Spring DI依赖注入的实现

x33g5p2x  于2021-03-14 发布在 Spring  
字(30.9k)|赞(0)|评价(0)|浏览(172)

上面对Ioc容器的初始化过程进行了分析,而这个初始化过程主要是在Ioc容器中建立BeanDefinition数据映射。但此过程并没有出现对Bean依赖关系进行注入,接下来,我们对Bean依赖关系注入进行分析。

依赖注入的时序图

AbstractBeanFactory.java

//--------------------------------------------------------------------------------------------
// 这里是对 BeanFactory 接口的实现,比如getBean接口方法
// 这些都是通过doGetBean来实现
// BeanFactory 接口 对应的5个实现,4个在这里,还有一个因为其特殊性并不是在这里重载或者实现的
//--------------------------------------------------------------------------------------------
public Object getBean(String name) throws BeansException {
        return doGetBean(name, null, null, false);
}

//这个方法是我的程序调用的
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
        return doGetBean(name, requiredType, null, false);    
}

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

public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
        return doGetBean(name, requiredType, args, false);
}

//这里是实际触发依赖注入的地方
protected <T> T doGetBean(
        final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
                throws BeansException {
        //指定名称获取被管理的Bean名称,并且指定名称中对容器的依赖
        //如果是别名,将别名转变为Bean的名称
        final String beanName = transformedBeanName(name);
        Object bean;

        //先缓存取得的Bean以及处理创建过的单件模式的Bean
        //单态模式的Bean只能创建一次
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) {
                if (logger.isDebugEnabled()) {
                        //如果指定名称的Bean在容器中已经存在,则直接返回
                        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 + "'");
                        }
                }
                //这里完成FactoryBean的相关处理,对于FactoryBean,BeanFactory已经有所提及,在后面会详细分析
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }
        else {
                //缓存中如果已经存在创建的单态模式,因为循环而创建失败,则抛出异常
                if (isPrototypeCurrentlyInCreation(beanName)) {
                        throw new BeanCurrentlyInCreationException(beanName);
                }
                
                //对容器中的BeanDefinition进行检查,检查能否在当前的BeanFactory中取得Bean.
                //如果在当前的工厂中取不到,则到父类的BeanFactory中去取
                //如果在父类中取不到,则到父类的父类中取
                BeanFactory parentBeanFactory = getParentBeanFactory();
                //当前容器的父容器存在,且当前容器中不存在指名的Bean
                if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                        String nameToLookup = originalBeanName(name);    //解析Bean的原始名
                        if (args != null) {
                                //委派给父类容器查找,根据指定的名称和显示参数
                                return (T) parentBeanFactory.getBean(nameToLookup, args);
                        }
                        else {
                                ///委派给父类容器查找,根据指定的名称和类型
                                return parentBeanFactory.getBean(nameToLookup, requiredType);
                        }
                }
                
                //创建的Bean是否需要进行验证,一般不需要
                if (!typeCheckOnly) {
                        //向容器指定的Bean已被创建
                        markBeanAsCreated(beanName);
                }

                try {
                        //根据Bean的名字获取BeanDefinition
                        final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                        checkMergedBeanDefinition(mbd, beanName, args);

                        //获取当前Bean的所有依赖的名称
                        String[] dependsOn = mbd.getDependsOn();
                        如果当前Bean有依赖Bean
                        if (dependsOn != null) {
                                for (String dependsOnBean : dependsOn) {
                                        if (isDependent(beanName, dependsOnBean)) {
                                                throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                                        "Circular depends-on relationship between '" + beanName + "' and '" 
                                                        + dependsOnBean + "'");
                                        }
                                        //把被依赖的Bean注册给当前依赖的Bean
                                        registerDependentBean(dependsOnBean, beanName);
                                        //触发递归
                                        getBean(dependsOnBean);    
                                }
                        }

                        //创建单态模式的Bean实例对象
                        if (mbd.isSingleton()) {
                                //使用一个内部匿名类,创建Bean实例对象,并且注册对所依赖的对象
                                sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                                        @Override
                                        public Object getObject() throws BeansException {
                                                try {
                                                        //创建一个指定Bean实例对象,如果有父级继承,则合并资对象
                                                        return createBean(beanName, mbd, args);
                                                }
                                                catch (BeansException ex) {
                                                        //清除显示容器单例模式Bean缓存中的实例对象
                                                        destroySingleton(beanName);
                                                        throw ex;
                                                }
                                        }
                                });
                                //获取给定Bean实例对象
                                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                        }
                        else if (mbd.isPrototype()) {  // 判断是否是原型模式
                                //原型模式会每次创建一个新的对象
                                Object prototypeInstance = null;
                                try {
                                        //回调方法,注册原型对象
                                        beforePrototypeCreation(beanName);
                                        //创建指定Bean对象实例
                                        prototypeInstance = createBean(beanName, mbd, args);
                                }
                                finally {
                                        //回调方法,Bean无法再次创建
                                        afterPrototypeCreation(beanName);
                                }
                                //获取给定Bean的实例对象
                                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
                        }
                        else {
                                String scopeName = mbd.getScope();
                                final Scope scope = this.scopes.get(scopeName);
                                //Bean定义资源中没有配置生命周期范围,则Bean不合法
                                if (scope == null) {
                                        throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
                                }
                                try {
                                        //这里使用一个匿名类,获取一个指定的生命周期范围
                                        Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
                                                @Override
                                                public Object getObject() throws BeansException {
                                                        beforePrototypeCreation(beanName);
                                                        try {
                                                                return createBean(beanName, mbd, args);
                                                        }
                                                        finally {
                                                                afterPrototypeCreation(beanName);
                                                        }
                                                }
                                        });
                                        //获取给定Bean的实例对象
                                        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;
                }
        }

        // 对创建的Bean进行类型检查,如果没有问题,就返回这个新建的Bean,这个Bean已经包含了依赖关系
        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());
                }
        }
        return (T) bean;
}

可以看出doGetBean是依赖注入的实际入口,它定义了Bean的定义模式,单列模式(Singleton)和原型模式(Prototype),而依赖注入触发的前提是BeanDefinition数据已经建立好的前提下。其实对于Ioc容器的使用,其实对于Ioc容器的使用,Spring提供了许多的参数配置,每一个参数配置实际上代表了一个Ioc实现特性,而这些特性的实现很多都需要在依赖注入的过程中或者对Bean进行生命周期管理的过程中完成。Spring Ioc容器作为一个产品,其真正的价值体现在一系列产品特征上,而这些特征都是以依赖反转模式作为核心,他们为控制反转提供了很多便利,从而实现了完整的Ioc容器。
:-: bean实例的创建和初始化:createBean

getBean()是依赖注入的起点,之后就会调用createBean(),createBean()可以生成需要的Bean以及对Bean进行初始化。但对createBean进行跟踪,发现他在AbstractBeanFactory中仅仅是声明,而具体实现是在AbstractAutowireCapableBeanFactory类里。

AbstractBeanFactory.java

protected abstract Object createBean(String beanName, RootBeanDefinition mbd, Object[] args)
			throws BeanCreationException;

AbstractAutowireCapableBeanFactory.java

protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
        if (logger.isDebugEnabled()) {
                logger.debug("Creating instance of bean '" + beanName + "'");
        }
        RootBeanDefinition mbdToUse = mbd;

        //判断创建的Bean是否需要实例化,以及这个类是否需要通过类来装载
        Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
        if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
                mbdToUse = new RootBeanDefinition(mbd);
                mbdToUse.setBeanClass(resolvedClass);
        }

        try {
                //校验和准备中的方法覆盖
                mbdToUse.prepareMethodOverrides();
        }
        catch (BeanDefinitionValidationException ex) {
                throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
                                beanName, "Validation of method overrides failed", ex);
        }

        try {
                //如果Bean配置了初始化前和后的处理器,则返回一个Bean对象
                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);
        }
        
        //创建Bean的入口
        Object beanInstance = doCreateBean(beanName, mbdToUse, args);
        if (logger.isDebugEnabled()) {
                logger.debug("Finished creating instance of bean '" + beanName + "'");
        }
        return beanInstance;
}

//Bean的真正创建位置
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
        // 这个BeanWrapper是用来封装创建出来的Bean对象的
        BeanWrapper instanceWrapper = null;
        // 如果是单态模式,就先把缓存中同名的Bean清除
        if (mbd.isSingleton()) {
                instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        //创建Bean,由createBeanInstance来完成
        if (instanceWrapper == null) {
                instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
        Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
        
        //调用PostProcessor后置处理器
        synchronized (mbd.postProcessingLock) {
                if (!mbd.postProcessed) {
                        applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
                        mbd.postProcessed = true;
                }
        }
        
        //缓存单态模式的Bean对象,以防循环引用
        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");
                }
                //匿名类,防止循环引用,可尽早的持有引用对象
                addSingletonFactory(beanName, new ObjectFactory<Object>() {
                        @Override
                        public Object getObject() throws BeansException {
                                return getEarlyBeanReference(beanName, mbd, bean);
                        }
                });
        }

        //初始化Bean的地方,依赖注入发生的地方
        //exposedObject在初始化完成后,会作为依赖注入完成后的Bean
        Object exposedObject = bean;
        try {
                //将Bean实例对象进行封装,并将Bean定义的配置属性赋值给实例对象
                populateBean(beanName, mbd, instanceWrapper);    
                if (exposedObject != null) {
                        //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);
                }
        }

        if (earlySingletonExposure) {
                //获取指定名称的单态模式对象
                Object earlySingletonReference = getSingleton(beanName, false);
                if (earlySingletonReference != null) {
                        //判断注册的实例化Bean和正在实例化的Bean是同一个
                        if (exposedObject == bean) {
                                exposedObject = earlySingletonReference;
                        }
                        //当前Bean依赖其他Bean,并且当发生循环引用时不允许创建新实例
                        else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
                                String[] dependentBeans = getDependentBeans(beanName);
                                Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
                                //获取当前Bean的所有依赖
                                for (String dependentBean : dependentBeans) {
                                        //对依赖bean进行检查
                                        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.");
                                }
                        }
                }
        }
        
        //注册完成依赖注入的Bean
        try {
                registerDisposableBeanIfNecessary(beanName, bean, mbd);
        }
        catch (BeanDefinitionValidationException ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
        }

        return exposedObject;
}

在对doCreateBean的追踪中我们发现Bean的创建方法createBeanInstance与BeanDefinition的载入与解析方法populateBean方法是最为重要的。因为控制反转原理的实现就是在这两个方法中实现的。
:-: 生成Bean中的对象:createBeanInstance

在createBeanInstance中生成了Bean所包含的Java对象,对象的生成有很多种不同的方法:工厂方法+反射,容器的autowire特性等等,这些生成方法都是由相关BeanDefinition来指定的。

AbstractAutowireCapableBeanFactory.java

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
        // 确认需要创建的Bean实例的类可以实例化
        Class<?> beanClass = resolveBeanClass(mbd, beanName);

        //这里使用工厂方法对Bean进行实例化
        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)  {
                //工厂发生进行实例化
                return instantiateUsingFactoryMethod(beanName, mbd, args);
        }

        //使用容器自动装配方法进行实例化
        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) {
                        //配置自动装配属性,使用容器自动实例化
                        return autowireConstructor(beanName, mbd, null, null);
                }
                else {
                        //无参构造方法实例化
                        return instantiateBean(beanName, mbd);
                }
        }

        //使用构造函数进行实例化
        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
        if (ctors != null ||
                        mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
                        mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
                //使用容器自动装配,调用构造方法实例化
                return autowireConstructor(beanName, mbd, ctors, args);
        }

        //使用默认构造函数对Bean进行实例化
        return instantiateBean(beanName, mbd);
}

/**
 * CGLIB是一个常用的字节码生成器的类库,他提供了一系列的API来提供生成和转换Java的字节码功能。
 * 在Spring AOP中也使用CGLIB对Java的字节码进行了增强。
 */ 
private InstantiationStrategy instantiationStrategy = new CglibSubclassingInstantiationStrategy();

protected InstantiationStrategy getInstantiationStrategy() {
        return this.instantiationStrategy;
}

/**
 * 最常见的实例化过程instantiateBean
 * 使用默认的实例化策略对Bean进行实例化,默认的实例化策略是
 * CglibSubclassingInstantiationStrategy,也就是用CGLIB来对Bean进行实例化
 */
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
        try {
                Object beanInstance;
                final BeanFactory parent = this;
                //获取系统的安全管理接口
                if (System.getSecurityManager() != null) {
                        beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
                                @Override
                                public Object run() {
                                        return getInstantiationStrategy().instantiate(mbd, beanName, parent);
                                }
                        }, getAccessControlContext());
                }
                else {
                        //实例化的对象封装
                        beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
                }
                BeanWrapper bw = new BeanWrapperImpl(beanInstance);
                initBeanWrapper(bw);
                return bw;
        }
        catch (Throwable ex) {
                throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
        }
}

我们对CglibSubclassingInstantiationStrategy进行跟踪,发现Spring中的CGLIB生成,是由SimpleInstantiationStrategy.instantiate方法来完成的,所以我们就直接看SimpleInstantiationStrategy.instantiate。

SimpleInstantiationStrategy.java

public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
        if (bd.getMethodOverrides().isEmpty()) {
                Constructor<?> constructorToUse;    //指定构造器或者生成的对象工厂方法来对Bean进行实例化
                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(
                                                        new PrivilegedExceptionAction<Constructor<?>>() {
                                                                @Override
                                                                public Constructor<?> run() throws Exception {
                                                                        return clazz.getDeclaredConstructor((Class[]) null);
                                                                }
                                                });
                                        }
                                        else {
                                                constructorToUse = clazz.getDeclaredConstructor((Class[]) null);
                                        }
                                        bd.resolvedConstructorOrFactoryMethod = constructorToUse;
                                }
                                catch (Exception ex) {
                                        throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                                }
                        }
                }
                //通过BeanUtils进行实例化,这个BeanUtils的实例化通过Constructor来完成
                return BeanUtils.instantiateClass(constructorToUse);
        }
        else {
                //使用CGLIB来实例化对象
                return instantiateWithMethodInjection(bd, beanName, owner);
        }
}

:-: 属性依赖注入实现:populateBean

Bean对象进行实例化以后。怎么把这些Bean对象之间的依赖关系处理好,以完成整个依赖注入,而这里就涉及到各种Bean对象依赖关系的处理过程了,而这些依赖关系都已经解析到了BeanDefinition。如果要仔细理解这个过程,我们必须从前面提到的populateBean方法入手。

AbstractAutowireCapableBeanFactory.java

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
        //这里取得BeanDefinition中的property值
        PropertyValues pvs = mbd.getPropertyValues();
        
        //实例对象为NULL
        if (bw == null) {
                //属性不能为空
                if (!pvs.isEmpty()) {
                        throw new BeanCreationException(
                                mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
                }
                else {
                        //实例对象为NULL,属性值也为空,不需设置,直接返回
                        return;
                }
        }

        //在设置属性之前调用Bean的PostProcessor后置处理器  
        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;
        }

        //开始进行依赖注入过程,先处理autowire的注入
        if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
                        mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
                MutablePropertyValues newPvs = new MutablePropertyValues(pvs);

                //根据Bean的名字或者类型自动装配处理
                if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
                        autowireByName(beanName, mbd, bw, newPvs);
                }
                
                //根据类型自动装配注入
                if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
                        autowireByType(beanName, mbd, bw, newPvs);
                }

                pvs = newPvs;
        }

        //检查日期是否持有用于单态模式Bean关闭时的后置处理器
        boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
        //Bean实例对象没有依赖,也没有继承
        boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
        if (hasInstAwareBpps || needsDepCheck) {
                //从实例对象中提取属性描述符
                PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                if (hasInstAwareBpps) {
                        for (BeanPostProcessor bp : getBeanPostProcessors()) {
                                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                                        InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                                        //使用BeanPostProcessor处理器处理属性值  
                                        pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                                        if (pvs == null) {
                                                return;
                                        }
                                }
                        }
                }
                if (needsDepCheck) {
                        //对要配置的属性进行依赖检查
                        checkDependencies(beanName, mbd, filteredPds, pvs);
                }
        }
        
        //对属性进行依赖注入
        applyPropertyValues(beanName, mbd, bw, pvs);
}

protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
        if (pvs == null || pvs.isEmpty()) {
                return;
        }

        //封装属性值
        MutablePropertyValues mpvs = null;
        List<PropertyValue> original;

        if (System.getSecurityManager() != null) {
                if (bw instanceof BeanWrapperImpl) {
                        //设置安全上下文,JDK安全机制  
                        ((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
                }
        }

        if (pvs instanceof MutablePropertyValues) {
                mpvs = (MutablePropertyValues) pvs;
                //属性值已经转换  
                if (mpvs.isConverted()) {
                        try {
                                //为实例化对象设置属性值  
                                bw.setPropertyValues(mpvs);
                                return;
                        }
                        catch (BeansException ex) {
                                throw new BeanCreationException(
                                        mbd.getResourceDescription(), beanName, "Error setting property values", ex);
                        }
                }
                //获取属性值对象的原始类型值  
                original = mpvs.getPropertyValueList();
        }
        else {
                original = Arrays.asList(pvs.getPropertyValues());
        }

        //获取用户自定义的类型转换  
        TypeConverter converter = getCustomTypeConverter();
        if (converter == null) {
                converter = bw;
        }
        //创建一个Bean定义属性值解析器,将Bean定义中的属性值解析为Bean实例对象  
        //的实际值  
        BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);

        //这里为解析值创建的一个副本,然后通过副本注入Bean
        List<PropertyValue> deepCopy = new ArrayList<PropertyValue>(original.size());
        boolean resolveNecessary = false;
        for (PropertyValue pv : original) {
                //属性值不需要转换 
                if (pv.isConverted()) {
                        deepCopy.add(pv);
                }
                //属性值需要转换  
                else {
                        String propertyName = pv.getName();
                        //原始的属性值,即转换之前的属性值  
                        Object originalValue = pv.getValue();
                        //转换属性值,例如将引用转换为IoC容器中实例化对象引用  
                        Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
                        //转换之后的属性值  
                        Object convertedValue = resolvedValue;
                        //属性值是否可以转换 
                        boolean convertible = bw.isWritableProperty(propertyName) &&
                                        !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
                        if (convertible) {
                                //使用用户自定义的类型转换器转换属性值  
                                convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
                        }
                        
                        //存储转换后的属性值,避免每次属性注入时的转换工作  
                        if (resolvedValue == originalValue) {
                                if (convertible) {
                                        //设置属性转换之后的值  
                                        pv.setConvertedValue(convertedValue);
                                }
                                deepCopy.add(pv);
                        }
                        //属性是可转换的,且属性原始值是字符串类型,且属性的原始类型值不是  
                        //动态生成的字符串,且属性的原始值不是集合或者数组类型  
                        else if (convertible && originalValue instanceof TypedStringValue &&
                                        !((TypedStringValue) originalValue).isDynamic() &&
                                        !(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
                                pv.setConvertedValue(convertedValue);
                                deepCopy.add(pv);
                        }
                        else {
                                resolveNecessary = true;
                                //重新封装属性的值
                                deepCopy.add(new PropertyValue(pv, convertedValue));
                        }
                }
        }
        if (mpvs != null && !resolveNecessary) {
                mpvs.setConverted();    //标记属性值已经转换过  
        }

        // 这里是依赖注入发生的地方
        try {
                bw.setPropertyValues(new MutablePropertyValues(deepCopy));
        }
        catch (BeansException ex) {
                throw new BeanCreationException(
                        mbd.getResourceDescription(), beanName, "Error setting property values", ex);
        }
}

对代码的分析,我们知道当BeanDefinition完成载入和解析后,就进入依赖注入,这里以属性注入为例。但真正依赖注入的地方却是BeanWrapper接口。我们对BeanWrapper接口进行跟踪

他在doCreateBean中最先被定义,然后进入createBeanInstance

恩,最后,进入instantiateBean

最后得到BeanWrapperImpl,就是说依赖注入是在BeanWrapperImpl里完成的。但是以相同的方法对BeanWrapperImpl进行解析,最后发现AbstractNestablePropertyAccessor.setPropertyValue方法最为重要。

而对BeanWrapper接口的跟踪,我们发现,BeanWrapper是一个组件接口,根据源码分析,其作用是封装主程序中的Bean实例。以此,我们对applyPropertyValues重新分析,发现BeanDefinitionValueResolver.resolveValueIfNecessary方法是依赖注入的另一个关键下面,我们就对其进行分析。

:-: 属性值的解析:resolveValueIfNecessary

BeanDefinitionValueResolver.java

public Object resolveValueIfNecessary(Object argName, Object value) {
        //对引用类型的属性进行解析
        if (value instanceof RuntimeBeanReference) {
                RuntimeBeanReference ref = (RuntimeBeanReference) value;
                //调用引用类型属性的解析方法
                return resolveReference(argName, ref);
        }
        //对属性值是引用容器中另一个Bean名称的解析
        else if (value instanceof RuntimeBeanNameReference) {
                String refName = ((RuntimeBeanNameReference) value).getBeanName();
                refName = String.valueOf(doEvaluate(refName));
                //从容器中获取指定名称的Bean  
                if (!this.beanFactory.containsBean(refName)) {
                        throw new BeanDefinitionStoreException(
                                        "Invalid bean name '" + refName + "' in bean reference for " + argName);
			}
                return refName;
        }
        //对Bean类型属性的解析,主要是Bean中的内部类  
        else if (value instanceof BeanDefinitionHolder) {
                BeanDefinitionHolder bdHolder = (BeanDefinitionHolder) value;
                return resolveInnerBean(argName, bdHolder.getBeanName(), bdHolder.getBeanDefinition());
        }
        else if (value instanceof BeanDefinition) {
                BeanDefinition bd = (BeanDefinition) value;
                String innerBeanName = "(inner bean)" + BeanFactoryUtils.GENERATED_BEAN_NAME_SEPARATOR +
                        ObjectUtils.getIdentityHexString(bd);
                return resolveInnerBean(argName, innerBeanName, bd);
        }
        //对集合数组类型的属性解析  
        else if (value instanceof ManagedArray) {
                ManagedArray array = (ManagedArray) value;
                Class<?> elementType = array.resolvedElementType;    //获取数组的类型  
                if (elementType == null) {
                        String elementTypeName = array.getElementTypeName();    //获取数组元素的类型  
                        if (StringUtils.hasText(elementTypeName)) {
                                try {
                                        //使用反射机制创建指定类型的对象  
                                        elementType = ClassUtils.forName(elementTypeName, this.beanFactory.getBeanClassLoader());
                                        array.resolvedElementType = elementType;
                                }
                                catch (Throwable ex) {
                                        throw new BeanCreationException(
                                                this.beanDefinition.getResourceDescription(), this.beanName,
                                                "Error resolving array type for " + argName, ex);
                                }
                        }
                        //没有获取到数组的类型,也没有获取到数组元素的类型,则直接设置数  
                        //组的类型为Object  
                        else {
                                elementType = Object.class;
                        }
                }
                //创建指定类型的数组  
                return resolveManagedArray(argName, (List<?>) value, elementType);
        }
        //解析list类型的属性值  
        else if (value instanceof ManagedList) {
                return resolveManagedList(argName, (List<?>) value);
        }
        //解析set类型的属性值  
        else if (value instanceof ManagedSet) {
                return resolveManagedSet(argName, (Set<?>) value);
        }
        //解析map类型的属性值  
        else if (value instanceof ManagedMap) {
                return resolveManagedMap(argName, (Map<?, ?>) value);
        }
        //解析props类型的属性值,props其实就是key和value均为字符串的map  
        else if (value instanceof ManagedProperties) {
                Properties original = (Properties) value;
                Properties copy = new Properties();    //创建一个拷贝,用于作为解析后的返回值  
                for (Map.Entry<Object, Object> propEntry : original.entrySet()) {
                        Object propKey = propEntry.getKey();
                        Object propValue = propEntry.getValue();
                        if (propKey instanceof TypedStringValue) {
                                propKey = evaluate((TypedStringValue) propKey);
                        }
                        if (propValue instanceof TypedStringValue) {
                                propValue = evaluate((TypedStringValue) propValue);
                        }
                        copy.put(propKey, propValue);
                }
                return copy;
        }
        //解析字符串类型的属性值  
        else if (value instanceof TypedStringValue) {
                TypedStringValue typedStringValue = (TypedStringValue) value;
                Object valueObject = evaluate(typedStringValue);
                try {
                        Class<?> resolvedTargetType = resolveTargetType(typedStringValue);     //获取属性的目标类型  
                        if (resolvedTargetType != null) {
                                return this.typeConverter.convertIfNecessary(valueObject, resolvedTargetType);
                        }
                        //没有获取到属性的目标对象,则按Object类型返回 
                        else {
                                return valueObject;
                        }
                }
                catch (Throwable ex) {
                        throw new BeanCreationException(
                                this.beanDefinition.getResourceDescription(), this.beanName,
                                "Error converting typed String value for " + argName, ex);
                }
        }
        else {
                return evaluate(value);
        }
}

private Object resolveReference(Object argName, RuntimeBeanReference ref) {
        try {
                String refName = ref.getBeanName();    //获取引用的Bean名称  
                refName = String.valueOf(doEvaluate(refName));
                
                //如果引用的对象在父类容器中,则从父类容器中获取指定的引用对象  
                if (ref.isToParent()) {
                        if (this.beanFactory.getParentBeanFactory() == null) {
                                throw new BeanCreationException(
                                        this.beanDefinition.getResourceDescription(), this.beanName,
                                        "Can't resolve reference to bean '" + refName +
                                        "' in parent factory: no parent factory available");
                        }
                        return this.beanFactory.getParentBeanFactory().getBean(refName);
                }
                //从当前的容器中获取指定的引用Bean对象,如果指定的Bean没有被实例化  
                //则会递归触发引用Bean的初始化和依赖注入  
                else {
                        Object bean = this.beanFactory.getBean(refName);
                        //将当前实例化对象的依赖引用对象  
                        this.beanFactory.registerDependentBean(refName, this.beanName);
                        return bean;
                }
        }
        catch (BeansException ex) {
                throw new BeanCreationException(
                        this.beanDefinition.getResourceDescription(), this.beanName,
                        "Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
        }
}

:-: 属性值得依赖注入:setPropertyValue

AbstractNestablePropertyAccessor的setPropertyValue有三个,但都指向最后一个:

AbstractNestablePropertyAccessor.java

protected void setPropertyValue(PropertyTokenHolder tokens, PropertyValue pv) throws BeansException {
        //PropertyTokenHolder主要保存属性的名称、路径,以及集合的size等信息  
        String propertyName = tokens.canonicalName;
        String actualName = tokens.actualName;

        //keys是用来保存集合类型属性的size  
        if (tokens.keys != null) {
                //将属性信息拷贝  
                PropertyTokenHolder getterTokens = new PropertyTokenHolder();
                getterTokens.canonicalName = tokens.canonicalName;
                getterTokens.actualName = tokens.actualName;
                getterTokens.keys = new String[tokens.keys.length - 1];
                System.arraycopy(tokens.keys, 0, getterTokens.keys, 0, tokens.keys.length - 1);
                Object propValue;
                try {
                        //获取属性值,该方法内部使用JDK的内省(Introspector)机制,调用getter(readerMethod)方法,获取属性的值
                        propValue = getPropertyValue(getterTokens);
                }
                catch (NotReadablePropertyException ex) {
                        throw new NotWritablePropertyException(getRootClass(), this.nestedPath + propertyName,
                                        "Cannot access indexed value in property referenced " +
                                        "in indexed property path '" + propertyName + "'", ex);
                }
                String key = tokens.keys[tokens.keys.length - 1];    //获取集合类型属性的长度
                if (propValue == null) {
                        if (isAutoGrowNestedPaths()) {
                                int lastKeyIndex = tokens.canonicalName.lastIndexOf('[');
                                getterTokens.canonicalName = tokens.canonicalName.substring(0, lastKeyIndex);
                                propValue = setDefaultValue(getterTokens);
                        }
                        else {
                                throw new NullValueInNestedPathException(getRootClass(), this.nestedPath + propertyName,
                                        "Cannot access indexed value in property referenced " +
                                        "in indexed property path '" + propertyName + "': returned null");
                        }
                }
                //注入array类型的属性值
                if (propValue.getClass().isArray()) {
                        PropertyHandler ph = getLocalPropertyHandler(actualName);    //获取属性的描述符  
                        Class<?> requiredType = propValue.getClass().getComponentType();    //获取数组的类型  
                        int arrayIndex = Integer.parseInt(key);    //获取数组的长度  
                        Object oldValue = null;
                        try {
                                //获取数组以前初始化的值 
                                if (isExtractOldValueForEditor() && arrayIndex < Array.getLength(propValue)) {
                                        oldValue = Array.get(propValue, arrayIndex);
                                }
                                //将属性的值赋值给数组中的元素
                                Object convertedValue = convertIfNecessary(propertyName, oldValue, pv.getValue(),
                                        requiredType, ph.nested(tokens.keys.length));
                                int length = Array.getLength(propValue);
                                if (arrayIndex >= length && arrayIndex < this.autoGrowCollectionLimit) {
                                        Class<?> componentType = propValue.getClass().getComponentType();
                                        Object newArray = Array.newInstance(componentType, arrayIndex + 1);
                                        System.arraycopy(propValue, 0, newArray, 0, length);
                                        setPropertyValue(actualName, newArray);
                                        propValue = getPropertyValue(actualName);
                                }
                                Array.set(propValue, arrayIndex, convertedValue);
                        }
                        catch (IndexOutOfBoundsException ex) {
                                throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
                                        "Invalid array index in property path '" + propertyName + "'", ex);
                        }
                }
                //注入list类型的属性值  
                else if (propValue instanceof List) {
                        PropertyHandler ph = getPropertyHandler(actualName);
                        Class<?> requiredType = ph.getCollectionType(tokens.keys.length);    //获取list集合的类型  
                        List<Object> list = (List<Object>) propValue;
                        int index = Integer.parseInt(key);    //获取list集合的size  
                        Object oldValue = null;
                        if (isExtractOldValueForEditor() && index < list.size()) {
                                oldValue = list.get(index);
                        }
                        Object convertedValue = convertIfNecessary(propertyName, oldValue, pv.getValue(),
                                        requiredType, ph.nested(tokens.keys.length));    //获取list解析后的属性值  
                        int size = list.size();    //为list属性赋值 
                        //如果list的长度大于属性值的长度,则多余的元素赋值为null  
                        if (index >= size && index < this.autoGrowCollectionLimit) {
                                for (int i = size; i < index; i++) {
                                        try {
                                                list.add(null);    
                                        }
                                        catch (NullPointerException ex) {
                                                throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
                                                                "Cannot set element with index " + index + " in List of size " +
                                                                size + ", accessed using property path '" + propertyName +
                                                                "': List does not support filling up gaps with null elements");
                                        }
                                }
                                list.add(convertedValue);
                        }
                        else {
                                try {
                                        list.set(index, convertedValue);
                                }
                                catch (IndexOutOfBoundsException ex) {
                                        throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
                                                "Invalid list index in property path '" + propertyName + "'", ex);
                                }
                        }
                }
                /注入map类型的属性值  
                else if (propValue instanceof Map) {
                        PropertyHandler ph = getLocalPropertyHandler(actualName);
                        Class<?> mapKeyType = ph.getMapKeyType(tokens.keys.length);    //获取map集合key的类型  
                        Class<?> mapValueType = ph.getMapValueType(tokens.keys.length);    //获取map集合value的类型 
                        Map<Object, Object> map = (Map<Object, Object>) propValue;
                        TypeDescriptor typeDescriptor = TypeDescriptor.valueOf(mapKeyType);
                        Object convertedMapKey = convertIfNecessary(null, null, key, mapKeyType, typeDescriptor);    //解析map类型属性key值  
                        Object oldValue = null;
                        if (isExtractOldValueForEditor()) {
                                oldValue = map.get(convertedMapKey);
                        }
                        //解析map类型属性value值
                        Object convertedMapValue = convertIfNecessary(propertyName, oldValue, pv.getValue(),
                                mapValueType, ph.nested(tokens.keys.length));
                        map.put(convertedMapKey, convertedMapValue);    //将解析后的key和value值赋值给map集合属性  
                }
                else {
                        throw new InvalidPropertyException(getRootClass(), this.nestedPath + propertyName,
                                "Property referenced in indexed property path '" + propertyName +
                                "' is neither an array nor a List nor a Map; returned value was [" + propValue + "]");
                }
        }
        //对非集合类型的属性注入  
        else {
                PropertyHandler ph = getLocalPropertyHandler(actualName);
                if (ph == null || !ph.isWritable()) {
                        if (pv.isOptional()) {
                                if (logger.isDebugEnabled()) {
                                        logger.debug("Ignoring optional value for property '" + actualName +
                                                "' - property not found on bean class [" + getRootClass().getName() + "]");
                                }
                                return;
                        }
                        else {
                                throw createNotWritablePropertyException(propertyName);
                        }
                }
                Object oldValue = null;
                try {
                        Object originalValue = pv.getValue();
                        Object valueToApply = originalValue;
                        if (!Boolean.FALSE.equals(pv.conversionNecessary)) {
                                if (pv.isConverted()) {
                                        valueToApply = pv.getConvertedValue();
                                }
                                else {
                                        if (isExtractOldValueForEditor() && ph.isReadable()) {
                                                try {
                                                        oldValue = ph.getValue();
                                                }
                                                catch (Exception ex) {
                                                        if (ex instanceof PrivilegedActionException) {
                                                                ex = ((PrivilegedActionException) ex).getException();
                                                        }
                                                        if (logger.isDebugEnabled()) {
                                                                logger.debug("Could not read previous value of property '" +
                                                                        this.nestedPath + propertyName + "'", ex);
                                                        }
                                                }
                                        }
                                        valueToApply = convertForProperty(
                                                propertyName, oldValue, originalValue, ph.toTypeDescriptor());
                                }
                                pv.getOriginalPropertyValue().conversionNecessary = (valueToApply != originalValue);
                        }
                        ph.setValue(object, valueToApply);
                }
                catch (TypeMismatchException ex) {
                        throw ex;
                }
                catch (InvocationTargetException ex) {
                        PropertyChangeEvent propertyChangeEvent =
                                new PropertyChangeEvent(this.rootObject, this.nestedPath + propertyName, oldValue, pv.getValue());
                        if (ex.getTargetException() instanceof ClassCastException) {
                                throw new TypeMismatchException(propertyChangeEvent, ph.getPropertyType(), ex.getTargetException());
                        }
                        else {
                                Throwable cause = ex.getTargetException();
                                if (cause instanceof UndeclaredThrowableException) {
                                        cause = cause.getCause();
                                }
                                throw new MethodInvocationException(propertyChangeEvent, cause);
                        }
                }
                catch (Exception ex) {
                        PropertyChangeEvent pce =
                                new PropertyChangeEvent(this.rootObject, this.nestedPath + propertyName, oldValue, pv.getValue());
                                        throw new MethodInvocationException(pce, ex);
                }
        }
}

相关文章