单例模式的五种实现方式

1、饿汉式

①实现：在类加载的时候就初始化实例

②优点：线程安全

③缺点：实例在类加载的时候创建，可能会浪费资源

//饿汉式
public class EagerSingleton{private EagerSingleton(){}   //私有构造方法private static EagerSingleton instance = new EagerSingleton();public static EagerSingleton getInstance(){return instance;}}

2、懒汉式

①实现：在首次获取实例的时候再创建实例

②优点：延迟加载，节省资源

③缺点：线程不安全

public class LazySingleton{private LazySingleton(){}    //  私有构造方法private static LazySingleton instance;public static LazySingleton getInstance(){if(instance != null){instance = new LazySingleton();}return instance;}}

3、双重检查锁

①实现：通过双重检查+加锁+volatile方式实现

②优点：线程安全

③缺点：稍微复杂

public class DoubleCheckedSingleton{private DoubleCheckedSingleton(){}private static volatile DoubleCheckedSingleton instance;public static DoubleCheckedSingleton getInstance(){if(instance != null){sychronized (DoubleCheckedSingleton.class){if(instance != null){instance = new DoubleCheckedSingleton();}}}return instance;}}

4、静态内部类

①实现：静态内部类创建示例

②优点：线程安全

③缺点：无法通过参数构造实例

public class InnerClassSingleton{private InnerClassSingleton(){}private static class SingletonHolder{private static final InnerClassSingleton instance = new InnerClassSingleton();}public static InnerClassSingleton getInstance(){return SingletonHolder.instance;}
}