Singleton singleton mode

（1） The first one is

Hungry Chinese style ： After class is loaded into memory , Just instantiate a single instance ,JVM Ensure thread safety , Simple and practical , Recommended ！ The only drawback ： Whether it's used or not , Class is instantiated when it is loaded （ When not in use , Why are you loading it ）.

final Must be initialized .

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");
    }
    
}

Test code ：

Because the construction method is private Of , Only through .getInstance() Method get object .

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

（2） The second kind

It has the same meaning as the first method

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");
    }

}

Test code ： 

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

（3） The third kind of  

lazy loading It's also called the sluggard style , Although it achieves the goal of on-demand initialization , But it brings thread insecurity .

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");
    }

}

Test code ：

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） A fourth

lazy loading It's also called the sluggard style , Although it achieves the goal of on-demand initialization , But it brings thread insecurity , Can pass synchronized solve , But it also leads to a decline in efficiency .

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");
    }

}

Test code ：

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） The fifth

lazy loading It's also called the sluggard style , Although it achieves the goal of on-demand initialization , But it brings thread insecurity , Can pass synchronized solve , But it also leads to a decline in efficiency .

public class Mgr05 {
    private static Mgr05 INSTANCE;

    private Mgr05() {
    }

    public static Mgr05 getInstance() {
        if (INSTANCE == null) {
            // Trying to improve efficiency by reducing synchronous code blocks , And then it doesn't work 
            synchronized (Mgr05.class) {
  
                INSTANCE = new Mgr05();
            }
        }
        return INSTANCE;
    }

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

}

Test code ：

public class Mgr05 {
    private static Mgr05 INSTANCE;

    private Mgr05() {
    }

    public static Mgr05 getInstance() {
        if (INSTANCE == null) {
            // Trying to improve efficiency by reducing synchronous code blocks , And then it doesn't work 
            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） 6 kinds of  

lazy loading, It's also called the sluggard style , Although it achieves the goal of on-demand initialization , But it brings thread insecurity .

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

    private Mgr06() {
    }

    public static Mgr06 getInstance() {
        if (INSTANCE == null) {
            // Double check 
            synchronized (Mgr06.class) {
                if(INSTANCE == null) {
                    
                    INSTANCE = new Mgr06();
                }
            }
        }
        return INSTANCE;
    }

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

}

Test code ：

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

    private Mgr06() {
    }

    public static Mgr06 getInstance() {
        if (INSTANCE == null) {
            // Double check 
            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） Seventh kinds

Static inner class mode ,JVM A guarantee , When loading external classes, internal classes are not loaded , This allows lazy loading .

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");
    }

}

Test code ：

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

（8） Eighth  

It can not only solve the problem of thread synchronization , It also prevents deserialization （ Perfect ）.

public enum Mgr08 {

    INSTANCE;

    public void m() {}

}

Test code ：

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