使用场景:线程池对象
实现:
1、饿汉式
2、懒汉式 -> synchronized锁
3、静态内部类实现延迟初始化 -> 静态内部类特性
4、枚举 -> Effective Java
5、双重检查锁+volatile -> 指令重排序问题
6、反序列化破坏单例 -> readObject();
7、多个类加载器破坏单例 -> 类加载原理
8、Spring如何创建单例 -> AbstractBeanFactory类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;
}