单例模式创建的几种方式

单例模式

饿汉式两种方式

/**
 * 单例
 * 静态常量饿汉式
 */
public class Demo1 {
    public static void main(String[] args){
        Singleton singleton2 = Singleton.GetInstance();
        Singleton singleton1 = Singleton.GetInstance();
        System.out.println(singleton1==singleton2);
    }

}
//静态常量饿汉式
class Singleton{
    private Singleton(){
    }

    private final static Singleton instance = new Singleton();

    public static Singleton  GetInstance(){
        return instance;
    }
}

/**
 * 单例
 * 静态常量饿汉式
 */
public class Demo2 {
    public static void main(String[] args){
        Singleton2 singleton2 = Singleton2.GetInstance();
        Singleton2 singleton1 = Singleton2.GetInstance();
        System.out.println(singleton1==singleton2);
    }

}

//静态常量饿汉式
class Singleton2{
    private Singleton2(){
    }

    private  static Singleton2 instance;
    static {
        instance = new Singleton2();
    }
    public static Singleton2 GetInstance(){
        return instance;
    }
}

懒汉式两种方式

/**
 * 单例双重检查
 * 线程安全懒汉式
 */
public class Demo5 {
    public static void main(String[] args) {
        Singleton5 singleton2 = Singleton5.GetInstance();
        Singleton5 singleton1 = Singleton5.GetInstance();
        System.out.println(singleton1 == singleton2);
    }
}

class Singleton5 {

    private static volatile Singleton5 instance;

    private Singleton5() {}

    public static Singleton5 GetInstance() {
        if(instance==null){
            synchronized (Singleton5.class){
                if(instance==null){
                    instance = new Singleton5();
                }
            }
        }
        return instance;
    }
}

/**
 * 单例静态内部类
 * 线程安全懒汉式
 */
public class Demo6 {
    public static void main(String[] args) {
        Singleton6 singleton2 = Singleton6.GetInstance();
        Singleton6 singleton1 = Singleton6.GetInstance();
        System.out.println(singleton1 == singleton2);
    }
}


class Singleton6 {

    private static  Singleton6 instance;

    private Singleton6() {}

    static class InnerClass{
        private static final  Singleton6 instance = new Singleton6();
    }
    public static Singleton6 GetInstance() {
        return instance = InnerClass.instance;
    }
}

使用场景

需要频繁创建与销毁的对象、工具类，如数据源，session工厂。