0
点赞
收藏
分享

微信扫一扫

Singleton 单例模式

秀儿2020 2022-02-16 阅读 63

(1)第一种

饿汉式:类加载到内存后,就实例化一个单例,JVM保证线程安全,简单实用,推荐使用! 唯一缺点:不管用到与否,类装载时就完成实例化(不用的时候,你装载它干啥)。

final必须初始化。

public class Mgr01 {
    private static final Mgr01 INSTANCE = new Mgr01();

    private Mgr01() {};

    public static Mgr01 getInstance() {
        return INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }
    
}

测试代码:

由于构造方法是private的,只能通过.getInstance()方法获取对象。

public static void main(String[] args) {
        Mgr01 m1 = Mgr01.getInstance();
        Mgr01 m2 = Mgr01.getInstance();
        System.out.println(m1 == m2);
}

(2)第二种

和第一种方法一个意思

public class Mgr02 {
    private static final Mgr02 INSTANCE;
    static {
        INSTANCE = new Mgr02();
    }

    private Mgr02() {};

    public static Mgr02 getInstance() {
        return INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }

}

测试代码: 

public static void main(String[] args) {
        Mgr02 m1 = Mgr02.getInstance();
        Mgr02 m2 = Mgr02.getInstance();
        System.out.println(m1 == m2);
}

(3)第三种 

lazy loading也称懒汉式,虽然达到了按需初始化的目的,但却带来线程不安全的问题。

public class Mgr03 {
    private static Mgr03 INSTANCE;

    private Mgr03() {
    }

    public static Mgr03 getInstance() {
        if (INSTANCE == null) {
          
            INSTANCE = new Mgr03();
        }
        return INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }

}

测试代码:

public class Mgr03 {
    private static Mgr03 INSTANCE;

    private Mgr03() {
    }

    public static Mgr03 getInstance() {
        if (INSTANCE == null) {
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            INSTANCE = new Mgr03();
        }
        return INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }

    public static void main(String[] args) {
        for(int i=0; i<100; i++) {
            new Thread(()->
                System.out.println(Mgr03.getInstance().hashCode())
            ).start();
        }
    }
}

(4)第四种

lazy loading也称懒汉式,虽然达到了按需初始化的目的,但却带来线程不安全的问题,可以通过synchronized解决,但也带来效率下降。

public class Mgr04 {
    private static Mgr04 INSTANCE;

    private Mgr04() {
    }

    public static synchronized Mgr04 getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new Mgr04();
        }
        return INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }

}

测试代码:

public class Mgr04 {
    private static Mgr04 INSTANCE;

    private Mgr04() {
    }

    public static synchronized Mgr04 getInstance() {
        if (INSTANCE == null) {
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            INSTANCE = new Mgr04();
        }
        return INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }

    public static void main(String[] args) {
        for(int i=0; i<100; i++) {
            new Thread(()->{
                System.out.println(Mgr04.getInstance().hashCode());
            }).start();
        }
    }
}

(5)第五种

lazy loading也称懒汉式,虽然达到了按需初始化的目的,但却带来线程不安全的问题,可以通过synchronized解决,但也带来效率下降。

public class Mgr05 {
    private static Mgr05 INSTANCE;

    private Mgr05() {
    }

    public static Mgr05 getInstance() {
        if (INSTANCE == null) {
            //妄图通过减小同步代码块的方式提高效率,然后不可行
            synchronized (Mgr05.class) {
  
                INSTANCE = new Mgr05();
            }
        }
        return INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }

}

测试代码:

public class Mgr05 {
    private static Mgr05 INSTANCE;

    private Mgr05() {
    }

    public static Mgr05 getInstance() {
        if (INSTANCE == null) {
            //妄图通过减小同步代码块的方式提高效率,然后不可行
            synchronized (Mgr05.class) {
                try {
                    Thread.sleep(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                INSTANCE = new Mgr05();
            }
        }
        return INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }

    public static void main(String[] args) {
        for(int i=0; i<100; i++) {
            new Thread(()->{
                System.out.println(Mgr05.getInstance().hashCode());
            }).start();
        }
    }
}

(6)第六种 

lazy loading,也称懒汉式,虽然达到了按需初始化的目的,但却带来线程不安全的问题。

public class Mgr06 {
    private static volatile Mgr06 INSTANCE; //JIT

    private Mgr06() {
    }

    public static Mgr06 getInstance() {
        if (INSTANCE == null) {
            //双重检查
            synchronized (Mgr06.class) {
                if(INSTANCE == null) {
                    
                    INSTANCE = new Mgr06();
                }
            }
        }
        return INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }

}

测试代码:

public class Mgr06 {
    private static volatile Mgr06 INSTANCE; //JIT

    private Mgr06() {
    }

    public static Mgr06 getInstance() {
        if (INSTANCE == null) {
            //双重检查
            synchronized (Mgr06.class) {
                if(INSTANCE == null) {
                    try {
                        Thread.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    INSTANCE = new Mgr06();
                }
            }
        }
        return INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }

    public static void main(String[] args) {
        for(int i=0; i<100; i++) {
            new Thread(()->{
                System.out.println(Mgr06.getInstance().hashCode());
            }).start();
        }
    }
}

(7)第七种

静态内部类方式,JVM保证单例,加载外部类时不会加载内部类,这样可以实现懒加载。

public class Mgr07 {

    private Mgr07() {
    }

    private static class Mgr07Holder {
        private final static Mgr07 INSTANCE = new Mgr07();
    }

    public static Mgr07 getInstance() {
        return Mgr07Holder.INSTANCE;
    }

    public void m() {
        System.out.println("m");
    }

}

测试代码:

 public static void main(String[] args) {
        for(int i=0; i<100; i++) {
            new Thread(()->{
                System.out.println(Mgr07.getInstance().hashCode());
            }).start();
        }
}

(8)第八种 

不仅可以解决线程同步,还可以防止反序列化(堪称完美)。

public enum Mgr08 {

    INSTANCE;

    public void m() {}

}

测试代码:

public static void main(String[] args) {
        for(int i=0; i<100; i++) {
            new Thread(()->{
                System.out.println(Mgr08.INSTANCE.hashCode());
            }).start();
        }
}
举报

相关推荐

0 条评论