index="0" ref="C" />
</bean>
<bean id="C" class="com.battcn.C">
  <constructor-arg index="0" ref="A" />
</bean>
setter 循環依賴

Spring 容器提早暴露了恰好完成的構造器注入,但未完成其餘步驟 (如 setter 注入bean)。只能解決單例做用域的 bean 循環依賴。

addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
prototype 範圍的依賴處理

Spring 容器沒法完成依賴注入,由於 Spring 容器不會緩存 prototype 做用域的 bean,所以沒法提早暴露一個建立中的 bean

<bean id="A" class="com.battcn.A" scope="prototype">
  <property name="B" ref="B" />
</bean>
<bean id="B" class="com.battcn.B" scope="prototype">
  <property name="C" ref="C" />
</bean>
<bean id="C" class="com.battcn.C" scope="prototype">
  <property name="A" ref="A" />
</bean>

總結

熬過幾個無人知曉的秋冬春夏,撐過去一切都會順着你想要的方向走...

說點什麼

全文代碼:https://gitee.com/battcn/battcn-spring-source/tree/master/Chapter4

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