单例模式，属于创建型模式的一种。在应用这个模式时，单例对象的类必须保证只有一个实例存在。
public class SingletonStatic { private static final SingletonStatic instance = new SingletonStatic(); private SingletonStatic(){ } public static SingletonStatic getSingleton(){ return instance; } }
特点：提前加载，线程安全
public class SingletonLazy { private volatile static SingletonLazy instance; private SingletonLazy(){} public static SingletonLazy getSingleton(){ if (instance == null){ synchronized (SingletonLazy.class){ if(instance == null){ //二次检查 instance = new SingletonLazy(); } } } return instance; } }
特点：懒加载，线程安全
ps: instance变量用volatile是为了杜绝 JVM 的指令重排对单例造成的影响，二次检查实现线程安全
public enum SingletonEnum { singletonFactory; private Singleton instance; private SingletonEnum(){//枚举类的构造方法在类加载时被实例化 instance = new Singleton(); } public Singleton getInstance(){ return instance; } } class Singleton{//需要获实现单例的类 public Singleton(){} }
增加一个标志变量，在构造函数中检查是否已被调用过，若已被调用过，将抛出异常，保证构造函数只被调用一次：
public class Singleton { private static Singleton instance; private static boolean isInstance = false; private Singleton() { synchronized (Singleton.class) { if (!isInstance) { isInstance = true; } else { throw new RuntimeException("单例模式受到反射攻击！已成功阻止！"); } } } public static Singleton getInstance() { if (instance == null) { instance = new Singleton(); } return instance; } }
（2）防止序列化
增加一个 readResolve 方法并返回 instance 对象。当 ObjectInputStream类反序列化时，如果对象存在 readResolve 方法，则会调用该方法返回对象。
public class Singleton implements Serializable { private static Singleton instance; private Singleton() {} public static Singleton getInstance() { if (instance == null) { instance = new Singleton(); } return instance; } private Object readResolve() { return instance; } }
参考文章：
https://juejin.im/post/6844904058583875592
https://juejin.im/post/6844903801111986190
https://blog.csdn.net/u011277123/article/details/104523638
https://blog.csdn.net/cselmu9/article/details/51366946
原文：https://www.cnblogs.com/yifanglai/p/13741532.html