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单例模式

某个类任何情况下只有一个实例，并提供一个全局访问点来获取该实例。Java6种单例模式：2种懒汉式，2种饿汉式 ，静态内部类 ，枚举类

懒汉式

synchronized+延迟加载

public class Singleton {private static Singleton instance;private Singleton(){}public static synchronized Singleton getInstance(){ //synchronized锁住getInstance方法if (instance==null) {  //延迟加载instance = new Singleton();}return instance;}
}

volatile+synchronized双检锁

public class Singleton {private volatile static Singleton instance; //volatile修饰实例private Singleton(){}public static  Singleton getInstance(){if (instance==null) {synchronized(Singleton.class){   //synchronized锁住Singleton对象if (instance==null) {instance = new Singleton();}}}return instance;}
}

饿汉式—类加载时触发实例化，避免了多线程同步问题

类加载时初始化静态实例

public class Singleton {private static Singleton instance = new Singleton(); //类加载时初始化静态实例private Singleton() {}public static Singleton getInstance() {return instance; //直接返回加载好的静态实例}
}

在静态代码块里初始化实例

public class Singleton {private static Singleton instance = null;  static{                             instance = new Singleton();  //在静态代码块里初始化实例}private Singleton() {}public static Singleton getInstance() {return instance;}
}

静态内部类方式—实现了延迟加载

调用静态内部类时，在静态内部类里初始化实例

public class Singleton {//定义静态内部类private static class InnerSingleton {private static final Singleton INSTANCE = new Singleton(); //初始化完成Singleton类静态实例}private Singleton() {}public static Singleton getInstance() {return InnerSingleton.INSTANCE; //返回静态内部类里的静态实例}
}

枚举类方式–既能避免多线程同步问题，又能防止反序列化重新创建对象

创建一个枚举，枚举成员为INSTANCE。使用枚举类.INSTANCE即可获取实例

package acc;public enum Singleton {INSTANCE; //定义1个唯一的成员INSTANCE//枚举类里的公共方法public  void  doSomething() {System.out.println("r2323t23");}
}public class TestSingleton {public static void main(String[] args) {//使用枚举类.INSTANCE即可获取实例Singleton.INSTANCE.doSomething();}
}