一次性讲清楚spring中bean的生命周期之三：bean是如何实例化的

在前面的两篇博文《一次性讲清楚spring中bean的生命周期之一：getSingleton方法》和《一次性讲清楚spring中bean的生命周期之二：FactoryBean的前世今生》中分析了spring中bean生命周期的过程中的getSingleton方法和getObjectForBeanInstance方法，今天来分析另外一个重要的方法createBean方法。分析的入口是AbstractBeanFacotry.doGetBean方法，如下图，

这就是本次分析的入口。下面看该方法的详细定义，AbstractAutowireCapableBeanFactory.createBean

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

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

        // Make sure bean class is actually resolved at this point, and
        // clone the bean definition in case of a dynamically resolved Class
        // which cannot be stored in the shared merged bean definition.
        Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
        if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            mbdToUse = new RootBeanDefinition(mbd);
            mbdToUse.setBeanClass(resolvedClass);
        }

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

        try {
            // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
            //给beanPostProcessor一个返回目标类代理类的机会
            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 {
            //执行doCreatBean方法
            Object beanInstance = doCreateBean(beanName, mbdToUse, args);
            if (logger.isTraceEnabled()) {
                logger.trace("Finished creating instance of bean '" + beanName + "'");
            }
            return beanInstance;
        }
        catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
            // A previously detected exception with proper bean creation context already,
            // or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
            throw ex;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(
                    mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
        }
    }

重点部分已用红色标出，下面具体来分析

bean生命周期前

在上面的方法定义中有这样一段代码，

// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
            Object bean = resolveBeforeInstantiation(beanName, mbdToUse);

就是说在进入bean的创建之前，开发者可以自己返回一个目标类的代理类，如果返回了那么便直接返回，不会继续向下执行。看该方法怎么实现的，AbstractAutowireCapableBeanFactory.resolveBeforeInstantiation

@Nullable
    protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
        Object bean = null;
        if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
            // Make sure bean class is actually resolved at this point.
            if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
                Class<?> targetType = determineTargetType(beanName, mbd);
                if (targetType != null) {
                    //应用BeanPostProcessor实例化前
                    bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
                    if (bean != null) {
                       //应用BeanpPostProcessor初始化后
                        bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
                    }
                }
            }
            mbd.beforeInstantiationResolved = (bean != null);
        }
        return bean;
    }

该方法重要的就是applyBeanPostProcessorsBeforeInstantiation和applyBeanPostProcessorsAfterInitalization方法。

applyBeanPostProcessorsBeforeInstantiation

@Nullable
    protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            //在spring中有InstantiationAwareBeanPostProcessor的实例，则执行其postProcessBeforeInstantiation方法
            if (bp instanceof InstantiationAwareBeanPostProcessor) {
                InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
                if (result != null) {
                    return result;
                }
            }
        }
        return null;
    }

该方法的主要作用是如果在spring中有InstantiationAwareBeanPostProcessor类型的beanPostProcessor的化，会执行其postProcessBeforeInstantiation方法，也就是我们可以实现InstantiationAwareBeanPostProcessor接口，并实现其postProcessBeforeInstantiation方法。默认情况下该接口的方法返回null，

@Nullable
    default Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException {
        return null;
    }

applyBeanPostProcessorsAfterInitalization

回过头来看另外一个方法，

@Override
    public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
            throws BeansException {

        Object result = existingBean;
        //遍历spring容器中所有的beanPostProcessor，执行其postProcessAfterInitialization方法
        for (BeanPostProcessor processor : getBeanPostProcessors()) {
            Object current = processor.postProcessAfterInitialization(result, beanName);
            if (current == null) {
                return result;
            }
            result = current;
        }
        return result;
    }

BeanPostProcessor接口中该方法默认返回如下

@Nullable
    default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

在spring容器中会有多少BeanPostProcessor这个后面会分析。

分析完了进入bean生命周期前的方法，也就是留个开发者一个后门，通过实现InstantiationAwareBeanPostProcessor接口中的postProcessBeforeInstantiation方法可以自定义返回一个目标类型的代理对象。现在回到createBean方法中，真正进入bean的生命周期，看doCreateBean方法，

doCreateBean

该方法的篇幅过长，仅保留关键代码，其他代码删除，请知悉，

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

        // Instantiate the bean.
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
            //实例化bean
            instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        final Object bean = instanceWrapper.getWrappedInstance();
        Class<?> beanType = instanceWrapper.getWrappedClass();
        if (beanType != NullBean.class) {
            mbd.resolvedTargetType = beanType;
        }

        ....

        // 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.isTraceEnabled()) {
                logger.trace("Eagerly caching bean '" + beanName +
                        "' to allow for resolving potential circular references");
            }
            //放到singletonFactory中解决循环依赖的问题
            addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
        }

        // Initialize the bean instance.
        Object exposedObject = bean;
        try {
            //属性注入
            populateBean(beanName, mbd, instanceWrapper);
            //初始化bean
            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);
            }
        }

        ....

        return exposedObject;
    }

看createBeanInstance方法

createBeanInstance

该方法的定义如下，

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable 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());
        }

        Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
        if (instanceSupplier != null) {
            return obtainFromSupplier(instanceSupplier, beanName);
        }
        //使用实例工厂或静态工厂的方式生成bean实例
        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;
                }
            }
        }
        if (resolved) {
            if (autowireNecessary) {
                return autowireConstructor(beanName, mbd, null, null);
            }
            else {
                return instantiateBean(beanName, mbd);
            }
        }

        // Candidate constructors for autowiring?
        //检测是否有重写的构造方法
        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
        if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
            return autowireConstructor(beanName, mbd, ctors, args);
        }

        // Preferred constructors for default construction?
        //首选默认的构造方法
        ctors = mbd.getPreferredConstructors();
        if (ctors != null) {
            return autowireConstructor(beanName, mbd, ctors, null);
        }

        // No special handling: simply use no-arg constructor.
        //使用无参构造方法
        return instantiateBean(beanName, mbd);
    }

从上面的方法中可以看出实例化bean，其实就是使用类的构造方法来进行实例化，这里看下instantiateBean方法的过程，

instantiateBean

看该方法的定义，

protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
        try {
            Object beanInstance;
            final BeanFactory parent = this;
            if (System.getSecurityManager() != null) {
                beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
                        getInstantiationStrategy().instantiate(mbd, beanName, parent),
                        getAccessControlContext());
            }
            else {
               //生成bean的实例
                beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
            }
            //构造一个beanWrapper对象
            BeanWrapper bw = new BeanWrapperImpl(beanInstance);
            initBeanWrapper(bw);
            return bw;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
        }
    }

从上面的代码可以看到该方法中重要的就是下面这句

beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);

看下getInstantiationStrategy()方法返回什么，

/**
     * Return the instantiation strategy to use for creating bean instances.
     */
    protected InstantiationStrategy getInstantiationStrategy() {
        return this.instantiationStrategy;
    }

返回的是一个属性，

/** Strategy for creating bean instances. */
    private InstantiationStrategy instantiationStrategy = new CglibSubclassingInstantiationStrategy();

是一个CglibSubclassingInstantiationStragegy对象，看其instantiate方法，由于CglibSubclassingInstantiationStrategy继承了SimpleInstantiationStrategy类，该方法在父类SimpleInstantiationStrategy中

@Override
    public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
        // Don't override the class with CGLIB if no overrides.
        //如果没有使用lookup或replaced，不使用CGLIB重写该类
        if (!bd.hasMethodOverrides()) {
            Constructor<?> constructorToUse;
            synchronized (bd.constructorArgumentLock) {
                constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
                if (constructorToUse == null) {
                    final Class<?> clazz = bd.getBeanClass();
                    if (clazz.isInterface()) {
                        throw new BeanInstantiationException(clazz, "Specified class is an interface");
                    }
                    try {
                        if (System.getSecurityManager() != null) {
                            constructorToUse = AccessController.doPrivileged(
                                    (PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
                        }
                        else {
                            constructorToUse = clazz.getDeclaredConstructor();
                        }
                        bd.resolvedConstructorOrFactoryMethod = constructorToUse;
                    }
                    catch (Throwable ex) {
                        throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                    }
                }
            }
            return BeanUtils.instantiateClass(constructorToUse);
        }
        else {
            // Must generate CGLIB subclass.
            //使用cglib生成子类
            return instantiateWithMethodInjection(bd, beanName, owner);
        }
    }

从上面可以看出有两种方式生成实例，使用反射的机制

return BeanUtils.instantiateClass(constructorToUse);

如果有lookup或replaced，使用cglib方式

return instantiateWithMethodInjection(bd, beanName, owner);

BeanUtils.instantiateClass（反射）

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
        Assert.notNull(ctor, "Constructor must not be null");
        try {
            ReflectionUtils.makeAccessible(ctor);
            return (KotlinDetector.isKotlinReflectPresent() && KotlinDetector.isKotlinType(ctor.getDeclaringClass()) ?
                    KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args));
        }
        catch (InstantiationException ex) {
            throw new BeanInstantiationException(ctor, "Is it an abstract class?", ex);
        }
        catch (IllegalAccessException ex) {
            throw new BeanInstantiationException(ctor, "Is the constructor accessible?", ex);
        }
        catch (IllegalArgumentException ex) {
            throw new BeanInstantiationException(ctor, "Illegal arguments for constructor", ex);
        }
        catch (InvocationTargetException ex) {
            throw new BeanInstantiationException(ctor, "Constructor threw exception", ex.getTargetException());
        }
    }

标红的代码就是生成的逻辑，可以看到是利用的java的反射机制，也就是使用Constructor类的newInstance方法。

instantiateWithMethodInjection（cglib）

该方法在CglibSubclassingInstantiationStrategy中

@Override
    protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
        return instantiateWithMethodInjection(bd, beanName, owner, null);
    }

@Override
    protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
            @Nullable Constructor<?> ctor, Object... args) {

        // Must generate CGLIB subclass...
        return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
    }

使用的是CglibSubclassCreator生成的实例，具体方式就是cglib生成代理类的方式，

public Object instantiate(@Nullable Constructor<?> ctor, Object... args) {
            Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
            Object instance;
            if (ctor == null) {
                instance = BeanUtils.instantiateClass(subclass);
            }
            else {
                try {
                    Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
                    instance = enhancedSubclassConstructor.newInstance(args);
                }
                catch (Exception ex) {
                    throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
                            "Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
                }
            }
            // SPR-10785: set callbacks directly on the instance instead of in the
            // enhanced class (via the Enhancer) in order to avoid memory leaks.
            Factory factory = (Factory) instance;
            factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
                    new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
                    new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
            return instance;
        }

分析完毕spring中单例bean的实例化过程。

总结

至此分析完了spring中单例bean的实例化过程，主要有两点，

1、类中有lookup或replaced方式，使用cglib的方式生成bean的实例；

2、类中无lookup或replaced方式，使用java反射机制Constructor生成实例；

推荐阅读

《一次性讲清楚spring中bean的生命周期之二：FactoryBean的前世今生 》

《一次性讲清楚spring中bean的生命周期之一：getSingleton方法》