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19--Spring从缓存中获取单例bean

19--Spring从缓存中获取单例bean

作者: 闲来也无事 | 来源:发表于2018-10-15 09:37 被阅读56次

    在之前的小节中已经分析了spring中的一些基本用法,xml解析,注册BeanDefinition等过程,IoC容器已经完成了初始化工作,但是bean却是我们第一次向IoC容器索取bean时才会被实例化,且根据bean的作用域如果bean的类型是单例bean的话,IoC容器会缓存该bean的实例,今天我们就来分析Spring从缓存中获取单例bean。

    引言

    我们知道对于单例bean,Spring在创建bean之后都会缓存bean的实例,以供下次使用,那么这些bean被缓存到哪里了呢?打开DefaultSingletonBeanRegistry,可以发现该类中有很多Map和Set的缓存,这些缓存都代表什么呢?看下源码:

    /** Cache of singleton objects: bean name to bean instance. */
    /** 缓存beanName和bean实例 key-->beanName,value-->beanInstance */
    private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
    
    /** Cache of singleton factories: bean name to ObjectFactory. */
    /** 缓存beanName和beanFactory key-->beanName,value-->beanFactory */
    private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
    
    /** Cache of early singleton objects: bean name to bean instance. */
    /** 缓存beanName和bean实例 key-->beanName,value-->beanInstance 该缓存主要为了解决bean的循环依赖引用 */
    private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
    
    /** Set of registered singletons, containing the bean names in registration order. */
    /** 缓存所有注册的单例beanName */
    private final Set<String> registeredSingletons = new LinkedHashSet<>(256);
    

    以上代码片段摘取了比较重要的缓存定义:

    • singletonObjects:缓存beanName和bean实例 key-->beanName,value-->beanInstance
    • singletonFactories:缓存beanName和beanFactory key-->beanName,value-->beanFactory
    • earlySingletonObjects:缓存beanName和bean实例 key-->beanName,value-->beanInstance 该缓存主要为了解决bean的循环依赖引用
    • registeredSingletons:缓存所有注册的单例beanName

    其他的对象暂时先不做介绍了,因为以上这些对象,我们将会在本小节中用到,大家先有所了解。

    1.测试用例

    在day01下的MyTest类中增加如下测试方法。该方法调用了两次getBean方法,第一次会创建新的Dog实例,而第二次则会从缓存中获取Dog实例。

    // 从缓存中获取单例bean
    @Test
    public void test16() {
        // 1、创建新的Dog实例
        Dog dog1 = xmlBeanFactory.getBean("dog1", Dog.class);
        // 2、从缓存中获取Dog实例
        dog1 = xmlBeanFactory.getBean("dog1", Dog.class);
        dog1.sayHello();
    }
    
    1.getBean方法简析

    getBean方法是IoC容器的核心方法之一,该方法的作用是从IoC容器获取bean的实例。打开该方法并跳转至doGetBean方法。

    protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType, @Nullable final Object[] args, boolean typeCheckOnly)
                throws BeansException {
    
        // 1、转换bean的名称,去掉&前缀,且如果bean有别名的话,优先使用别名
        final String beanName = transformedBeanName(name);
        Object bean;
    
        // 2、从缓存中获取bean
        Object sharedInstance = getSingleton(beanName);
        if (sharedInstance != null && args == null) {
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }
    
        // 3、如果未能从缓存中获取到bean,则要重新创建bean
        else {
            // Fail if we're already creating this bean instance: We're assumably within a circular reference.
            // 判断指定的原型模式的bean是否当前正在创建(在当前线程内),如果是->则抛出异常(Spring不会解决原型模式bean的循环依赖)
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }
    
            // Check if bean definition exists in this factory.
            // 检测bean definition是否存在beanFactory中
            BeanFactory parentBeanFactory = getParentBeanFactory();
            // 如果当前BeanFactory中不包含给定beanName的beanDefinition定义,且父beanFactory不为空,则去父beanFactory中再次查找
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // 将name转换为原始beanName
                // 因为这里的name已经经过beanName的规范处理,例如:&myBean-->规范-->myBean
                // 所以当我们再次去父beanFactory查找时,要将beanName再次转换为原始的beanName,myBean-->回转-->&myBean
                String nameToLookup = originalBeanName(name);
                // 下面会递归调用各种getBean的方法重载,从当前bean的父factoryBean中加载bean
                if (parentBeanFactory instanceof AbstractBeanFactory) {
                    return ((AbstractBeanFactory) parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);
                }
                else if (args != null) {
                    // 参数不为空,则委托parentBeanFactory使用显式参数调动
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else if (requiredType != null) {
                    // 参数为空,则委托parentBeanFactory使用标准的getBean方法获取bean
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
                else {
                    // 否则委托parentBeanFactory使用默认的getBean方法
                    return (T) parentBeanFactory.getBean(nameToLookup);
                }
            }
    
            // 如果当前bean不是用于类型检查,则将该bean标记为已经被创建或者即将被创建
            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }
    
            try {
                // 合并beanDefinition,如果指定的bean是一个子bean的话,则遍历其所有的父bean
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                // 校验合并的beanDefinition,如果验证失败,则抛出异常
                checkMergedBeanDefinition(mbd, beanName, args);
    
                // 确保初始化当前bean所依赖的bean。
                String[] dependsOn = mbd.getDependsOn();
                if (dependsOn != null) {
                    // 循环所有的依赖bean,并递归实例化
                    for (String dep : dependsOn) {
                        if (isDependent(beanName, dep)) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                        }
                        // 注册依赖
                        registerDependentBean(dep, beanName);
                        try {
                            // 实例化依赖的bean
                            getBean(dep);
                        }
                        catch (NoSuchBeanDefinitionException ex) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                        }
                    }
                }
    
                // 创建单例bean
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, () -> {
                        try {
                            // 创建bean
                            return createBean(beanName, mbd, args);
                        }
                        catch (BeansException ex) {
                            // Explicitly remove instance from singleton cache: It might have been put there
                            // eagerly by the creation process, to allow for circular reference resolution.
                            // Also remove any beans that received a temporary reference to the bean.
                            destroySingleton(beanName);
                            throw ex;
                        }
                    });
                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                }
    
                // 创建原型模式bean
                else if (mbd.isPrototype()) {
                    // It's a prototype -> create a new instance.
                    Object prototypeInstance = null;
                    try {
                        beforePrototypeCreation(beanName);
                        prototypeInstance = createBean(beanName, mbd, args);
                    }
                    finally {
                        afterPrototypeCreation(beanName);
                    }
                    bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
                }
    
                else {
                    String scopeName = mbd.getScope();
                    final Scope scope = this.scopes.get(scopeName);
                    if (scope == null) {
                        throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
                    }
                    try {
                        Object scopedInstance = scope.get(beanName, () -> {
                            beforePrototypeCreation(beanName);
                            try {
                                return createBean(beanName, mbd, args);
                            }
                            finally {
                                afterPrototypeCreation(beanName);
                            }
                        });
                        bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                    }
                    catch (IllegalStateException ex) {
                        throw new BeanCreationException(beanName,
                                "Scope '" + scopeName + "' is not active for the current thread; consider " +
                                "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
                                ex);
                    }
                }
            }
            catch (BeansException ex) {
                cleanupAfterBeanCreationFailure(beanName);
                throw ex;
            }
        }
    
        // Check if required type matches the type of the actual bean instance.
        if (requiredType != null && !requiredType.isInstance(bean)) {
            try {
                T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
                if (convertedBean == null) {
                    throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
                }
                return convertedBean;
            }
            catch (TypeMismatchException ex) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Failed to convert bean '" + name + "' to required type '" +
                            ClassUtils.getQualifiedName(requiredType) + "'", ex);
                }
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
        }
        return (T) bean;
    }
    

    该方法的处理流程分支较多,我们先来分析从缓存中获取单例bean的过程。

    2.transformedBeanName转换bean的名称分析

    该方法的作用是:转换bean的名称,去掉&前缀,且如果bean有别名的话,优先使用别名

    protected String transformedBeanName(String name) {
        // 转换并规范beanName
        return canonicalName(BeanFactoryUtils.transformedBeanName(name));
    }
    

    该方法分为两步执行,转换beanName和规范beanName。

    • 转换beanName
      该方法的作用是返回bean的真实名称,去掉FactoryBean引用前缀&符号
    /**
     * 返回bean的真实名称,去掉FactoryBean引用前缀
     * Return the actual bean name, stripping out the factory dereference
     * prefix (if any, also stripping repeated factory prefixes if found).
     * @param name the name of the bean
     * @return the transformed name
     * @see BeanFactory#FACTORY_BEAN_PREFIX
     */
    public static String transformedBeanName(String name) {
        String beanName = name;
        //beanName前缀为&,循环截取直至所有&被去掉
        while (beanName.startsWith(BeanFactory.FACTORY_BEAN_PREFIX)) {
            beanName = beanName.substring(BeanFactory.FACTORY_BEAN_PREFIX.length());
        }
        return beanName;
    }
    
    • 规范规范beanName
    /**
     * 确定原始名称,将别名解析为规范名称。
     * Determine the raw name, resolving aliases to canonical names.
     * @param name the user-specified name 用户指定的beanName
     * @return the transformed name 转换后的beanName
     */
    public String canonicalName(String name) {
        String canonicalName = name;
        // Handle aliasing...
        String resolvedName;
        do {
            //从别名缓存Map中获取对应beanName
            resolvedName = this.aliasMap.get(canonicalName);
            if (resolvedName != null) {
                canonicalName = resolvedName;
            }
        } 
        while (resolvedName != null);
        return canonicalName;
    }
    
    3.从缓存中获取单例bean
    • getSingleton
    /**
     * 返回在给定名称下注册的(原始)单例对象。
     * 检查已经实例化的单例,并允许对当前创建的单例的早期引用(解决循环引用)。
     * Return the (raw) singleton object registered under the given name.
     * <p>Checks already instantiated singletons and also allows for an early
     * reference to a currently created singleton (resolving a circular reference).
     * @param beanName the name of the bean to look for 要查找的bean的名称
     * @param allowEarlyReference whether early references should be created or not 是否应该创建早期引用
     * @return the registered singleton object, or {@code null} if none found 返回已经注册单例bean,如果未获取到则返回null
     */
    @Nullable
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        // 1、从缓存中获取bean
        Object singletonObject = this.singletonObjects.get(beanName);
        // 2、未能获取到bean,但是允许对当前创建的单例的早期引用(解决循环引用)
        // isSingletonCurrentlyInCreation-->判断指定的单例bean是否当前正在创建(Spring只解决单例bean的循环依赖问题)
        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
            synchronized (this.singletonObjects) {
                // 从earlySingletonObjects获取提前曝光的bean
                singletonObject = this.earlySingletonObjects.get(beanName);
                // 未能获取到提前曝光的bean且当前的bean允许被创建早期依赖
                if (singletonObject == null && allowEarlyReference) {
                    // 从缓存中获取BeanFactory
                    ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        // 通过getObject()方法获取bean,注意:通过此方法获取的bean不是被缓存的
                        singletonObject = singletonFactory.getObject();
                        // 将获取到的singletonObject缓存至earlySingletonObjects
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        // 从singletonFactories移除bean
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return singletonObject;
    }
    

    该过程中包含了对bean循环依赖的处理,但是无需理会该点,我们会在接下来的章节中详细介绍循环以来的处理。

    前面已经介绍过singletonObjects对象缓存beanName和bean实例 key-->beanName,value-->beanInstance,测试用例中调用了两次getBean方法,当第二次调用时我们可以看到singletonObjects已经缓存了Dog的实例。那么该方法也就能够返回Dog的实例。

    image.png
    • getObjectForBeanInstance
      无论是从缓存获取单例bean还是新创建的单例bean,在bean实例化之后都会调用getObjectForBeanInstance方法,该方法的作用就是判断到底是应该返回一个普通的bean的实例还是返回FactoryBean的实例。
    protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
        // 1、判断bean是否factoryBean
        // Don't let calling code try to dereference the factory if the bean isn't a factory.
        if (BeanFactoryUtils.isFactoryDereference(name)) {
            // 当前bean是factoryBean,且beanInstance是NullBean的实例,则返回beanInstance
            if (beanInstance instanceof NullBean) {
                return beanInstance;
            }
            // 当前bean是factoryBean,但是不是FactoryBean的实例,则抛出异常
            // 因BeanFactoryUtils.isFactoryDereference(name)-->只是从bean名称上进行了判断,我们通过getBean("&myBean")可以人为将一个非factoryBean当做factoryBean
            // 所以这里必须要判断beanInstance是否为FactoryBean的实例
            if (!(beanInstance instanceof FactoryBean)) {
                throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
            }
        }
    
        // 现在我们有了bean实例,它可能是一个普通的bean,也可能是一个FactoryBean。
        // 如果它是FactoryBean,我们使用它创建一个bean实例,除非调用者实际上需要工厂的引用。
        // Now we have the bean instance, which may be a normal bean or a FactoryBean.
        // If it's a FactoryBean, we use it to create a bean instance, unless the
        // caller actually wants a reference to the factory.
        /**
         * 下面这句话稍微有些绕,首先判断beanInstance是FactoryBean的实例,然后又加了一个非的条件,将判断结果反置
         * 再加一个或条件,判断该bean的name是否有&引用,这样一来就可以判断是返回bean的实例还是返回FactoryBean对象
         * 例1:我们通过getBean("&myBean"),假设myBean实现了BeanFactory接口,那么myBean肯定是FactoryBean的实例
         * 此时将第一个判断条件置否,再去判断bean的name是否包含了&符,如果是的话,那么就返回FactoryBean对象本身
         *
         * 例2:我们通过getBean("myBean"),假设myBean是一个普通的bean,那么它肯定不是FactoryBean的实例,
         * 那么该bean跟FactoryBean无任何关系,直接返回其实例即可
         *
         */
        if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
            return beanInstance;
        }
    
        Object object = null;
        // 如果beanDefinition为null,则尝试从缓存中获取给定的FactoryBean公开的对象
        if (mbd == null) {
            object = getCachedObjectForFactoryBean(beanName);
        }
        // 未能从缓存中获得FactoryBean公开的对象,则说明该bean是一个新创建的bean
        if (object == null) {
            // Return bean instance from factory.
            FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
            // rootBeanDefinition为null,但是在beanDefinitionMap中缓存了对应的beanName
            // Caches object obtained from FactoryBean if it is a singleton.
            if (mbd == null && containsBeanDefinition(beanName)) {
                //合并beanDefinition(包括父类bean)
                mbd = getMergedLocalBeanDefinition(beanName);
            }
            // 如果beanDefinition不为null,则要判断该beanDefinition对象是否通过合成获得,
            // 如果不是,则说明该beanDefinition不由有程序本身定义的
            boolean synthetic = (mbd != null && mbd.isSynthetic());
            // 从给定的FactoryBean中获取指定的beanName对象
            object = getObjectFromFactoryBean(factory, beanName, !synthetic);
        }
        return object;
    }
    

    从代码中可以看到当执行到

    if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
       return beanInstance;
    }
    

    便可以确定当前获取的是普通的bean实例,关于获取FactoryBean的过程,我们留在以后分析。

    至此,已经从缓存中获取到了单例bean的实例。

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