【原理】單例模式

使用場景：線程池對象

實現：

1、餓漢式

2、懶漢式 -> synchronized鎖

3、靜態內部類實現延遲初始化 -> 靜態內部類特性

4、枚舉 -> Effective Java

5、雙重檢查鎖+volatile -> 指令重排序問題

6、反序列化破壞單例 ->  readObject();

7、多個類加載器破壞單例 -> 類加載原理

8、Spring如何創建單例 -> AbstractBeanFactory類getSingleton()方法

/**
 * 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
 */
@Nullable
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;
}