Spring框架中的单例模式（源码解读）

Spring框架中的单例模式（源码解读）

转载：http://www.cnblogs.com/chengxuyuanzhilu/p/6404991.html

单例模式是一种常用的软件设计模式。通过单例模式可以保证系统中一个类只有一个实例。

注：Spring源码的版本4.3.4

Spring依赖注入Bean实例默认是单例的。

Spring的依赖注入（包括lazy-init方式）都是发生在 AbstractBeanFactory 的 getBean 里。 getBean 的 doGetBean 方法调用 getSingleton 进行bean的创建。lazy-init方式(lazy-init=“true”)，在用户向容器第一次索要bean时进行调用；非lazy-init方式(lazy-init=“false”)，在容器初始化时候进行调用。

同步线程安全的单例核心代码：

/**
     * Return the (raw) singleton object registered under the given name.
     * 
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
     * @param allowEarlyReference whether early references should be created or not
     * @return the registered singleton object, or {@code null} if none found
     */
    protected Object getSingleton(String beanName, boolean allowEarlyReference) {
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
            synchronized (this.singletonObjects) {
                singletonObject = this.earlySingletonObjects.get(beanName);
                if (singletonObject == null && allowEarlyReference) {
                    ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
                    if (singletonFactory != null) {
                        singletonObject = singletonFactory.getObject();
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                    }
                }
            }
        }
        return (singletonObject != NULL_OBJECT ? singletonObject : null);
    }

从上面代码可以看到，spring依赖注入时，使用了 双重判断加锁 的单例模式。

首先从缓存singletonObjects（实际上是一个map）中获取bean实例，如果为null，对缓存singletonObjects加锁，然后再从缓存中获取bean，如果继续为null，就创建一个bean。

这样双重判断，能够避免在加锁的瞬间，有其他依赖注入引发bean实例的创建，从而造成重复创建的结果。

在这里Spring并没有使用私有构造方法来创建bean，而是通过 singletonFactory.getObject() 返回具体beanName对应的ObjectFactory来创建bean。

我们一路跟踪下去，发现实际上是调用了 AbstractAutowireCapableBeanFactory 的 doCreateBean 方法，返回了BeanWrapper包装并创建的bean实例。

（ObjectFactory主要检查是否有用户定义的BeanPostProcessor后处理内容，并在创建bean时进行处理，如果没有，就直接返回bean本身）

见如下代码：

/**
     * Actually create the specified bean. Pre-creation processing has already happened
     * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
     * 
Differentiates between default bean instantiation, use of a
     * factory method, and autowiring a constructor.
     * @param beanName the name of the bean
     * @param mbd the merged bean definition for the bean
     * @param args explicit arguments to use for constructor or factory method invocation
     * @return a new instance of the bean
     * @throws BeanCreationException if the bean could not be created
     * @see #instantiateBean
     * @see #instantiateUsingFactoryMethod
     * @see #autowireConstructor
     */
    protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
            throws BeanCreationException {

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

        // Allow post-processors to modify the merged bean definition.
        synchronized (mbd.postProcessingLock) {
            if (!mbd.postProcessed) {
                try {
                    applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
                }
                catch (Throwable ex) {
                    throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                            "Post-processing of merged bean definition failed", ex);
                }
                mbd.postProcessed = true;
            }
        }

        // Eagerly cache singletons to be able to resolve circular references
        // even when triggered by lifecycle interfaces like BeanFactoryAware.
        boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                isSingletonCurrentlyInCreation(beanName));
        if (earlySingletonExposure) {
            if (logger.isDebugEnabled()) {
                logger.debug("Eagerly caching bean '" + beanName +
                        "' to allow for resolving potential circular references");
            }
            addSingletonFactory(beanName, new ObjectFactory<Object>() {
                @Override
                public Object getObject() throws BeansException {
                    return getEarlyBeanReference(beanName, mbd, bean);
                }
            });
        }

instanceWrapper = createBeanInstance(beanName, mbd, args); 创建bean实例返回给BeanWrapper

addSingletonFactory 增加beanName和ObjectFactory的键值对应关系。

getEarlyBeanReference(beanName, mbd, bean); 获取bean的所有后处理器，并进行处理。如果是SmartInstantiationAwareBeanPostProcessor类型，就进行处理，如果没有相关处理内容，就返回默认的实现。

/**
     * Obtain a reference for early access to the specified bean,
     * typically for the purpose of resolving a circular reference.
     * @param beanName the name of the bean (for error handling purposes)
     * @param mbd the merged bean definition for the bean
     * @param bean the raw bean instance
     * @return the object to expose as bean reference
     */
    protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
        Object exposedObject = bean;
        if (bean != null && !mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            for (BeanPostProcessor bp : getBeanPostProcessors()) {
                if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
                    SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
                    exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
                    if (exposedObject == null) {
                        return null;
                    }
                }
            }
        }
        return exposedObject;
    }