Spring4.3.x 容器中bean的创建过程（2）——实例化Bean

概述

在Spring4.3.x 容器中bean的创建过程（1）——走进初始化bean的主要战场一篇中我们找到了初始化bean的主要场所在AbstractAutowireCapableBeanFactory类中的doCreateBean方法中，在这个方法中首先调用createBeanInstance方法创建bean的实例；然后调用populateBean方法设置bean的属性；接着调用initializeBean方法执行Bean后处理器和InitializingBean对象的afterPropertiesSet方法以及init-method方法；最后调用registerDisposableBeanIfNecessary方法注册bean的销毁方法。它的每一步都比较繁琐，所以我把他们分开来探讨，在这一篇我们只讨论第一步bean的实例化过程中会遇到哪些情况，以及如何处理各个情况。

创建对象有两种方式，其一是直接使用构造器创建对象；其二是使用工厂模式间接创建对象，这中方式又分为静态工厂、工厂方法和抽象工厂3类。在spring中，从纯XML配置的角度出发有四种方式，第一种是使用默认构造器，第二种是使用带参构造器，第三种是使用静态工厂，第四种是使用实例工厂。

    <!--默认构造器创建bean-->    <bean id="studentServiceDefault" class="com.chyohn.service.StudentService">        <property name="name" value="默认构造器"/>    </bean>    <!-- 指定构造器创建bean-->    <bean id="studentServiceArgs" class="com.chyohn.service.StudentService">        <constructor-arg name="name" value="带参构造器"/>    </bean>    <!-- 静态工厂创建bean -->    <bean id="studentServiceStaticF" class="com.chyohn.factory.StudentServiceFactory" factory-method="createSudentService" />    <!-- 实例工厂创建bean -->    <bean id="studentServiceFactoryBean" class="com.chyohn.factory.StudentServiceFactory"/>    <bean id="studentServiceSimpleF" factory-bean="studentServiceFactoryBean" factory-method="createSudentService">        <constructor-arg name="name" value="简单工厂"/>    </bean>

Spring中不管是工厂方法还是抽象工厂，只要需要实例化工厂对象的都需要通过第四种配置方式创建bean。下面我们看看Spring如何创建这四种配置方式的bean。

实例化bean

AbstractAutowireCapableBeanFactory类中的doCreateBean方法调用它的createBeanInstance方法实例化bean并返回bean的包装器BeanWrapper对象，源码如下。

    protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {        // 获取bean的Class对象        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)  {            // 使用指定的工厂方法创建bean示例            return instantiateUsingFactoryMethod(beanName, mbd, args);        }        // 再一次创建已经创建过的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);            }        }        // 获取bean的构造器        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);        if (ctors != null ||                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {            // 通过带参构造器创建bean对象            return autowireConstructor(beanName, mbd, ctors, args);        }        // 通过无参构造器创建bean对象        return instantiateBean(beanName, mbd);    }

createBeanInstance方法所做的事情非常清晰，它根据bean的定义选择bean采用何种方式创建，这个方法把bean的创建分成3种方式，其一是通过指定的工厂创建bean，这个包括使用实例工厂或静态工厂；其二是通过带参构造器创建bean；其三是通过无参构造器创建bean。下面我们分别探讨这三种方式。

1. 使用工厂创建bean

createBeanInstance方法调用AbstractAutowireCapableBeanFactory对象的instantiateUsingFactoryMethod方法通过使用工厂方法创建bean实例的入口，源码如下。

    protected BeanWrapper instantiateUsingFactoryMethod(            String beanName, RootBeanDefinition mbd, Object[] explicitArgs) {        return new ConstructorResolver(this).instantiateUsingFactoryMethod(beanName, mbd, explicitArgs);    }

instantiateUsingFactoryMethod方法首先创建一个ConstructorResolver对象后，调用这个对象的instantiateUsingFactoryMethod方法来实例化bean。instantiateUsingFactoryMethod方法的源码如下。

    public BeanWrapper instantiateUsingFactoryMethod(            final String beanName, final RootBeanDefinition mbd, final Object[] explicitArgs) {        BeanWrapperImpl bw = new BeanWrapperImpl();        // 向BeanWrapper对象中添加ConversionService对象和属性编辑器PropertyEditor对象        this.beanFactory.initBeanWrapper(bw);        Object factoryBean;        Class<?> factoryClass;        boolean isStatic;        String factoryBeanName = mbd.getFactoryBeanName();        if (factoryBeanName != null) {            if (factoryBeanName.equals(beanName)) {                throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,                        "factory-bean reference points back to the same bean definition");            }            // 获取工厂实例            factoryBean = this.beanFactory.getBean(factoryBeanName);            if (factoryBean == null) {                throw new BeanCreationException(mbd.getResourceDescription(), beanName,                        "factory-bean '" + factoryBeanName + "' (or a BeanPostProcessor involved) returned null");            }            if (mbd.isSingleton() && this.beanFactory.containsSingleton(beanName)) {                throw new IllegalStateException("About-to-be-created singleton instance implicitly appeared " +                        "through the creation of the factory bean that its bean definition points to");            }            factoryClass = factoryBean.getClass();            isStatic = false;        } else {            // 静态工厂创建bean，必须要提供工厂的全类名            if (!mbd.hasBeanClass()) {                throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,                        "bean definition declares neither a bean class nor a factory-bean reference");            }            factoryBean = null;            factoryClass = mbd.getBeanClass();            isStatic = true;        }        Method factoryMethodToUse = null;        ArgumentsHolder argsHolderToUse = null;        Object[] argsToUse = null;        // 获取工厂方法的参数        if (explicitArgs != null) {            argsToUse = explicitArgs;        } else {            Object[] argsToResolve = null;            synchronized (mbd.constructorArgumentLock) {                factoryMethodToUse = (Method) mbd.resolvedConstructorOrFactoryMethod;                if (factoryMethodToUse != null && mbd.constructorArgumentsResolved) {                    // Found a cached factory method...                    argsToUse = mbd.resolvedConstructorArguments;                    if (argsToUse == null) {                        argsToResolve = mbd.preparedConstructorArguments;                    }                }            }            if (argsToResolve != null) {                argsToUse = resolvePreparedArguments(beanName, mbd, bw, factoryMethodToUse, argsToResolve);            }        }        // 获取工厂方法，和工厂方法所需要的参数        if (factoryMethodToUse == null || argsToUse == null) {            // 工厂对象可能是一个被代理的对象，那么需要获取正确的工厂类全名称            factoryClass = ClassUtils.getUserClass(factoryClass);            // 获取所有待定的工厂方法            Method[] rawCandidates = getCandidateMethods(factoryClass, mbd);            List<Method> candidateSet = new ArrayList<Method>();            for (Method candidate : rawCandidates) {                if (Modifier.isStatic(candidate.getModifiers()) == isStatic && mbd.isFactoryMethod(candidate)) {                    candidateSet.add(candidate);                }            }            Method[] candidates = candidateSet.toArray(new Method[candidateSet.size()]);            AutowireUtils.sortFactoryMethods(candidates);            ConstructorArgumentValues resolvedValues = null;            boolean autowiring = (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR);            int minTypeDiffWeight = Integer.MAX_VALUE;            Set<Method> ambiguousFactoryMethods = null;            int minNrOfArgs;            if (explicitArgs != null) {                minNrOfArgs = explicitArgs.length;            } else {                ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();                resolvedValues = new ConstructorArgumentValues();                minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);            }            LinkedList<UnsatisfiedDependencyException> causes = null;            for (Method candidate : candidates) {                Class<?>[] paramTypes = candidate.getParameterTypes();                if (paramTypes.length >= minNrOfArgs) {                    ArgumentsHolder argsHolder;                    if (resolvedValues != null) {                        try {                            String[] paramNames = null;                            ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();                            if (pnd != null) {                                paramNames = pnd.getParameterNames(candidate);                            }                            argsHolder = createArgumentArray(                                    beanName, mbd, resolvedValues, bw, paramTypes, paramNames, candidate, autowiring);                        } catch (UnsatisfiedDependencyException ex) {                            if (this.beanFactory.logger.isTraceEnabled()) {                                this.beanFactory.logger.trace("Ignoring factory method [" + candidate +                                        "] of bean '" + beanName + "': " + ex);                            }                            if (causes == null) {                                causes = new LinkedList<UnsatisfiedDependencyException>();                            }                            causes.add(ex);                            continue;                        }                    } else {                        // 如果在配置的时候，参数个数必须要与方法的参数个数相同                        if (paramTypes.length != explicitArgs.length) {                            continue;                        }                        argsHolder = new ArgumentsHolder(explicitArgs);                    }                    int typeDiffWeight = (mbd.isLenientConstructorResolution() ?                            argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));                    if (typeDiffWeight < minTypeDiffWeight) {                        // 查找到完全匹配的工厂方法                        factoryMethodToUse = candidate;                        argsHolderToUse = argsHolder;                        argsToUse = argsHolder.arguments;                        minTypeDiffWeight = typeDiffWeight;                        ambiguousFactoryMethods = null;                    } else if (factoryMethodToUse != null && typeDiffWeight == minTypeDiffWeight &&                            !mbd.isLenientConstructorResolution() &&                            paramTypes.length == factoryMethodToUse.getParameterTypes().length &&                            !Arrays.equals(paramTypes, factoryMethodToUse.getParameterTypes())) {                        // 查找到多个可匹配的方法                        if (ambiguousFactoryMethods == null) {                            ambiguousFactoryMethods = new LinkedHashSet<Method>();                            ambiguousFactoryMethods.add(factoryMethodToUse);                        }                        ambiguousFactoryMethods.add(candidate);                    }                }            }            if (factoryMethodToUse == null) {                // 没有查找到可执行的工厂方法，抛出异常                if (causes != null) {                    UnsatisfiedDependencyException ex = causes.removeLast();                    for (Exception cause : causes) {                        this.beanFactory.onSuppressedException(cause);                    }                    throw ex;                }                List<String> argTypes = new ArrayList<String>(minNrOfArgs);                if (explicitArgs != null) {                    for (Object arg : explicitArgs) {                        argTypes.add(arg != null ? arg.getClass().getSimpleName() : "null");                    }                } else {                    Set<ValueHolder> valueHolders = new LinkedHashSet<ValueHolder>(resolvedValues.getArgumentCount());                    valueHolders.addAll(resolvedValues.getIndexedArgumentValues().values());                    valueHolders.addAll(resolvedValues.getGenericArgumentValues());                    for (ValueHolder value : valueHolders) {                        String argType = (value.getType() != null ? ClassUtils.getShortName(value.getType()) :                                (value.getValue() != null ? value.getValue().getClass().getSimpleName() : "null"));                        argTypes.add(argType);                    }                }                String argDesc = StringUtils.collectionToCommaDelimitedString(argTypes);                throw new BeanCreationException(mbd.getResourceDescription(), beanName,                        "No matching factory method found: " +                        (mbd.getFactoryBeanName() != null ?                            "factory bean '" + mbd.getFactoryBeanName() + "'; " : "") +                        "factory method '" + mbd.getFactoryMethodName() + "(" + argDesc + ")'. " +                        "Check that a method with the specified name " +                        (minNrOfArgs > 0 ? "and arguments " : "") +                        "exists and that it is " +                        (isStatic ? "static" : "non-static") + ".");            } else if (void.class == factoryMethodToUse.getReturnType()) {                // 查找到的工厂方法的返回类型为void，抛出异常                throw new BeanCreationException(mbd.getResourceDescription(), beanName,                        "Invalid factory method '" + mbd.getFactoryMethodName() +                        "': needs to have a non-void return type!");            } else if (ambiguousFactoryMethods != null) {                // 查找到多个匹配的工厂方法，抛出异常                throw new BeanCreationException(mbd.getResourceDescription(), beanName,                        "Ambiguous factory method matches found in bean '" + beanName + "' " +                        "(hint: specify index/type/name arguments for simple parameters to avoid type ambiguities): " +                        ambiguousFactoryMethods);            }            if (explicitArgs == null && argsHolderToUse != null) {                // 没有传入参数且                argsHolderToUse.storeCache(mbd, factoryMethodToUse);            }        }        try {            // 实例化bean            Object beanInstance;            if (System.getSecurityManager() != null) {                final Object fb = factoryBean;                final Method factoryMethod = factoryMethodToUse;                final Object[] args = argsToUse;                beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {                    @Override                    public Object run() {                        // 通过执行工厂方法来创建bean示例                        return beanFactory.getInstantiationStrategy().instantiate(                                mbd, beanName, beanFactory, fb, factoryMethod, args);                    }                }, beanFactory.getAccessControlContext());            } else {                // 通过执行工厂方法来创建bean示例                beanInstance = this.beanFactory.getInstantiationStrategy().instantiate(                        mbd, beanName, this.beanFactory, factoryBean, factoryMethodToUse, argsToUse);            }            if (beanInstance == null) {                return null;            }            // 让BeanWraper对象保存bean实例            bw.setBeanInstance(beanInstance);            return bw;        } catch (Throwable ex) {            throw new BeanCreationException(mbd.getResourceDescription(), beanName,                    "Bean instantiation via factory method failed", ex);        }    }

instantiateUsingFactoryMethod方法首先获取工厂对象，然后获取工厂方法及其参数，最后调用InstantiationStrategy对象的instantiate方法来创建bean实例。Spring bean工厂默认的InstantiationStrategy对象是CglibSubclassingInstantiationStrategy，这个instantiate方法在CglibSubclassingInstantiationStrategy的父类SimpleInstantiationStrategy中有实现，代码如下。

    @Override    public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner,            Object factoryBean, final Method factoryMethod, Object... args) {        try {            // 设置方法的访问权限为可访问            if (System.getSecurityManager() != null) {                AccessController.doPrivileged(new PrivilegedAction<Object>() {                    @Override                    public Object run() {                        ReflectionUtils.makeAccessible(factoryMethod);                        return null;                    }                });            } else {                ReflectionUtils.makeAccessible(factoryMethod);            }            Method priorInvokedFactoryMethod = currentlyInvokedFactoryMethod.get();            try {                currentlyInvokedFactoryMethod.set(factoryMethod);                // 执行工厂方法，并返回实例                return factoryMethod.invoke(factoryBean, args);            } finally {                if (priorInvokedFactoryMethod != null) {                    currentlyInvokedFactoryMethod.set(priorInvokedFactoryMethod);                } else {                    currentlyInvokedFactoryMethod.remove();                }            }        } catch (IllegalArgumentException ex) {            throw new BeanInstantiationException(factoryMethod,                    "Illegal arguments to factory method '" + factoryMethod.getName() + "'; " +                    "args: " + StringUtils.arrayToCommaDelimitedString(args), ex);        } catch (IllegalAccessException ex) {            throw new BeanInstantiationException(factoryMethod,                    "Cannot access factory method '" + factoryMethod.getName() + "'; is it public?", ex);        } catch (InvocationTargetException ex) {            String msg = "Factory method '" + factoryMethod.getName() + "' threw exception";            if (bd.getFactoryBeanName() != null && owner instanceof ConfigurableBeanFactory &&                    ((ConfigurableBeanFactory) owner).isCurrentlyInCreation(bd.getFactoryBeanName())) {                msg = "Circular reference involving containing bean '" + bd.getFactoryBeanName() + "' - consider " +                        "declaring the factory method as static for independence from its containing instance. " + msg;            }            throw new BeanInstantiationException(factoryMethod, msg, ex.getTargetException());        }    }

instantiate方法最主要的目的，就是通过java反射执行工厂方法并返回创建好的实例。

2. 使用带参构造器创建bean

createBeanInstance方法调用AbstractAutowireCapableBeanFactory对象的autowireConstructor方法是使用带参构造器创建bean实例的入口，源码如下。

    protected BeanWrapper autowireConstructor(            String beanName, RootBeanDefinition mbd, Constructor<?>[] ctors, Object[] explicitArgs) {        return new ConstructorResolver(this).autowireConstructor(beanName, mbd, ctors, explicitArgs);    }

autowireConstructor方法首先创建一个ConstructorResolver对象后，调用这个对象的autowireConstructor方法来实例化bean。autowireConstructor方法的源码如下。

    public BeanWrapper autowireConstructor(final String beanName, final RootBeanDefinition mbd,            Constructor<?>[] chosenCtors, final Object[] explicitArgs) {        BeanWrapperImpl bw = new BeanWrapperImpl();        // 向BeanWrapper对象中添加ConversionService对象和属性编辑器PropertyEditor对象        this.beanFactory.initBeanWrapper(bw);        Constructor<?> constructorToUse = null;        ArgumentsHolder argsHolderToUse = null;        Object[] argsToUse = null;        if (explicitArgs != null) {            argsToUse = explicitArgs;        } else {            Object[] argsToResolve = null;            synchronized (mbd.constructorArgumentLock) {                constructorToUse = (Constructor<?>) mbd.resolvedConstructorOrFactoryMethod;                if (constructorToUse != null && mbd.constructorArgumentsResolved) {                    // 从缓存中获取构造器和参数                    argsToUse = mbd.resolvedConstructorArguments;                    if (argsToUse == null) {                        argsToResolve = mbd.preparedConstructorArguments;                    }                }            }            if (argsToResolve != null) {                argsToUse = resolvePreparedArguments(beanName, mbd, bw, constructorToUse, argsToResolve);            }        }        if (constructorToUse == null) {            // 相应的构造器不能从缓存中获取时            // 获取正确的构造器及其参数            boolean autowiring = (chosenCtors != null ||                    mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR);            ConstructorArgumentValues resolvedValues = null;            int minNrOfArgs;            if (explicitArgs != null) {                minNrOfArgs = explicitArgs.length;            } else {                ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();                resolvedValues = new ConstructorArgumentValues();                minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);            }            Constructor<?>[] candidates = chosenCtors;            if (candidates == null) {                Class<?> beanClass = mbd.getBeanClass();                try {                    candidates = (mbd.isNonPublicAccessAllowed() ?                            beanClass.getDeclaredConstructors() : beanClass.getConstructors());                } catch (Throwable ex) {                    throw new BeanCreationException(mbd.getResourceDescription(), beanName,                            "Resolution of declared constructors on bean Class [" + beanClass.getName() +                            "] from ClassLoader [" + beanClass.getClassLoader() + "] failed", ex);                }            }            AutowireUtils.sortConstructors(candidates);            int minTypeDiffWeight = Integer.MAX_VALUE;            Set<Constructor<?>> ambiguousConstructors = null;            LinkedList<UnsatisfiedDependencyException> causes = null;            for (Constructor<?> candidate : candidates) {                Class<?>[] paramTypes = candidate.getParameterTypes();                if (constructorToUse != null && argsToUse.length > paramTypes.length) {                    // 找到匹配的构造器，这终止循环                    break;                }                if (paramTypes.length < minNrOfArgs) {                    // 当前循环的构造器参数个数小于要求的最小参数个数，那么不匹配，继续从匹配下一个                    continue;                }                ArgumentsHolder argsHolder;                if (resolvedValues != null) {                    // 没有提供构造器参数时执行此段代码                    // 这段代码的作用是获取构造器的参数，可能需要自动装配，比如@Autowire、@Value注解的参数                    try {                        // 从@ConstructorProperties注解中获取构造器的参数名称列表                        String[] paramNames = ConstructorPropertiesChecker.evaluate(candidate, paramTypes.length);                        if (paramNames == null) {                            // 通过ParameterNameDiscoverer对象获取构造器的参数名称列表                            ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();                            if (pnd != null) {                                paramNames = pnd.getParameterNames(candidate);                            }                        }                        // 获取构造器参数值列表                        // 在这一步会先把未初始化的参数初始化                        argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw, paramTypes, paramNames,                                getUserDeclaredConstructor(candidate), autowiring);                    } catch (UnsatisfiedDependencyException ex) {                        if (this.beanFactory.logger.isTraceEnabled()) {                            this.beanFactory.logger.trace(                                    "Ignoring constructor [" + candidate + "] of bean '" + beanName + "': " + ex);                        }                        if (causes == null) {                            causes = new LinkedList<UnsatisfiedDependencyException>();                        }                        causes.add(ex);                        continue;                    }                } else {                    // 当前循环的构造器参数个数必须等于提供的参数个数，否则继续查找下一个                    if (paramTypes.length != explicitArgs.length) {                        continue;                    }                    argsHolder = new ArgumentsHolder(explicitArgs);                }                int typeDiffWeight = (mbd.isLenientConstructorResolution() ?                        argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));                if (typeDiffWeight < minTypeDiffWeight) {                    // 完全匹配构造器                    constructorToUse = candidate;                    argsHolderToUse = argsHolder;                    argsToUse = argsHolder.arguments;                    minTypeDiffWeight = typeDiffWeight;                    ambiguousConstructors = null;                } else if (constructorToUse != null && typeDiffWeight == minTypeDiffWeight) {                    // 多个可匹配的构造器                    if (ambiguousConstructors == null) {                        ambiguousConstructors = new LinkedHashSet<Constructor<?>>();                        ambiguousConstructors.add(constructorToUse);                    }                    ambiguousConstructors.add(candidate);                }            }            if (constructorToUse == null) {                // 没有找到匹配的构造器                if (causes != null) {                    UnsatisfiedDependencyException ex = causes.removeLast();                    for (Exception cause : causes) {                        this.beanFactory.onSuppressedException(cause);                    }                    throw ex;                }                throw new BeanCreationException(mbd.getResourceDescription(), beanName,                        "Could not resolve matching constructor " +                        "(hint: specify index/type/name arguments for simple parameters to avoid type ambiguities)");            } else if (ambiguousConstructors != null && !mbd.isLenientConstructorResolution()) {                // 查找到多个构造器                throw new BeanCreationException(mbd.getResourceDescription(), beanName,                        "Ambiguous constructor matches found in bean '" + beanName + "' " +                        "(hint: specify index/type/name arguments for simple parameters to avoid type ambiguities): " +                        ambiguousConstructors);            }            if (explicitArgs == null) {                // 没有提供构造器参数的情况，保存到缓存中以方便下次构造的时候直接取用                // 其他情况参数个数和类型变动性大、不确认因素多，没有必要存缓存                argsHolderToUse.storeCache(mbd, constructorToUse);            }        }        try {            // 拿到构造器对象和其所需参数后            // 实例化bean            Object beanInstance;            if (System.getSecurityManager() != null) {                final Constructor<?> ctorToUse = constructorToUse;                final Object[] argumentsToUse = argsToUse;                beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {                    @Override                    public Object run() {                        // 通过构造器实例化bean                        return beanFactory.getInstantiationStrategy().instantiate(                                mbd, beanName, beanFactory, ctorToUse, argumentsToUse);                    }                }, beanFactory.getAccessControlContext());            } else {                // 通过构造器实例化bean                beanInstance = this.beanFactory.getInstantiationStrategy().instantiate(                        mbd, beanName, this.beanFactory, constructorToUse, argsToUse);            }            // 让BeanWraper对象保存bean实例            bw.setBeanInstance(beanInstance);            return bw;        } catch (Throwable ex) {            throw new BeanCreationException(mbd.getResourceDescription(), beanName,                    "Bean instantiation via constructor failed", ex);        }    }

autowireConstructor方法首先获取构造器的参数列表对象和构造器对象，最后执行初始化策略器InstantiationStrategy对象的instantiate方法，这个方法的实现在SimpleInstantiationStrategy类中，源码如下。

    @Override    public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner,            final Constructor<?> ctor, Object... args) {        if (bd.getMethodOverrides().isEmpty()) {            // 没有配置lookup-method或者replaced-method标签或者@Lookup注解            if (System.getSecurityManager() != null) {                // use own privileged to change accessibility (when security is on)                AccessController.doPrivileged(new PrivilegedAction<Object>() {                    @Override                    public Object run() {                        ReflectionUtils.makeAccessible(ctor);                        return null;                    }                });            }            // 通过BeanUtils直接使用构造器对象实例化bean            return BeanUtils.instantiateClass(ctor, args);        } else {            // 生成CGLIB创建的子类对象            return instantiateWithMethodInjection(bd, beanName, owner, ctor, args);        }    }

如果bean的配置没有lookup-method或者replaced-method标签或者@Lookup注解，则直接使用构造器创建bean，否则调用SimpleInstantiationStrategy类或者其子类的instantiateWithMethodInjection方法。这个方法在SimpleInstantiationStrategy类的实现如下。

    protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner,            Constructor<?> ctor, Object... args) {        throw new UnsupportedOperationException("Method Injection not supported in SimpleInstantiationStrategy");    }

下面是CglibSubclassingInstantiationStrategy类实现的instantiateWithMethodInjection方法的源码。

    @Override    protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner,            Constructor<?> ctor, Object... args) {        // 通过CGLIB生成一个子类对象        return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);    }

CglibSubclassCreator类是CglibSubclassingInstantiationStrategy的私有静态类，下面是它的instantiate方法源码。

        public Object instantiate(Constructor<?> ctor, Object... args) {            // 通过Cglib创建一个代理类            Class<?> subclass = createEnhancedSubclass(this.beanDefinition);            Object instance;            if (ctor == null) {                // 没有提供构造器对象，通过BeanUtils工具类使用默认构造器创建一个bean实例                instance = BeanUtils.instantiateClass(subclass);            } else {                try {                    // 获取代理类对应的构造器对象，并实例化bean                    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);                }            }            // 为了避免memory leaks异常，直接在bean实例上设置回调对象            Factory factory = (Factory) instance;            factory.setCallbacks(new Callback[] {NoOp.INSTANCE,                    new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),                    new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});            return instance;        }

CglibSubclassCreator类对象通过调用它的createEnhancedSubclass方法来创建一个Cglib代理的子类，源码如下。

        private Class<?> createEnhancedSubclass(RootBeanDefinition beanDefinition) {            Enhancer enhancer = new Enhancer();            enhancer.setSuperclass(beanDefinition.getBeanClass());            enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);            if (this.owner instanceof ConfigurableBeanFactory) {                ClassLoader cl = ((ConfigurableBeanFactory) this.owner).getBeanClassLoader();                enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(cl));            }            enhancer.setCallbackFilter(new MethodOverrideCallbackFilter(beanDefinition));            enhancer.setCallbackTypes(CALLBACK_TYPES);            return enhancer.createClass();        }

3. 使用无参构造器创建bean

createBeanInstance方法调用AbstractAutowireCapableBeanFactory对象的instantiateBean方法是使用无参构造器创建bean实例的入口，源码如下。

    protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {        try {            Object beanInstance;            final BeanFactory parent = this;            if (System.getSecurityManager() != null) {                beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {                    @Override                    public Object run() {                        return getInstantiationStrategy().instantiate(mbd, beanName, parent);                    }                }, getAccessControlContext());            } else {                beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);            }            // 创建BeanWrapper对象            BeanWrapper bw = new BeanWrapperImpl(beanInstance);            // 初始化BeanWrapper,向BeanWrapper对象中添加ConversionService对象和属性编辑器PropertyEditor对象            initBeanWrapper(bw);            return bw;        } catch (Throwable ex) {            throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);        }    }

instantiateBean方法通过调用InstantiationStrategy对象的instantiateBean方法创建bean实例，这个instantiateBean方法在SimpleInstantiationStrategy类中的实现代码如下。

    @Override    public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {        // Don't override the class with CGLIB if no overrides.        if (bd.getMethodOverrides().isEmpty()) {            // 没有配置lookup-method或者replaced-method标签或者@Lookup注解            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 {                        // 获取默认构造器对象                        if (System.getSecurityManager() != null) {                            constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor<?>>() {                                @Override                                public Constructor<?> run() throws Exception {                                    return clazz.getDeclaredConstructor((Class[]) null);                                }                            });                        } else {                            constructorToUse =  clazz.getDeclaredConstructor((Class[]) null);                        }                        // 缓存默认构造器对象                        bd.resolvedConstructorOrFactoryMethod = constructorToUse;                    } catch (Throwable ex) {                        throw new BeanInstantiationException(clazz, "No default constructor found", ex);                    }                }            }            return BeanUtils.instantiateClass(constructorToUse);        } else {            // 生成CGLIB创建的子类对象            return instantiateWithMethodInjection(bd, beanName, owner);        }    }

关于生成CGLIB创建的子类对象调用调用的方法和使用带参构造器实例化bean一节的一样，这里就不重复说了。

总结

（1）Spring通过实例化策略接口InstantiationStrategy的派生类创建bean，这个接口的源码如下。

public interface InstantiationStrategy {    /**     * 使用默认构造器创建bean实例     * @param bd bean定义信息     * @param beanName bean名称，可以为空     * @param owner bean工厂     * @return     * @throws BeansException     */    Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner)            throws BeansException;    /**     * 通过指定的构造器创建bean实例     * @param bd bean定义信息     * @param beanName bean名称，可以为空     * @param owner bean工厂     * @param ctor 构造器对象     * @param args 构造器所需参数     * @return      * @throws BeansException     */    Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner,            Constructor<?> ctor, Object... args) throws BeansException;    /**     * 根据指定的工厂方法创建bean实例     * @param bd bean的定义信息     * @param beanName bean名称，可以为空     * @param owner bean工厂     * @param factoryBean 工厂对象，如果为静态工厂，此参数为null     * @param factoryMethod 工厂方法对象     * @param args 工厂方法所需参数     * @return      * @throws BeansException      */    Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner,            Object factoryBean, Method factoryMethod, Object... args) throws BeansException;}

Spring默认的实例bean策略类为CglibSubclassingInstantiationStrategy。

（2）使用了lookup-method或者replaced-method标签或者@Lookup注解的bean，Spring使用Cglib的Enhancer类创建它们的代理子类。