單例模式是一種常用的軟件設計模式。在它的核心結構中只包含一個被稱爲單例的特殊類。通過單例模式可以保證系統中一個類只有一個實例
注：Spring源碼的版本4.3.4
Spring依賴注入Bean實例默認是單例的，我們由此展開。
Spring的依賴注入（包括lazy-init方式）都是發生在AbstractBeanFactory的getBean裏。getBean的doGetBean方法調用getSingleton進行bean的創建。lazy-init方式，在容器初始化時候進行調用，非lazy-init方式，在用戶向容器第一次索要bean時進行調用
同步線程安全的單例核心代碼：
/**
* 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
* @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);
}從上面代碼可以看到，spring依賴注入時，使用了雙重判斷加鎖的單例模式，首先從緩存中獲取bean實例，如果爲null，對緩存map加鎖，然後再從緩存中獲取bean，如果繼續爲null，就創建一個bean。這樣雙重判斷，能夠避免在加鎖的瞬間，有其他依賴注入引發bean實例的創建，從而造成重複創建的結果。
在這裏Spring並沒有使用私有構造方法來創建bean，而是通過singletonFactory.getObject()返回具體beanName對應的ObjectFactory來創建bean。我們一路跟蹤下去，發現實際上是調用了AbstractAutowireCapableBeanFactory的doCreateBean方法，返回了BeanWrapper包裝並創建的bean實例。
（ObjectFactory主要檢查是否有用戶定義的BeanPostProcessor後處理內容，並在創建bean時進行處理，如果沒有，就直接返回bean本身）
見如下代碼：
534行創建bean實例返回給BeanWrapper
563行addSingletonFactory增加beanName和ObjectFactory的鍵值對應關係。
/**
* Actually create the specified bean. Pre-creation processing has already happened
* at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
* <p>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);
}
});
}getEarlyBeanReference獲取bean的所有後處理器，並進行處理。如果是SmartInstantiationAwareBeanPostProcessor類型，就進行處理，如果沒有相關處理內容，就返回默認的實現。
/**
* 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;
}