singleton

package singleton;

public class Singleton1 {
    //私有化构造方法
    private Singleton1() {
        System.out.println("this is a singleton pattern");
    }

    //通过自身创建一个实例对象
    private static Singleton1 instance = new Singleton1();

    //获取此对象的方法
    public static Singleton1 getinstance() {
        return instance;
    }
}
懒汉式：会造成多线程不安全的问题

package singleton;

//懒汉式
public class Singleton2 {
    private static Singleton2 instance;

    private Singleton2() {
    }

    ;

    public static Singleton2 getInstance() {
        if (instance == null) {
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            instance = new Singleton2();
        }
        return instance;
    }
}

package singleton;

public class Main {
    public static void main(String[] args) {
         for (int i=0;i<100;i++){
             new Thread(()->
                     System.out.println(Singleton2.getInstance().hashCode())
                     ).start();
         }
    }
}
双重检查

package singleton;

//双重检查
public class Singleton3 {
    private static Singleton3 instance;

    private Singleton3() {
    }

    ;

    public static Singleton3 getInstance() {
        //判断是否为空
        if (instance == null) {
            synchronized (Singleton3.class) {

                if (instance == null) {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    instance = new Singleton3();
                }
            }
        }
        return instance;
    }
}

package singleton;

public class Main {
    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> System.out.println(Singleton3.getInstance().hashCode())).start();
        }
    }
}


另外一种方式：

package singleton;

public class Singleton4 {
    private Singleton4(){};
    private static class instanceGet{
       private static final Singleton4 instance = new Singleton4();
    }
    public static Singleton4 getInstance(){
        return instanceGet.instance;
    }
}

最后的枚举单例：
枚举没有构造方法

package singleton;

public enum Singleton5 {
    INSTANCE;
    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(() -> System.out.println(Singleton5.INSTANCE.hashCode())).start();
        }
    }
}