设计模式---单例模式

一、五种单例模式的实现方式

1.饿汉模式

2.饿汉枚举类型

3.懒汉式

4.双检锁懒汉式

5.内部类懒汉式

二、JDK 中单例的体现

一、五种单例模式的实现方式

1.饿汉模式

public class Singleton1 implements Serializable {private Singleton1() {if (INSTANCE != null) {throw new RuntimeException("单例对象不能重复创建");}System.out.println("private Singleton1()");}private static final Singleton1 INSTANCE = new Singleton1();public static Singleton1 getInstance() {return INSTANCE;}public static void otherMethod() {System.out.println("otherMethod()");}public Object readResolve() {return INSTANCE;}
}

构造方法抛出异常是防止反射破坏单例
readResolve() 是防止反序列化破坏单例

2.饿汉枚举类型

public enum Singleton2 {INSTANCE;private Singleton2() {System.out.println("private Singleton2()");}@Overridepublic String toString() {return getClass().getName() + "@" + Integer.toHexString(hashCode());}public static Singleton2 getInstance() {return INSTANCE;}public static void otherMethod() {System.out.println("otherMethod()");}
}

枚举饿汉式能天然防止反射、反序列化破坏单例

3.懒汉式

public class Singleton3 implements Serializable {private Singleton3() {System.out.println("private Singleton3()");}private static Singleton3 INSTANCE = null;// Singleton3.classpublic static synchronized Singleton3 getInstance() {if (INSTANCE == null) {INSTANCE = new Singleton3();}return INSTANCE;}public static void otherMethod() {System.out.println("otherMethod()");}}

其实只有首次创建单例对象时才需要同步，但该代码实际上每次调用都会同步
因此有了下面的双检锁改进

4.双检锁懒汉式

public class Singleton4 implements Serializable {private Singleton4() {System.out.println("private Singleton4()");}private static volatile Singleton4 INSTANCE = null; // 可见性，有序性public static Singleton4 getInstance() {if (INSTANCE == null) {synchronized (Singleton4.class) {if (INSTANCE == null) {INSTANCE = new Singleton4();}}}return INSTANCE;}public static void otherMethod() {System.out.println("otherMethod()");}
}

为何必须加 volatile：

INSTANCE = new Singleton4() 不是原子的，分成 3 步：创建对象、调用构造、给静态变量赋值，其中后两步可能被指令重排序优化，变成先赋值、再调用构造
如果线程1 先执行了赋值，线程2 执行到第一个 INSTANCE == null 时发现 INSTANCE 已经不为 null，此时就会返回一个未完全构造的对象

5.内部类懒汉式

public class Singleton5 implements Serializable {private Singleton5() {System.out.println("private Singleton5()");}private static class Holder {static Singleton5 INSTANCE = new Singleton5();}public static Singleton5 getInstance() {return Holder.INSTANCE;}public static void otherMethod() {System.out.println("otherMethod()");}
}